Associations in mixtures of hydrophobically modified polymer and surfactant in dilute and semidilute aqueous solutions.: A rheology and PFG NMR self-diffusion investigation

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 approximate to 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 D-ME There is no difference in the trend of D-ME 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 (D-net approximate to 10(-14) m(2) 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).