Effect of Surfactant on Dynamic and Viscoelastic Properties of Aqueous Solutions of Hydrophobically Modified Ethyl(hydroxyethyl)cellulose, with and without Spacer

Abstract

Dynamical, rheological, and some structural properties of 1 wt % aqueous solutions (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 processes were slowed at moderate surfactant concentrations due to enhanced hydrophobic associations. The time correlation data obtained from the dynamic light scattering (DLS) experiments showed the existence of two relaxation modes, one single exponential at short times followed by a stretched exponential at longer times. The slow relaxation time, as well as the rheological counterpart (the longest relaxation time), revealed an optimum in intermolecular hydrophobic interactions for the HM4-EHEC/SDS and HM0-EHEC/SDS systems at a surfactant concentration of ca. 4 mm, while the corresponding concentration for the EHEC/SDS system is ca. 15 mm. The dynamical and rheological features were found to be strongly dependent upon the level of surfactant addition, 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 observed for the EHEC/SDS system. Under conditions of high association strength, the angular dependence of the slow mode of the three systems is much stronger than that of the fast diffusive mode and decreases at high SDS concentrations. This wave vector dependence of the slow relaxation time as well as other dynamic features can be rationalized in the framework of the coupling model of Ngai. At high surfactant concentrations, the influence of hydrophobic association was essentially lost, and the behavior is virtually the same for the three polymers. The picture that emerges from this study is that the associated polymer network undergoes a structural reorganization from a heterogeneous structure at low surfactant concentrations to a more homogeneous network at high levels of surfactant addition.