Effect of Silica on the Structure of Cetyltrimethylammonium Bromide

The association structures of cetyltrimethylammonium bromide (CTAB) in the presence of fumed silica nanoparticles were studied by temperature-modulated differential scanning calorimetry (TMDSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and powder X-ray diffraction. CTAB association structures were found for the adsorption of surfactant on the silica surface namely: a monolayer, a second layer that completed a bilayer, and multilayer structures. The silica surface was found to lower both the melting and crystallization temperatures of the CTAB tails. Higher decomposition temperatures were observed for the near-surface structures compared to bulk CTAB. The enthalpy and entropy changes for CTAB during the melting and crystallization indicate that the CTAB molecules underwent significant structural changes, from surface to bulk-like structures. The enthalpies of melting and crystallization over a wide range of compositions were well modeled with the contributions from a surface monolayer (very small enthalpy of melting, less than 5 J/g, which increased linearly with adsorbed amount), a second layer corresponding to completion of a bilayer (small enthalpy of melting, around 58 J/g), and multilayers whose enthalpy increased exponentially from the bilayer to near the bulk value with an exponential constant of 1.2 mg/m(2).