Dynamic light scattering on semidilute aqueous systems of ethyl (hydroxyethyl) cellulose. Effects of temperature surfactant concentration, and salinity

摘要

The effects of temperature, surfactant, and salt on the phenomena of association and thermoreversible gelation of a semidilute aqueous solution of ethyl (hydroxyethyl) cellulose (EHEC) are studied by using 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 fast relaxation mode is always diffusive and yields a collective diffusion coefficient D_c, which is temperature dependent and increases with increasing surfactant addition. The slow relaxation time is associated with disengagement of chains or clusters, and this quantity increases during gelation and enhanced association. This parameter, which exhibits similar features as the dynamic viscosity, can be used to characterise the degree of intermolecular association of EHEC systems. The general picture that emerges from this study is that gels are formed in the presence of low concentrations of sodium dodecyl sulphate at elevated temperatures. An high surfactant concentrations the network structure is disrupted, and the slow relaxation time decreases. At moderate levels of surfactant addition, addition of salt is found to strengthen the intermolecular associations.