Associations in Mixtures of Hydrophobically Modified Polymer and Surfactant in Dilute and Semidilute Aqueous Solutions. A Rheology and PFG NMR Self-Diffusion Investigation

Abstract

Viscosity (dilute regime) and surfactant and polymer NMR self-diffusion (dilute and semidilute regime) measurements were carried out on aqueous solutions of ethyl(hydroxyethyl)cellulose (EHEC) and of a hydrophobically modified analogue, with (HM4-EHEC) and without (HM0-EHEC) a spacer, in the presence of various amounts of sodium dodecyl sulfate (SDS). Strong polymer−surfactant interactions were revealed. From the binding isotherm the critical aggregation concentration (cac ≈ 3 mm) was obtained for the EHEC/SDS system. By comparing the results from the binding isotherms with those from the rheological measurements, molecular interactions could be correlated to macroscopic properties. The polymer self-diffusion results suggest that the spin-echo attenuation can be described by a log-normal distribution (distribution of self-diffusion coefficients), giving a median polymer self-diffusion coefficient DME. There is no difference in the trend of DME between EHEC and the hydrophobically modified analogues at various SDS concentrations. The NMR signal decays of highly viscous samples of the modified analogues are initially described by a distribution mode followed by a single exponential (slow component). The effect of hydrophobic modification is seen in the existence of a slowly diffusing component (Dnet≈ 10-14 m2 s-1) (not observed for the unmodified analogue) that is related to a strong and long-lived network, at least of the order of the NMR time scale (ca. 0.5 s).