Effects of the nanoassociation of hexadecyltrimethoxysilane precursors on the sol-gel process

Modifications to the refractive indices of meso-structured organic-inorganic films caused by variations in the mole fraction of precursors in ethanolic solutions were investigated. The refractive indices were dependent on the mole fraction of C16TMS (hexadecyltrimethoxysilane) and of the C16TMS/TMOS (tetramethoxysilane) (1/1) mixture in ethanol. The dependency was determined to be nonlinear, and the phenomenon was attributed to self-assembly caused by the long alkyl groups (C-16) on the C16TMS. Changes in the maximum decreasing rate of d(n)/d(x) values [(d(n)/d(x))(max)-for d(n)/d(x) estimation, the curves from Fig. 1 were associated with a polynomial; using a dedicated program, d(n)/d(x) was calculated; maximum values of d(n)/d(x) were taken into account and were included in Table 1)] were used to distinguish the behavior of alcoholic precursor mixtures. In the case of using pyrene as a fluorescent probe, the ratio between two peaks from the pyrene emission spectra (I-1/I-3) strongly decreased as hydrophobic micro-surroundings formed due to the self-assembly process. The UV-VIS spectra of a cationic dye solution, R6G, was studied because dilute solutions of the dye in equilibrium form measurable ratios of dimers and monomers. The absorbance was modified as micro-surroundings with different polarities were formed. These three methods demonstrated that nano-structuration was present prior to the occurrence of the sol-gel process. The combination of C16TMS with other alkyltrialkoxysilane precursors having hydrocarbon chain lengths between C-1 and C-8 provided further evidence for a nonlinear dependence of the refractive indice and fluorescence spectra of pyrene. The properties of the final hybrids obtained by the sol-gel process were significantly affected by the precursor hydrocarbon chain lengths. DSC, XRD and FTIR measurements were used to show the plasticizing phenomena of C-16 as other alkyltrialkoxysilanes (i.e., C-1-C-8) were added.