Effect of anilines as a synthesis component on the hydrophobicity of silica

A. C. TURALLAS, G. ROMANELLI, P. VÁZQUEZ

STUDIES IN SURFACE SCIENCE AND CATALYSIS

Elsevier

Año: 2005 vol. 155 p. 511 - 516

0167-2991

Introduction In the field of polymeric systems an important goal is to reinforce the polymer matrix and to improve mechanical and thermal properties. Inorganic additives are introduced into polymer systems as fillers or as reinforcing agents. Among the inorganic materials, the silica receives considerable interest because different degrees of hydrophobic/hydrophilic are determining its dispersion in the polymeric matrix. The silica can have hydrophobic properties when its surface is functionalized. This process requires the presence of silanol groups at the surface. There are many types of silica differing in their surface properties, in particular, the density of silanol groups and the types of silanols (isolated, hydrogen bonded, geminal) vary by techniques of preparation of silica. The purpose of this investigation is to know the grafting process on silica prepared by sol-gel. The principle of the investigation work consists in comparing the grafting of anilines, with different active groups, on silica to obtain different surface degree of hydrophobicity. For a better understanding of these solids, the characterization of nature species were determined by FT-IR, XRD, DTA-TGA and surface area (SBET).   Experimental   Silica preparation by sol-gel technique. Pure silica was prepared according to a home-adapted method, using TEOS, ethanol and acetic acid as catalyst. Funcionalized silica. The silica was prepared by addition of different anilines (these were prepared in our lab, named as A42 and A45, respectively, and present in sheme 1) to a suspension of silica in refluxing toluene, stirred for 5 h. The solid was filtered, washed in a soxhlet apparatus with diethylether and dichloromethane and then dried at 393 K (1).   Results and Discussion   Figure 1 shows the FT-IR spectra of silica and functionalized silica with A42 (A45) anilines at room temperature (r.t.) and at 120 °C. The main differences between functionalized silicas spectra and that of the pure silica are due to: the N-H     O N H O N H OCH3 A45 A42     Scheme 1     stretching band (3300-2600 cm-1, broad peak, overlapped with the stretching bands of silanol of silica and -CH2- groups), overtone band (1975 cm-1). Two possible vibrational bands: asymmetrical ?NH3+ bending band (1600 cm-1) and the symmetrical ?NH3+ bending band (1500 cm-1). The silica framework vibrational bands (1200 shoulder, 1054 very strong and 940, 790, 570 and 440 cm-1 were consistent with previous report (2). The Si-C stretching band (1147 cm-1) overlapped with the strong 1200 and 1050 cm-1 vibrational bands of the Si-O-Si framework. On the other hand, the grafting of silica surface decreases the surface area, these results suggest that a considerable grafting of silanol groups takes place at the work conditions used. Figure 1. FT-IR spectra of functionalized silica   The mode of anchorage of the organic compounds is of prime importance for the characteristics of the final functionalized silica. In the Table 1 are presented the values of water and ethanol to determine the degree of hydrophobicity. This degree is very high but it  evidence the preservation of the hydrophilic patches of the surface silica. This hypothesis has been taken into account during the functionalization procedure in solvent as toluene, perhaps in polar solvents this behaviour is made by other alternative routes. The final mechanism for our systems is in study and these results will be analyse according to different possible mechanisms in order to discriminate between them.   Table 1. Degree of hydrophobicity   Water (ml) Etanol (ml) Silica 0.33 0.14 Silica A42-r.t 0.12 0.06 Silica A42-120 0.04 0.03 Silica A45-r.t 0.28 0.17 Silica A45-120 0.19 0.20