A DIFFERENTIAL SCANNING CALORIMETRY STUDY OF KAPPA-CARRAGEENAN IN THENACL NAI/CSI/CSCL SYSTEMS AND ANALYSIS BY POISSON-BOLTZMANN CALCULATIONS/

The dependence on salt composition of the coil-helix-aggregate transit ion of kappa-carrageenan has been studied experimentally and theoretic ally. The transition enthalpy on cooling has been measured by differen tial scanning calorimetry (DSC). The salt composition was varied syste matically using mixtures of the four salts NaCl, NaI, CsI, and CsCl, w hich includes both aggregating and nonaggregating conditions. One anio n or cation have been exchanged for another, keeping the total salt co ncentration at 0.1 M. An electrostatic model, based on the nonlinear P oisson-Boltzmann equation solved in cylindrical symmetry, was used to describe the specific binding of cesium and iodide ions to the kappa-c arrageenan helix. This modeling resulted in a semiquantitative descrip tion of the variation of the ion binding and the charge density in the different salt mixtures. It was found that the model can reproduce th e transition enthalpies in the nonaggregated systems very well while i n the aggregated systems the model predictions deviates markedly from the experimental results. This deviation, interpreted as an aggregatio n enthalpy, varies with the extent of thermal hysteresis. The hysteres is occurs only when the charge density is lower than the charge densit y for the bare kappa-carrageenan helix (without bound ions). Another i nteresting observation is that the width of the DSC peak obtained on h eating increases drastically when the aggregation occurs. Finally, the uniaxial gel strength was found to depend linearly on the extent of t he hysteresis.