Alkanediyi-α,ω-bis(dimethylalkylammonium bromide) surfactants -: 10.: Behavior in aqueous solution at concentrations below the critical micellization concentration:: An electrical conductivity study

Ion pairing and premicellar association have been often invoked to explain results obtained in studies of aqueous solutions of ionic dimeric surfactants (gemini surfactants), mainly by means of surface tension and electrical conductivity, at concentrations below the critical micellization concentration (cmc). The present work was undertaken in an attempt to find out under which conditions these effects come into play. For this purpose the electrical conductivity of solutions of many dimeric surfactants of the type spacer-alpha,omega-bis(alkyldimethylammonium bromide) have been measured. The alkyl chain contained m = 10-18 carbon atoms. The spacer group was either an alkanediyl with s carbon atoms (m-s-m surfactants) or a xylylene (m-xylyl-m surfactants). The results show that ion pairing occurs in solutions of m-s-m dimers with m less than or equal to 10, mostly as a result of their high cmc values. The results for 12-s-12 dimers with s less than or equal to 10 and for 12-xylyl-12 showed no evidence of either ion pairing or premicellar association. Premicellar association was present for 12-s-12 dimers with s greater than or equal to 12, for m-8-m dimers with m greater than or equal to 14, and for 16-xylyl-16. It showed through a positive curvature of the specific conductivity versus concentration plot and the presence of a maximum in the equivalent conductivity vs (concentration)(0.5) plot at concentrations below the cmc. The free energy associated with the premicellar association of m-8-m dimers has been estimated from the available cmc and micelle ionization degree data. (C) 2002 Elsevier Science.