7308
J. Phys. Chem. 1992, 96, 7308-7315
Mechanism of Hydration of Sodium Silicate Glass In a Steam Environment: ?gSiNMR and ab Initio Molecular Orbital Studies Takashi Uchino,* Tetsuo Sakka, Yukio Ogata, and Matae Iwasaki Institute of Atomic Energy, Kyoto University, Uji, Kyoto-Fu 611, Japan (Received: March 20, 1992)
?Si cross polarization magic angle spinning (CP/MAS) nuclear magnetic resonance spectra of hydrated 33.3Na20.66.7Si02 (mol %) glasses with up to 28.2 wt % water have been measured. We have found that the observed spectra consist of four components. These components are assigned to Si04tetrahedra (1) with three bridging oxygens and one hydroxyl group (chemical shift 6 = -96 ppm), (2) with three bridging oxygens and one nonbridging oxygen (6 = -89 ppm), (3) with two bridging oxygens, one hydroxyl group, and one nonbridging oxygen (6 = -82 ppm), and (4) with one bridging oxygen, two hydroxyl groups, and one nonbridging oxygen (6 = -76 ppm). It was found that the chemical shift and the line widths for all the 2%i signals generally become more negative and narrower, respectively, as the concentration of water increases. We have also carried out ab initio molecular orbital calculations on clusters modeling four types of interaction between water and the glass network at the Hartree-Fock STO-3G level: interactions between molecular water and sodium ion, molecular water and nonbridging oxygen, molecular water and bridging oxygen, and Na(0H) and silanol. The calculations revealed that the interaction between =Si--OH and Na(0H) has the largest binding energy, indicating that the Na(0H) complexes exist stably in the glass network. From these observed and calculated results, we present a possible mechanism of the reaction between water and sodium silicate glasses.
1. Introduction
Conventionally melted silicate glasses generally contain 0.014.1 wt % water.' It is known that even such a small amount of water does affect various properties of glass such as viscosity, thermal expansion, chemical resistance, and mechanical and electrical For example, an increase in water content increases the expansion coefficient and decreases the viscosity and the chemical durability. Atmospheric water also plays an important role in the crack initiation of SiOz glass.' On the other hand, the concentration of water in glass must be as low as possible to produce a good quality optical fiber because attenuation loss in an optical fiber originates from overtone absorption of dissolved water.* Moreover, it has newly been found that a large amount of water (a few to over 30 wt %) can be incorporated into the glass structure by hydrating glasses in an autoclave under a steam atmosphere?JO By a careful choice of both composition and autoclave parameters (temperature and percent saturation), glasses with a wide range of water content can be made. Such "hydrated glasses" exhibit plasticity even at room temperature and are characterized by interesting mechanical and electrical properties. Outside the area of glass science and technology, geologists have also studied the reactions between silicate glasses and water since water is one of the important volatiles in igneous processes." Thus water in glasses has been examined extensively by various authors in different fields of science, so the structural environment of dissolved water and its effect upon the glass structure have received considerable attention. Recent infrared and 'H NMR s t u d i e ~ ~ * ' ~have J ~ -provided '~ useful information about the state of water in glass: (1) water in silicate glasses is present as both hydroxyl groups (Si-OH) and water molecules (H20); (2) the concentration of hydroxyl groups levels off at higher (>3 wt %) total water content; (3) the proportion of molecular water, therefore, increases with increasing total water content greater than 3 wt 8. This indicates that at high total water content (>3 wt %) water infiltrates into glasses predominantly in the form of H 2 0 . In spite of this extensive research, the present knowledge about the mechanism of the water attack to the glass structure and about the bonding configurations of dissolved water is still vague. This kind of knowledge, however, will be indispensable to a better understanding of the effect of dissolved water upon the glass structure. Hence, in this work, we employ the following two methods to investigate the unsolved problems mentioned above: 29Sicross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy and ab initio molecular orbital (MO) calculations. 29SiNMR has been newly and extensively used to analyze the local structure of silicate g l a ~ s e s . ' ~ -It' ~enables determination 0022-3654/92/2096-7308$03.00/0
of the Q(X) distribution for glasses with nonbridging oxygens (in the Q(X) nomenclature Q means silicon in Cfold coordination with oxygen, n and 4 - n indicate the number of bridging and nonbridging oxygens, respectively, and X is the atom attached to the nonbridging oxygen). Moreover, the combination of CP and MAS provides 29SiNMR spectra with a high degree of structural resolution for the samples with protons such as silica gels.2*22 We hence apply % CP/MAS NMR to sodium silicate glasses (33.3 mol % Na20) with various water contents and establish significant changes in the 29SiNMR spectra with hydration. Ab initio MO calculations are also useful to investigate the local scale structure of the glass network. In previous we have demonstrated that the geometric and electronic structure of alkali silicate gbw are satisfactory reproduced by the MO calculations on small clusters such as HbSi2O7MX(x = 0, 1,2,4; M = Li, Na, K). This result supports the well-accepted assertion that the properties of silicates are governed by the short range structure of the Si-0-Si n e t w ~ r k . ~In~ ,this ~ ~ work, we calculate the equilibrium geometries of the clusters H$i2@Na2~nHzO modeling hydrated sodium silicate glasses and obtain interaction energies between water and the glass network. The calculated results are used to interpret the changes in 29SiNMR spectra with water content. On the basis of these observed and calculated mults,we preaent a possible reaction mechanism of water with sodium silicate glasses in a steam atmosphere. 2. Experimental and Calculational Procedure 2.1. Sample Preparation. The glass used in this study had the composition (mol %) of 33.3NaZ0*66.7SiO2. Samples were
prepared by melting reagent-grade Na2C03and Si02 powders in a Pt crucible at 1500 O C for about 3 h. The molten glass was annealed at 480 O C for several hours before being cooled at room temperature. The dry glasses were ground to coarse powders and hydrated in an autoclave. Hydration was camed out in saturated steam at 120, 130, and 150 OC, and thereby glasses containing 28.2, 22.0, and 15.2 wt % water, respectively, were obtained. During the hydration process water infiltrated uniformly through the glass particles to form a lump of homogeneous hydrated glass. To obtain lower-water-content glasses, the hydrated glasses were partially dehydrated by heating at 180 and 300 OC in an electric furnace, which resulted in glasses containing 11.4 and 3.0 wt % water, respectively. The water content was determined by the weight loss after heating the samples at 400-500 OC. 2.2. 29si CP/MAS NMR Experbeds. For the measurement of 29Si NMR spectra the hydrated glasses were ground into Q 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 7309
Hydration of Sodium Silicate Glass
(a) complex I
Figure 1. Conformation of the cluster HSi2O7Na2. Optimized bond lengths (in angstroms) and angles (in degrees) are also shown.
powders. 29SiCP/MAS NMR spectra of the powdered samples were recorded on a JEOL JNM-X400 NMR spectrometer operating at 79.3 MHz by using contact time of 50 ps-10 ms and a pulse repetition time of 10 s. Magic-angle spinning was routinely carried out at -6.0 kHz with a zirconia rotor. The 29Sichemical shifts reported in this study were referenced to the chemical shift of poly(dimethylsi1ane) (PDMS); the 29Sichemical shift 6 of PDMS was taken to equal -34.0 ppm. The effect of water absorption from the air atmosphere upon the hydrated samples may be negligible because an apparent weight gain of the samples was not observed after the measurement of the spectra. In Si-H cross-polarization experiments the efficiency of the CP process is determined by the relative values of several NMR relaxation times and the experimental contact time, tc.26*27This is because the 29SiCP NMR signal is generated by using the energy and relaxation properties of protons under the fulfillment of the wcalled Hartmann-Hahn condition.26 Hence, the CP rate varies depending on the number of attached or nearby protons as well as molecular motion. When the Hartmann-Hahn condition is satisfied, the silicon magnetization, M(tc),is given by2* M(tc) =
exp(-rc/ TlpH) - exp(-tc/ TSiH + tc/TlpSi’) TSiH) 1/ TsiH 1/ Tl,sit - 1/ TlrH (1)
+
where Mois the maximum silicon magnetization available in a matched spin-lock experiment with no dissipative process, TlpH is the proton rotating-frame spin-lattice relaxation time constant, TsiHis the proton-silicon matched spin-lock cross-polarization time constant, and TIPSit is the silicon rotating-frame spin-lattice relaxation time constant in the presence of dipolar decoupling of the protons. Since TlpSil>> Ts~H,M(tc) is approximately given by exp(%/ TlpH) - exp(-tc/ TSiH) M(tc) ( M O / %H) TsiH- T , ~ ~ (2) or M(rc)
-
(b) compbx II
e 0 987
w$w
(C) complex 111 H
1476-
1
em,
u
# 1678
h
1404
A
u
Ow-
Na Figure 2. Optimized geometries of the complexes studied: (a) interaction between molecular water and sodium ions, complex I; (b) interaction between molecular water and nonbridging oxygens, complex 11; (c) interaction between N a ( 0 H ) and Si-OH, complex 111. Angles are in degrees and lengths in angstroms. complex IV
0
M*(exp(-tc/TIpH) - exp(-tc/TSiH)j
(3)
where (4)
Thus,the values of TlpHand Tsm are obtained from a plot of M(tc) vs tc. To acquire these relaxation parameters, we have carried out variablecontact-time experiments for the samples containing 15.2 and 28.2 wt % water. 2.3. MO Cdculatiom. We have previously shown that the optimized geometries of H&i207Na, (x = 1,2,4) at the minimal STO-3G level satisfactorily reproduce the observed bond lengths and angles in sodium silicate glasses with 20, 33.3, and 50 mol % Na20, respectively.238 We hence use the H4Si207Na2cluster (see Figure 1) as a model for dry 33.3Na20.66.7Si02 glass. In the cluster, the dangling bonds of “surface oxygen” atoms are terminated by hydrogen atoms; this is a common way to eliminate the surface unsaturated bonds of small The hydrogen atoms are thus used as pseudoatoms to replace the solid with a finite cluster model. The structure of water-containing glass was estimated by interacting H 2 0 with H4Si207Na2cluster as shown in Figures 2 and 3. Our primary purpose is not to obtain quantitative results on these simplified models but to see some tendencies. We,
Figure 3. Optimized geometries of the complex modeling the interaction between molecular water and bridging oxygen in hydrated sodium silicate glass, complex IV. Angles are in degrees and lengths in angstroms.
therefore, employed the minimal STO-3G basis set3I for the geometry optimization. All ab initio MO calculations were cam4 out by using the GAuSSIANEZ computer program32at the Hartree-Fock (HF) level, and the geometries of the clusters were optimized by means of the gradient method.33 The HF/STO-3G geometries were then used for single point calculations with the split-valence 3-21G basis set augmented by polarization d functionS on Si,)4 which will be called the 3-21G+d(Si) basis set from now on. It has been well established that such Si d functions are necessary to obtain realistic charge distribution in the clusters containing Si-0 bond~.~~JO The molecular charge distributions were obtained from a Mulliken population analysis.3s 3. Results 3.1. NMR Measurements. 3.1.1. General spectrpl Features. Figure 4 shows 29SiCPf MAS NMR spectra of hydrated sodium silicate glasses containing 28.2, 22.0, 15.2, 11.4, and 3.0 wt %
water. Chemical shifts are given with the convention that algebraically larger values correspond to lower shieldings. All these spectra were acquired with the same contact time of 2 ms and were plotted at different vertical gains. The observed spectra were decomposed into a sum of the appropriate number of Gaussians;
7310 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992
11.4 15.2 22.0 28.2
73 76 77 78
Chemical shift.
7.0 8.3 7.9 1.4
5.6 11.7 11.0 21.7
80 82 83 85
Full width at half maximum.
8.4 8.1 8.1 5.6
Uchino et al.
36.2 38.5 40.0 35.7
87 89 91 91
10.3 9.6 9.9 7.6
46.9 38.5 42.1 34.5
96 96 98 96
14.6 15.0 13.3 8.6
11.3 11.3 6.9 8.1
Relative intensities
a
I
\
I \
wv
r l l r r r r l i r r l l l l l l l l r I l r i r r r r r r r m . . l l l l
-50 -60 O'-
-6C
-9ti
-:go
-
:10 -CX
Cross-polarization MAS 29Si spectra of hydrated 33.3Na2066.7Si02 glasses as a function of total water content: (a) 3.0 wt %, (b) 11.4 wt %, (c) 15.2 wt %, (d) 22.0 wt %, (e) 28.2 wt 4%; repetition time = 10 s; contact time = 2 ms; number of scans, -8000 per spectrum.
Figure 4.
several examples of decomposed spectra are shown in Figure 5. To find the best fit to the experimental spectra, each frequency position, line width, and intensity was adjusted to minimize the root-mean-square error between observed and calculated spectrum. We see that the observed spectra are composed of four individual Gaussians; these are labeled a, b, c, and d, from left to right in Figure 5. We can see that the a and b signals become more prominent with increasing total water content. The ?Si chemical shifts (6), relative intensities, and full-widths at half-maximum (fwhm) of each component are listed in Table I. 3.1.2. A s s i g " t of % CP/MAS i NMR Signals. Previous %i MAS NMR studies on nonhydratul sodium silicate glasa1*J9 have shown that when NazO is incorporated into the SiOz structure, silicons bonded to four other Si by bridging oxygens (Q),Q", are progressively transformed into silicons in which one oxygen is assoCiated with one sodium atom, @(Na); at 33.3 mol 4% NazOonly @(Na) species (-89 ppm) are present, and beyond
.
n-,
-50
.-.----. .-. ... - ~ , ~, . -.- -.... .._ ~
-60
0'
I_
~
-80
1
~
*
~
90
T_
-:oo
~
___
Tn_
-!:o
.-.PH, -120
5. Examples of dcumvoluted aaur-polarization MAS '9si speara: (a) 3.0 wt R, water, (b) 22.0 wt % water, (c) 28.2 wt % water. The dashed lincs correspond to the individual Gaussians to simulate the ob served spectra.
this composition Q3(Na) units are progressively replaced by Q2(Na, Na) units (-77 ppm) until complete replacement is achieved at 50 mol % NazO. This indicates that the nonbridging oxygens (0,b) formed from the reaction =Sidb-Si= + NazO 2=Si--O,bNa (5) are distributed homogeneously in the glasses. In this study we used the glass with the composition 33.3Na0.66.7SiO2. Hence,
-
The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 7311
Hydration of Sodium Silicate Glass
4r-----l t
a
0 -76 ppm
io000
0 -82 ppm e000
\ L 6000
\L 4000
3000 2000
" 0
2
1000
600
4
6
0
10
12
contact time (ms) Figure 7. 29Sisignal intensity for the sample with 15.2 wt % water as a function of matched spin-lock contact time (in as). Solid lines indicate the results of fitting cq 4 to the observed data.
300 200
TABLE Ik %i-'H Cram Polvizrtioa Relaxation Timg Tsm (ma) rad Rota Spio-hfflce Rehxrtioa Timg TI, (m)for Hydrated !hdiInm
silicate clucras
100
50
TSiH
TlpH
TSiH
TIpH
0.40 0.29
6.54 4.93
0.68 0.30
10.48 15.90
2000
4.1. 3.13. Rehxrtioa
6000
!OOO 600 300
200
100
P 50
-60 O'-
TlpH
4.09 3.84
4000
D A
-50
TSiH
0.31 0.24
water and nonbridging oxygens in the sodium silicate glass. In this work, we attribute the -95 ppm signal to Q3(H), the Si moiety with one OH and three bridging oxygens, the -82 ppm signal to Q?(Na, H), the Si moiety with one OH, one O n d a , and two bridging oxygens, and the -76 ppm signal to Q1(Na, H, H) (-76 ppm), the Si moiety with two OH, one OnbNa,and one bridging oxygen. The mechanism of formation of these Q3(H), Q?(Na, H), and Q'(Na, H, H) units will be discussed in section
b
-40
wt 4% 15.2 28.2
-80 -90 -!OO-!!0-!20-!30
Figme 6. Cross-polarizationMAS 2%ispectra of hydrated sodium silicate g b m as a fuactions of matched spin-lock contact time (in ps): (a) 15.2 wt % water, (b) 28.2 wt 4% water.
the component located at -89 ppm (labeled c in Figure 5) is assigned to Q3(Na) units. The other signals at -76, e-82, and -95 ppm (labeled a, b, and d in Figure 5, respectively) are not seen in the spectrum of dry 33.3Na2066.7Si02glass. Furthermore, these signals are not seen in the 29si NMR spectra of hydrous albite glasses (NaAlSi30s*H20),s in dry albite glass there is no nonbridging oxygen locally charge balanced by Na'. These results indicate that the -76, -82, and 4 5 ppm signals observed in this study are due to Q species formed from the reaction between dissolved
Results of variablacontact-time experiments are given in Figure 6; Figure 7 illustrates the dependence of the amplitude of the 29Sisignals on the cross polarization contact time for the sample with 15.2 wt 5% water. According to eq 3, we see that the initial rise on the left side of each plot in Figure 7 is due to the growth of 29Simagnetization generated by the spin flips between the silicon and proton spins (characterized by T s ~ Hand ) , the later decline is an indication of the dccay of the proton magnetization in the spin-locked state (as determined by T i C ~ )Unfortunately, . in our ex riments the intensities of the peak at -95 ppm assigned to Q (H) were too weak to obtain reasonable TlCH and Tsixvalues. The results of fitting eq 3 to the observed data are summarized in Table II; the data were analyzed by using the SALS (statistical analysis with least squares fitting) program." As shown in Figure 7, eq 3 was satisfactorily fitted to these observed data, indicating the validity of the application of eq 3 to the present data. A close examination of these relaxation parameters will be given later. 3.2 MO c.lcuhtion& It is reasonable to assume that in alkali silicate glasses nonbridging oxygen-alkali bonds are most s u b j d to water attack; it is hence of great interest to determine the atomic arrangement of the @i-onbNa-H20 complex. For this purpose, we obtained equilibrium geometries of three possible H4Si2O7Na2.2H20complexes at the STO-3G level (see Figure 2); one in which molecular water is associated with Na (complex I) and the others in which molecular water acts as a proton donor in a hydrogen bond to the nonbridging oxygen (complexes I1 and 111). In complexes I1 and I11 the Na atom is located adjacent to the nonbridging oxygen and to the oxygen in H20, respectively, and each water molecule is constrained to be coplanar with the =Si-Onb-Na plane. Table I11 summarizes the principal o p t i m i d parameters for these complexes and compara them with
73"
Uchino et al.
7312 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 TABLE III: Optimized Eond Lolrgths and Bond Angles for Model Clusters at the HF/STO-3G Level
interatomic
dist, A bond angle, deg Si-Oh Si-Omh S i U S i av 0-Si-0
H4Si20,Na2 complex I complex I1 complex 111 complex IV
H6Si207
1.584 1.585 1.593 1.619 1.576 1.656
1.612 1.613 1.609 1.604 1.640 1.596
109.34 109.32 109.39 109.40 109.32 109.45
139.4 139.5 137.8 140.4 132.8 142.2
TABLE Tv: Muwkea Atomic chrpes a d Eond OIerhp PopulrHons for Model Clusters at the 3-21G+d(Si)//STO-X Level bond overlap
atomic charges Q(Si)
Q(ob)
Q(0.b)
populations QWa) n(Si-Ob) @*d
H4Si207- +1.403 -0.825 -0.879 +0.725 Na2 complex I complex I1 complex 111 complex IV
(OH)$iOSi(OH)3
+1.402 +1.461 +1.433 +1.393 +1.450
-0.827 -0.816 -0.815 -0.882 -0.801
-0.865 -0.942 -0.845 -0.858 -0.725
+0.613 +0.720 +0.729 +0.627
0.672
0.859
0.671 0.680 0.694 0.587 0.702
0.852 0.780 0.810 0.873 0.702
TABLE V &&Water-Molecde Interaction Energies for Model Complexes (W/mol)
complex
complex
complex
I
I1
111"
interaction energy -109.96
-71.93
-125.79
complex IV -48.82
"Binding energy between Na(0H) and Si(0H). the STO-3G geometry of H,$i207Na2 with no molecular water. We see that the Onb-HO bond length (1.040 A) in complex I11 is considerably shorter than the O n b H - Obond length (1.359 A) in the same cluster; the former bond length shows a typical value of hydroxyl groups. For this reason, the calculated structure for complex I11 should be regarded as an equilibrium geometry for the hydrogen bond between a i - O H and Na(0H) rather than between -i-Onb and (H20 + Na). We also notice that the S i - O b bond length in complex I1 is shorter than that in the cluster H4Si207Na2,indicating that the S i U S i network is strengthened as a result of the interaction between molecular water and nonbridging oxygen. In accordance with this increase in Sibond strength, the 0, atoms become less negative and the S i - O b overlap populations increase (see Table IV), indicating the increase in the covalent character. In complex I, however, we cannot see such an apparent change in the structure of the Si-Si network. It may hence safely be said that hydration of Na hardly affects the glass structure. The energies of interaction of water in these complexes (AE) are also of interest. The values of AE presented iq Table V are differences between energies, as products minus reactants. Table V shows that the interaction energy derived from complex I11 (the =SOH-NaOH interaction energy) is larger than those derived from complexes I (the Na-H20 interaction energy) and I1 (the Onb-H20 interaction energy), suggesting that the Na(0H) molecule exists stably through hydrogen bond with W i O H . We, therefore, propose that the conventional H-Na exchange reaction between molecular water and Na in sodium silicate glasses m i - O n b N a+ H 2 0
-
ei-OH
-
+ Na+ + OH-
(6)
should be expressed as follows
--Si-OnbNa + H 2 0 @i-OH.-(OH)-Na+ (7) Kohn et al.36have also pointed out that such a Na(0H) complex is considered to exist in hydrous albite glasses. Furthermore, to investigate the effect of the interaction between molecular water and bridging oxygen in hydrated glasses, we calculated the equilibrium geometry of complex IV (see Figure
3). which models the interaction between Ob in complex I and H20. Interactions between molecular water and bridging oxygens in complexes I1 and I11 are less probable because the bridging oxygen in complexes I1 and I11 is less negative compared with that in complex I (see Table IV). To reduce the number of optimization parameters and to retain high symmetry, the following constraints among variables were imposed in the optimization procedure: (1) the hydrogen atom interacting with the bridging oxygen is located in the Si-Ob-Si plane and on the bisector of the external Si-Ob-Si angle; (2) the hydrogen bond angle, U H - Q , is fmed at 180'; (3) the dihedral angle between the planes H+H(center) and Si-Ob-Si is fixed at 90°. Op timized bond lengths and angles for complex IV are listed in Figure 3 and Table 111. From Table 111, we see that the Si-Ob bond length and the San bond blength in complex IV are longer and shorter than the correspond Si-0 bond lengths in complex I. This clearly demonstrates that the Si-Ob bond is weakened while the Si-Onb bond is enhanced as a result of the interaction between molecular water and ob. Our recent infrared studies of hydrated sodium silicate glasses3*have shown that with increasing total water contents especially over -10 wt % the S i U S i antisymmetric stretching band (-1100 cm-I) decreases in intensity and shifts to lower wavenumbers while the Si-Onbstretching band (-950 cm-I) shifts to higher wavenumbers. From the observed results, we have proposed that in glasses with over -10 wt % water, molecular water can interact with bridging oxygen to weaken the Si-Ob bonds and to enhance the Si-Onbbonds.38 This proposal is indeed consistent with the structural changes derived from the MO calculations mentioned above. We also calculated the interaction energy between molecular water and bridging oxygen (see Table V). From Table V, one sees that this interaction energy (-48.82 kJ/mol) is considerably small compared with those obtained from complex I (-109.96 kJ/mol) and complex I1 (-71.39 kJ/mol). Furthermore, it should be noted that previously calculated AE values for the SiOH-H20 interaction (-22 to -40 k J / m ~ l ~ are ~ ~comI) parable to that for the H 2 M binteraction obtained here. The relation between bonding environment of molecular water and its interaction energy will be discussed again in section 4.1.2. 4. Discussion 4.1. Mechanism of Reaction of Water with Sodium Silicate Glass. The reactions of water vapor with sodium silicate glasses He proposed that the were discussed previously by reaction mechanism could be written in three steps. The first is the conventional H-Na exchange reaction as expressed in eq 6. Charles considered that thus formed hydroxyl ion diffuse freely through the S i U S i network, reacting with a bridging oxygen as follows:
Wi-O-Siz
+ OH-
-
Wi-OH
+ =Si+-
(8)
The nonbridging oxygen formed from reaction 8 can then react with another water molecule ei-0-
+ H20
-
=%-OH
+ OH-
(9)
Accordiig to these reactions, the hydration reactions will proceed autocatalytically; that is, acceleration of water vapor corrosion of alkali silicate glasses should generally be expected. Contrary to the expectation, such increase in hydration rate is not actually observed for glasses containing alkali 0xides?~9" This means that the mechanism of hydration mentioned above will require some improvement. Hence, in this subsection, we reconsider the reaction between water and glass and present a new reaction mechanism on the basis of the present 29SiNMR results and ab initio MO calculations. 4.1.1. At Low Tobl Water Cmtents (-3 wt 8).As mentioned previously, the proportion of molecular water begins to increase with increasing total water content greater than -3 wt %. The molecular water thus introduced will be attracted by the negatively or positively charged areas in the glass structure; that is, water m o l d e s in alkali silicate glaaw will be hydrogen bonded to the hydroxyl groups ( = S i - O H and OH-), closely bound to the alkali ions (as in complex I, see Figure 2), and/or hydrogen bonded to the nonbridging oxygens (as in complex 11). Previous differential thermogravimetric analysis (DTG) and differential thermal analysis (DTA) have shown that two types of water can exist in alkali silicate g l a s ~ e s : 'water ~ ~ ~ ~which is completely liberated under 200 OC (loosely bonded water) and water which begins to be liberated over 200 OC (tightly bonded water). Furthermore, in our recent infrared spectra of hydrated sodium silicate glasses,38two bending vibration bands due to molecular water can be seen at 1760 and 1670 cm-'; the former and the latter bands were assigned to the bending vibration modes of the tightly and the loosely bonded molecular water, respectively. The bonding environment of these two types of molecular water can be evaluated by comparing the interaction energies M listed in Table V. From the values of M, we attribute the loosely bonded water to the water molecules hydrogen bonded to bridging oxygens (M = -48.82 kJ/mol) and/or silanols (MF:-22 to -40 k . I / m ~ l ~and ~ ' )the tightly bonded water to the water molecules interacting with nonbridging oxygens (M = -71.39 kJ/mol) and/or sodium ions (M = -109.96 kJ/mol). Thus one can easily 0 0 see that to introduce a large amount of molecular water stably into glasses, polar chemical bonds composed of nonbridging oxygens and alkali ions are needed, as has been confirmed experi0 0 0 0 I I I I mentally. As mentioned in section 4.1.1, the mobility of the Na+ ions 02(Na,H)** * 02(Na,H)) (03(Na)-03(H)* (0H)Na' increases with increasing total water content over -3 wt %, namely, However, the next equilibrium reaction by the introduction of molecular water. For example, the NMR spectrum of a Na20-Si02 glass with 32.5 wt % water showed that I I I I the mobility of Na+ at 100 "C in the glass is comparable to that ? ? ? ? in liquid water at room t e m p e r a t ~ r e .Bartholomew' ~~ proposed NaO~~$i-O-S[i-OH***(OH)-Na+ NaO,b-S[i-O,b*** HO-&OH (12) that this anomalous increase in the Na+ mobility is due to the I I I I 0 0 0 Na 0 hydration of the sodium ion. We, therefore, consider that with I I I I increasing water content over -3 wt % the Na(0H) molecules (03(Na)-03(H)* (OH)-Na* t 02(Na,Na)* O*(H.H)) diffuse more easily through the glass and, accordingly, more Si+Si bridges are broken to form Si-OH groups according to is not expected to occur because it has been established that the Na+ ions in silicate glasses tend to distribute homogeneou~ly.'~J~ reaction 11. As a result of this increase in the concentration of Si-OH groups, the following reaction as well as reaction 11 is These considerationsare indeed consistent with our assignment expected to occur: of the %i CP/MAS NMR signals. We have already shown that the signals at 4 6 , -89, and -82 ppm are assigned to Q3(H), Q3(Na), and Q2(Na, H) units, respectively. Referring to Figure 5 , one sees that 29SiCP/MAS spectrum of the sample with 3.0 wt % water consists mainly of three components (6 = 4 6 , -89, -82 ppm). According to our assignment, the observed three sigials are easily understood in terms of the reactants (Q3(H) and Q3(Na)) and the product (Q2(Na, H)) of reaction 11. However, (d(Na)-d(Na,H)-d(H)* *(OH)-Na* 03( Na)-Q'(Na,H,H)* 02(Na,H) the presence of these three signals cannot be explained in terms (13) of reaction 12; if reaction 12 had occurred, four 29Sisignals due to @(H), Q3(Na), @(Na, Na), and @(H, H) might be observed That is, Q'(Na,H,H) uNts are newly formed. In accordance with even for the samples with low water contents. this expectation, we see that the signal at -76 ppm assigned to Furthermore, the nonbridging oxygen formed from reaction 11 the Q1(Na, H, H) units (labeled a) becomes enhanced with inis probably hydrogen bonded to the adjacent = S i - O H group. creasing total water content over -10 wt % (see Figure 4). Miiller-Warmuth et aI.4' also proposed the presence of W i - O H It should also be noted that the total concentration of hydroxyl groups closely associated with M i - O n b Mgroups (M = alkali) groups ( W i - O H and OH-) does not change after reactions 11 by analyzing the NMR line shapes of alkali silicate glasses with and 13; the total concentration of hydroxyl groups is dependent low water content. It is not likely, therefore, that thus formed on the initial concentration of the complex, =Si-OH--(OH)nonbridging oxygen can react with another water molecule as Na+. The reaction mechanism proposed here is hence in agreement with the observed results that at high total water contents written in reaction 9; which explains the reason why the actual has Little significance in the hydration mechanism of alkali silicate glasses as evidenced by the fact that fused quartz and silica are hardly hydrated under normal autoclave condition^.^^ For this reason in alkali silicate glasses it is expected that hydration reactions proceed as a result of the interaction between H 2 0 and diOnbNa. As mentioned above, Charles42proposed that OH- ions formed from the H-Na exchange reaction (reaction 6) diffuse freely to react with bridging oxygens. However, our calculations have shown that the hydroxyl ions are hydrogen bonded to the terminal Si-OH groups (see reaction 7). Furthermore, do re mu^^^ suggested that sodium and hydroxyl ions diffuse as NaOH molecules through the glass structure since any separation of these ions would require a great deal of energy. This allows us to expect that the mobility of the NaOH molecules is lower than that of the Na+ ions in dry glasses and, therefore, is not so high as to diffuse freely through the glass network. In fact, the expectation is supported by the previous electrical conductivity measurements by Tomozawa and his co-workers?6 They have found that the conductivity decreases initially with increasing total water content to a minimum at 3-4 wt % water and then increases with further increase in water content. From the results, they concluded that the addition of hydroxyl groups (not molecular water) reduces the number of sodium charge carriers. According to the above consideration, the observed decrease in the number of sodium charge carriers can be attributed to the immobilized OH- ions adjacent to the Na+ ions. The subsequent increase in conductivity beyond the minimum with the introduction of larger amounts of water will be discussed in section 4.1.2. For the above reasons we consider that some of the hydroxyl ions formed from reaction 7 are attached to the adjacent silanols, and therefore, only a part of the OH- ions will react with bridging oxygens. That is, the following equilibrium is considered to exist: I I
A b
7314 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992
(>=3 wt %) the concentration of hydroxyl groups levels off and water enters the glass structure predominantly in the form of H20. In conclusion, (1) in glasses at higher (>=3 wt 9%) total water contents the mobility of the Na(0H) molecules increases by the introduction of molecular water, (2) the mobile Na(0H) complex can then react with a bridging oxygen, and (3) as a result, Q2(Na,H) and Q’(Na,H,H) units are formed according to reactions 11 and 13. 4.2. Interpretationof 29si NMR Spectra Figures 4 and 5 and Tables I and I1 show that not only relative intensities of the four 29SiCP/MAS signals but also other parameters such as 6, fwhm, TipH,and TsiH vary systematically as the concentration of water increases. In this subsection, we discuss these changes in spectral parameters in the light of the mechanism of hydration mentioned above. 4.2.1. chemical Shift. It has been established empirically that the 29SiNMR signal becomes less shielded (or less negative) with the increase of electron density around the test atomqS0Although the chemical shifts do not always correlate with the ab initio total charges in a linear manner,s1calculationsof the Si atomic charges of related clusters will give a clue to the cause of observed changes in 29Sichemical shifts. Table IV shows that the atomic charge on Si in complex I (+1.402) is almost the same with that in the cluster with no molecular water, H4SiZO7Na2(+1.403), while the Si atomic charge in complex I1 (+1.461)is slightly larger than the latter one. This indicates that when molecular water is hydrogen bonded to a nonbridging oxygen, the atomic charge on the Si atom to which the nonbridging oxygen is attached increases. Such an increase in the Si atomic charge will result in an increased shielding at 29Si. For this reason, the increased shielding of the Q3(Na), Q2(Na, H), and Ql(Na, H, H) resonances with increasing total water content (seeTable I) is most likely caused by the interaction between molecular water and nonbridging oxygen. In Table IV we also compare Mulliken atomic charges for complex I11 and H6Si,07; the former cluster models the hydrogen bonds between S i - O H and NaOH while the latter cluster models the isolated S i - O H group. The atomic charge on Si in &Si207 (+1.450)is larger than that in complex I11 (+1.433). We have proposed in section 4.1.1that at low ( methyl carbon (rotating) > protonated methine carbon > methylene carbon > methyl carbon (~tatic).~’This general rule appears to be applicable to the present TsiH values for the sample with 15.2 wt % water; the TsiH values decrease in the order Q3(Na) > Q2(Na, H) > Ql(Na, H, H). It should be noted, however, that for the sample with a higher water content (28.2 wt % water) the Q2(Na,H) and Q3(Na) signals have similar T s i ~values of -0.3 ms. This is probably due to the progress of hydration of the nonbridging oxygen in Q3(Na) units, because the nonbridging oxygen-water interaction as shown in Figure 2 also forms @(Na) silicons removed by two bonds (including hydrogen bond) from the nearest proton like Q2(Na,H) silicons with one directly attached hydroxyl group.
Hydration of Sodium Silicate Glass On the other hand, TIC^ in solids usually represent an average value of the relaxationbehavior over the ensemble of near-neighbor protons. Table I1 shows that the TipHvalues for @(Na, H) and Qi(Na, H, H) are substantially shorter than those for Q3(Na). The result suggests that considerable proton exchange between hydroxyl and molecular water is involved in the mechanism of rotating-frame 'Hrelaxation. Such shorter TICH values ( 3 6 ms) have also been reported for silica gels in which molecular water is adsorbed on the surface, although the T l p values ~ for silica gels heated under vacuum at temperatures over 100 OC are remarkably long (60-1 10 ~ S ) . ~ O 5. Conclusions We have measured 29SiCP/MAS NMR spectra of hydrated 33.3Na2066.7Si02glasses with up to 28.2 wt 5% water. The observed spectra consist of four components (6 -76, -82, -89, and -96 ppm). These signals can assigned to Q1(Na, H, H), Q2(Na, H), Q'(Na), and Qi(H) species; the Q1(Na, H, H) and Q2(Na, H) peaks become more prominent as the concentration of water increases. We have also carried out ab initio MO calculations on small clusters modeling various typa of interaction between water and the glass network at the HF/STO-3G level and have obtained the equilibrium geometries and the interaction energies for the clusters. From the calculations of interaction energies, it has been proposed that the loosely bonded molecular water is attributed to water molecules hydrogen bonded to bridging oxygens and/or silanols and the tightly bonded molecular water to water molecules interacting with nonbridging oxygens and/or alkali ions. On the basis of these observed and calculated results, we have presented a possible mechanism of reaction between water and alkali silicate glasses: (1) at low total water contents (
