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 hydro phobically modified analogue (HM-EHEC) in water in the presence of var ious 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 hydrophobicall y modified polymer is less soluble than the unmodified EHEC at low con centrations of SDS, while at higher surfactant concentrations the solu bility properties of the two polymers are practically the same. The re sulting enthalpic titration curves from the calorimetric measurements on 0.25% polymer solutions at 25 degrees C consist of a pronounced end othermic peak (moderate SDS concentration) followed by a shallow exoth ermic one at higher surfactant concentrations. The prominent endotherm ic peak observed for both systems indicates a strong polymer-surfactan t interaction. From the shape of the curves, the onset of the saturati on stage of the binding process was estimated to be 20 mm for both pol ymers. The degree of surfactant binding to the polymer was found, at m oderate SDS concentrations, to be the same for EHEC and HM-EHEC. The l ight-scattering results for both the EHEC/ SDS and HM-EHEC/SDS systems revealed a decreasing angular dependence of the reduced inverse scatt ered intensity function S(0)S(q) (q is the wave vector) with increasin g surfactant concentration. It is observed at moderate levels of surfa ctant addition that the correlation length xi decreases more strongly for the EHEC/SDS system than for the HM-EHEC/SDS system. By plotting t he static light scattering data in the form S(xi)/S(0) versus the dime nsionless 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 sy stems.