Variable Temperature FT-IR Studies of n-Alkyl Modified Silica Gels

Abstract

Silica gels, chemically modified with n-alkyl chains of various lengths (CnH2n+1, n = 8, 9, 10, 12, 16, 18, 22, and 30), in the dry state were examined over a broad temperature range (123−353 K) via infrared spectroscopy. Here, for the first time a comprehensive variable temperature FT-IR study is available dealing with the conformational order of alkyl modified silica gels. The conformational state of the alkyl chains in their bound state was monitored via various conformation-sensitive vibrational bands. A qualitative statement about the conformational order was possible based on the analysis of the symmetric and antisymmetric CH2 stretching bands. Likewise, CH2 wagging bands were used to identify and determine the relative amountsi.e., integral values over the whole chainsof gauche-trans-gauche, double gauche, and end gauche conformers. The amount of gauche conformers at a specific site in the alkyl chains was accessible via the analysis of CD2 rocking bands of samples bearing selectively deuterated alkyl chains. These latter data are consistent with those obtained from a complementary analysis of the CD2 stretching vibrational bands. The present study lucidly demonstrates that the conformational state of the chemically attached alkyl chains strongly depends on temperature, actual chain length, and chain position. The total number of gauche conformers per chain of the systems examined here varies between 2.4 and 5.1, depending on the actual sample temperature and alkyl chain length. The total number of gauche conformers is found to vary linearly with temperature. Shorter alkyl chains, such as octyl and decyl chains, are found to possess a high degree of conformational disorder. The observed increase in alkyl chain length is accompanied by a decrease in conformational disorder, most probably due to a better chain packing. For the C18 modified system, selectively deuterated samples were available. Here, at room temperature the percentage of gauche conformers for positions C-4, C-6 and C-12 are 40%, 20%, and 25%, respectively, reflecting a pronounced position dependence for the degree of conformational order. The present data on the conformational alkyl chain order are discussed in conjunction with studies on other n-alkyl modified silica gels as well as related systems involving n-alkyl segments in various environments, such as pure hydrocarbons, biological membranes, and self-assembled monolayers (SAMs). On the bases of the results from the present variable temperature FT IR study, overall structures of the alkyl chains, chemically attached to the silica surface, are proposed.