Application of ordered mesoporous silica in adsolubilisation of alcohols by conventional and gemini cationic surfactants

Abstract

Application of ordered porous silica in adsolubilisation of primary and aromatic alcohols by conventional and gemini cationic surfactants has been tested in aqueous media at 298 K. Benzyldimethyldodecylammonium bromide (BDDAB) and two alkanediyl-α,ω-bis(dodecyldimethylammonium bromide) surfactants (C 12C SC 12) with ethanediyl (C 2) or hexanediyl (C 6) spacer were adsorbed onto microporous SiAl32 d18 or mesoporous SiAl32 d50 aluminosilicate of the MCM-41 type (the framework Si:Al ratio is equal to 32; d18 or d50 refers to the mean pore diameter in angstroms). The reversibility of adsorption of both surfactant ions, i.e. the surface-active cation and its counter-ion, the ion exchange with sodium cations, the evolution of some other parameters with surfactant adsorption, such as the pH of the equilibrated supernatant, the electrophoretic mobility of the solid particles and the differential molar enthalpy of displacement, were first studied taking BDDAB and purely siliceous Si d29 as an example of the adsorption system. Adsolubilisation of 1-butanol (C 4OH) by BDDAB adsorbed onto SiAl32 d18 and SiAl32 d50 was quantified either as a function of the equilibrium alcohol concentration at a fixed surfactant content of 12 mmol kg −1 or as a function of the surfactant concentration from a 10 mmol kg −1 NaBr solution at a fixed alcohol content of 10 mmol kg −1. The uptake of phenol (PhOH) by C 12C SC 12–SiAl32 d50 systems was monitored as a function of the equilibrium surfactant concentration at a fixed alcohol content of 1.5 mmol kg −1. The individual adsorption isotherms of the surface-active cation and alcohol additive were supplemented by the measurements of the differential molar enthalpy of displacement in systems with and without additive. Transfer of solubilisate molecules from the aqueous phase to the interfacial aggregates contributes to the overall exothermicity of the displacement phenomenon. Compared to non-porous silica supports, ordered mesoporous aluminosilicates are much more efficient as adsolubilisation host systems for low molecular weight alcohols.