Binding of alkylpyridinium chloride surfactants to sodium polystyrene sulfonate

Binding of cationic surfactants to anionic polymers is well studied. However, the surfactant binding characteristics at very low concentration near the start of binding and at high concentration, where charge compensation may occur, are less well known. Therefore, the binding characteristics of hexadecyl-pyridinium chloride (C16PC), dodecyl-pyridinium chloride (C12PC) and decyl-pyridinium chloride (C10PC) to sodium polystyrene sulfonate (SPSS) are investigated over a wide concentration range. The equilibrium surfactant concentration is measured with a surfactant-selective membrane electrode. The electrode shows good performance for the three surfactants, also in presence of SPSS. The observed critical micelle concentrations (CMCs) of the surfactants as a function of the ionic strength agree well with literature values and the CMC is not significantly affected by the presence of SPSS. For each surfactant, binding isotherms are obtained from depletion studies at three different NaCl concentrations. For C16PC and C12PC the binding isotherms at different NaCl concentration have a common intersection point (CIP) at relatively high surfactant concentration. For C10PC the experimental accuracy is too small to observe a CIP. The CIP closely corresponds with the bound amount at the iso-electric point (IEP) of the surfactant–SPSS complex. At the IEP/CIP the charge of SPSS is compensated by bound surfactant ions. In region between the CIP and the CMC the polyelectrolyte charge is overcompensated by surfactant ions. Similar behavior has been found before with the same surfactants and humic acid as polyelectrolyte. At very low surfactant concentrations the C10P+ ions bind on isolated sites along the SPSS chain, the C12P+ ions form some dimers and the C16P+ ions form, already at the lowest concentration that could be measured, aggregates. At somewhat higher concentrations also C10PC and C12PC start to form small aggregates along the SPSS chain. The concentration at which this occurs is the critical aggregation concentration (CAC). Similarly as the CMC the CAC is a function of the surfactant chain length and the ionic strength. The size of the small aggregates increases with increasing chain length, but hardly depends on the ionic strength. With increasing surfactant concentration the formation of the aggregates along the SPSS chain continues. Electrode calibration at very low concentrations C16PC and C12PC can be improved when the SPSS–surfactant complex is used to buffer the solutions.