Calorimetric study of phenol adsolubilisation by cationic surfactants adsorbed on a flat silica surface or confined within small mesopores of powdered MCM-41 aluminosilicates

Adsolubilisation of phenol by cationic surfactants adsorbed on a flat silica surface or confined within small mesopores of powdered MCM-41 aluminosilicates has been studied at room temperature by means of titration calorimetry. In aqueous solution of dodecyltrimethylammonium bromide (DTAB), the transfer of phenol to the outer portions of the surfactant micelles was considered to account for the slight depression of the CMC value and for the marked exothermic effect adding to that of the micellisation process. For the system containing non-porous X015M and DTAB, the determination of surfactant adsorption and phenol adsolubilisation isotherms was supplemented by calorimetric measurements of the differential enthalpy changes accompanying both phenomena. The displacement process was more exothermic when carried out in the presence of phenol. The uptake of aromatic molecules in the DTAB-X015M system was markedly increased compared with the adsorption of phenol onto bare silica surface. Aluminosilicate-encapsulated surfactant aggregates were prepared using two alkyltrimethylammonium bromides, i.e. C12 and C16, as the solid-structure-directing agents. The phenol adsolubilisation isotherms in both aluminosilicate–surfactant systems and the related enthalpy curves were determined over a wide range of phenol molality in the equilibrium bulk solution. For the two adsolubilisation host systems both with porous and non-porous substrate, phenol was shown to be localised on at least two different types of adsolubilisation sites, in order of decreasing thermal effect of transfer from the aqueous phase.