J. CHEM. SOC. FARADAY TRANS., 1992, 88(21), 3197-3200 3 I97

Influence of Water in the Reaction of y-Aminopropyltriethoxysilane with Silica Gel A Fourier-transform Infrared and Cross-polarisation Magic-angle-spinning Nuclear Magnetic Resonance Study

Karl C. Vrancken," Pascal Van Der Voort, lwan Gillis-D'Hamers and Etienne F. Vansant Laboratory of Inorganic Chemistry, University of Antwerp, Universiteitsplein I , B-2610 Wilrijk, Belgium Piet Grobet Laboratory of Surface Analysis, Faculty of Agricultural Sciences, Catholic University of Leuven, Card. Mercierlaan 92, B300 I Heverlee, Belgium

Silica gel has been modified with y-arninopropyltriethoxysilane under varying conditions, controlling the influ- ence of water in the different modification stages. Diffuse reflectance infrared Fourier-transform (DRIFT) spectra revealed the influence of surface water in the reaction stage and of air humidity in the curing stage. These results were confirmed and refined by 29Si and ' 3C cross-polarisation magic-angle-spinning nuclear magnetic resonance (CPMASNMR) spectroscopy. Combining the results of both techniques, four modification structures present on the silica surface are proposed, depending on the conditions used.

Organosilane-modified substrates find a wide variety of applications in science and industry.' One of the most com- monly used is silica gel coated with y-a! ninopropyltrieth- oxysilane [APTS, (CH,CH20),SiCH2CH2CH2NH2]. The applications of this compound include chromatography,2 trace-metal c~ncen t r a t ion ,~ ,~ composite synthesis5 and protein imm~bi l iza t ion .~*~ Because of the widespread use, there is much interest in the reaction mechanism and in the parameters controlling the coating efficiency. Many surface analysis techniques have therefore been applied.* In order to study the chemical structure and interactions of the surface compound, FTIR has been recognised as a very powerful

Complementary information on the chemical environment of the Si and C atoms in the coating can be obtained from CPMASNMR.'4 -' Furthermore studies with X-ray photoelectron spectroscopy (XPS),' 8,1 secondary ion mass spectrometry (SIMS)20 and atomic emission spectros- copy (AES)21 techniques have been reported. Because of the large variety in reaction parameters and the different condi- tions used in each of these studies, no straightforward general reaction scheme could be derived.

The presence of water in the system is one of the most important parameters in the reaction sequence. Water induces hydrolysis of the ethoxy groups, leading to silanol groups which can combine to form a siloxane linkage between two silane molecules, with production of a new water molecule (reaction 1).

2ESi-OEt + 2 H 2 0 --* 2=Si-OH + 2EtOH

+ H2O (1) SSi-OH + HO-Sie -+ ESi-O-SiG

In order to control this parameter, modification procedures in dry organic solvents have been set up.',

Experimental A 1% v/v mixture of APTS (Al100, UCAR) in toluene (p.a., Merck) was reacted with 1 g of Kieselgel 60 (Merck). Sample- specific conditions are indicated in Table 1. The toluene and APTS were predried on molecular sieves. After filtration, the modified substrate was cured for 20 h in air at 383 K or at 423 K under vacuum. For one sample a precuring in air for 2 h was performed.

DRIFT spectra were recorded on a Nicolet 20 SXB FTIR spectrometer, with 4 cm- resolution, equipped with the Spectra-Tech 'Collector' diffuse reflectance accessory, after diluting the sample with 95% of oven-dried KBr. Band inte- gration was performed with standard Nicolet software after conversion to Kubelka-Munk units. Values were ratioed to the 1938-1766 cm-' Si-0 reference band."

The 29Si (79.5 MHz) and the I3C (100.6 MHz) CPMASNMR were performed on a Bruker 400 MSL spec- trometer. The 29Si C P experiments were performed using a contact time of 5 ms, a recycle time of 3 s, a spinning rate of 3.5 kHz and a number of scans between 3000 and 8000. In the 13C CPMAS experiment the contact time was 2.5 ms, the recycling time 3 s, the spinning rate 4 kHz and the number of scans 10000.

Results and Discussion

The modification proceeds in three stages. (i) By thermally pretreating the silica gel the degree of hydration of the surface was varied. When heated at 673 K the surface was completely dehydrated.22 (ii) The pretreated silica gel was allowed to react for 2 h with an APTS solution. The water content of the solvent was varied in order to study its influ- ence on the modified structure. (iii) After the substrate had been filtered off it was treated thermally overnight to stabilise the coating (curing). It is in this stage that most of the chemi- cal bonds with the substrate surface are formed.

Table 1 of the siloxane band (1350-870 cm- ')

Sample preparing conditions and ratioed integrated values

curing conditions SiOSi peak

sample pretreatment solvent (20 h) Surface

1 none C7Hu 383 K, air 102.9 2 673 K, air H*O 55.30 3 673 K, air 1% H,O H,H, 383 K, air 46.94 4 673 K, air C,", 383 K, air 33.78 5 673 K, air C7Hn 423 K, vac ~

423 K, vac -

7 673 K, vac C7Hu 383 K, air (2 h)

383 K, air

6 673 K, vac C7Hn

+ 423 K, vac

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3198 J. CHEM. SOC. FARADAY TRANS., 1992, VOL. 88

The influence of water is important in all three reaction stages and will be discussed separately.

Reaction on Hydrated Silica Gel

Hydrated silica gel was modified with APTS and studied by DRIFT and CPMASNMR. The IR spectrum of the modified silica (Fig. 1) shows silane NH, CH and Si-0-Si bands together with silica lattice and surface vibrations, as pre- viously described by Chiang et a1.' The influence of water can be investigated in studying the CH, and Si-0-Si band areas, because hydrolysis and subsequent polymerisation will decrease the former and increase the latter. In practice the asymmetrical CH, stretching (5 = 2978 cm- ') and the Si-0-Si stretching (1350-870 cm- ') bands are most suited.

The position of the NH, vibrational modes indicates a per- turbation of the NH, group. Hydrogen bonding shifts the maxima of the stretching vibration bands to below 3370 and 3310 cm-l, respectively.

The absence of the CH, band and the magnitude of the Si-0-Si band together with the position of the NH, vibra- tion modes (3367, 3300 cm-') indicate the formation of a polymerised aminopropylsiloxane coating with NH, groups interacting with Si-OH groups by means of hydrogen bonding (Fig. 2, structure I). These observations are con-

1 ' . I

4000 3500 3000 2500 2000 1500 1000 500

wavenumber/cm-

Fig. 1 toluene solution dried in air

DRIFT spectrum at hydrated silica reacted with APTS in

category ( i ) surface water, water solvent

R' 0 R' \ I \ I

I \ I \ Si Si I

0 0 0 O-Si-

= silica surface ,H

R'= (CH2)3tN 0%

A j i

category (ii) modification of dry silica gel (a ) air curing

R' 0 R' \ I \ \ I \ I \ I Si H Si\ Si Si

I \ I \ f \

I I I I I I I

7 CHBCH~O J(CH& - N ' 0 R'

0 0 0 0 CH3CH20 0 0 0

(b ) vacuum curing

CH3CH20 R' \ I

I \ Si IV

0 0

Fig. 2 Bonding models of APTS on silica gel

firmed by the ,'Si and 13C CPMASNMR spectra of the modified compound [Fig. 3(a) and (b)]. Concerning the number of siloxane bonds around the central Si atom, the 29Si peaks at -68.2 and -59.5 ppm are assigned to (a) tri- dentate and (b) bidentate bonded Si in the APTS molecule, re~pective1y.l~ The peak corresponding to (c) the mono- dentate form is negligible, while (d) the hydrolysed mono- dentate form is absent.

R R

(c 1 ( d ) -52.9 ppm -49.2 ppm

(a 1 (b 1 -67 ppm -59 ppm

The 29Si spectrum [Fig. 3(a)] indicates the coating to be hydrolysed and crosslinked. After deconvolution of the recorded spectrum into a sum of Gaussian curves, the differ- ent contributing signals can be integrated separately. The ratio of the integrated values can be used to determine the relative population of the different surface structures present." From the integration data the bi- dentate : tridentate is calculated to be 40 : 60. The presence of the monodentate form can be neglected in comparison to the amount of the other two forms. Because three covalent bonds with the surface can be excluded due to steric reasons, the tridentate form implies polymerisation of the APTS mol- ecules.

The 13C spectrum [Fig. 3(b)] indicates a total loss of ethoxy groups as no peaks are observed at 57.6 ppm (-OCH,-) and 15.9 ppm (-OCH,CH,). The three observed peaks have been assigned14 to the propyl carbons (a-CH = 9.5 ppm, B-CH = 26.3 ppm, y-CH = 43.3 ppm). The 13C shift value of the a-CH is an indication of the stability of the aminosilane coating on the silica surface. An increase in the APTS coating stability shifts the a-CH peak to a higher shift value (APTS solution a-CH = 7.9 ppm).16 The observed

R'-H Or Et

50 40 30 20 10 0 6

A

' ' .- -40 -60 -80 -100 -120 -140

6

Fig. 3 29Si CPMASNMR spectrum; (h) 13C CPMASNMR spectrum

Hydrated silica, modified in toluene solvent, air cured. (a)

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J. CHEM. SOC. FARADAY TRANS., 1992, VOL. 88 3199

peak position of the a-CH (9.5 ppm) indicates covalent attachment of the silane molecules to the silica surface.

Surface water can cause hydrolysis and polymerisation of the APTS coating, leading to a coating with 60% of the mol- ecules having a structure of type I (Fig. 2). The residual 40% occur as the type I1 structure in the hydrolysed form.

Water in the SolventSilica System

Various experiments were performed on dehydrated silica gel, using different amounts of water in the reaction phase. Polymer formation is evaluated by integrating the Si-0-Si stretching band. The results are summarised in Table 1. By ratioing the integrated value to the internal reference band (1938-1766 cm- ‘)lo variations in the silica surface (Si-0-Si) contribution to the different samples are ruled out. Integrated values show a clear increase with increasing water content of the solvent. It is therefore obvious that the amount of water is the determining factor in the polymeris- ation process. Additionally, the DRIFT spectrum of the sample reacted in toluene solvent (sample 4) shows residual ethoxy groups, whereas the aqueous samples (samples 2, 3) do not have the CH, band. Comparing the Si-0-Si peak surface of the dehydrated, aqueous-modified sample (2) to the hydrated sample reacted in toluene (sample 1) shows an increase in the amount of polymerised silane molecules on the silica surface for the latter sample. Therefore it can be concluded that the polymerisation takes place on the silica surface, i.e. after an adsorption of the aminosilane molecules. The surface effect can be explained by the interaction of the silane NH, group with the substrate surface. In water solvent the hydrolysed aminosilane molecules are believed to be sta- bilised by internal hydrogen bonding of the amino group to the silane hydroxyls. When the amino group is H bonded to a surface hydroxyl group this stabilisation disappears and the silane silanols can condense to form a siloxane linkage. When the reaction is performed with hydrated silica in a dry solvent (sample l), the hydrolysis only takes place at the silica surface and can immediately be followed by the condensation reac- tion. In both cases, structures of type I are formed.

Reaction in a dry solvent with dehydrated silica (sample 4) minimises the hydrolysis and polymerisation in the reaction stage. Bonding takes place by direct condensation of the silane silicon with a silica surface hydroxyl, with loss of ethanol.

Air Humidity in the Curing Stage

In order to study the influence of air humidity in the curing stage, post-reaction curing was performed in air at 383 K (sample 4) and in vacuum at 423 K (sample 5). In both samples dehydrated silica was reacted in a dry solvent, mini- mising the influence of hydrolysis in the first two modifi- cation stages. DRIFT spectra of both samples (Fig. 4) show a higher ethoxy content for the vacuum-cured sample. Because of the higher curing temperature a higher degree of covalent bonding to the silica surface, i.e. a lower ethoxy content, is expected for this sample.I7 The observed loss of ethoxy groups can be attributed to the influence of air humidity in the curing stage, inducing hydrolysis, possibly followed by oligomerisation, even for samples reacted in dry conditions. The polymerisation, however, is reduced, com- pared to the reaction in the presence of water. The existence of silane silanols, oligomers and residual ethoxy groups can be concluded (Fig. 2, structure I1 and 111). Only when the reaction and the curing stage are performed without any water present (sample 5 ) hydrolysis can be prevented, leading

3050 2920 2790

wavenum ber/cm ~ ’ Fig. 4 DRIFT spectra of APTS-modified silica gel: (a) air cured; (h) vacuum cured

These results are again confirmed by CPMASNMR mea- surements. Vacuum-pretreated vacuum-cured samples were prepared, with minimal water contact. The influence of air humidity in the curing stage was tested by precuring one sample in air for 2 h (sample 7). I3C NMR spectra are pre- sented in Fig. 5. Ethoxy peaks are visible at 57.6 ppm (-OCH,-) and at 15.8 ppm (-OCH,CH,). The ethoxy content of the air precured sample [Fig. 5(a)] is clearly reduced, compared to the non-precured sample [Fig. 5(b)]. The 29Si spectra, however, show no clear difference in relative species population. Both samples have 60% of the surface species in the bidentate form, with 20% each in monodentate and tridentate form. The bidentate species, however, covers molecules with R’ = H as well as R’ = CH,CH,. A decrease in the ethoxy content with constant population of the biden- tate species indicates the conversion of the -OCH,CH, into

I

70 60 50 40 30 20 10 0 6

Fig. 5 CPMASNMR spectra of dry treated APTS-modified to a type IV surface structure. silica: (a) 2 h, 383 K air precured; (b) 423 K-vacuum cured

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3200 J. CHEM. SOC. FARADAY TRANS., 1992, VOL. 88

an -OH group, without further condensation. This conver- sion should be attributed to the influence of the air humidity. The coating in sample 7 mainly has the type I1 structure. The stability of the OH group is caused by a hydrogen bond with the NH, group of a neighbouring silane molecule or by the isolated character of these groups at the surface. Air humidity therefore, appears to have a clear influence on the hydrolysis of surface-bonded APTS molecules.

Conclusion Besides the influence of surface water, the contribution of air humidity to the hydrolysis of APTS molecules adsorbed on the silica surface has been characterised. Short time exposures to humid air cause partial hydrolysis of the modified layer. After an extensive hydrolysis by surface water, aminosilane molecules condense to form an aminopropylpolysiloxane layer. Only when all three modification stages (pretreating, reaction, curing) are performed in completely dry conditions, can hydrolysis of ethoxy groups be prevented. The structures formed under the various modification conditions are sum- marised in Fig. 2.

The authors wish to thank Mrs. H. Geerts of K.U.L. for kindly performing the CPMASNMR measurements and for valuable discussions. K. C. Vrancken is indebted to the NFWO/FNRS as a research assistant. I. G.-D’H. acknowl- edges the financial support of the IWONL/IRSIA.

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Paper 2/03237F; Received 19th June, 1992

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