Thermodynamics of interaction between some cellulose ethers and SDS by titration microcalorimetry - I. EHEC and HPMC

The interaction between certain nonionic cellulose ethers (ethyl hydroxyeth yl cellulose and hydroxypropyl methyl cellulose) and sodium dodecyl sulphat e (SDS) has been investigated using isothermal titration microcalorimetry a t temperatures between 25-50 degrees C. The observed heat how curves have b een interpreted in terms of a plausible mechanism of the interaction of the substituent groups with SDS monomers and clusters. The data have been rela ted to changes occuring in the system at the macro- and microscopic levels with the addition of surfactants and with temperature. The process consists predominantly of polymer-surfactant interactions initially and surfactant- surfactant interactions at the later stages. A phenomenological model of th e cooperative interaction (adsorption) process has been derived, and earlie r published equilibrium binding data have been used to recover binding cons tants and Gibbs energy changes for this process. The adsorption enthalpies and entropies have been recovered along with the heat capacity change. The enthalpic cost of confining the nonpolar regions of the polymers in surfact ant clusters is high, but the entropy gain from release of hydration shell water molecules as well as increased freedom of movement of these nonpolar regions in the clusters gives the process a strong entropic driving force. The process is entropy-driven initially and converts to being both enthalpy and entropy-driven at high SDS concentrations. An enthalpy-entropy compens ation behavior is seen. Strongly negative heat capacity changes have been o btained resulting from the transfer of nonpolar groups from aqueous into no npolar environments, as well as a reduction of conformational domains that the chains can populate. Changes in these two components cause the heat cap acity change to become less negative at the higher binding levels. The syst em can be classified as exhibiting nonclassical hydrophobic binding at the later stages of binding. (C) 1999 Academic Press.