The Use of FAB-MS to Study Characters of Silicic Acids
and Silicates in Sodium Silicate Solution
Masa-aki MUROYA
*1, Mihoyo FUJITAKE
*2, Kazuhiko YAGUCHI
*3,
Hiroshi YAMADA
*4and Hitoshi KOSHIMIZU
*4Abstract
Fast atom bombardment mass spectrometry(FAB-MS) has been used to study the structural forms and characters of the silicic acid and metal adduct-silicic acid species in the sodium silicate solution. Many signals were observed in the range of m/z 0-1000, and those signals were analyzed by an electrochemical and colloidal point of views used 28Si. Many characteristic species, which were traced on the presences of about 60 kinds of the silicic acids and metals adduct silicic acids, were observed by this FAB-MS study. In those species, the hydrated silicic acid monomer, dimer, cyclic tetramer, Ca-adduct silicic acid monomer, and Na-adduct silicic acid trimer(Si3(OH)4O6Na4) have the character of the weak electrolyte like feature. On the other hand, the hydrated silicic acid linear-tetramers, linear- and cyclic-pentamers, linear- and cyclic-hexamers, and Na- and Ca-adducts poly-silicic acids are presented to be the oligomers which are the lyophilic colloids forming the polyhedral geometric structures. The hydrated Si3(OH)4O6Na4 silicic acid trimer played an important role as the inhibitor for the polymerization action by the self-condensation between the active orthosilicic acid monomers. The isotope effect was observed in the silicic acid dimer.
1. Introduction
The sodium silicate solution is particularly interesting, for they exist in the mixture system in NaHSiO3 and colloidal silica[1-4]. The silica particles in solution system are in the form of particles from 0.8 to 2.0nm in diameter which corresponded to monomer and oligomers, and the surface of the particles is saturated with adsorbed sodium ions[5]. These particles are in equilibrium with smaller monomeric and oligomeric silicate ions which are also combined with sodium ions[5]. The observation result of 29Si NMR revealed that the building units as polymers for liquid silicates was imagined from that three kinds of broad resonance lines and the singlet sharp resonance lines which are the end-group(E, Q1), middle-group(M, Q2), trifunctional branching(T, Q3), tetrafunctional branching(Q, Q4), and neso-group(N, Q0)[6]. From a different point of view described above, F. Kohlrausch and H. Ukihashi concluded that a sodium silicate solution is constituted of hydrated colloid which has highly viscosity and emulsion properties from the result of electric conductivity measurement[7,8]. In addition, it was reported that four kinds of hydrated Na+, H
3SiO4, H2SiO42, and OH species were contributed to electric conductions to be charge carriers[9,10], and the radii of ion sizes in these
*1Professor Emeritus of Osaka Electro-Communication University, 7-6-8 Higashi-tokiwadai, Toyono-Chyo, Osaka, Japan, 563-0103.
*2Osaka University of Pharmaceutical Sciences, 4-20-1, Nasahara, Takatsuki, Osaka, Japan, 569-1094. *3Team MIRAI in Fuji-Silysia Chemical, Ltd., 1846, kozohji Kasugai, Ichi, Japan, 487-0013.
*4Osaka Labo. Site, Fuji Chemical, Co. Ltd., 3-2-33, Higashi-noda, miyakojima, Osaka, Japan, 534-0024.
大阪電気通信大学 研究論集 （自然科学編） 第 55 号 大阪電気通信大学 研究論集 （自然科学編） 第 55 号
hydrated silicic acid anions are estimated to 3.6-4.6Å[11]. Furthermore, the presence of colloidal silicic acid in a sodium silicate solution is confirmed by method of light scattering by UV range, and then the molecular weights are estimated to be about 150-1500[12-14]. These studies are result in knowledge of primary importance to a sodium silicate solution, but further details are obscure for the properties and structural forms of the silicic acids and metals adduct silicic acid species.
In recent years, mass spectrometry(MS) was applied to the speculation of species in a solution, and then the silicate oligomers were detected in an aqueous solution[15]. FAB-MS and ESI-MS analyses revealed that the presence of a number of silicic acid anions and complex species consisting of silicic acid were found in some kind of solutions[16,17].
In this work, FAB-MS analysis was directly applied to the sodium silicate solution. The goal of this investigation was to detect smaller silicic acids, colloidal silicic oligomers as building units[6], various silicic acids, and metal adducts-silicic acid and to determine the characteristics of these species via electrochemical analyses and using a colloidal approach. Moreover, the dependency of the signals on the molar ratios is detected, and the suitable inhibition factor and/or the contribution specie for a polymerization of silicic acid are tried to be found. In addition, the detection of isotope effect in silicic acid species is also one of objects in this study[18,19].
2. Experimental procedure
Materials;
Four kinds of the sodium silicate solutions were used as samples in this experiment. The molarratios of these samples, respectively, are the values of 2.11, 3.19, 3.40, and 3.75, the chemical compositions and containing impurities of these samples were determined by chemical and ICP analysis, and these values were shown in Table 1. These samples were obtained from Fuji Chemical Co. and the samples were used in this experiment without further purification. Glycerol, which is reagent grade, was used to be the matrix for FAB-MS measurement[20,21].
Table 1. Chemical compositions and impurities contained in the sodium silicate solutions used in this experiment. Composition Impurity Concentration (%) Concentration (ppm) n*1 SiO2 Na2O H2O Ca K Fe Al Mg Ti Zr 2.11 26.03 12.83 61.24 21.2 444.9 38.68 123.5 0.44 42.42 30.03 3.18 29.25 9.49 61.26 12.64 510.8 42.17 116 0.802 49.18 29.18 3.40 25.29 7.66 67.05 10.19 105.5 36.18 123.55 6.52 39.67 8.78 3.75 25.85 7.11 67.04 0.37 128.3 15.21 72.89 2.498 34.95 19.63 *1molar ratio
Experimental procedures;
In the case of FAB-MS measurement in this experiment, the fluidity of thesample was solidified by using glycerol. Using glycerol, we achieved slight dehydration of the sample, but it was not a problem, as confirmed via infrared spectroscopy. FAB-MS experiment was carried out as follows; the sample mixture was fit on the target, the bulb was sealed off after highly evacuated bulb, and the fast atom Xe was irradiated to the sample. Xe was generated from Xe+ ions which were accelerated to 6keV. Ions evolved from the sample on target were analyzed by the spectrometer. FAB-MS was carried out by using the
apparatus of JMS-700(2)(JEOL Ltd., Tokyo, Japan) in this experiment[20,21]. Using in this apparatus, a signal was detected to be 2 times of S/N ratios, and this signal intensity corresponded to trace level. In FAB-MS observation, the signals based on negative and positive charges are observed, and then negative charge was mainly detected in this work. And the intensities of signals have been classified for convenience into six categories which are vs(very strong), s(strong), m(medium), w(weak), vw(very weak), and trace, respectively.
3. Results and Discussion
3.1 FAB-MS Spectrum of the sodium silicate solution
FAB-MS spectrum observed at 6keV for the sodium silicate solution prepared at the molar ratio of 3.40 was shown in Fig.1. Many signals appeared at various intensities in the range of m/z from 0 to 1000. The vs-signal appeared in the position of m/z 205. The signal is not assigned to the silicic acid and metals adduct silicic acid anions and is assigned to the dimerized Mg-adduct glycerol anion(C3(OH)2H5OMgOC3H5(OH)O̼).
Fig.1. Signal pattern observed in the sodium silicate solution prepared by the molar ratio at 3.40. Four kinds of the m-level signals appeared in m/z 133, 173, 225, and 229, respectively. In these signals, 173 and 133, respectively, originated from the silicic acid anion species[17], but 225 and 229 signals, respectively, are not related to the silicic acid anion species. To interpret and assign the signals, the vertical axis in Fig.1 was magnified 2.1 times to the intensity of the m/z 173 signal, which was defined as 100%. The general treatment of data was the same as that described above.
3.2 Origin and generation mechanism of signal
The sodium silicate solution can be regarded as an electrolyte solution, and then an electrochemical approach to such solution system can be considered that such a treatment is reasonable. However, the electrochemical treatment for this solution resulted in problems because of the strong and thick electrolyte solution. However, it is thought that the electrochemical and colloidal treatments are possibly brought to the interpretation of the signal feature.
In order to speculate the generation mechanism of anion from the sodium silicate solution media, a useful model was stipulated to be for example, and the illustrations for the silicic acid monomer(Si(OH)4) were shown in Fig.2. The orthsilicic acid monomer(Fig.2 (a)) which are hydrated to H2O, and such hydrated silicic acid is surrounded by cloudily ionic atmosphere(ionic atmosphere) which consists of H+, H
3O+, and hydrated Na+ marked at symbols Ɣ.In such a system, an electric attraction and repulsion forces are acted to between the hydrated tetrahedral center ion and ionic atmosphere, and the electrolyte like system is kept to be a stable electrochemical equilibrium state. In this system, a potential, ij, at the any point P is a function of only the distance, r, from central ion, as is illustrated in Fig.2(a). In this case, Poisson-Boltzmann equation[22]can be applied to the system, a potential ij at P-point existed in space at a distance r from i-specific ion was as follows;
1/r2ād/drÂ(r2dij/dr)= ȡ/İ
0İi (1)
where, ȡ, is charge density in solution. The potential, ij, at P-point;
ij=(zie/4ʌİ0İir)exp(br)§(zie/4ʌİ0İir)(zieb/4ʌİ0İi) (2)
where, zie is charge of the i-specific ion, İ0 and İi are dielectric constants in vacuum and specific dielectric
constants in media, respectively. 1/b is Debye length which corresponded to thickness of the ionic atmosphere surrounding i-ion(tetrahedral hydrated Si(OH)4). In the equation (2), the first term in right-side terms is a potential ij at P-point existed in space at a distance r from i-specific ion which is postulated ion, and the second term is a potential formed by counter ionic atmosphere surrounding postulated center ion.
Fig.2. Schematic illustration of generation mechanism in the silicic acid anions by the fast atom Xe bombardment for the silicic acid monomer(a and a’).
When the fast atom Xe is irradiated on such solution system, the hydrated center ion is set off by removing from the ionic atmosphere inside as a result of the disorder in system zone, and then such hydrated center ion is escaped to be anion(Si(OH)3O̼) to outer side of the solution system. If the center ion is the colloidal silicic acid, the ionic atmosphere corresponding to the electric double layer. On the basis of the view, the facility of anion escape is distinctly reflected to the signal intensity. For example, a postulate center ion is alkali metal such as is Na case, the ion is hydrated in silicate solution inside, and the hydration coordinate number is estimated to be about 4±1 for such a cation[23]. No signal corresponded to such a specie was detected in the
m/z 23. Probably the hydrated Na specie is very difficult for the escape from the silicate solution because of the
interaction between the hydrated Na cation and ionic atmosphere is very strong.In the other case, the intensity of m/z 173 signal, which is assigned to dimeric silicic acid anion[17], is strong compared with those of the other anion signals. It is suggested that the escape of dimer silicic acid anion is most easy compared with those of the other anions. If the facility of escape depended upon anion size, the intensity of dimer anion is expected to be weak than that of the monomer, but the intensity of dimer anion(m-level) is stronger than that of the monomer(vw-level). In this case, the anion size of hydrated dimer is probably larger than that of hydrated monomer which is estimate to about 3.6-4.6Å[11]. This is one of questions. It has probably been believed that the silicic acid monomer is completely solvated by hydration to solvent water molecule. Four kinds of dipole-dipole interactions formed between four hydroxyls of tetrahedral silicic acid monomer and four solvent water molecules, as shown in Fig.2(a). Then the ionic atmosphere is not easily allowed for the escape of hydrate monomer. On the other hand, the intensity of m/z 173 signal(dimer silicic acid anion(so called hydrate dimer)) is vs-level, and it intensity is suggested that the escape of hydrate dimer is easy compared with that of the hydrate monomer. This means that the value of second term of right side in equation (2) and the solvation affinity to water for the hydrate dimer, are small compared with these of the case in hydrate monomer.
It has become apparent that the facility of escape is generally decided by three kinds of factors which are a density of ionic atmosphere(1/b), a strength of interaction between center silicic acid and ionic atmosphere, and a solvation affinity to water of a silicic acid. One of these factors, the influence of density of ionic atmosphere on the signal intensity can be extracted from already observed data. It is thought that the ionic valency of dimer silicic acid anion probably reflected to signal intensity. In order to confirm of this matter, the relative intensity against as a function of m/z was plotted, and the result was shown by the symbolic mark ی in Fig.3. In this case, if the anions of different ionic valency are occurrence from dissociations of one by one proton which is released by the fast atom Xe bombardment, these signals corresponded to m/z 173, 172, 171, and 170, and various charging dimer anions are generally formed as follows; Si2(OH)6O ̼ Hn+disso. ĺ Si2(OH)lOmn + Hn+,
where the univalent(m/z 173) is of n=1, l=5, and m=2. In the cases of n=2, 3, 4, l=4, 3, 2, and m=3, 4, 5, these anions corresponding to those of bivalent Si2(OH)4O32(m/z 172), trivalent Si2(OH)3O43(m/z 171), and tetravalent Si2(OH)2O54(m/z 170), respectively. The intensity value of 100 at m/z 173 steeply decreased with decreasing in m/z, and such a value reached to about 15 at m/z 172, as seen in Fig.3. This phenomenon is indicated that the charge density of univalent anion(m/z 173) is less than those of bivalent(172) and other anions(171, 170).These results suggested that the interaction between the univalent anion and the ionic atmosphere is fairly weaker than those of the cases in the bivalent and other polyvalent anions. Therefore the univalent anion is easily produced by the fast atom Xe bombardment, and escape of such a uivalent anion is easily than those of the bivalent or other polyvalent anions. In addition to this fact, it is suggested that the
escape to silicic acid anion is dependent on the degrees of ionic atmosphere density(1/b) and/or electrical charging of anions species. The dependency of the anion charges on the signal intensities was also found in the monomer silicic acid anions. As can be seen from these results, the signal reflected the characteristics of the anion, which was assumed to be dependent on the composition.
Fig.3. Influence of charge density on signal intensity for the silicic acid dimer anions.
In order to see validity and reliability of the positions of m/z in signals and those intensities detected at 6keV, the experiment was carried out by 4keV, and it result was shown in symbol Ŷ of Fig.3. Where, the signal intensities at 4keV were relatively corrected to that of 6keV which was referred. The signal positions and those relative intensities observed at 4keV(Ŷ)are almost in agreement with those of 6keV(ی), and the complementarily distribution of data between 6keV and 4keV were obtained by this experiment.
3.3 Signals related to silicic acid anions
The observed signals and assignments related to the silicic acid anions are shown in Table 2.
These anion species were divided into three anion groups: a single bond
(>Si<͐), a double
bond
(O=Si<͐), and alkaline. Signal appeared at m/z 77 and 95,which were assigned to monomeric
silicic acid, specifically, meta- and ortho-silicic acid, respectively. The 77-signal appeared with a
m-level intensity, which signal originated from the
Si(OH)O2Ѹ (O=Si(OH)O) anion,and the anion was
derived from metasilicic acid.
In addition, the 77-signal also appeared by that this signal occurred via dehydration process in Si(OH)3O(m/z 95)H2O ĺ O=Si(OH)Oí(m/z 77)[24], moreover, the 77-signal is also agreement with that of the mass of Al(OH)2Oí anion. Al in such anion contain as impurity in the starting material shown in Table 1. It is not clear that the 77-signal corresponded to either O=Si(OH)Oanion or Al(OH)2Oí anion. If the m/z 77-signal originated to two kinds of anions, the matter is suggested the escape facilities of these anions are analogous to be same order. When 77-signal corresponded to the Al(OH)2O anion, the detection of such anion suggested that the Al(OH)3(gibbsite) is contained in the sample solution. Furthermore, single signal corresponded to two or above anions species will be reported together with results on the other signals which are the m/z 311, 467, 155, 233, and 389, as seen in Table 2.Table 2. Signals(m/z, intensities) corresponded silicic acid anions,and the assignments of signals.
Signal Intensity Anion specie Anion Silicic acid Overlap anion
m/z 6keV
>Si< 䈈, Si䇴O single bond only in siloxan chain
95 13.5 w Si(OH)3O䇴 monome Si(OH)4 㻌㻌㻌㻌㻌㻌㻌㻌䇷
173 100 vs Si2(OH)5O2䇴 dimer Si2(OH)6O 㻌㻌㻌㻌㻌㻌㻌㻌䇷 251 4.2 vw Si3(OH)7O3䇴 trimer Si3(OH)8O2 㻌㻌㻌㻌㻌㻌㻌㻌䇷 329 1.7 vw Si4(OH)9O4䇴 linear tetramer Si4(OH)10O3 㻌㻌㻌㻌㻌㻌㻌㻌䇷
311 17.3 w Si4(OH)7O5䇴 cyclic tetramer Si4(OH)8O4 Si3(OH)7Al(OH)O4ʷ 407 2.0 vw Si5(OH)11O5䇴 linear pentamer Si5(OH)12O4 㻌㻌㻌㻌㻌㻌㻌㻌䇷
389 5.7 vw Si5(OH)9O6䇴 cyclic pentamer Si5(OH)10O5 㻌㻌㻌㻌㻌㻌㻌㻌䇷 485 3.3 vw Si6(OH)13O6䇴 linear hexamer Si6(OH)14O5 㻌㻌㻌㻌㻌㻌㻌㻌䇷
467 2.4 vw Si6(OH)11O7䇴 cyclic hexamer Si 3+2+1 Si6(OH)12O6 Si5(OH)11Al(OH)O6䇴 O=Si< 䈈, Si=O contained double bond in siloxane chain
77 36.3 m Si(OH)O2䌦 monomer Si(OH)2O Al(OH)2Oʷ
155 12.3 w Si2(OH)3O3ʷ dimer Si2(OH)4O2 Si(OH)3Al(OH)O2ʷ 137 4.5 vw Si2(OH)O4ʷ dimer Si2(OH)2O3 䇷
233 9.3 vw Si3(OH)5O4ʷ trimer Si3(OH)6O3 Si2(OH)5Al(OH)O3ʷ 215 21.0 w Si3(OH)3O5ʷ trimer Si3(OH)4O4 䇷
197 9.4 vw Si3(OH)O6ʷ trimer Si3(OH)2O5 䇷
311 17.3 w Si4(OH)7O5ʷ linear tetramer Si4(OH)8O4 Si3(OH)7Al(OH)O4䇴 293 6.3 vw Si4(OH)5O6ʷ linear tetramer Si4(OH)6O5 䇷
275 trace Si4(OH)3O7ʷ linear tetramer Si4(OH)4O6 䇷 257 trace Si4(OH)O8ʷ linear tetramer Si4(OH)2O7 䇷
389 5.7 vw Si5(OH)9O6ʷ linear pentamer Si5(OH)10O5 Si4(OH)9Al(OH)O5ʷ 371 3.2 vw Si5(OH)7O7ʷ linear pentamer Si5(OH)8O6 䇷
353 6.8 vw Si5(OH)5O8ʷ linear pentamer Si5(OH)6O7 䇷 335 5.2 vw Si5(OH)3O9ʷ linear pentamer Si5(OH)4O8 䇷 317 7.0 vw Si5(OH)O10ʷ linear pentamer Si5(OH)2O9 䇷 449 3.4 vw Si6(OH)9O8䇴 cyclic hexamer 3x2 Si6(OH)10O7 䇷 Alkaline
30 0.9 trac Li2O䇴 ʊ ʊ 䇷
62 1.8 vw Na2Oʷ ʊ ʊ 䇷
94 2.9 vw K2O䇴 ʊ ʊ 䇷
In many signals, the interesting signals related to silicic acid anions appeared in the m/z 95(vw), 311(vw), and 329(trace), and these signals are assigned to the orthosilicic acid monomer(95,Si(OH)3Oí), cyclic tetramer(311,Si4(OH)7O5í), and linear tetramer(329,Si4(OH)9O4) anions, respectively. In these anions, the
mass of cyclic tetramer Si4(OH)7O5í anion is also in agreement with that of the linear tetramer O=Si4(OH)7O5 anion. Further, 311-signal also corresponded to Al-adduct silicic acid anion as showing in Table 2. The exact assignment for the 311-signal is undecided yet in any event. However, the detection of 311-signal suggested that these three kinds of anions are produced by the fast atom Xe bombardment. In addition to these tetramer, the m/z 329 signal also corresponded to the mass of linear silicic acid tetramer anion, Si4(OH)9O4.
Compare the two linear anions, the intensity of linear tetramer anion, Si4(OH)9O4(329 trace) is less than that of the linear tetramer anion of O=Si4(OH)7O5(311 vw). It is suggested that the escape of the O=Si4(OH)7O5 anion(311) from the sodium silicate solution is 10 times easy compared with that of the Si4(OH)9O4 anion(m/z 329), by the two kinds of reasons, the O=Si< double bond is existent in the O=Si4(OH)7O5 anion and the solvation affinity to water of O=Si4(OH)8O4 silicic acid is less than that of Si4(OH)10O3 silicic acid. In other example, the intensity of orthosilicic acid anion((Si(OH)3O-), m/z 95) is less than that of metasilicic acid anion((O=Si(OH)O-), m/z 77), therefore, the presence of O=Si< double bond will result in molecular ion and it promotes the anion escape. Fig.4 shows the comparison of anions having double bond with cyclic(symbols Ŷ) and without double bond(䕺). The intensities of cyclic anions(symbols Ŷ㻕 except for the cyclic-hexamer(467-signal), which is single bond only, were generally strong compared with those of the linear anions(symbols 䕺). On the other hand, the intensities of linear anions are smaller values, the escape of such linear anions are not easy by the reason that of the linear anions are governed by strong electrical atmospheres which are electric double layer in colloidal system. If the linear anions may have been the character of oligomers as colloidal mode, the problem will now be discussed from a slightly different point of view, that is, this difficulty makes it necessary to consideration of a colloidal stability in the linear silicic acids based on the applications of DLVO theory and of Schultze-Hardy rule to such a system[25-28]. If the linear silicic acids are present as colloidal substances, the electrical double layer probably formed on surrounding such hydrate silicic acids. Therefore, it is thought that the escape of the linear tetramer sillicic acid anions is fairly difficult than that of cyclic tetramer silicic acid anions. It is presumed that an anion surrounding electric atmosphere is the ionic atmosphere for the cyclic species and is the electric double layer as colloidal mode for the linear species. From this point of view, it was recognized that the silicic acid species, which are the monomer, dimer, and cyclic tetramer, are fitted to the characters as weak electrolyte like substances, but the linear silicic acid species, which are tetramer, pentamer, and hexamer, are probably fitted to the oligomer as colloidal mode.It is thought that these poly-siloxane structures of oligomer silicic acids probably corresponded to the building units observed 29NMR spectrum[6].
The other linear- and cyclic-silicic acid anions described above were detected to be the signals corresponded to the tetramers, pentamers, and hexamers, as seen in Table 2. And those anion signals appeared at vw-level intensities, and it is assumed that those anions are probably colloid like oligomers.
Fig.4.Intensities of cyclic and linear anions species vs m/z.
3.4 Signals of the metal adduct silicic acid anions
Many signals corresponded to Na- and Ca-adduct silicic acid anions appeared in the range of m/z 0-1000, and m/z of those signals and intensities were listed in Tables 3 and 4. The relation between the intensities of those signals and m/z was plotted, and the result was shown in Fig.5. The way in which the values of the intensity distributions for m/z in signals for the Na-adduct silicic acid anions(symbols Ŷ) differed compared with those of the cases of Ca-adduct anions(Ƈ). Very weak(vw) intensities are found on the Na-adduct silicic acid anions except for m/z 339 signal. Compare the intensities of Na-adduct monomer silicic acid anion with that of Ca-adduct monomer, the interactions between the univalent Na adduct anions(m/z 117, 139) and these ionic atmospheres are fairly strong than that of bivalent Ca adduct anion(133). In addition, the interaction between Na-adduct silicic acid and solvent water, on the basis of dipole-dipole interaction between the hydroxyl dipole in Na-adduct silicic acid and the dipole moment of solvent water molecule, are also fairly strong than that of the Ca-adduct anions, and then the escapes of both these Na-adduct anions(m/z 117, 139) are resulted in very lower level. Then further mode of the interaction will be discussed to the arranging hydroxyl number in anions. The strength of this interaction reflected to the signal intensity, the intensity of m/z 117 signal(3.4) corresponded to the Si(OH)2O2Na anions(plural hydroxyls) is less than that of 139 signal(8.1) corresponded to the Si(OH)O3Na2 anion(singular hydroxyl), therefore, the facility of anion escape is dependent on the strength of interaction between anion and solvent water. In other various Na-adduct silicic acid anions except for m/z 117 and 139, the signal intensities of the Na-adduct silicic acids, which are the tetramers, pentamers, and hexamers for linear and cyclic anions, are uniformly vw-levels. It is suggested that the Na-adduct silicic acids are probably composed of form of the poly-silicic acid or oligomers as colloidal mode. From their general results, it may be concluded that the poly-siloxane also corresponded to the building unit observed on 29NMR spectrum[6].
Table 3. Signals(m/z, intensities), and the assignments.
Signal / m/z Intensity Anion specie
Anion
S
ilicic acidNa-adduct silicic acid anion
117 3.4 vw Si(OH)2O2Na䇴 monomer Si(OH)3ONa
139 8.1 vw Si(OH)O3Na2䇴 monomer Si(OH)2O2Na2
161 trace SiO4Na3䇴 monomer Si(OH)O3Na3
195 7.6 vw Si2(OH)4O3Na䇴 dimer Si2(OH)5O2Na 217
11.4 w Si2(OH)3O4Na2䇴 dimer Si2(OH)4O3Na2 239 trace Si2(OH)2O5Na3䇴 dimer Si2(OH)3O4Na3 261 6.4 vw Si2(OH)O6Na4䇶 dimer Si2(OH)2O5Na4 273 trace Si3(OH)6O4Na䇴 trimer Si3(OH)7O3Na 317 7.0 vw Si3(OH)4O6Na3䇴 trimer Si3(OH)5O5Na3 339 41.2 m Si3(OH)3O7Na4䇴
trimer
Si3(OH)4O6Na4 361 6.8 vw Si3(OH)2O8Na5䇴 trimer Si3(OH)3O7Na5 295 9.2 䡒w Si3(OH)5O5Na2䇴trimer
Si3(OH)6O4Na2 333 5.6 vw Si4(OH)6O6Na䇴 cyclic tetramer, Si2x2 Si4(OH)7O5Na 351 4.5 vw Si4(OH)8O5Na䇴 linear tetramer Si4(OH)9O4Na 355 6.8 vw Si4(OH)5O7Na2䇴 cyclic tetramer, Si2x2 Si4(OH)6O6Na2 373 3.1 vw Si4(OH)7O6Na2䇴 linear tetramer Si4(OH)8O5Na2 377 4.0 vw Si4(OH)4O8Na3䇴 cyclic tetramer, Si2x2 Si4(OH)5O7Na3 395 4.5 vw Si4(OH)6O7Na3䇴 linear tetramer Si4(OH)7O6Na3 399 5.6 vw Si4(OH)3O9Na4䇴 cyclic tetramer, Si 2x2 Si4(OH)4O8Na4 417 6.7 vw Si4(OH)5O8Na4䇴 linear tetramer Si4(OH)6O7Na4 411 6.1 vw Si5(OH)8O7Na䇴 cyclic pentamer, Si 3+2 Si5(OH)9O6Na 429 3.6 vw Si5(OH)10O6Na䇴 linear pentamer Si5(OH)11O5Na 433 6.4 vw Si5(OH)7O8Na2䇴 cyclic pentamer, Si 3+2 S䡅5(OH)8O7Na2 451 2.6 vw Si5(OH)9O7Na2䇴 linear pentamer Si5(OH)10O6Na2 455 4.2 vw Si5(OH)6O9Na3䇴 cyclic pentamer, Si 3+2 Si5(OH)7O8Na3 471 2.1 vw Si6(OH)8O9Na䇴 cyclic hexamer(1), Si 3x2 Si6(OH)9O8Na 489 2.6 vw Si6(OH)10O8Na䇴 cyclic hexamer(2), Si 3+2+1 Si6(OH)11O7Na 507 2.8 vw Si6(OH)12O7Na䇴 linear hexamer Si6(OH)13O6NaTable 4. Signals(m/z, intensities), and the assignments.
Signal / m/z Intensity Anion specie Anion Silicic acid Overlap anion Ca-adduct silicic acid
133 92.7 vs Si(OH)O3Ca䇴 monomer Si(OH)2O2Ca 㻌㻌㻌㻌㻌㻌㻌㻌䇷
211 㻌㻌㻌㻌trace Si2(OH)3O4Caʷ dimer Si2(OH)4O3Ca ʊ
249 12.5 w Si2(OH)O6Ca2䇴 dimer Si2(OH)2O5Ca2 㻌㻌㻌㻌㻌㻌㻌㻌䇷
327 2.0 vw Si3(OH)3O7Ca2䇴 trimer Si3(OH)4O6Ca2 ʊ 289 12.8 w Si3(OH)5O5Ca䇴 trimer Si3(OH)6O4Ca, 㻌㻌㻌㻌㻌㻌㻌㻌䇷
361 6.8 vw Si3(OH)O9Ca3ʷ trimer Si3(OH)2O8Ca3 Si3(OH)2O8Na5䇴
349 trace Si4(OH)5O7Caʷ cyclic tetramer, Si 2x2 Si4(OH)6O6Ca ʊ
405 3.4 vw Si4(OH)5O8Ca2䇴 linear tetramer Si4(OH)6O7Ca2 ʊ
425 5.7 vw Si4(OH)O11Ca3䇴 cyclic tetramer, Si 2x2 Si4(OH)2O10Ca3 ʊ
443 4.0 vw Si4(OH)3O10Ca3䇴 linear tetramer Si4(OH)4O9Ca3 ʊ
481 12.0 w Si4(OH)O12Ca4䇴 linear tetramer Si4(OH)2O11Ca4 ʊ
427 2.3 vw Si5(OH)7O8Ca䇴 cyclic pentamer, Si 3+2 Si5(OH)8O7Ca ʊ
445 2.7 vw Si5(OH)9O7Ca䇴 linear pentamer Si5(OH)10O6Ca ʊ
465 3.0 vw Si5(OH)5O10Ca2䇴 cyclic pentamer, Si 3+2 Si5(OH)6O9Ca2 ʊ
483 3.2 vw Si5(OH)7O9Ca2䇴 linear pentamer Si5(OH)8O8Ca2 ʊ
503 4.9 vw Si5(OH)3O12Ca3䇴 cyclic pentamer, Si 3+2 Si5(OH)4O11Ca3 ʊ
521 5.2 vw Si5(OH)5O11Ca3䇴 linear pentamer Si5(OH)6O10Ca3 ʊ
487 2.0 vw Si6(OH)7O10Ca䇴 cyclic hexamer(1), Si 3x2 Si6(OH)8O9Ca ʊ
505 2.8 vw Si6(OH)9O9Ca䇴 cyclic hexamer(2), Si 3+2+1 Si6(OH)10O8Ca ʊ
523 7.4 vw Si6(OH)11O8Caʷ linear hexamer Si6(OH)12O7Ca ʊ
525 4.6 vw Si6(OH)5O12Ca2䇴 cyclic hexamer(1), Si 3x2 Si6(OH)6O11Ca2 ʊ
543 4.0 vw Si6(OH)7O11Ca2䇴 cyclic hexamer(2), Si 3+2+1 Si6(OH)8O10Ca2 ʊ
561 10.7 w Si6(OH)9O10Ca2䇴 linear hexamer Si6(OH)10O9Ca2 ʊ
Other metal-adduct and overlap anions
117 3.4 vw SiO4K3ʷ monomer Si(OH)O3K Si(OH)2O2Na䇴
133 92.7 vs Si(OH)2O2K䇴 monomer Si(OH)3OK Si(OH)O3Ca䇴
117 3.4 vw Si(OH)O3Mgʷ monomer Si(OH)2O2Mg Si(OH)2O2Na䇴
217 11.4 w Si2(OH)O6Mg2ʷ dimer Si2(OH)2O5Mg2 Si2(OH)3O4Na2䇴
449 3.4 vw Si3(OH)5O5Mgʷ trimer Si3(OH)6O4Mg Si6(OH)9O8䇴
417 6.7 vw Si4(OH)O12Mg4ʷ linear tetramer Si4(OH)2O11Mg4 Si4(OH)5O8Na4䇴
517 1.8 vw Si5(OH)O15Mg5䇴 㻌linear pentamer Si5(OH)2O14Mg5
77 36.3 m Al(OH)2O䇴 monomer Al(OH)3 Si(OH)O2䇴
311 17.3 w Si3(OH)8O4Alʷ trimer Si3(OH)9O3Al Si4(OH)7O5䇴
193 㻌㻌㻌㻌㻌 2.1 vw Si(OH)3OTi(OH)2O䇴 monomer Si(OH)3OTi(OH)3 ʊ
236 㻌 1.1 vw Si(OH)3OZr(OH)2O䇴 monomer Si(OH)3OZr(OH)3 ʊ
Fig.5. Comparison of the relative intensities for the Ca- and Na-adduct silicic acid anions.
The outstanding signal appears in m/z 339 which is m-level intensity, the intensity of signal is most strong within many Na-adduct anions, and the signal was assigned to the trimeric Na-adduct silicic acid anion which corresponded to Si3(OH)3O7Na4̼. Besides this trimer anion, four kinds of other trimer anions were observed in
m/z 273(trace), 295(vw), 317(vw), and 361(vw), respectively, and the assignments of these signals were shown
in Table 3. In order to see the character of m/z 339-signal, the intensity of 339-signals is plotted as a function of Na2O concentration, and it result was shown in marked at symbol 䕺 of Fig.6 together with the signal intensity of m/z 95(Ŷ) which is Si(OH)3O monomer anion. The value of signal intensity of m/z 339 is about 44 at 7.1% Na2O, it value proportionally decreased with increasing of Na2O concentration until about 9.5%, and then such a intensity decreased till 12.8%. The intensity dependency of m/z 339 on the Na2O concentrationindicated that the affinity of the hydrated Si3(OH)4O6Na4 silicic acid to the solvent water increased; in addition, the restriction power of the ion atmosphere to the center Si3(OH)4O6Na4 species alsoincreased with increase in the Na2O concentration.
In contrast to the 339-signal, the intensity of m/z 95-signal corresponded monomer silicic acid anion is very weak(vw) with the same concentration of 7.1% Na2O described above. The intensity of 95-signal proportionally increased with increasing of Na2O concentration until about 9.5% Na2O, and the intensity of 95-signal slightly increased in the region of about from 9.5 to 12.8% Na2O. In the region of Na2O concentration between about 7.1 and 9.5%, the intensity dependency of the 95-signal on the Na2O concentration are proportionally opposite direction compared with that of 339-signal intensity, and the facilities of escape in these anions are also inversely proportional to the Na2O concentration. These results suggested that the freedom of hydrated Si3(OH)4O6Na4 silicic acid trimer specie is relatively high when Na2O concentration is relatively lower state which is about 7.1-9.5 Na2O%, but the freedom was depressed by the increasing Na2O concentration at about above 9.5% Na2O. In the case of the hydrated Si(OH)4 silicic acid monomer, the freedom of this monomer was depressed by under the condition of about 7.1-9.5% Na2O, but such freedom is enhanced as a result of the freedom of hydrated Si3(OH)4O6Na4 silicic acid trimer specie depressed in the region of about 9.5 and above Na2O%. When the freedom of monomer is enhanced, the frequency of polymerization between monomers is also enhanced, and the intensity enhancement of monomer is discontinued as a result of probability occurrences in the number of monomer anions are reduced. These phenomena are commonly analogous to that of the behavior of specific conductivity investigated by H. Ukihashi[8,11], and it is thought that one of some kind of charge carriers corresponded to the hydrated silicic acid monomer. Perhaps it is thought that the Si3(OH)4O6Na4 specie detected in this experiment corresponded to the contribution specie for a control of the electrical conduction in a sodium silicate solution. From these results, the hydrated silicic acid monomer has a strongly active character, such activity is reduction by the behavior of high freedom in the Si3(OH)4O6Na4 specie, but the activity of silicic acid monomer is heightened by emancipation from reduction state by inactivation of the active Si3(OH)4O6Na4. In such Na2O higher concentration region, it is thought that the active silicic acid monomers are rapidly polymerized by the condensation reaction between active silicic acid monomers. Therefore, the hydrated Si3(OH)4O6Na4 silicic acid trimer specie appears to play an important role in the polymerization action as a inhibitor for self-condensations of the active silicic acid monomers in the sodium silicate solution inside.
The interesting signal was observed in m/z 62 position. This signal corresponded to the mass of Na2O, the intensity of the signal is trace-level in spite of high concentration of Na2O, which is one of important components in the sodium silicate solution, and such 62-signal may be assigned to Na2O̼ anion. On the other hand, the molar refractive index of a sodium silicate solution is measured by the sodium D-line(589nm), and the observation value is slightly larger than that of the theoretical value[29]. Generally, the free or isolated Na2O is not presence in a sodium silicate solution[1,29]. If such a deviation arise from the existence of the free or isolated Na2O, the 62-signal is possibly generated from Na2O specie. It seems reasonable to assume that the concentration of the free or isolated Na2O specie in the sodium silicate solution is well not defined. Furthermore, the concentration of the free or isolated Na2O cannot be estimated by FAB-MS analysis. On the species of K2O̼ and Li2O̼, these signals detected in m/z 94 and 30 as shown in Table 2, and the presences of K2O and Li2O species can be presumed in the sodium silicate solution.
On the other hand, the signal appeared in m/z 133 with vs intensity, this signal was assigned to the Ca-adduct silicic acid monomer anion corresponded to Si(OH)O3Ca[17], but the m/z of it signal is also correspondence
with the mass of monomer Si(OH)2O2K anion. In these anion species, Ca and K substances, respectively, are impurities contained in the sample. The assignment of 133-signal is overlapped to two kinds of different anions, but the detections of these signals probably suggested the presence and occurrence of these anions. Then the solvation affinity to water of the Ca-adducted silicic acid is less than that of the K-adducted silicic acid by the reason of hydration interaction between Si(OH)2O2Ca(bi-OH dipoles) and the dipole moment of solvent H2O is less than that of Si(OH)3OK(tri-OH dipoles). The restriction of Si(OH)2O2Ca by the ionic atmosphere is loose compared with that of Si(OH)3OK, and it is suggested that the case of escape from Si(OH)2O2Ca to Si(OH)O3Ca anion is easily than that of the case of Si(OH)2O2K anion. Probably, it is thought that the signal of m/z 133 is predominantly generation from mainly Si(OH)O3Ca anion. Although the 133-signal is chiefly derived from the Ca-adduct silicic acid anion, it anion is present as two types structures which are CaO2Si(OH)O and O=Si(OH)OCaO. When the O=Si(OH)OCaOH specie was ionized to O=Si(OH)OCaO + H+ by action of the fast atom Xe bombardment, the ionicity of CaO
2Si(OH)O specie is probably slightly stronger than that of O=Si(OH)OCaO specie which is molecular ion having double bond of sp2 orbital in
O=Si< of O=Si(OH)OCaO anion. Therefore, the O=Si(OH)OCaO anion is preferentially escaped to outer side to be m/z 133 signal.
One may compare the monomer with dimer for the Ca-adduct silicic acid anions(䕺), the intensity of Ca-monomer anion(m/z 133, vs) drastically decreases in about 1/7 as seen in 249-signal(dimer), as seen in Fig.5. It is thought that the differences of signal intensities between the monomer and dimer may be attributed to the differences of the ionicity and structural factor of these species. The w-level intensity observed at m/z 249 is also reproducible for other signals of the trimer(m/z 289)-, tetramer(481)-, and hexamer(561)-Ca-adduct anion species(so called group w), but the signals of trimer(327) and linear pentamer Ca-adduct silicic acid anions(445, 465, and 521) are appearance with the vw-level intensities(group vw), therefore, the escapes of group vw Ca-species are difficult than those of the group w anion species. These results suggested that the facility of escape for the linear Ca-adduct silicic acid anions are governed by the electrical double layer restriction together with the polyhedral geometric siloxane structures which are probably an asymmetrical conformation and a twist of siloxane chain. In addition, the signal intensities of the cyclic-tetramer, -pentamer and hexamer Ca-adduct silicic acids were also at vw-levels which indicated that the escape of those anions was very difficult. These results were also suggested that those cyclic Ca-adduct silicic acids and the linear Ca-adduct poly-silicic acid are probably present to be the oligomers as colloidal mode in the sodium silicate solution inside.
3.5 Dependency of the signals on molar ratios
The signals corresponded to the silicic acid anions described above 3.3 and metal adduct silicic acid anions described 3.4, respectively, were also detectable in the molar ratios of 2.11, 3.19, and 3.75 as same in the case of 3.40. The relation between relative intensities and molar ratios were examined, and then the signal intensities of those anions were almost not dependent on the molar ratios, except for the m/z 95 and 339 signals. This result suggested that the signal intensity is not dependent on the molar ratios and concentrations of SiO2 and Na2O.
3.6 Signals related to impurities and elementary substances
According to the literature[30], many impurity substances contained in a commercially sodium silicate solution, and those substances and concentrations were shown in literature column of Table 5 which was
reproduced from the literature[30].On the sodium silicate solution used in this work, seven kinds of impurity substances were detected by ICP analysis as shown in Table 1, but the presences of many kinds of impurity substances can be expected for the sample. It has been tried to search the signals corresponded to masses of the impurity species, and the results were listed in Table 5. Many signals corresponded to those impurity substances comparable to the literature[30] were detected in the sample, but the signals corresponded to N and F cannot be detected in the samples of all molar ratios. In the process of signal search, it has been found that many signals corresponded to elementary substances were found, and the result was listed in Table 6. Those elementary substances corresponded to impurities contained in the sample.
Table 5. Impurities and signals.
Impurity Intensity Substances Concentration / ppm Search m.r. 3.40*2
literature*1 m.r. 3.40*2 signal / m/z F 6.7-9.5 䇷 19 unkown Cl 130-1900 䇷 35 0.5 trace SO4 <160-1700 䇷 96 2.6 vw N 0.1-44 䇷 14 unkowen P <18 䇷 31 1.5 vw K ʗ 105.50 39 䚷䚷 0.9 trace Ca 1-76 10.19 40 3.9 vw Ba <0.2-2.8 䇷 137 4.5 vw Sr <0.2-1.5 䇷 88 unkown Mg 4-26 6.52 24 3.9 vw Mn 0.1-1.8 䇷 55 5.7 vw Hg <0.3-2.5 ppb 䇷 201 8.6 vw Bi <25 䇷 209 14.4 w Ni <0.3 䇷 59 48.6 s V 0.3ʷ0.8 䇷 51 0.8 trace Sn <60 䇷 119 9.5 w As <1 䇷 75 2.1 vw Al 50-220 123.55 27 1.2 vw Fe 36-120 36.18 56 0.4 trace Zn 0.6-2.8 䇷 65 2.4 vw Cu <0.6-1. 䇷 64 1.4 vw Se <20 䇷 79 6.0 vw Cd 10-21 ppb 䇷 112 2.7 vw Co <0.3 䇷 59 48.6 s Sb <15 䇷 122 unkown Pb 0.2-0.6 䇷 207 44.8 s Cr 0.3ʷ10 䇷 52 0.7 trace Ti 0.2-0.6 39.67 48 0.8 trace Zr 䇷 8.78 91 >100 vs *1The values were reproduced from the literature [30].
㻌㻌 *2The values in this column were found in this experiment, as is listed in Table 1. 䇷
Table 6.Signal searchs of elementary substances.
Searched Atomic Atomic Searched Relative intensity Overlap anion elementary number mass signal m.r.*1 3.40 m.r.*1 2.11-3.73 specie 1 specie 2
substances m/z S 16 32.07 32 2.5 vw 1 - 6 vw 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Sc 21 44.96 45 10.3 w unkown 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Ga 31 69.7 70 1.4 vw 5 - 7 vw 䇷 䇷 Ge 32 72.64 73 unkown 12 - 25 w - m 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Br 35 79.9 80 1.1 vw 2 vw 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Rb 37 85.47 85 2.5 vw 4 - 16 vw 䂛 m 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Y 39 88.91 89 unkown 21 - 63 m - s 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Nb 41 92.91 93 0.7 trace 3 - 6 vw 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Mo 42 95.94 96 2.6 vw 2 - 5 vw SO4 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Tc 43 99 99 8.7 vw 5 - 22 vw - m 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Ru 44 101.1 101 unkown 1 - 11 vw - w 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Rh 45 102.9 103 2.4 vw 5 - 15 vw - w 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Pd 46 106.4 106 0.7 trace 2 - unkown Fe(OH)2O䇴 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷
Ag 47 107.9 108 2.0 vw 0.4 - 4 trace-vw 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 In 49 114.8 115 0.01 trace 16 unkown - m Ti(OH)3O䇴 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷
Te 52 127.6 128 1.6 vw 1 - 5 vw 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 I 53 126.9 127 7.9 vw 27 - 29 m 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Cs 55 132.9 133 92.7 vs >100 vs Si(OH)O3Ca䇴 Si(OH)2O2K䇴
Hf 72 178.5 179 unkown 4 unkown - vw 䇷 䇷 Ta 73 180.9 181 unkown unkown Si(OH)O3Sr䇴 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷
W 74 183.8 184 unkown 2 unkown - vw 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Re 75 186.2 186 7.5 vw 6 - 9 vw 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Os 76 190.2 190 2.4 vw 5 vw 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Ir 77 192.2 192 1.5 vw 3 trace - vw 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Pt 78 195.1 195 7.6 vw 11 - 24 m Si(OH)4O3Na䇴 Fe2(OH)3O䇴 Au 79 197 197 9.4 vw 3 - 30 vw - m 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Tl 81 204.4 204 2.1 vw 2 - 4 vw 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 Pb 82 207.21 207 44.8 s >100 vs 䇷 㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌䇷 *1molar ratio
Interestingly, a few representative members of homologous series were detected by the search, the signal intensities of alkali metals(K, Rb, and Cs) and halogenous substances(Cl, Br and I), respectively, increased with increasing of the periodic order, and then the signal reflected an ionization energy of alkali metal and an electron affinity of halogen. In addition to these signals, the signals corresponded to transition metals for atomic number below 79 were also found in line spectra, but the no signals are found for corresponded to Hf, Ta, and W. Moreover, a signal search was carried out for the specific metal adduct silicic acid, and these anions were found to be Mg, Al, Ti, Fe, and Zr because these metals had a relatively high concentration in the sample used for this experiment, as shown in Table 1. These metals adduct species were detected to be the signals corresponded to Mg-, Al-, Ti-, and Zr-adduct monomer silicic acid anions, as shown in Table 4. These metals adduct species are probably formed from the active monomer silicic acid and those hydrated metals. The signal corresponded to Fe-adduct silicic acid anion was obscure, and it is suggested that the formation of it anion specie is probably doesn’t occur in the sample.
3.7 Isotope effect on signal
The isotope effect was found for the signals which corresponded to the silicon in the silicic acid anions. In Si(OH)3OSi(OH)2O- (silicic acid dimer) anion, the intensity of m/z 173(28Si) signal is vs-level, but the intensities of 174(29Si) and 175(30Si) signals were vw and trace, respectively. The differences of signal intensities in three kinds of the silicon isotopes in dimer silicic acid are approximately correspondence to natural abundance of silicon isotopes[18,19], therefore, isotope effect reflected to the signal intensity.
4. Conclusions
Many signals based on negative charges were observed in the region of m/z 0-1000 for the sodium silicate solution prepared with molar ratio at 2.11-3.75. By the analysis on the basis of an electrochemical and colloidal standpoints for those signals, the anion species related to about 60 kinds of the silicic acids and metals adduct silicic acid were found, moreover, those related to about 55 kinds of impurity substances were also found.
The hydrate silicic acid monomer, dimer, cyclic tetramer, and Na-adduct silicic acid trimer(Si3(OH)4O6Na4) have the characters to be weak electrolyte like feature. However, the hydrate silicic acids corresponded to poly-silicic acid and Ca- and Na-adduct silicic acids composed of poly-siloxanes, which include linear and cyclic tetramers, pentamers, and hexamers, with correspondence to the oligomer, which is one of the formers that have polyhedral geometric structures. It may be concluded that such oligomer is correspondence to the building unit observed 29NMR spectrum[6].
The characteristic signals were found in the m/z 95 and 339 which were assigned to the silicic acid monomer(Si(OH)3O-) and Na-adduct silicic acid trimer(Si3(OH)3O7Na4̼) anions. And the intensities of these anion signals were inversely dependent on Na2O concentration. It was suggested that the hydrate Si3(OH)4O6Na4 silicic acid trimer specie acts as the inhibitor to polymerization of the active hydrate silicic acid monomers.
Isotope effect has been detected during a study of FAB-MS for the dimer silicic acid anion, but other silicic acid anions were obscure by reason that those signal intensities are relatively very weak.
Acknowledgment
The work described herein was supported by the Fuji Chemical Co. The authors wish to acknowledge the Team 21 in Fuji Chemical Co.
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Our Surgical Navigation System based on Depth–
Depth Matching of Virtual and Real Images
Hiroshi Noborio, Katsuhiko Onishi, Masanao Koeda, Kaoru Watanabe
Department of Computer Science, Osaka Electro-Communication University, Osaka, Japan
Abstract—We propose a surgical navigation system aimed at conducting depth–depth matching (DDM) between virtual and real organ images. The depth image of virtual organs modeled using stereolithography data derived from the Z-buffer of a GPU. In contrast, the depth image of real organs is obtained through an arbitrary depth camera. Therefore, in DDM, we need only non-combinatorial L subtractions and additions between virtual and real 2D depth images with pixel number of L, which is approximately 100,000. The most popular iterative closest point (ICP) algorithm in the point cloud library consumes a considerable amount of time for checking the coincidence of two kinds of point clouds of whole organs. This could be because (1) the ICP needs combinatorial M × N calculation of the Euclidean distances of 3D cloud points (where M and N are usually near 100,000) and (2) considering that a real organ is obstructed by the patient’s body, the directions from which it is captured by a camera are restricted to the top view or near a shadowless lamp.
I. INTRODUCTION
In this research, we developed a novel liver surgical navigation system, key concept of which is the DDM of virtual and real liver images in the narrow part (Fig. 1). In surgical navigation, a liver (organ) is usually obstructed by the patient’s body, and its narrow opening is gradually changed. Therefore, in our research, we used only the narrow part as a kind of varying landmark captured from one direction, which is usually from the top view or near the shadow-less lamp. Based on the narrow view, we built a tracking system such that the virtual liver (organ) follows its real counterpart (organ). For this purpose, we propose DDM of the virtual and real depth images in the narrow part. For the virtual liver image to coincide with the real liver image, we investigated the orientation and position of the virtual liver in the six-degrees-of-freedom (DOF) space in 3D Euclidean coordination. In the search, we prepare a huge of neighbor directions for moving virtual liver to check the coincidence. Therefore, DDM should not be time consuming. The depth image of the virtual liver, modeled by STL data, was derived from the Z-buffer of a GPU. In contrast, the depth image of the real liver was captured by an arbitrary depth camera.
Figure 2. Left: DDM in 3D translation movement. Right: DDM in 3D orientation movement.
Figure 1. By minimizing the sum of square differences between real and virtual depths in all the pixels, we are seeking for overlapping position and orientation between real and virtual livers. /HIW: No-obstruction case.
Therefore, our DDM technique requires only K × L subtractions between the virtual and real depths in K × L image pixels (where K and L are selected for some depth camera (Fig. 2). Both are usually near 1000). This calculation is relatively faster than using the most popular ICP algorithm in PCL for checking the coincidence of two kinds of point clouds of whole objects> @.
The ICP is relatively time consuming because it needs M × N combinatorial calculations of 3D Euclidean distances (M and N are usually near 100,000; Fig. 3. The addition and subtraction of the 2D depths is relatively faster than the Euclidean distance calculation of 3D cloud points (Table I).
TABLE I. COMPARISON BETWEEN DDM AND ICP
DDM ICP
View area 2D part based on occlusion
3D overall area based on nonocclusion Number of
calculations Sequential at each pixel Combination of two points Calculation
method Subtraction
Multiplication for Euclidean distance Number of
cameras One Multiple
Figure 4. By using the color image, we can precisely overlap a virtual organ with its real organ by changing from green and red to blue via yellow (Source: Noborio [11] (2015)).
Figure 3. Left: Matching between two crowds based on the combinational shortest Euclidean distance calculation is very hard because the number of crowd points is too large. Right: Correspondence between two
II. DDMTECHNIQUE AND ITS APPLICATIONS
Our concept of DDM has been explained in our previous study [10]. The main benefit of DDM is to identify translational and orientational movements by using a specified organ shape. Thus, the cutting shape of an organ or its tumor and blood vessels by a scalpel can easily be achieved by using DDM.
Before using DDM, we should adjust the initialization such that virtual- and real-depth images coincide with each other by using a visual initial identification tool. By using the tool, we can precisely overlap a virtual organ with its real counterpart by watching pixel colors in the depth image (Fig. 4). For each pixel, we can identify the difference between virtual and real depths [11].
Many studies have used several kinds of steepest descendent algorithms for selecting the best neighbor position/orientation to move [12,13] (Fig. 5). We propose a steepest descendent algorithm to select neighbors, whose numbers are defined by six DOF with 1–3 neighbors and 2 positive- and negative-direction candidates or the presence of 36−1, 56−1, and 76−1 candidates around the present candidate (Fig. 6). Finally, as Six DOF consist of three
translational degrees and three rotational degrees, our algorithm is designed for selecting the best translational neighbor point from one 3D space and independently selecting the best orientation neighbor point from the other 3D space [12,13] (Fig. 7).
Simultaneously, images are selected as the minimum, median, or average values in their distribution. In addition, the number of images, M, is simultaneously changed to 10, 50, and 100, and the number of pixels, N, is selected randomly. As a result, when using the algorithm with 26 or 728 neighbors, the median-image-average-pixel type of the DDM algorithms is better than that of the others for all the combinations of M and N with respect to speed and accuracy. In particular, the combinations of (M,N) = (10,100) and (50,10) in a system with 26 and 726 neighbors, respectively, are the best for achieving the optimal accuracy [12,13] (Fig. 8).
Further, we attempted to achieve as many experimental results as possible based on the most commonly used depth cameras, which are Kinect v1 and v2. The depth sensor in Kinect v1 uses the “Light Coding” method that reads the emitted infrared (IR) patterns and obtains depth information from the pattern distortion. For this reason, the depth sensor was divided into an IR projector that emits an IR pattern (left) and an IR camera that reads the pattern (right). A color camera was mounted between the depth sensors [14].
Figure 5. Flowchart of our posituion/orientation regiastration method based on digital neigobors (Source: Noborio [10] (2014)).
Figure 6. The least descendent algorithm always selects the best neighbors of the present points (=position/orientation) by using the evaluation value. (a),(b),(c), the left panels show a 1 DOF search space with distances of 1, 2, and 3, respectively. The right panels show 6 DOF search space with distances of 1, 2,
The depth sensor of Kinect v2 employs the “time of flight” method, which obtains the depth information since the emitted IR light is reflected and returned. The depth sensor, which is not visible from the outside, is equipped with an IR camera (left) and a projector (right) that emits pulse-modulated IR light next to the color camera [15].
Presently, we are testing the performance for developing depth sensor, RealSense D435, based on depth sensor, RealSense R300, which were broken down well. The Intel RealSense Depth Camera D400 series is a stereo vision depth camera that can measure depth. Equipped with two depth sensors, an RGB sensor, and an IR projector, it operates with a USB power supply. The D435 used in this study has a global shutter and a wide viewing angle, providing high-resolution depth information when a moving object must be measured or when the device itself moves. It also minimizes blind spots and covers a larger area than the previous versions.
In a real open surgery, an organ is always obstructed by a patient’s body. Therefore, only a part of the organ can be captured by the sensor. For this reason, a real organ should be followed by its virtual organ via the part of surface. In general, when a surgeon cuts an organ, a complicated shape is achieved. With the support of the complicated concave
Figure 8. Our algorithm randomly selects a set of N number of pixels in each image and then evaluates the average, median, or minimum of difference distribution between real and virtual depths. Furthermore, we select the average, median, or minimum of evaluation values in M images. These two randomizations escape from local minima of 6D motion space in our 2D
DDM (Source: Watanabe [13] (2015)). Figure 7. The least descendent algorithm always
selects the best points neighboring the present point by using the evaluation value. This figure
shows three translational DOF and three rotational DOF search spaces, with distance of 1
(Source: Watanabe [13] (2015)).
Figure 9. Upper: (a), (b), (c) Strobe shot of actual liver surgery video. Bottom: Occlusion situation. (a) The whole experimental apparatus and (b) the figure which shows the experimental apparatus from the side. The height from the highest part of the liver to the occlusion is 0.02 m. (c) A view of the experimental apparatus
from directly above. The occlusion was made from a black plastic board cut out from a 0.1 m or 0.09 m diameter circle, and the initial position of the depth images of the incised real and virtual livers was adjusted
shape, the quality following a virtual organ with its real one increases. Therefore, even if the open part is very narrow, the following improves in our navigation system [16-18] (Fig. 9).
In our proposed system, we used the steepest descendent algorithm based on DDM change in the digitalized 6D space defined by three translational DOF and three rotational DOF. Next, in order not to enter into a local minimum, we use the simulated annealing algorithm [19].
However, recently, the digitalized 6D potential field was determined to reach the global minimum without any local minima in a wider area [20]. Owing to this global property, the steepest descendent algorithm always selects the coincidence point between real and virtual organs with respect to three-DOF position and three-DOF orientation.
Moreover, the liver is a rheology object with nonlinear viscous and elastic properties. Therefore, it is flexibly deformed and its position/orientation is quickly changed during surgery [21]. Dealing with such a rheological object is difficult, and requires the use of computer graphics in virtual reality, mixed reality, and augmented reality.
As mentioned earlier, we recently determined that the digital search function for the superposition point is globally unimodal (Fig. 10). Accordingly, we constructed an intra-operative surgery navigator that accurately superimposes the virtual and real organs not only with respect to position/posture but also its shape.
As shown in Fig. 10, the steepest descent method based on DDM is relatively stable in position/orientation identification. In our surgical navigation, the sampling time, which consists of sensing (e.g., 90 fps for RealSense D435), matching, and investing, is too small; therefore, the shape deformation is also very small. For these reasons, deformation matching according to DDM can be achieved after that. The investigation may sometimes be conducted using a multicore GPU (Fig. 11).
Finally, to design an organ surgical navigation system, we calibrated the virtual and real livers as well as the virtual and real Cavitron ultrasonic surgical aspirator (CUSA) scalpels (Fig. 12). In the first stage, we used MicronTracker 3 provided by ClaroNav Co. to identify several special artificial markers [22-24]. However, as the marker tracing vision system is extremely expensive, in the second stage of our experiments, we used the ArUco Markers instead [25,26]. V.CONCLUSIONS
To overlap many point clouds captured from many cameras, researchers used ICP of the PCM. However, as the number of cloud points is extensive, combinatorial calculation was employed to minimize the sum of Euclidean distances between two cloud points. In addition, a target object, such as an organ, cannot be omnidirectionally captured from multiple cameras during a surgery. Therefore, in 2014, the DDM approach was proposed to match a real organ with its virtual organ. This approach is based on one view and does not have any combination and multiplication calculation. In this paper, we explained many algorithms and experimental extensions of the DDM approach. Finally, we briefly introduce our DDM-based surgical navigation system.
Figure 10. Digital potential field defined by (a) XY rotational DOF, (b) XZ rotational DOF, and (c) YZ rotational DOF. All field shapes are simply concave whose bottom is the coincident point,
where the real organ overlaps its virtual counterpart (Source: Numata [20] (2019)).
Figure 11. Organ deformation matching by DDM after organ position/orientation matching was
ACKNOWLEDGMENT
This study was partly supported by 2014 in-Aid for Scientific Research (B) (No. 26289069) and 2017 Grants-in-Aid for Scientific Research (C) (No. 17K00420) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Further support was provided by the 2014 Cooperation Research Fund from the Graduate School at Osaka Electro-Communication University. Finally, we would like to thank Editage (www.editage.com) for English language editing.
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Figure 12. Overall surgical navigation system with a scraper, which is calibrated by many precise artificial landmarks captured by Micron Tracker 3 (Source: Doi [22] (2015)).
![Figure 5. Flowchart of our posituion/orientation regiastration method based on digital neigobors (Source: Noborio [10] (2014))](https://thumb-ap.123doks.com/thumbv2/123deta/7853133.1235065/22.892.285.595.126.332/figure-flowchart-posituion-orientation-regiastration-digital-neigobors-noborio.webp)
