title: Are hydrotropes distinct from surfactants?
abstract: The physicochemical properties of a homologous series of sodium p-n-alkylbenzoates have been investigated. The objective was to determine whether there is a clear transition point from hydrotropic to surfactant-like behavior with increasing alkyl chain length n, so as to shed clear light on the aggregation mechanism of so-called "hydrotropes". Electrical conductivity measurements were used for a first estimation of the critical aggregation concentrations (cac). As for classical surfactants, log(cac) depends on alkyl chain length n, but two branches of behavior were observed: one having a gradient typical of long chain fatty acid salts and the other with a more shallow dependence. Surface tension (γ) measurements of high purity aqueous solutions were used to generate limiting headgroup areas A(cac), which were in the range (40-50 Å(2)) being consistent with monolayer formation. Small-angle neutron scattering conclusively shows that the lower chain length homologues (classed as hydrotropes) exhibit sharp transitions in aggregation as a function of bulk concentration, typical of regular surfactants. As such, there is little to suggest from this study that hydrotropes differ in association behavior from regular surfactants.
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, United Kingdom.
authors.forename: Marios Hopkins
authors.initials: MH
authors.lastname: Hatzopoulos
authors.forename: Julian
authors.initials: J
authors.lastname: Eastoe
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Dowding
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Rogers
authors.forename: Richard
authors.initials: R
authors.lastname: Heenan
authors.forename: Robert
authors.initials: R
authors.lastname: Dyer