Salt influence in the polymer-surfactant interaction in solution. A fluorescence probe investigation of the EHEC/SDS/ water system

Abstract

The effect of small amounts of salt on the interaction between two fractions of ethyl(hydroxy)ethyl cellulose, EHEC, and sodium dodecyl sulfate, SDS, has been investigated by means of steady-state fluorescence measurements. The two polymer fractions display different properties in hydrophobicity expressed as different cloud points. The results are discussed in relation to hydrodynamic (viscosity) and thermodynamic (equilibrium dialysis) properties. The micropolarity as sensed by the probe pyrene shows that the polymers begin to interact with SDS at a lower concentration in the presence of salt. The average aggregation numbers of polymer-bound clusters, Np, were obtained by fluorescence-quenching data in combination with equilibrium dialysis experiments. Np was found to increase in the presence of salt for the EHEC fraction with a high cloud point (CP). The polymer with a low CP displays higher Np in the presence of salt at low SDS concentrations, but exhibit lower Np at higher SDS concentrations than in the salt-free system. The microviscosity index as determined by intramolecular excimer formation of l,3-di(l-pyrenyl)propane (P3P) is highest for the lowest Np and there is a correlation with Np in the presence as well as absence of salt for both EHEC fractions. It is found that when the same fractional amount of SDS is bound to the polymers, 10–20% of the value of saturation, the increase in macroviscosity occurs and the microviscosity shows high rigidity.