J. Phys. Chem. 1995,99, 3711-3714
3711
Influence of Water on Adsorption of Triethylsilanol onto Silica R. Denoyel* and P. Trens Centre de Thermodynamique et de Microcalorim6trie du CNRS, 26, Rue du 141 Eme RIA, 13331 Marseille Cedex, France Received: October 28, 1994; In Final Form: December 17, I994@
The adsorption of triethylsilanol (TES) on silica from a heptane solution has been studied in situ by microcalorimetry with a special attention to the water content of the adsorbent. In a first step, controlled rate transformation analysis, which allows one to distinguish between physisorbed and chemisorbed water, is used to prepare silica samples with a known surface concentration of water. Adsorption isotherms and displacement enthalpies (of the solvent by the solute) are then determined. A decrease of both affinity and displacement enthalpy is observed when the water surface concentration on silica is decreased. Moreover, irreversibility of adsorption (attributed to the formation of siloxane bonds) is observed only when physisorbed water is present: the siloxane bond formation seems to be catalyzed by the surface water.
Introduction Organosilane-modified silicas find a variety of important applications. For instance, the suitability of high performance liquid chromatography (HPLC) for the analysis and separation of simple organic compounds as well as complex biomolecules has been possible through the development of silanization chemistry during the past 3 decades. Also, silane coupling agents are used to improve the interfacial bonding between the reinforcing material and the resin matrix in composites. This is why adsorption of silane coupling agents such as y-(aminopropy1)triethoxysilane ( A P S ) onto silica or others materials was found worth being studied by NMR,' GPC,2 FTIR,3-5 ellipsometry, X P S , or calorimetry6 These molecules present a relatively complex adsorption behavior: for example, in aqueous solution, A P S gives rise after hydrolysis,to alcohol and trisilanol molecules which can themselves poly~ondensate.~One expects the trisilanol molecule to be the one adsorbed on silica from aqueous s ~ l u t i o n . ~In, ~the same manner, adsorption from an organic solvent is often described as the hydrolysis of the APS molecule by the surface water before subsequent adsorption of the silanol form, through condensation with surface silanol group^?,^^,'^ In order to understand the adsorption mechanism of such a complex molecule, we decided to carry out the thermodynamic study of a single silanol function on silica. This is why the present study is based upon the determination of adsorption isotherms and upon microcalorimetry experiments of the adsorption of triethylsilanol (TES) on silica from heptane. In a recent paper, Azzopardi and Arribartil have evidenced, by FTIR,the formation of siloxane bond between TES and silica. However, their experiments were carried out with a silicon internal reflection element (ATR procedure) whose hydration state is difficult to control. Indeed, the silica surface is covered with hydroxyl functions (surface silanols) and physisorbed water whose surface densities depend on the preparation procedure and are expected to play a major role in the adsorption from organic solutions. This role was already stressed for adsorption of hydrolyzable silanes: and it is therefore interesting to examine it for the case of nonhydrolyzable silanols like TES. In such a study, there is a special need for highly reproducible hydration states of the surface. As a consequence, we decided to investigate the influence of moisture on the adsorption of
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracrs, February 15, 1995. 0022-365419512099-3711$09.0010
TES by using controlled transformation rate thermal analysis (CRTA) to prepare reproducible surface state.
Experimental Section Methods. In the CRTA equipment used here for the preparation of the surface, the rate control is based upon the determination of the rate of gas (i.e. here, water vapor) evolution as a sample is evacuated. Appropriate heating of the sample allows the rate of outgassing to remain constant. This equipment was already described as a controlled rate evolved gas detection system12 or CR-EGD. After the thermal treatment, the silica samples are transferred under dry nitrogen, in a glovebox, either into the test tubes needed for adsorption isotherm determination or into the microcalorimetric cell. The adsorption isotherms are determined by the solution depletion method. After centrifugation of the test tubes, the equilibrium concentrations are determined in the supematents by Fourier transformation infrared spectroscopy (Nicolet 205). Infrared spectra are recorded using a transmission flow cell with ca. 3-mm optical path length. Typically, 120 scans at 4-cm-I resolution are recorded for each spectrum. The infrared peak used to measure TES adsorption is the -SiOH sharp peak at 3703.5 cm-'. For the silica samples themselves, the infrared spectra are determined using a diffuse reflectance accessory (Collector DRIFT, Spectra-Tech) equipped with a controlled environment chamber, (Model 0030-100, Spectra-Tech) able to be heated up to 550 "C under a vacuum of 0.1 Pa. This cell is equipped with two ZnSe windows and a water cooling apparatus. The enthalpies of displacement are measured with a microcalorimeter already described,13 which allows either batch or liquid flow experiments to be carried out. Most of the experiments presented hereafter were made in the batch mode where a feed solution of silane is added step by step into the microcalorimetriccell where the silica is maintained in suspension by a stimng system. In most cases, it is only possible to get the integral enthalpies of displacement as a function of coverage. Experiments carried out with the same apparatus showed that the heat of dilution evolving during the adsorption experiment can be neglected. The liquid flow mode is used to determine the reversibility of the adsorption. In each case, we make two adsorption-desorption cycles. The thermal difference 0 1995 American Chemical Society
3712 J. Phys. Chem., Vol. 99, No. 11, 1995
Denoyel and Trens
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Figure 1. CRTA curve for Spherosil. Weight loss versus tempmature.
between the two adsorption steps allows one to estimate the reversibility of adsorption. Systems. The silicas used in this study are Spherosil XOB015 provided by Sepracor and Aerosil 200 provided by Degussa. The Bmauer-Emmet-Teller (BET) specific surface areas, determined by nitrogen adsorption at 77 K, are 25.5 i2 and 200 f 25 mz g-l, respectively. The mean pore width of Spherosil is 300 nm (mercury porosimetry), and its particle size ranges between 40 and 100 pm. Aerosil is a well-defined nonporous silica with a mean particle size around 12 nm. Heptane (98%) were supplied by Aldrich.
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Results and D i s d o n Surface Characterization. In order to investigate the reaction of TES with silica, it is impoaant to evaluate the surface density of both hydroxyl groups and water before adsorption. In the case of Spherosil, the temperature versus weight loss curve obtained by CRTA is shown in Figure I . This type of curve is well described in the 1iterat~re.l~ Typically part 1 (which ranges between 25 and 150 "C) is ascribed to the loss of physisorbed water and part 2 to the dehydroxylation process (loss of chemisorbed water). From the mass of the sample before and after thermal treatment, and with the provisional assumption that most hydroxyl functions are located on the surface, we can calculate the density of the hydroxyl groups on the surface (below 500 OC, there is no change of the specific surface areal4). The surface density of physisorbed water derived from the weight loss between 25 and 250 "C is 13 pmol m-z which is close to the calculated surface density of the monolayer (15.8 pmol tK2). The density of hydroxyl groups derived fromthe weight loss between 250 and IO00 "C is 35-40 OWnmz, i.e. much higher than the densities quoted in the literature for these types of samples: between 3 and 7 OWnmz, the latter being the maximum theoretical v a l ~ e . ~ ~Ou .r ' ~ assumption that most hydroxyl groups are initially on the surface does not hold for this silica, which therefore contains a large portion of internal hydroxyl groups (situated in the closed porosity for example). Finally, this curve provides two pieces of informations: (i) if one only wishes to eliminate part of the physisorbed water, the outgassing temperature must be lower than 200 "C; (ii) if one wishes to eliminate all the physisorbed water, an outgassing temperature above 300 OC is needed, with the risk of starting the dehydroxylation of the surface. Indeed, the transition between dehydration and dehydroxylation takes place between 200 and 300 "C and the inflection point of the CRTA curve (around 250 "C) can be considered as the best point of separation. Figure 2 shows the CRTA curve of Aerosil. The transition between the deparhue of physisorbed water and the dehydroxylation occurs aronnd 150 "C. Here, the surface density of
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Figure 3. Infrsred spectra of Spherod and Aerosil obtained by DRIFT. hydroxyl groups derived from the weight loss above 150 'C is close to that proposed by the literature: 4.0 OWnmz.L4 The curve also showed that a heat treatment at 140 "C is sufficient for eliminating most of the physisorbed water (which amounts to a tenth of a monolayer). We present in Figure 3 the infrared spectra of Spherosil and Aerosil. The Spherosil does not possess any isolated silanol since there is not the characteristic sharp band situated at 3745 cm-l, which appears very clearly on the spectrum corresponding to the Aerosil. This is a confirmation of the high OH content determined by thermal analysis; moreover the large number of internal OH is evidenced here by the strong peak around 3700 cm-l. Even upon the evacuation at 200 OC, no peak corresponding to isolated SiOH appears: only a decrease of the broad 3600-2800-~m-~band (corresponding to bound adsorbed silanol and adsorbed water) is adsorbed. This more hydrophilic character of Spherosil, as compared to Aerosil, was also evidenced in the past from the comparison of immersion enthalpies in water." Adsorption from Solution. The adsorption isotherms of TES on Spherosil are reported in Figure 4. The adsorption isotherm corresponding to the untreated Spherosil shows a high affinity of the TES molecule for the silica surface. The initial slope is indeed quasi-infinite, and the final plateau is well defined. The average area per molecule calculated from the plateau surface concentration is 0.92 nm2,whereas 0.45 nmz would theoretically be possible for a close packing. After thermal treatment the clearest trend is a decrease of the initial slope, i.e. of the affinity of the TES molecule for the surface. The adsorption isotherms of TES on Aerosil are reported in Figure 5. Two experiments were carried out: one with the pristine sample, the other one after outgassing at 140 OC (removing of physisorbed water). Two observations can be made: (i) the affinity of the TES molecule for the Aerosil surface is much lower than that for
Influence of H2O on Adsorption of TES onto Silica
. I . Phys. Chem., Vol. 99, No. 11, 1995
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Figure 4. Adsorbed isotherms of TES on untreated Spherosil, outgassed at 140 or 300 "C.
the temperature range used for both samples. At 140 "C, most physisorbed water has been eliminated in the case of Aerosil, whereas a heat treatment of at least 250 "C is necessary to obtain the same dehydration in the case of Spherosil. It is then remarkable that (i) untreated samples are "similar" (as indicated by the enthalpic curve), (ii) a heat treatment at 140 "C has a large effect on Aerosil and a small one on Spherosil, and (iii) a heat treatment at 300 "C makes the Spherosil similar to the Aerosil outgassed at 140 "C. These features stress the influence of surface water on the adsorption of the silanol function. They show a change of the adsorption mechanism depending on the presence of molecular water on the surface. If the interactions between the ethyl group and the surface are neglected (which is reasonable here, since the enthalpy of displacement of an alkyl chain by another one is small, especially when they are shod8), the adsorption of the TES molecule can be described by one of the following "reactions", for which one of the possible configurations is given in Figure 7: =)
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Figure 5. Adsorbed isotherms of TES on untreated Aerosil or outgassed at 140 "C. 100
1
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0 40
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Figure 6. Enthalpies of displacement of heptane by TES on Spherosil or Aerosil. (A) Untreated Spherosil; (B) Spherosil outgassed at 140 "C; (C) Spherosil outgased at 300 "C; (D) untreated Aerosil; (E) Aerosil outgassed at 140 "C.
the Spherosil surface; (ii) here, the heat treatment also decreases the affinity for the surface even if the situation is not clear at very low coverage. This variation of affinity with the type of silica and its water content is also stressed by the microcalorimetric results reported in Figure 6. They clearly show a decrease of the displacement enthalpy of the heptane by the solute as the solid sample is outgassed. The enthalpies of displacement are the same for the two untreated samples, showing that the same kind of "reaction" occurs at the two interfaces. As a consequence, the observed difference between the adsorption isotherms could only be due to the surface density of sites involved in the reaction (it must be noted as a reminder that both the OH surface density and the water content of Aerosil are much lower than those of Spherosil). As underlined in the previous paragraph, the main effect of the thermal treatment is to eliminate the physisorbed water in
3713
H bond formation with a surface silanol
(1)
H bond formation with the adsorbed water condensation with a surface silanol giving rise to a siloxane bond (111)
The experimental results of the present work would then show a change of the distribution of the TES molecules between these different states as the superficial water is removed. Keeping in mind that siloxane bonds were recently evidenced by FTIR' in the case of irreversibly adsorbed TES, we have also carried out microcalorimetric liquid flow experiments in order to check the reversibility of adsorption. They show that irreversibility only occurs with the untreated Spherosil (Aerosil is not suitable for this kind of experiment). Among the various adsorption states quoted above (Figure 7), the siloxane bonds should be those leading themselves most easily to irreversibility. The main result of our investigation is therefore that siloxane bridge formation is possible only if physisorbed water molecules are present on the surface. Quantitatively, we get only 10% of irreversibility for the untreated sample. If we consider that siloxane bonds are formed only if surface water is present, this is a weak effect compared with that obtained for enthalpies of displacement (Figure 6). This discrepancy may find two explanations: the siloxane bond is not completely irreversible - t h e water surface content is lower in the case of a liquid flow experiment than in the case of a batch experiment Indeed, in the batch experiment, where the system is closed, the surface density of water is determined by the preparation of the solid (residual water from the solvent is negligible), whereas, in the liquid flow experiment, the surface density of water is fixed by its bulk equilibrium concentration which is both very low and unknown in the organic solvent. As shown by ref 19, flowing a dry solvent on a silica gel may remove most of the physisorbed water and is equivalent to a heat treatment at 140 "C. If our interpretations are correct, we then show the catalytic effect role of water in the formation of the siloxane bonds since this formation only occurs if water is present and leads to the formation of a new water molecule: -SiOH
+ (Et),SiOH
H20
+ -SiOSi(Et),
(A)
3714 J. Phys. Chem., Vol. 99,No. 11, 1995
Denoyel and Trens CIHJ
Figure 7. Possible interactions between T!3, surface water. and silica.
This influence of water on the siloxane bond formation has been evidenced recently by Vigil et al." in the case of sintering between two silica surfaces. It would now be interesting to derive the enthalpies of formation of both the siloxane bond and the H bond (Figure 6). All the enthalpy curves show a decrease as a function of coverage which is an indication of surface heterogeneities or of several reactions occurring at the same time. For example, the adsorbed phase on the untreated samples (which contains water) may contain both a siloxane bond and a H bond. On the other hand, the enthalpy of formation of a H bond may depend on the kind of silanol involved in the reaction (isolated, geminal, or vicinal silanol). The simplest method is to compare the curves at low coverage where the most energetic sites are involved. Moreover, up to a coverage of 0.4 pmol m-2, displacement enthalpies are almost constant, indicating a single mechanism of adsorption. As a consequence, assuming that only H bonds are created in the absence of physisorbed water, we get from Figure 6 AH = -40 kJ mol-I. With the same assumptions, the enthalpy change corresponding to reaction A (which only occurs with untreated samples) would be -75 kJ mol-'. However, the final state is not well defined since we do not know what happens to the water molecule prodnced. It is probable, since curves shown in Figure 6 correspond to a batch experiment, that the water molecule is itself adsorbed by the silica. This readsorption is possible because the surface is not fully covered by the TES molecules (see above paragraph on adsorption isotherms). As a consequence, it is not possible to firmly ascribe an enthalpy value to the siloxane bond formation. A value of -19.5 kJ mol-l (derived by Brunaner et al.2' from the heat of dissolution of silica samples with different states of dehydroxylation) is quoted by Iler,Zz whereas a calculation from the enthalpies of formati0n2' (assuming that the Si-0 bond energy, estimated from the C-0 bond energy, is the same in the silanol and siloxane groups) gives -25 kT mol-I. These values are consistent with ow results since the readsorption of water may be itself highly e~othermal.2~ Conclusion The main information bronght by the present work is the catalytic role played by the surface water in the chemisorption of triethylsilanol on a silica surface. This catalytic behavior has been evidenced in the past only for alkoxy- or chlorosilanes, which need to be hydrolyzed by water before reacting with the surface. The formation of a siloxane bond seems indeed to only
occw when some physisorbed water is present on the surface. Moreover, the water molecule produced during the condensation is probably adsorbed by the silica surface, bringing an exothermal contribution to the overall free energy of the process.
Acknowledgment. This work has received financial snppott from Saint Gobain Recherche. We thank Dr.H.Arribart from the Laboratoire Mixte Saint Gobain-CNRS and Drs. P. Chartier and E. Dallies from Saint Gobain Recherche for frnitful discussion.
References and Notes (1) De Haan, 1. W.; Van Den Bogaert, H. M.;Ponjee, I. I.; Van de Ven, L. I. M.3. Colloidlnlerfoce Sci. 1986. 110 (2). 591. (2) Nishiyama. N.; Shick, R.; Ishida, H. 3. Colloidlntefoce Sci. 1991, 143 (1). 413. (3) Ishida H.; Kcenig, J. L. 3. Colloid lnte@ce Sci. 1978, 64,555. (4) Ishida, H.; Kcenig, J. L. 3. Colloid lnterfoce Sci. 1978, 64,565. ( 5 ) Chiang, C.-H.; Koenig, I. L. 3. Colloid lntmface Sci. 1981.83 (2). 71. (6) Kelly, D. J.; Leyden, D. E. 3. Colloid lnterfnee Sci. 1991, 147 (1). 213.
(7) Flueddemann. E. P.In Si& Coupling Agents; Plenum Press: New York, London, 1991; pp 55-63. (8) Trens, P.;Denoyel. R. Langrnuir 1993, 9, 519. (9) Vrancken, K. C.; Van der Vmrt, P.;GiIlis d'Hamers, I.; Vansant, E. F. 3. Chem. Sac.,Faraday Tram. 1992.88 (21), 3197. (IO) Vrancken, K. C.; Casteleyn, E.; Possemien, K.; Van der V m t t P.; Vansant, E. F. 3. Chem. Soc., Famdoy Tmnr. 1993. 89 (W, 2037. (11) Azzopardi, M. 1.; Ani& H. J. Adhesion 1993, 6, 230. (12) Rouqueml. Themehim. Acta 1989. 144,209. (13) Davy, L.; Deuoyel. R.; Rouquerol, 1. 3. Colorimctrie Analyse Thcmique; Association Franpise de Calorim6hie et Analyse Thermique: Marseille, 1990; Vals. XX-XXI (Roceedings of the Calorimetry and Thermal Analysis Days, held at Clermont-Ferrand. May 1990). (14) Legrand. P.;Hommel, H.; Tuel, A.; Vidal, A,; Balm, H.; Pap-, E.; Levitz, P.; Czernichowski, M.; Erre, R.; Van Damme, H.; Gallas, 1. P.; Hemidy, J. F.; Lavalley, I. C.; B m s , 0.; Bumeau, A,; Grillef Y. Ad". Colloid lntcrface Sci. 1990. 33,91. (15) Zhuravlev, L. T. Longmuir 1987, 3, 316. (16) Unger, K. K. Porous Silica. J. Chromtogr. Ubr. 16. pp 6-11. (17) Denoyel, R.; Rouqueml, F.; Rouqueml, J. In F d a m e n t l s of Adsorption; Liapis, A. J., Ed.; Engineering Foundation and American Institute of Chemical Engineers: New York, 1987. (18) Johnson, 1.; Denoyel, R.; Rouquerol, 1.; Everett. D. H. Colloidr S u e 1990.49, 133. (19) Scott, R. P. W.; Traiman, S. 3. Chromtogr. 1980, 1% (2). 193. (20) VigJ, G.; Xu,2.;Steinberg. S.; Israelachvili. 1.3. Colloidlnterfnce Sci. 1994, 165, 367. (21) Bmnauer, S.;Kanfro, D. L.; Weise. C . H. Con. 3. Chem. 19%. 34.
1483.
(22) Uer, R. K. The Chemistry of Silicn; Wiley: New York, 1979 pp 645-646. (23) CODATA Task Group. J. Chem. Thcnnodyn. 1578, 10,903. (24) Fubini, B. T h e m h i m . Acta 1988,135, 19.
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