Thermodynamics of the Air-Water Interface of Mixtures of Surfactants with Polyelectrolytes, Oligoelectrolytes and Multivalent Metal Electrolytes.

A full comparison of results from binding isotherms and surface tension (ST) measurements on polyelectrolyte (PE)-surfactant (S) mixtures, especially the polymer dependence, shows up clear distinctions between the behaviour of two representative PE-S systems, poly(sodium styrenesulfonate) (NaPSS) with dodecyltrimethylammonium bromide (C12 TAB) and poly(dimethyl-diallyl-ammonium chloride) (PDMDAAC) with sodium dodecylsulfate (SDS) in 100 mM NaCl. The surfactant-monomer binding constant in NaPSS-C12 TAB is an order of magnitude greater than that in the PDMDAAC-SDS system. This results in the ST behaviour being dominated largely by non-cooperativity in the former and by cooperativity in the latter. This leads to the surface activity in PDMDAAC-SDS being at its highest when the average bound fraction is low, but being lost completely as saturation approaches. A full analysis is also given of how this is altered in the mixture of PDMDAAC-SDS with the nonionic surfactant hexethylene glycol monododecyl ether. In contrast, the much stronger interaction in NaPSS-C12 TAB leads to non-surface active complexes at low bound fractions while the strongest surface activity occurs near the maximum bound fraction, close to or at precipitation. In the PDMDAAC-SDS system the ST drops to a low value at low surfactant concentrations but then increases to a high value just before precipitation occurs, which, combined with the increasing surface activity of free surfactant, results in a sharp ST peak. In the NaPSS-C12 TAB system the ST does not drop until the system is at or close to precipitation and it then stays on a plateau until the point at which free surfactant takes over. Thermodynamics does not allow large step changes in the ST and it is suggested that where these have been observed they are the result of self-depletion of PE or PE-S complexes on the finely divided precipitate of the complex and are therefore not representative of full surface equilibrium.