Adsorption mechanism of HTAB-octanol mixture onto silica sensor by QCM-D and MD simulation

Adsorption of a mixture of cationic surfactant, Hexadecyl Trimetehyleammonium Bromide (HTAB) and octanol on silica surface has been studied at various surfactant and octanol concentrations by QCM-D (Quartz Crystal Microbalance with Dissipation monitoring) and Molecular Dynamics (MD) simulations. In the QCM-D studies, frequency, and dissipation values with HTAB alone gradually and significantly increased from -2 to -18 Hz and 0.18 x 10-6 to 1.2 x 10-6, respectively. However, addition of octanol to HTAB was shown to achieve better adsorption properties due to the co-adsorption effect. The frequency values with increasing octanol concentrations at constant HTAB concentration of 5 x 10-4 M raised them to -10 to -20 Hz with the favorable help of hydrophobic forces. Dissipation values, on the other hand, were significantly improved in the presence of alcohol due to chain length contribution. In addition, increase in contact angle values with increasing octanol concentration at constant HTAB concentration supports the QCM-D results. Experimental studies were then corroborated by Molecular Dynamics (MD) simulations to clarify morphological and structural properties of surfactant in the presence and absence of octanol. MD simulations demonstrated that interaction of HTAB with the silica surface is much more conducive due to the presence of octanol in the medium.