A quantitative model based on the hydrophobic interaction concept has been presented for the polymer-surfactant complex formation and checked by potentiometric measurements using Au/Hg/HglDS and sodium-ion selective glass electrodes. The interaction of sodium dodecyl sulfate (NaDS) with poly-(vinyl alcohol) (PVA). poly-(vinylpyrrolidone) (PVP) and poly-(ethylene oxide) (PEO) has been investigated. The polymer surfactant complex formation is characterized by the cooperativity of the process, i.e., by the formation of surfactant aggregates (subunits), which are smaller than the ordinary micelles, being distributed along the polymer chain. The aggregation number of subunits for the PVA-NaDs complex is 12 and it increased with the counter-ion concentration. In the presence of 0.1 M excess NaNO3 the aggregation numbers are between 40 and 50 for all the polymer-NaDS complexes concerned here. The standard free energy change of the transfer of DS-ions into surfactant complex subunits are very close to that of the micelle formation, which confirms that the main driving force of the polymer-surfactant complex formation is the micelle-like aggregation of the surfactant ions. The energy contribution arising from the interaction between the subaggregates being in the same polymer chain seems to be insignificant comparing to that of the sub-aggregate formation. The binding of the surfactants to the polymer (or the increase of the complex concentration) with increasing amount of surfactants is interrupted by the free micelle formation, because the surfactant monomer activity reaches its upper limit. In the case of the PVA-NaDS system the degree of binding is far from saturation but, at the beginning of the micelle formation, the PVP-NaDS and PEO-NaDS system approach it. The presenteòi treatment explains adequately the shape of binding isotherms of ionic surfactants by polymers, e.g. as observed by equilibrium dialysis and surface tension measurements.
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