Time dependent association phenomena in dilute aqueous mixtures of a hydrophobically modified cellulose derivative and an anionic surfactant

Associations in dilute solutions of hydrophobically modified ethyl (hydroxyethyl) cellulose (HM-EHEC) in the presence of an anionic surfactant (sodium dodecyl sulfate (SDS)) are studied with the aid of dynamic light scattering. The autocorrelation functions revealed the existence of two relaxation modes: a single exponential at short times, followed by a stretched exponential at longer times. The slow relaxation time is affected by both polymer and surfactant concentrations. In very dilute polymer solutions the slow relaxation time increases with time at moderate levels of surfactant addition. At higher surfactant concentrations the fast and the slow relaxation time are constant with respect to time. The results are interpreted in terms of a change in the interaction between polymer coils with adsorbed surfactant molecules. The conjecture is that depending on the polymer concentration two scenarios are possible. At low polymer concentrations the polymer–surfactant clusters associate as time passes, forming aggregates. At higher polymer concentrations the polymer–surfactant clusters undergo a conformational transition with time. In both cases a phase separation at long time after mixing is found.