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

The effects of temperature, surfactant, and salt on the phenomena of associ ation and thermoreversible gelation of a semidilute aqueous solution of eth yl (hydroxyethyl) cellulose (EHEC) are studied by using dynamic light scatt ering. The autocorrelation functions revealed the existence of two relaxati on modes: a single exponential at short times, followed by a stretched expo nential at longer times. The fast relaxation mode is always diffusive and y ields a collective diffusion coefficient D-c, which is temperature dependen t and increases with increasing surfactant addition. The slow relaxation ti me is associated with disengagement of chains or clusters, and this quantit y increases during gelation and enhanced association. This parameter, which exhibits similar features as the dynamic viscosity, can be used to charact erise the degree of intermolecular association of EHEC systems. The general picture that emerges from this study is that gels are formed in the presen ce of low concentrations of sodium dodecyl sulphate at elevated temperature s. At high surfactant concentrations the network structure is disrupted, an d the slow relaxation time decreases. At moderate levels of surfactant addi tion, addition of salt is found to strengthen the intermolecular associatio ns. (C) 1999 Elsevier Science B.V. All rights reserved.