THESURFACE STRUCTURE OF POROUS S~LICAS
the corresponding refractory compounds of the group IV transition metal^.^^^ Acknowledgments. The authors wish to thank Mrs.
394 1
E. Cisney for obtaining the X-ray diffraction data. We thank J. G. Davis for his aid in performing the vapor pressure meaeurements.
The Surface Structure of Porous Silicas
by L. R. Snyder and J. W. Ward Research Center, Union oil Company of Californk, &ea, California (Receiued June 97, 1966)
A variety of experimental techniques (notably selective silanization and infrared spectroscopy) have been applied to a wide range of high surface area (139-824 mz//g) silica samples in an effort to characterize the nature of their surfaces and to identify the types of surface groups which participate most effectively in adsorption on and reaction with the silica surface. A previously overlooked surface hydroxyl type (so-called “reactive hydroxyls”) constitutes the strongest site for the adsorption of aromatic hydrocarbons such as fluoranthene. These hydroxyls also react most rapidly with trimethylchlorosilane and dimethyldichlorosilane. Reactive hydroxyls appear to consist of an adjacent pair of strongly hydrogen bonded surface hydroxyls. The proposal of DeBoer and Vleeskens that the silica surface can be annealed by thermal treatqent now appears incorrect. The silica surface varies widely among various samples. Reactive hydroxyls predominate on fine pore eilicas and are virtually absent from coarse pore samples.
The surface properties of silicas are important for both practical and theoretical reasons. The nature of the silica surface and its interactions with various adsorbates and reactants have been intensively studied (see review of Hockey’ and subsequent work by Davydov, et d 2 ) .On the basis of these studies, a fairly simple, reasonably consistent Dicture has emerged concerning the types of groups present on the silica surface and their respective roles in determining various surface phenomena. It is commonly accepted that the surface of a hydrated silica is covered with hydroxyl groups which are attached to silicon atoms, and that these surface hydroxyls may be classified into two distinct types: “free” hydroxyls which give rise to a narrow absorption band in the infrared near 3750 cm-l, and hydrogen bonded (“bound”) hydroxyl groups which are characterized by a broad absorption band in the infrared between 2800 and 3700 cm-l. A major role has been accorded the free hydroxyls in adsorption
on and reaction with the silica surface. Thus infrared Ftbsorption studies suggest that the selective adsorption of polar niolecules and aromatic hydrocarbons on silica occurs primarily upon free hydroxyls. a - 6 The esterification of surface hydroxyls by extended reaction with trimethylchlorosilane (TMCS) and dimethyldichlorosilane (DMDCS) has also been shown to involve primarily free hydroxyls, with a t most partial reaction of bound hydroxyls.2 The greater importance of free hydroxyls in adsorption and reaction appears (1) J. A. Hockey, Chem. Z d . (London), 57 (1965). (2) V. Y. Davydov, A. V. Kiselev, and L. T. Zhuravlev, Trans. Faraday Soc., 60, 2254 (1964); RUSE.J . Phys. Chem., 38, 1108 (1964). (3) R. 5. McDonald, J . Am. Chem. SOC.,79, 850 (1957). (4) M.R. Basila, J . C h . Phys., 35, 1151 (1961). (5) G. A. Galkin, A. V. Kiselev, and V. I. Lygin, Trans. Faraday SOC.,60,431 (1964). (6) A. N. Sidorov and I. E. Neimark, &as. J. Phus. Chem., 38, 1518 (1964).
Volume 70, Number 19 December 1966
L. R. SNYDER AND J. W. WARD
3942
superficially reasonable (see discussion of Kiselev, et aL2v7). Upon heating silicas above 200O, surface hydroxyls begin to condense to form siloxane bonds (Si-0-Si) or other oxide groups with loss of water. Siloxane groups appear unimpcrtant in determining the specific adsorption properties of hydrated silicas; a fully dehydrated silica surface shows greatly reduced adsorption of both polar and unsaturated molecule~.~JJ The variation of the silica surface among different samples has also received much attention. McDonalds has noted that the nature of the hydroxyl groups and the relative proportions of bound and free hydroxyls vary among different samples. Similarly, DeBoer and Vleeskens'O and Hockey' have claimed that the surface of a fully hydrated silica can exhibit differences in hydroxyl concentration and in crystallinity or order. Kiselev and co-workers, on the other hand, have presented evidence that the surfaces of all but fine pore silicas are generally similar. For several maximally hydrated silicas and aerosils with surface areas between 39 and 750 m2/g, it was observed2 that the concentrations of surface hydroxyls are approximately constant for a given drying temperature between 200 and 1000". Similarly, Kiselev, et al.,7v* found that relative adsorption and heats of adsorption on silica of various hydrocarbons (both saturated and unsaturated) and polar compounds are the same per unit area of surface for silicas of widely different origin and surface area. Only in the case of very fine pore silicas do differences in adsorption characteristics appear. These were attributed to increased adsorbent-adsorbate contacts which are possible in very fine pores. I n the present study, several different experimental techniques (infrared spectroscopy, selective silanization, liquid phase adsorption, chemical and physical analysis, and thermal treating) were applied to the characterization of eight widely different silica samples. The data obtained suggest a substantially revised view of the structure of the silica surface, the variability of the surface between different samples, the role of different surface groups in various adsorption and surface reaction phenomena, and the susceptibility of the silica surface to thermal treatment (e.g., annealing). Experimental Section
Materials. Eight different laboratory or commercial silica Were investigated. These had the origin, geometrical characteristics, and ahminum "I tents listed in Table 1." The Davison silicas ameared to be hydrogels which had not been heated high enough to result in significant loss of structural water (see later discussion) and were used as received. The bulk density of the starting Cab-0-Si1 was increased by L
The Journal of Physical Chemistry
A
mixing with water, drying a t l l O o , crushing, and sieving. The various gel preparations were not initially heated above 115' and were therefore assumed to be in a fully hydrated state. The particle sizes of all the samples of Table I fell in the 60-200 mesh range. Surface areas were determined by the BET method using nitrogen adsorption (16.2 A2 assumed for N2 molecular area). ~~
Table I : History and Properties of the Silica Samples Studied in the Present Investigation
Sample no.
I I1 I11 IV
V VI VI1 VI11
Sample description Cab-0-Sil" Davison Code 62b Ethyl silicate] pH loc Ethyl silicate, pH < I c Davison MSb Sodium silicate] M o d Sodium silicate, blankd Davison Code 12b
Surface Pore area, diameter, mp/g
A
139 319 380 496 770 792 824 748
805 241 243 120 111 70 64 41
% A1
0.16 0.01 0.01 0.16
0.11
a Cabot Corp. W. R. Grace & Go., Davison Chemical Division. Prepared by precipitation of hydrolyzed ethyl silicate from solution of indicated pH. Prepared by precipitation of sodium silicate with HCl;" MO refers to the presence of methyl orange in the precipitation step.
The various silanes, TMCS, DMDCS, and hexamethyldisilazane (HMDS) were obtained from Applied Science Laboratories, Inc. (State College, Pa.) and were used as received. Silica-Silane Reactions. Silane-silica reactions were carried out in a simple glass flow apparatus at 195 5" using a nitrogen flow of 2 ml/sec as carrier gas and purge. The silica samples (3 to 6 g) contained in a detachable U-tube with glass wool plugs were first dried to constant weight in the apparatus (nitrogen flow) for removal of physically bound water. Incremental additions of the desired silane were made through a rubber septum by means of a syringe, with conditioning of the silica after each silane addition for 15 min (Le., to constant weight) to remove reaction products and unreacted silane. The weights of dry
*
(7) A. V. Kiselev, Y. S. Nikitin, R. S. Petrova, K. D. Shcherbakova, and Y. I. Yashin, Anal. Chem., 36, 1526 (1964). ( 8 ) A. V. Kiselev, "The Structure and Properties of Porous Materials." D. H. Everett and F. 5. Stone, Ed., Butterworth and Co. Ltd., lg5. (9) R.S. McDonald, J. Phys. Chem., 62, 1168 (1958). (10) J. H.DeBoer and J. M. Vleeskens, PTOC.Koninkl. Ned. Akad. Wetenschap., ser.B , 61, 2 (1958).
(11) F.H.Dickey, J. mYs. Chem., 59,695 (1955).
THESURFACE STRUCTURE OF POROUS SILICAS
3943
silica and of reacted silica after each silane addition were obtained to yield a plot of silica weight increase vs. silane nddition. Liquid Phase Adsorption Studies. The linear isotherm adsorption of the aromatic hydrocarbon fluoranthene (CI6Hl0) by various silica samples (dried at 195" in a nitrogen stream to constant weight) from 5 vol yo benzene-pentane was obtained in a standard procedure. A 160 X 10-mm column was packed with the silica to be tested, 10 ml of the latter solvent was passed through the column and rejected, 25 p1 of 0.1% fluoranthene in benzene was charged to the column, and elution with solvent was continued at 3-5 cm3/min. Fractions (5 cc) were collected and measured for ultraviolet absorbance at 287 mp. An equivalent retention volume R_" (equal to the distribution coefficient of fluoranthene in this system: K in cm3/g) was obtained in the usual way.12 Infrared Spectroscopic Measurements. The infrared spectra were determined with a Perltin-Elmer Model 221G double-beam spectrophotometer. The silicas, after various treatments, were compacted into 1-in. diameter plates of thickness 10 mg/cm2 at 3000 psi. The transparency of the plates was such that the instrument conditions recommended by the manufacturer for normal operation could be used. The reference beam was attenuated by suitable screens. The silica samples were evacuated for 2 hr a t each temperature investigated in a one-piece Pyrex-quartz cell. Hambleton, et a1.,13 noted that the properties of pressed disks differ from those of the original solid, and attributed this to the formation (by pressing) of surface regions inaccessible to adsorbing species, i.e., simple blocking of original pores. Since adsorption experiments were not performed on our pressed disks, this potential complication was of no significance.
Water Determinations. The water contents of various silicas were desired after removal of physically adsorbed water. The samples of interest were first dried at 105-110" in air for 4 hr or more as recommended by DeBoer, et u1.,14then in a stream of nitrogen at the same temperature to constant weight. An additional 5 to 10% of the total water in the sample after air drying was lost in the nitrogen drying. The water contents of the dried samples were then determined gravimetricaaly by dehydration of the samples a t 1250" in a stream of dry air. Replicate determinations agreed within *7%. Use of a Meker burner (recommended by DeBoer, et all4) gave water contents which averaged 10% low, as is also shown by Zhuravlev, et al. l6
Results The esterification reactions of the silanes HMDS, TMCS, and DMDCS with the silica surface are assumed2to proceed as
+
HMDS: (CH3)3Si-NH-Si(CH3)3
+ NH3
2+Si-OH +2+Si-O-Si(CH3)3 TMCS: (CH3)3Si-C1
+ +Si-OH
+
+Si-O-Si(CH3)3 DMDCS: (CH&SiC12
+ +Si-OH
(1)
+ HC1
(la)
+
+Si-O-SiC1(CH3)2
+ HC1
(lb)
In the case of DMDCS, the possibility of reaction with two adjacent surface hydroxyls (diesterification) also exists +Si-0
+
\ /
+
(CH3)zSiC12 23Si-OH + Si(CH3)2 2HC1 (2) +Si-0
When physically adsorbed water is present on the silica surface, corresponding reactions of silane and water can be visualized. A difference of opinion exists1,l6 concerning the relative concentrations of molecular water on silicas heated above 115". Under the conditions used in the present silanization studies, however, it seems unlikely that significant amounts of
0
1
I
2
4
I
I
I
I
I
6 8 IO 12 14 m Moles/g ADDED SILANE
Figure 1 . Reaction of silica sample VI1 with hexamethyldisilazane.
I
16
I
(12) L. R. Snyder, J . Chromatog., 6, 22 (1961). (13) F. H. Hambleton, J. A. Hockey, and J. -4.G. Taylor, Nature, 208, 138 (1965). (14) J. H. DeBoer, J. H. Hermans, and J. M. Vleeskens, Proc. Koninkl. Ned. Akad. Wetenschap., Ser. B, 61, 45 (19583. (15) L. T. Zhuravlev, A. V. Kiselev, V. P. Naidina, and A. L. Polyakov, Russ. J . Phys. Chem., 37, 1216 (1963). (16) J. J. Fripiat and J. Uytterhoeven, J . Phgs. Chem., 66, 800 (1962).
Volume 70. h'umber 12 December 1966
3944
water are present on the surface of the reacting silicas (compare ref 16). This is supported by TGA studies.” The unimportance of competing water-silane reactions was confirmed by noting that uptake of silane by silica was independent of temperature over the interval 110-250”. The reaction of HRIDS with the samples of Table I appears to give rapid, complete coverage of the silica surface. A typical reaction curve is shown in Figure 1, where sample weight increase is plotted os. the amount of added silane. The dashed line through the origin (“100% reaction”) is the calculated curve for complete reaction of added silane according to eq 1 (with loss of KH3). After an initial, rapid reaction of silane and silica, a much slower, secondary reaction is apparent. Extrapolation of this secondary uptake of silane back to the point P gives the approximate weight increase AW (in milligrams) of the sample corresponding to completion of the initial fast reaction. The total concentration (milliequivalents per gram) of trimethylsilyl (TIIS) groups introduced into the silica surface during the initial fast reaction (St) is A W I 7 2 W ) ; W is the weight (grams) of starting, dry silica. The determination of St was reproducible within =t4Y0,Values of X t for the silicas of Table I are listed in Table 11, along with derived values ut of the apparent molecular area of a ThIS group for each sample. hdditional ut values for some modified silicas are shown in Table 111. The experimental ut values for these 13 samples (56 A2 with a standard deviation of =t4 A2) are reasonably constant and fall within the range of values indicated for complete coverage of the silica surface. (The standard deviations of BET surface area determinations (=k7%) and St determinations (h4%) suggest an experimental uncertainty of ut of i S % or + 5 A2.) Thus close packing of TAIS groups on any surface (van der Waals separation)lg gives ut equal to 51 L42;for a spacing of TMS groups as in normal physical adsorptionll@ut equals 63 A2. The reaction of TJICS with the samples of Table I is similar to HRIDS reaction in showing an initial fast reaction followed by slow secondary reaction. I n the case of samples I, 11, 111, VII, and VIII, the rate of the initial fast reaction is sufficiently greater than that of the secondary slow reaction to permit an accurate determination of the amount of TRSCS taken up by the silica during the initial fast reaction (SJ, similar to the determination of St (see Figure 2 ) . For samples IV, V, and VI, the distinction between the rates of initial and secondary reaction was less pronounced and the determination of S, more ambiguous. The determination of S, in the latter cases was improved The Journal of Physical Chemistry
L. R. SNYDERAND J. W. WARD
Table 11: Characterization of Samples of Table I by Means of Selective Silanization and Adsorption from Solution -K, cms/gbTMCS Sample
I 11 111 IV V VI VI1
VI11
P S i l a n i z a t i o n dataa-St S, SJSt
0.50 0.87 1.16 1.42 2.36 2.24 2.21 2.24
0.01
0.02 0.03 0.03 0.06 0.21 0.32 0.40 0.71
0.03 0.03 0.09 0.50
0.72 0.88 1.60
Orig. ut
47 61 54 58 57 59 62 55
fd
sample
0.07 0.21 0.24 0.30 0.77
3.2 3.2 5.4 8.9 16.0 17.1 18.8 26.1
produot
1.4 1.5 3.6 4.6 4.7 4.3 3.5 1.2
a For completion of initial fast reaction with silane; St, total uptake (mequiv/g) HMDS; S,, total uptake (mequiv/g) TMCS; ut, experimental molecular area of TMS group (A*) calculated from St and sample surface area; fd, fraction diesterification in reaction with DMDCS. * Linear isotherm distribution coefficient for adsorption of fluoranthene from 5 % benzene~ pentane.
-
-
ADDED SILANE
m Moles/g
I
I
I
I
I
I
.3 .4 .5 .2 m Moles/o ADDED SILANE Figure 2. Reaction of silica with trimethylchlorosilane and dimethyldichlorosilane: (a) sample VIII; ( b ) sample 11; 0,TMCS; m, DMDCS. .I
(17) K.R. Lange, J. Colloid Sci., 20, 231 (1965). (18)L. Pauling, “The Nature of the Chemical Bond.” Cornell University Press, Ithaca, N. Y., 1940, Chapter V. (19) L. R. Snyder and E. R. Fett, J. Chromatog., 18, 461 (1965).
THESURFACE STRUCTURE OF POROUS SILICAS
3945
Table I11 : Properties of Rehydrated and Annealed Silicas'
H20
Surface area, mP/g
S,! mequiv/g
2.42 5.80 5.93
217 714 668
0.02 0.50 1.60
0.90 3.32 2.97
129 520 436
0.04
% Sample
I1
V VI11
I1
v
VI11
0.46
. . .f
St, mequiv/g
-Apparent at
Rehydrated samplesd 0.69 52 55 2.15 57 1.95
Stb
CalcdC
&/St
7.4
6.8 5.2 6.1
6.9 5.4 5.5
0.03 0.23 0.82
5.5 6.0
Annealed samples' 0.39 55 1.48 58
4.7 4.2 4.6
. . .f
OH-
SA"
4.5 4.4
...
0.10 0.31
...
Determined from BET surface area. Determined from surface area estimated from St. Assumes OH (surface) = 4.8; bulk Contacted with liquid water a t 95" for 16 hr, then dried (see procedure of DeBoer, et aZ.9. ' Heated a t 890" for = 0.7%/g. 16 hr then rehydrated as in d (see procedure of DeBoer and VleeskensIo). Silane reactions were quite slow, preventing the accurate measurement of St 01 8,.
H20
as described below. Values of S, for the various silicas studied are listed in Table 11; these values are repeatable within about *0.02 mequiv/g. S, is always less than St, from which it follows that TMCS is less reactive than HMDS, and a particularly reactive portion of the silica surface is involved in the addition of the initial S, equivalents of TMCS to the sample. The ratio S,/St, which represents the fraction of reactive surface for each silica, varies widely for the samples of Table I1 (0.02 6 Sr/St 6 0.71). The variation in the extent of initial reaction with TMCS of these samples is further illustrated in the reduced plots of Figure 3, where AW per unit of surface (Awl WSt) is plotted vs. added TMCS per unit of surface (mequiv TMCS/ WSt). The reaction of DMDCS with the samples of Table I1 closely resembles reaction with TMCS. DMDCS uptake in the initial fast reaction averages only 80% of S,, however. Addition of TMCS to DMDCS reacted sample gives an initial rapid uptake of TMCS until total silane uptake equals SI (corrected for secondary reaction), followed by the normal secondary uptake of TMCS. As seen in Figure 2, addition of DMDCS to TMCS treated silicas gives a somewhat slower secondary uptake of silane. Thus DMDCS appears to react rapidly with the same part of the silica surface which is especially reactive toward TMCS, but at a slightly slower rate. This contrasts with the extended reaction of these two silanes with silica,2where DMCDS appears more reactive. The extent of diesterification in the initial uptake of 0.8 SI equivalent of DMDCS was calculated for several of the samples of Table I1 from the chloride contents of the reacted samples (the initial silicas all had negligible chloride contents, as did TMCS-reacted samples). The fraction of
3;
5
4
50
w
u
ca 2
m
a 0 2
40
m
LL
0
k 30 z 3
a W
a w
cn
20
a
W LT
0
z
3
IO
a
0
2
m I 2 3 SILANE ADDED PER UNIT OF SILICA SURFACE (meq. TMCS/W.St)
Figure 3. Reactions of different silica samples with.trimethylchlorosilane.
diesterification f d in the total reaction of DMDCS with the silicas of Table I1 is seen to vary widely (0.07 6 fd 0.77) and to parallel values of Sr/St. That portion of the silica surface which reacts rapidly with TMCS also preferentially adsorbs aromatic hy-
180 mz/g, vs. 500 m2/g for DeBoer and Vleeskens sample), both Davydov, et u Z . , ~ and Fripiat and Uytterh~even'~ have observed that bulk hydroxyls begin to leave the sample at a temperature of 500-600", a major part of the bulk hydroxyls can be removed at temperatures between 700 and 800", and virtually complete removal of bulk hydroxyls occurs a t temperatures above 800 to 900". Similarly, DeBoer and VleeskenslO found that heating their silica at 450" for an extended time (without intermediate hydration) gave little reduction in n o H (6.2 to 5.8), heating at 650" gave substantial reduction (to 5.0), and heating a t 890" gave complete "annealing" ( n o H equal to 4.6). Hockey's assumption that bulk hydroxyls cannot be lost from a silica at 450" rests exclusively upon observations on silica glasses.21p24 This comparison seems basically unsound, since water is much more easily lost from the comparatively open structure of a hydrogel than from a solid sample of fused silica. Whereas bulk hydroxyls are not lost from silica glasses at 700" under vacuum,z4silica hydrogels give up much of the bulk hydroxyls under these conditions (cf. above). Davydov, et U Z . , ~ have in fact observed extensive loss of bulk hydroxyls from one hydrogel (39 mz/g surface area) at temperatures below 400". The observations of both Hockey' and DeBoer and Vleeskenslo can thus be explained on the basis of bulk
(24) A.
(1961).
J. Moulson and J. P. Roberts, Trans. Faraday
Soc., 57, 1208
THESURFACE STRUCTURE OF POROUS SILICAS
FREQUENCY cm-1
Figure 9. Infrared spectra of annealed and rehydrated silicas after heating to 330"; a, sample 11; b, sample V; e, sample VIII.
3951
and Vleeskens'O) for the rehydrated samples of Table 111, and these calculated values are seen to be in good agreement with the experimental noH values. The similar calculation of the apparent noH value of the silica studied by DeBoer and Vleeskens'O gives a value of 5 . 5 , vs. 6.2 experimental. The annealing process is seen in Table I11 to have essentially no effect on the S, values of samples I1 and V,25suggesting little change in silica surface type as a result of annealing. Similarly, the infrared spectra of annealed samples 11,V, and VI11 (Figure 9) differ little from the spectra of the unannealed samples (Figure 6). There is an apparent loss of bound hydroxyls from sample 11, which can be attributed to the simple removal of bulk hydroxyls during annealing. Samples V and VIII, which according to Hockey's proposal should show the greatest change in surface hydroxyl type upon annealing, show essentially no change in infrared spectra. We conclude that the annealing of silicas as described by DeBoer, et al., serves to remove bulk hydroxyls and to reduce surface area, but does not significantly alter the nature of the remaining surface hydroxyls. Reactive hydroxyls appear to survive the process of annealing and rehydration intact, while other hydroxyl types (and their associated surfaces) are partially destroyed. This is reasonable since reactive hydroxyls should be preferentially removed at low temperatures, while irreversible destruction of surface (and associated hydroxyls) by hydroxyl condensation occurs primarily at high temperatures.26 Rehydration then serves to regenerate surface hydroxyl groups with the exception of those hydroxyl types which are involved in surfaceto-surface condensation at high temperatures. The Origin of Surface Diferences. On the basis of the preceding discussion, it seems clear that the postulate of DeBoer and Vleeskens'O and of Hockey' concerning the origin of surface differences aniong various silica samples must be modified. These differences cannot be reconciled in terms of varying concentrations of total surface hydroxyls (cf. annealed silicas of Table 111). However, the corollary suggestion that differences in surface type reflect differing degrees of surface regularity or crystallinity appears reasonable. On this basis, it is proposed that large-pore diameter silicas such as samples I, 11, and I11 are relatively crystalline, fine pore samples such as sample VI11 are essentially amorphous, and silicas of intermediate pore size possess
hydroxyl removal, without any need to invoke surface annealing or alteration of surface structure. The proposal of DeBoer and Vleeskens'O and of Hockey' with regard to the nature of the "annealing" process was further studied, using samples 11, V, and VIII. These samples were first rehydrated according to the procedure of DeBoer, et al.,'* in order to ensure complete surface coverage by hydroxyls, then annealed (and again rehydrated) by the method of DeBoer and Vleeskens. '0 The water contents (and apparent noH values) and S, and St values of both the rehydrated and annealed samples were obtained, along with the infrared spectra of the annealed samples. These data are summarized in Table I11 and Figure 9. The decrease in the absorbance of the free hydroxyl group is due to loss of surface area. Annealing and rehydration lowers noH for all three samples to an approximately constant value of 4.5. This is similar to the DeBoer and Vleeskens'O average value (4.6) and verifies that the annealing of these samples was complete. The n O H values of the unannealed samples are higher, as found by DeBoer and Vleeskens,lo but show a correlation with the infrared spectra of these samples (Figure 6) which is the reverse of that predicted by Hockey. The data of Davydov, et C L ~ . , ~ (25) Srand St values for annealed sample VI11 could not be measured suggest an average bulk hydroxyl content (as water) accurately, due to greatly reduced silane reaction rates. This was apparently a result of partial blocking of the fine pore network by for silica hydrogels equal to 0.7%, and an average sintering, with a resulting decrease in the rate of silane diffusion. surface n O H value equal to 4.8. This permits the esti(26) J. H. DeBoer and Vleeskens, Proc. Koninkl. ,Ved. Akad. Wetenmation of apparent noH values (Le., as per DeBoer schap., Ser. B , 6 0 , 234 (1957). Volume 70,Number 12 December 1966
3952
B. E. CONWAY AND R. E. VERRALL
an intermediate structure. Hockey' and others have noted that the surface of a crystalline silica will be covered exclusively by free hydroxyls, with a separation between adjacent hydroxyls of 5.0 A (as in the P-tridymite structure). With perturbation of an initially crystalline structure and randomization of the positions of surface hydroxyls, the average spacing between nearest hydroxyl neighbors must tend to decrease, and an increasing number of hydroxyls will be sufficiently close to permit hydrogen bonding. Reactive hydroxyls appear to comprise the closer, more tightly bound surface hydroxyls, and their concentration should increase regularly with decreasing silica crystallinity and de-
creasing concentration of free hydroxyls. The parallelism of surface crystallinity and average pore diameter probably reflects a dependence of each of these properties on some basic aspect of the original silica synthesis. Thus it seems likely that those factors which promote silica crystallinity during its synthesis will likewise favor large crystallite size and increased silica pore diameter.
Acknowledgment. The authors are grateful to W. P. Cummings of W. R. Grace and Company for making available pore size distribution data on samples I1 and VIII.
Partial Molar Volumes and Adiabatic Compressibilities of Tetraalkylammonium and Aminium Salts in Water. I.
Compressibility Behavior
by B. E. Conway and R. E. Verrall' Department of Chemistry, Unicersity of Ottawa, Ottawa, Canada
(Received J u l y 6 , 1966)
Differential ultrasonic velocity measurements have been carried out on a series of aqueous solutions of symmetrical tetra-n-alkylammonium salts, corresponding salts of protonated methylamines, and the neutral methylamines themselves. The apparent molal adiabatic compressibilities q5K(s) have been derived and the values a t infinite dilution estimated. The dependence of upon the coordination of the N + center by H 2 0 molecules and by Me groups has been considered in relation to the variation of with molecular weight in the homologous series of symmetrical R4N+ salts from R = Me to R = n-Bu. Effects due to electrostriction and structure promotion are considered as a function of alkyl substitution a t the N center.
Introduction In recent years, considerable interest2-11 has arisen concerning the behavior of tetraalkylammonium salts in aqueous solution, particularly with regard to their behavior, and in water* A apparent structure-promoting review has also been given.12 E l s e ~ h e r e ~we~ 'have ~ examined the additivity of partial ionic volumes
v
The Journal of Physical Chemistry
of symmetrical homologous ions in this series, the concentration dependence2* of for various corresponding salts, and deduced the individual ionic contributions
v
(1) Work carried out in partial fulfillment of the requirements for the Ph.D. degree in the University of Ottawa, Ottawa, Canada. (2) W. Y. Wen and S. Saito, J . Phys. Chem., 68, 2639 (1964). (3)w. y. wen and s. Saito, &id., 69, 3569 (1965). (4) B. J. Levien, Australian J . Chem., 18, 1161 (1965).
