The effect of electrolyte on the micellar properties of an anionic-nonionic detergent in aqueous solution

Abstract

The effect of sodium chloride on the micellar properties of an anionic-nonionic detergent C16H33(OCH2·CH2)7OSO3Na has been investigated and the results have been compared with previous measurements on the nonionic analogue C16H33(OCH2·CH2)7OH. Light scattering and viscosity measurements showed that, over the electrolyte concentration range studied (0–1.0M NaCl), the micelles were very much smaller (m.m.w.=74,800 in 0.1M NaCl) than those of the nonionie analogue and exhibited no similar variation of size, shape or hydration with temperature, indicating that addition of electrolyte caused insufficient shielding of the micellar charge to induce nonionic behaviour. Viscosity results suggested a reduction in the micellar hydration with increase in electrolyte concentration to a limiting value of 0.28 g H2O/g of detergent for salt concentrations greater than 0.01M. A possible explanation of this effect is proposed. The effect of electrolyte on the c.m.c. was determined from surface tension measurements and was described by the equation, log c.m.c.=−6.4−0.54 log (c.m.c.+salt conc.) Comparison with data reported for sodium hexadecylpolyoxyethylene sulphates with shorter ethylene oxide chain lengths indicated a decrease in the c.m.c. in salt-free systems with increase in the chain length, the data obeying the equation, log c.m.c.=−3.5−0.20n wheren is the number of ethylene oxide groups in the chain.