Effect of surfactant on dynamic and viscoelastic properties of aqueous solutions of hydrophobically modified ethyl(hydroxyethyl)cellulose, with and without spacer

Dynamical, rheological, and some structural properties of 1 wt % aqueous so lutions (semidilute regime) of ethyl(hydroxyethyl)cellulose (EHEC) and of a hydrophobically modified analogue, with (HM4-EHEC) and without (HM0-EHEC) spacer (the spacer consists of four EO groups), in the presence of various amounts of sodium dodecyl sulfate (SDS) have been investigated by different experimental methods. Both the dynamical and rheological relaxation proces ses were slowed at moderate surfactant concentrations due to enhanced hydro phobic associations. The time correlation data obtained from the dynamic li ght scattering (DLS) experiments showed the existence of two relaxation mod es, one single exponential at short times followed by a stretched exponenti al at longer times. The slow relaxation time, as well as the rheological co unterpart (the longest relaxation time), revealed an optimum in intermolecu lar hydrophobic interactions for the HM4-EHEC/SDS and HM0-EHEC/SDS systems at a surfactant concentration of ca. 4 mm, while the corresponding concentr ation for the EHEC/SDS system is ca. 15 mm. The dynamical and rheological f eatures were found to be strongly dependent upon the level of surfactant ad dition, with at first an increase and then a decrease in the values of the parameters. The values are highest for the HM0-EHEC/SDS system, followed by those representing the HM4-EHEC/SDS system, and the lowest values were obs erved for the EHEC/SDS system. Under conditions of high association strengt h, the angular dependence of the slow mode of the three systems is much str onger than that of the fast diffusive mode and decreases at high SDS concen trations. This wave vector dependence of the slow relaxation time as well a s other dynamic features can be rationalized in the framework of the coupli ng model of Ngai. At high surfactant concentrations, the influence of hydro phobic association was essentially lost, and the behavior is virtually the same for the three polymers. The picture that emerges from this study is th at the associated polymer network undergoes a structural reorganization fro m a heterogeneous structure at low surfactant concentrations to a more homo geneous network at high levels of surfactant addition.