Heat capacities and volumes of suspensions in the presence of surfactants

Density and heat capacity measurements of water-surfactant-silica tertiary systems were carried out as functions of the silica and the surfactant concentrations at 298 K. From these properties, the apparent molar volume and heat capacity of the surfactant in a given water+silica mixture were evaluated. As well, the apparent specific volume and heat capacity of silica, at a given concentration, in the water + surfactant mixtures as functions of the surfactant concentration were calculated. Some surfactants (decyltrimethylammonium bromide, sodium octanoate, sodium decanoate, sodium dodecanoate, sodium decylsulfate, sodium perfluorooctanoate and N,N-dimethyidodecylamine-N-oxide) were chosen to study the effect of the head-group and the hydrophobicity. As a general feature, the hydrophobicity of the surfactant does not essentially play a role on the silica-surfactant interactions whereas the surfactant head-group does. Finally, the apparent specific properties of silica are very sensitive in detecting the destabilization of suspensions whereas the apparent molar properties of the surfactant are not. (C) 2004 Elsevier B.V. All rights reserved.