The determination of the structure of assemblies of hexadecyltrimethoxysilane (HDTMS) on silica particles were made using temperature–modulated differential scanning calorimetry (TMDSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and powder X-ray spectroscopy. At small adsorbed amounts (&lt; 0.6 mg/m2), HDTMS molecules were directly bound to the silica surface through covalent bonding, and their aggregates were primarily amorphous. Direct attachment to the silica surface was found to significantly lower the enthalpies for both melting and crystallization of HDTMS. The enthalpies for the melting of directly bound HDTMS molecules increased linearly with their adsorbed amount until 0.6 mg/m2. Compared to bulk HDTMS, these bound HDTMS molecules on silica decomposed at higher temperatures. At larger adsorbed amounts of 0.6 mg/m2 or more, e.g. "multilayer region", both melting and crystallization enthalpies for the HDTMS were found to increase exponentially, and eventually, approach the bulk enthalpies. The significant enthalpy changes for the melting/crystallization transitions indicated that the adsorbed HDTMS molecules underwent structural changes, from directly bonded on the surface to bulk-like with an exponential constant of about 1.6 mg/m2.

Development of structure in adsorbed hexadecyltrimethoxysilane on silica

256th National Meeting (2018)

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