Titration microcalorimetry of poly(vinylpyrrolidone) and sodium dodecylbenzenesulphonate in aqueous solutions

Microcalorimetric titrations are carried out on solutions containing the an ionic surfactant sodium dodecylbenzene-sulphonate (SDBS), and mixtures of S DBS and the uncharged polymer poly(vinylpyrrolidone) (PVP). Measurements ar e taken at different temperatures. Micellisation of SDBS is driven by hydro phobic bonding. The interaction enthalpy of mixed PVP/SDBS systems shows cl early a consecutive endothermic and exothermic region with increasing surfa ctant concentration. The endothermic part can be looked upon as an incremen tal binding isotherm and reflects the number of surfactant molecules involv ed in the association process. The exothermic region features inverse hydro phobic bonding behaviour. This is related to the flexible nature of the ads orbent, i.e. the polymer. Electrostatic repulsion between neighbouring surf actant molecules causes at increased surfactant concentrations structural r earrangements of the polymer-surfactant complexes. This is accompanied by l osing inter- and intrachain linking and entropy gain since the expanded com plexes can move more freely. Additional surfactants continue to adsorb on t he vacant hydrophobic adsorption sites. The influence of the initial amount of polymer and the electrolyte concentration support our proposals. (C) 19 99 Elsevier Science B.V. All rights reserved.