Adsorption of Triethylamine and Water Vapor and the ... - Science Direct

Adsorption of Triethylamine and Water Vapor and the Modification of Silica Surface by Gaseous Trimethylchlorosilane A . V. K I S E L E V ,

B. V. K U Z N E T S O V ,

AND S. N . L A N I N

Laboratory of Adsorption and Chromatography, Department of Chemistry, M. V. Lomonosov State University of Moscow, 117234 Moscow, USSR

Received July 6, 1978; accepted September 11, 1978 Vapor-phase modification of silica surfaces with different surface concentrations of hydroxy groups, Ctonwas carried out with trimethylchlorosilane (TMCS) at 400°C and heats of adsorption of TMCS, triethylamine (Et3N), and water vapor on the silicas were investigated. The ~on values of an initial (unmodified) silica sample treated in vacuo at 500° and 1000°C were about 3.2 and 0.8/zmole/m 2, respectively. In the latter case, by replacing all the remaining surface OH groups with trimethylsiloxy (-OSiMe3) groups it was possible to obtain a completely hydroxy-free silica surface with aosiMe:,~ 0.8/zmole/m2. The silica with t~OH~ 3.2/zmole/m2, thrice-treated with TMCS at 400°C, had C~OSiM,:,~ 2.7 /~mole/m~ and contained a minor amount of remaining OH groups (aoa ~ 0.5 tzmole/m~). Rehydroxylation of both these OSiMez-modified silica samples gave t h e same results: aOHbetween 0.6 and 0.8/xmole/m2. The sample with aosiMe:,~ 2.7/zmole/m z exhibited the lowest adsorption capacity in respect of EtsN and water. A procedure is proposed for detel'mining aOH from heats of Et3N adsorption dependance on surface coverage and from isotherms of EtsN adsorption. This procedure enabled the degrees of hydroxylation and modification with OSiMe3 groups to be quantitatively characterized for the silica samples in question. The IR spectra were in good agreement with these results. The chemistry of the surface of amorp h o u s s i l i c a a n d t h e r e a c t i o n s o f this s u r f a c e with different chemical modifiers were cons i d e r e d in a n u m b e r o f e a r l i e r p a p e r s ; ( e . g . , in r e v i e w s o f ( 1 - 4 ) ) . U s u a l l y t h e c h e m i c a l m o d i f i c a t i o n s a r e c a r r i e d o u t in t h e l i q u i d p h a s e (4). T h e v a p o r - p h a s e m o d i f i c a t i o n o f silicas w i t h a l k y l c h l o r o s i l a n e s p r e a d s o r b e d o n a p a r t i a l l y d e h y d r o x y l a t e d (at 500 a n d 1000°C in v a c u o ) s i l i c a s u r f a c e e f f e c t i v e l y p r e v e n t s p o s s i b l e s i d e r e a c t i o n s d u e to h y drolytic instability and oxidizability of the reagents. Furthermore, the use of a monohalogensilane, such as trimethylchlorosil a n e ( T M C S ) , as t h e m o d i f y i n g a g e n t p r e v e n t s t h e f o r m a t i o n o f p o l y m e r i c films on the surface and thereby gives reproducible and easily interpretable results. The present s t u d y is d e v o t e d to t h e i n v e s t i g a t i o n o f t h e a d s o r p t i o n p r o p e r t i e s a n d h y d r o l y t i c stabil-

ity o f silica m o d i f i e d b y r e a c t i o n o f its surface hydroxyl groups with TMCS yielding d i f f e r e n t c o n c e n t r a t i o n s o f g r a f t e d trimethylsiloxy (-OSiMe3) groups on the surface. It w a s s h o w n e a r l i e r (5) t h a t t h e r e a c t i o n between the surface hydroxyl groups and pure gaseous TMCS could take place comp l e t e l y o n l y at 400°C a n d f o l l o w i n g a t r i p l e t r e a t m e n t , e a c h s t a g e l a s t i n g a b o u t 2.5 hr. T h e r e f o r e , the s u r f a c e c o n c e n t r a t i o n o f t h e g r a f t e d g r o u p s , aOSiMez, m u s t a l w a y s c o r r e s p o n d to t h e s u r f a c e c o n c e n t r a t i o n o f h y d r o x y l g r o u p s on t h e initial silica, aOH, p r o vided that the surface area per each OH g r o u p , ~oOH = 1/O~oH,d o e s n o t e x c e e d a v a l u e c o r r e s p o n d i n g to t h e m i n i m a l s u r f a c e a r e a per one adsorbed TMCS molecule. Using t h e m o l e c u l a r size in l o o s e l y p a c k e d T M C S , i . e . , t a k i n g a T M C S m o l e c u l e to o c c u p y an 148

0021 9797/79/040148-09502.00/0 -

Copyright © 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal o[ Colloid and lnterjace Science, Vol. 69, No. 1, March 15, 1979

149

A D S O R P T I O N ON M O D I F I E D S I L I C A 90-

~v, k,J/m01e

oo~

70

o.

t

~,A

2

0J

5 6

L :35.6 50

2

¢

6

oL ~.t~a , ium°le/m2

Fro. 1. The relationships between the differential heat of adsorption, 0v, of triethylamine vapor at 19-20°C and its surface concentration, aEt:,.~, on aerosilogels pretreated in vacuo at t e m p e r a t u r e s : (1) 200, (2) 500, (3) 700, (4) 1000, and (5) 1050°C. Curves 6 and 7 refer to silica gels e v a c u a t e d at 700 and 1000°C, respectively (7). Herein and below, L is heat o f c o n d e n s a t i o n and black dots indicate desorption.

area ~o,, = 48 ~ (bounded by a circumscribed circle having a radius corresponding to its van der Waals dimensions), we obtained the surface concentration o~T~cs of 3.4 /xmole/m 2. Determination from the TMCS vapor adsorption isotherm of its rnonolayer capacity on hydroxylated surface by the B E T method yielded monolayer capacity values O~,,,TMCSof 2.8 (6) and 3.5 ttmole/m 2 (7). On the basis of these results for the silica used in the modification reaction, we had chosen the value of % , = 3.0 ixmole/m 2 corresponding to ¢o = 55 ~2 per a hydroxyl group. This concentration of the surface O H groups corresponds to silica evacuation at a temperature of about 500°C (8). The minimum surface concentration of hydroxyl groups on silica depends on the thermal stability of its porous skeleton, and in the case of macroporous silica aerosilogel, which withstands heating to 1100°C in vacuo without sintering (9), is about 0.5 txmole/m 2. 1. E X P E R I M E N T A L D E T A I L S

To obtain the aforementioned limiting values of the surface concentrations of

grafted OSiMe3 groups, a laboratory sample of macroporous silica, aerosilogel (9), was used to investigate the surface-modifying reactions with TMCS. The specific surface area of the initial aerosilogel sample after treatment with heated water vapor was SKr = 106 m2/g (¢o,~ = 21.5 A2). The adsorbates, apparatus, conditions, and procedures were the same as those employed in (5) and (7). The infrared spectra were recorded by means of a UR-20 Zeiss spectrometer with NaC1 or LiF prisms. The sample was placed in a portable device with an NaCl-windowed cell for spectrometric investigation of the adsorption (10). The weight of the aerosilogel tablet pressed under 30 kg/mm 2 was about 20 mg/cm 2. 2. R E S U L T S A N D D I S C U S S I O N

2.1. Degree of Surface Hydroxylation and Triethylamine Adsorption The OLOHvalue was controlled by the method we already used (5, 7), i.e., by adsorption o f triethylamine, molecules which are strong proton acceptors and which tend to form strong hydrogen bonds with the silanol groups of the silica surface (11) but are

8 &t

6

02 c~5

4

2

0

400

500

{200

T,°C FIG. 2. The effect of silica t r e a t m e n t t e m p e r a t u r e on the surface concentration of hydroxyl groups. C u r v e 1 obtained by deutero-exchange m e t h o d (8). C u r v e 2, from Fig. 1 for g/v = 60 kJ/mole; curve 3, the s a m e for qv = 50 kJ/mole; curve 4, the triethylamine vapor adsorption isotherm values at P/Po = 0.02. Journal odColloid and InterJhce Science, Vol. 69, No. 1, March 15, 1979

150

KISELEV, KUZNETSOV, AND LANIN 6

c~ ttr,., ,/umde/m 2

~f

y

~. 2

6 ~i

0.2

0.4

7

0.6

p/po FIG. 3. Triethylamine vapor adsorption isotherms at 19-20°C on aerosilogels evacuated at temperatures from 200 to 1050°C. Notations are the same as in Fig. 1.

incapable of causing specific adsorbate-adsorbate intermolecular interactions. Figure 1 is a plot of the differential heat of adsorption of triethylamine, 0v, as a function of its average surface concentration, ozE~~, on aerosilogel (curves 1-5) and silica gel (curve 6) (7) pretreated in vacuo at different temperatures. As the number of the hydroxyl groups on the silica surface decreases with increasing calcination temperature, the number of sites for strong specific EtaN molecule adsorption decreases and the values of OLEt:,N corresponding to a high Ov drop. Gradual removal of hydroxyl groups from the silica surface by treatment in vacuo at temperatures of 500, 700, 900, 1000, and 1050°C entails, first, narrowing the range of OtEtaN values corresponding to high and approximately constant c)v values and, second, some decrease in this high value of 0v. At high degrees of surface dehydroxylation, this leads to a continuous reduction of qv values to that close to the triethylamine heat of condensation (curves 4 and 5 in Fig. 1). The curves indicate that heating of silica in vacuo to 1000°C was ineffective in removing all the hydroxyl groups, while further temperature increase results in extensive sinterJournal of Colloid and lnterJace Science, Vol. 69, No. 1, March 15, 1979

ing. The CeEt3Nvalue corresponding to the bend point on the qv vs aEt:,n curve (the ordinate of this point varies in the narrow range of 50 to 60 kJ/mole) can be taken equal to the surface concentration of remaining hydroxyl groups. Indeed, for samples treated in vacuo at temperatures above 400°C, when only free hydroxyl groups remain on the surface (12), the corresponding values of aEt3N are consistent with those of aOH determined by the mass-spectrometrically controlled deutero-exchange method (8) (see Fig. 2). At vacuum treatment temperatures below 400-500°C, the triethylamine heat of adsorption method yields lower aoa values, since the dimensions of the EhN molecule are large compared to those of the hydroxyl group. At lower dehydroxylation temperatures, the number of hydroxyl groups per unit surface area of silica is much more than the number of Et3N molecules that can be adsorbed. If we determine the Et3N vapor adsorption values near to relative vapor pressure P/Po = 0.02 from the corresponding isotherm of adsorption (Fig. 3, curves 1-7), a rather reliable evaluation of the aon values (cf. Table I and curve 4 in Fig. 2) would also be provided. The results of Figs. 1-3 and Table I show that the number of hydroxyl groups can be evaluated by considering the relationships between the differential triethylamine heats of adsorption and OLEtaNvalues or the adsorption isotherms near to P/Po--0.02. The evaluations made according to these methods of the Et3N vapor heat of adsorption and adsorption isotherm are in this case quite consistent with the results obtained by deutero-exchange and reactions with LiCH3 or MgICH3 (references listed in Ref. (8)). It should be noted, that the results of Table I obtained by determining from the Et3N vapor adsorption values at P/Po = 0.02 for smaller OZor~are in better agreement with the results obtained according to the qv versus O~Et3Ncurves at Ov = 50 kJ/mole. For higher aOH this agreement is closer at qv = 60 kJ/ mole. However, in attempts to determine

A D S O R P T I O N ON M O D I F I E D S I L I C A

151

~MoaSiCl , /umo/e/rnz

b [I

cz=

501~

t

omA'~2

4

0.2

0.4

~6

0/po

c~e~s~ct , /u~ole/~'z

FIG. 4. The heats (a) and isotherms (b) of trimethylchlorosilane vapor adsorption at 19-20°C on aerosilogels treated in vacuo at 500°C and then 1000°C (2) and on aerosil pretreated in air at 500°C and then in vacuo at 450°C (3) (these last results were reported in (6)).

very small surface concentrations (like those on TMCS-modified samples), the error may be as high as 50% or more. Unfortunately, none of the heretofore used

methods has a sufficient sensitivity in the region of low Son to produce reliable and comparable data with those obtained by the Et3N vapor adsorption method.

TABLE I Determination of Hydroxyl Group Concentration of Silica Surfaces by Triethylamine Adsorption cqm(tzmole/m2)

Vacuum treatment temperature (V), TMCS modification temperature (M), rehydroxylationtemperature (R) (°C) Sample

V

1 2 3 4 5 6~ 7° M2 M4 M6 ~ M7 a MR2 MR2A MR4 MRM2A

200 500 700 1000 1050 700 1000 500 1000 700 1000 500 500 1000 500

M

400 400 310 310 400 400 400 400

V

200 200 200 200 200 200 200 200

R

100 100 100 100

V

200 400 200 400

M

400

From heats of adsorption at two different Ov(kJ/mole)

V

60

50

From isotherms of adsorption at p/po = 0,02

200

3.4 3.1 2.4 0.7 0.5 2.0 0.8 0.1 0 0.4 0.1 0.6 0.3 0.6 0.05

3.7 3.4 2.9 1.0 0.8 2.3 1.0 0.3 0 0.7 0.2 0.8 0.6 0.8 0.2

3.4 3.2 2.8 1.0 0.9 2.6 1.2 0.4 0.2 0.9 0.5 0.7 0.6 1.0 0.4

a The experimental data for samples 6 and 7 (silica gel) have been borrowed from Ref. (7). Journal of Colloid and lnterjace Science, Vol. 69, No. 1, March 15, 1979

152

KISELEV, KUZNETSOV, AND LANIN

gOIr~ ~v'kJ/m°le k

(3.

oe t

,0I/L I II~"

,,.

4

I I\I

o,

5

300

,

~

'

/,

,

&

ot~ ~ , q , f m o l e / m ~

2

•

0

0.2

1";-

0.4

'

d6

'

O/Oo

Fro. 5. The heats (a) and isotherms (b) of triethylamine vapor adsorption at 19-20°C on various aerosiloge! specimens pretreated as specified in Table I. 1--specimen M2; 2 - - M 4 ; 3 - - M R 2 ; 4 - - M R 4 ; 5--MR2A; 6--MRM2A.

2.2. Surface Modification by Reaction with TMCS and Rehydroxylation and Reversible TMCS Adsorption The following discussion of the modification results and investigation of the hydrolytic stability of silica surface, as well as all the adsorption parameters and heat of adsorption measurements, will refer only to samples 2 and 4 of Table I, i.e., to aerosilogel samples evacuated at 500 and 1000°C. Investigation of the T M C S vapor heats of adsorption and adsorption isotherms at 1920°C showed that TMCS adsorption, unlike that of saturated hydrocarbons, is affected by the presence of h y d r o x y l groups, though in a lesser degree than adsorption of Et3N (Fig. 4). The probable reason is the formation of a weak hydrogen bond (10) ~-~SiOH . . . C1SiMe3 (the difference between the heats of adsorption on samples evacuated at 500 and 1000°C is small, no more than 6 - 7 kJ/mole). F r o m Fig. 4 it can also be seen that no chemical reaction involving formation o f strong covalent bonds has taken place as yet, since both the isotherms and the curves of the differential heats of TMCS vapor adsorption are reversible for both silica samples. No chemJournal of Colloid and lnterfizce Science, Vol. 69, No. I, March 15, 1979

ical reaction takes place on heating the silicas with TMCS vapor up to at least 210°C. Thus, for example, if about 3.5/xmole/cm 2 o f TMCS vapor is readsorbed on sample 2 (Fig. 4), the system is heated to between 65 and 69°C in a closed calorimetric vessel and then cooled, and the adsorption branch of the isotherm is determined anew. The result is practically the same reversible isotherm (the black triangles in Fig. 4) up to o~ ~ 0.14 /xmole/m 2. Repeating the procedure including heating silica in the TMCS vapor to 210°C, we have obtained the same reversible isotherm (the black overturned triangles in the same figure). Only after the silica was heated to about 310°C in the TMCS vapor did the pressure in the apparatus grow (on account of evolving HCI) and the reaction between T M C S and the surface hydroxyl groups remaining after the first evacuation at 1000°C got under way to a noticeable extent. The trimethylchlorosilane heat of adsorption at 19°C (Fig. 4a) rapidly decreases from 59 to about 40 kJ/mole as oz~e3SiClincreases to about 0.14 /xmole/m 2 and then slowly tends to the heat of condensation. In the C)v range of 34 to 40 kJ/mole, the 0v versus C~Me:,SiClrelationship on silica gel pre-evacuated at 1000°C, as well as the correspond-

ADSORPTION ON MODIFIED SILICA

ing TMCS vapor adsorption isotherm, are reversible. The difference between the TMCS adsorption isotherms obtained by the authors from those reported in (6) is most probably due to the differing OLOHvalues used and silica treatment conditions. The area occupied by a TMCS molecule on the surface, calculated by the BET equation from isotherm 1 in Fig. 4, is 45 A2 and is in satisfactory agreement with the area of 48 ]~2 calculated by projecting the van der Waals model onto the surface in case of loose packing, i.e., having an area bounded by the circumscribed circle. Triple repetition (2.5 hr each time) of the chemical reaction between the TMCS vapor with the hydroxyl groups at 400°C yielded modified samples M2 and M4 (cf. Table I). These modified samples were then used to study the adsorption isotherms and the differential heats of adsorption of the surface silanol groups indicating adsorbate that we had chosen for the purpose (Et3N). Curve 1 of Fig. 5a shows that the initial heat of triethylamine adsorption on the M4 sample is as little as about 40 k J/mole and even shows a minor increase, as aEt:, N increases, rather than a decrease as was the case with the initial silica samples (cf., for example, Fig. 1). The initial portion of the triethylamine adsorption isotherm (Curve 1 in Fig. 5b) for the same sample is almost rectilinear, which, in this case, is an indication of a very weak adsorbate-adsorbent interaction, with the adsorbate-adsorbate interaction being quite detectable against its background (compare Fig. 3). An evaluation of the concentration of the surface hydroxyl groups by allowing triethylamine to be adsorbed on this sample and then considering both the heat of adsorption curves and Et3N vapor adsorption values at P/Po = 0.02 taken from the corresponding isotherms of adsorption (Fig. 5) gives aou = 0. The initial triethylamine heats of adsorption on sample M2 (Curve 2) are somewhat higher, since an increase of ao~ prevents a portion of the

153

hydroxyl groups from reacting with TMCS because of blocking by a denser grafted layer of the trimethylsiloxane groups (cf. also (13)). Therefore, in order to involve all the surface hydroxyl groups in the modification reaction, it is necessary to reduce the O~oH of the initial sample (taking into account the possible rotation of the OSiM% group, as well as the virtual, but not averaged, distribution of the surface hydroxyl groups). In reality, a part of these groups on sample M2 could not be modified by comparatively low concentrations of the modifying agent. To overcome these hindrances, a considerable increase of the TMCS vapor pressure might be necessary during the modification reaction at 400°C. It is interesting to note that the Et~N vapor adsorption isotherm on sample M2 (Curve 1 in Fig. 5b), more convex at first, becomes smoother as it crosses the similar isotherm for sample M4 (Curve 2). A lower triethylamine adsorption at medium relative pressure is indicated. The same effect is evident from the heat of adsorption curves (curves 1 and 2 in Fig. 5a). This minor decrease of the adsorption and heat of adsorption at medium relative vapor pressure values is apparently due to the increasing concentration of the trimethylsiloxane groups screening off the silicon-oxygen skeleton of silica and thereby inhibiting the intermolecular interactions between the triethylamine molecules and this skeleton. On sample M2, the concentration of the remaining OH groups (not involved in reaction with TMCS), determined according to triethylamine adsorption, is between 0.3 and 0.7/xmole/m2, and the concentration of the grafted OSiM% groups was about 2.8/zmole/mz. It should be noted that the triethylamine adsorption isotherm and heats of adsorption curves on samples M2 and M4 are somewhat irreversible over the entire measurement range. This irreversibility is more pronounced on the sample pretreated in vacuo at 500°C. The reason is that the diffusion Journal of Colloid and Interface Science, Vol. 69, No. 1, March 15, 1979

154


300D

3200

:5400

3600

3000

4000

('-3 O.5

~2,3

~.0

~4

e~

~3

D FIG. 6. IR spectra of aerogel evacuated at 1000°C and then modified with trimethylchlorosilane. 1--aerosilogel evacuated at 1000°C; 2 - - s a m e after following modification with trimethylchlorosilane at 400°C; 3 - - s a m e following rehydroxylation with water vapor at 100°C, P/Po = 1 and treatment in vacuo at 200°C; and 4 - - a f t e r following second modification with trimethylchlorosilane at 400°C.

of the sufficiently branched Et3N molecules toward the silicon-oxygen skeleton of silica is prevented by the already grafted OSiMe3 groups; however, by virtue of the adsorption forces, it has a higher rate during adsorption, when the pressure of Et3N vapor increases, than during desorption back to the vapor phase. Rehydroxylation of silicas modified with TMCS at 400°C with water vapor at 100°C over a period of 3 hr leads to the fact the Et3N vapor adsorption isotherms and heats of adsorption curves become irreversible, the heat of adsorption becoming higher for the same surface concentrations. This change in adsorption properties results in the inability of the triethylamine molecules to diffuse toward the silicon-oxygen skeleton, but are further adsorbed on the newly formed hydroxyl groups. On rehydroxylated samples MR-2 and MR-4, the initial Et3N vapor heats of adsorption are particularly markedly increased (compare Fig. 5a and Table I). This is also indicative of the appearance of free hydroxyl groups capable of reacting with Et3N to form the hydrogen bond. It is interesting that, in spite of the same initial Journal of Colloid and Interface Science, Vol. 69, No. 1, March 15, 1979

heats of adsorption, the triethylamine adsorption isotherm on sample MR-4 with a lower aos~,:~ attains somewhat higher values upon rehydroxylation, while on sample MR-2 which has a higher aosiMe:,, it remains almost unchanged. This result corresponds to heats of adsorption at coverages of about a half monolayer (OZEt~,~= 1 -- 3/zmole/m2). The treatment of the latter sample in vacuo at a higher temperature (400°C, sample MR-2A) followed by its second modification by TMCS (sample MRM-2A) very insignificantly affects the triethylamine adsorption isotherm for these samples (see Fig. 5b). Only in its initial portion is the Et~N vapor adsorption isotherm on the remodified sample smoother. These changes correspond to the decrease of the initial triethylamine heats of adsorption and, therefore, are attributable to the decreasing concentration of the remaining hydroxyl groups on silica surfaces (cf. Table I). The above-described alterations of the chemistry of surface and adsorption properties of identically treated samples were monitored by IR spectroscopy. Figure 6 illustrates the IR spectra of the initial aerogel evacuated at 1000°C (curve 1) and then

A D S O R P T I O N ON M O D I F I E D S I L I C A

8o. ~,, kJ/mole

155

~w~o,Nm°l~/m~

'

/,'""l

,Jy

71

40

t0 2

4

6

d~o ,/umole/m'

0

02

,' o.4

0.6

0,8

4D

P/Po

Fie. 7. T h e heats (a) and i s o t h e r m s (b) of water vapor adsorption at 19-20°C on trimethylchlorosilane-modified silicas and d e h y d r o x y l a t e d silica gel. 1 - - s i l i c a gel d e h y d r o x y l a t e d in vacuo at 900°C (14). Modified silicas: 2 - - a e r o s i l (15); 3 - - a e r o s i l o g e l after heating in v a c u o at 500°C, 4 - - t h e s a m e heated at 1000°C.

modified by reaction with TMCS at 400°C (curve 2), rehydroxylated (curve 3), and remodified by a second reaction with TMCS (curve 4). All these spectra were obtained for one and the same tablet which was treated and the spectra taken in the aforementioned sequence without connecting to the atmosphere. The results of Fig. 6 show that rehydroxylation does not reduce the intensity of the 2969- and 2912-cm -1 (see Fig. 6, curve 3) bands relating to the grafted OSiMe3 groups. Thus, the bonds of these groups with silica are not hydrolyzed. Around 3750 cm -1, rehydroxylation of the samples modified at 400°C with TMCS results in a band of very small intensity. Remodification with TMCS at 400°C completely removes this band, at the same time increasing a little the intensity of the 2969cm -1 band (see Fig. 6, curve 4). It should be noted that the intensity of the absorption band in the 3750-cm -1 region of close, for the rehydroxylated sample, to the background level (cf. Fig. 6, curves 3 and 4), so that a reliable control of the concentration ~ remaining hydroxyl groups on the T?vfCS-modified samples can be effected only by the triethylamine vapor adsorption method (compare Fig. 5 and Table I). Nevertheless, the spectral pattern gives

sufficient evidence that no hydrolysis of the grafted trimethylsilyl groups occurs. The hydrolytic stability and hydrophobic properties of modified samples were estimated by measuring the water vapor differential heats of adsorption and adsorption isotherms. The results are presented in Fig. 7 and compared with those reported in (14) for a strongly dehydroxylated silica gel sample and aerosilogel most completely modified with liquid-phase trimethylchlorosilane (15). It is seen from Fig. 7a that neither dehydroxylation nor modification by boiling with pure (not donor-doped (16)) TMCS (12, 15) provides a surface where the intermolecular interactions with water are weaker than between the water molecules themselves, since the differential heats of water vapor adsorption are higher than its heat of condensation (15). A much lower water vapor heat of adsorption value was obtained for samples M2 and M4 modified with TMCS vapor at 400°C. Curves 3 and 4 of Fig. 7a lie below the water vapor heat of condensation, which attests to the formation of a much denser layer of the grafted OSiMe~ groups in high temperature reactions with TMCS vapor. The same is indicated by the water vapor adsorption isotherms which are compared in Fig. 7b. Of special interest is Journal of CoUoid and Interface Science, Vol. 69, No. 1, March 15, 1979

156


curve 4 obtained for water vapor adsorption on silica surface containing the maximum number of grafted OSiMea groups. The same type of water vapor adsorption isotherm was obtained on a Cab-o-Sil surface modified with hexamethyldisilazane by Zettlemoyer and Hsing (17). It is only from this sample (M2) that water can be quite easily desorbed, and the isotherm has a desorption branch which is close to the adsorption one. However, even in this case it is impossible to desorb at 1920°C all the adsorbed water. The surface concentration of water retained on silica is rather high and amounts to 2 /zmole/m 2. This water is partially chemisorbed, i.e., takes part in the formation of the ~ S i O H groups (it rehydroxylates the silica surface) and is adsorbed at a greater rate than Et3N by virtue of its penetrating to the places inaccessible to the larger Et3N molecules. However, upon rehydroxylation at 100°C for 3 hr followed by treatment in vacuo at 400°C aoH, it is only about 0.6-0.8 t~mole/ m2, i.e., 2 to 3 times less than the quantity of water remaining after desorption at 20°C (see Fig. 5 and Table I). The water vapor adsorption isotherm (Fig. 7b) shows that, despite the high hydrolytic stability of the grafted (at 400°C) trimethylsiloxane coating (Fig. 6), sample M2 is still capable of retaining large quantities of water.

Journal of Colloid and Interface Science, Vol. 69, No. 1, March 15, 1979

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