Titration microcalorimetry of adsorption processes in aqueous systems

Procedures are described for analysing enthalpograms characterising adsorption by macromolecules in solution recorded using a titration microcalorimeter. The experimental procedure involves injecting small aliquots of a solution containing adsorbate into a sample cell containing a solution of macromolecular adsorbent. Treatments based on both Langmuir and Frumkin adsorption isotherms are described. The procedures are illustrated by application of the derived equations to the interaction of micelles of sodium dodecylsulfate(aq) and of sodium decylsulfate(aq) with the water-soluble polymer, PVP. The dominant features in the recorded enthalpograms are described using equations developed from the Frumkin equation. In both cases the adsorption is endothermic attributed to hydrophobic interactions between polymer and surfactant. However, an important feature of the analysis is the characterisation of adsorbate–adsorbate interactions using enthalpic interaction parameters. The enthalpograms are characterised by three composition ranges: (i) micelle deaggregation and weak interaction of monomers with polymer, (ii) micelle adsorption on to the soluble polymer up to surface saturation and (iii) micelle dilution.