Effect of Surfactant on Structural and Thermodynamic Properties of Aqueous Solutions of Hydrophobically Modified Ethyl(hydroxyethyl)cellulose

Solution properties of ethyl(hydroxyethyl)cellulose (EHEC) and a hydrophobically modified analogue (HM-EHEC) in water in the presence of various amounts of sodium dodecyl sulfate (SDS) have been investigated by cloud point (CP) measurements, titration microcalorimetry, and static light scattering. The CP experiments revealed that the hydrophobically modified polymer is less soluble than the unmodified EHEC at low concentrations of SDS, while at higher surfactant concentrations the solubility properties of the two polymers are practically the same. The resulting enthalpic titration curves from the calorimetric measurements on 0.25% polymer solutions at 25 degrees C consist of a pronounced endothermic peak (moderate SDS concentration) followed by a shallow exothermic one at higher surfactant concentrations. The prominent endothermic peak observed for both systems indicates a strong polymer-surfactant interaction. From the shape of the curves, the onset of the saturation stage of the binding process was estimated to be 20 mm for both polymers. The degree of surfactant binding to the polymer was found, at moderate SDS concentrations, to be the same for EHEC and HM-EHEC. The light-scattering results for both the EHEC/ SDS and HM-EHEC/SDS systems revealed a decreasing angular dependence of the reduced inverse scattered intensity function S(0)S(q) (q is the wave vector) with increasing surfactant concentration. It is observed at moderate levels of surfactant addition that the correlation length xi decreases more strongly for the EHEC/SDS system than for the HM-EHEC/SDS system. By plotting the static light scattering data in the form S(xi)/S(0) versus the dimensionless parameter q xi a universal picture emerged. In the regime q xi > 1, the power law S(q) similar to q(-1.6) was observed for both systems