Salt effects on the interaction of poly(ethylene oxide) and sodium dodecyl sulfate measured by conductivity

The salt effect of NaCl on solutions of poly(ethylene oxide) (PEG) and sodium dodecyl sulfate (SDS) was investigated conductimetrically. The variation of the two breakpoints in the specific conductivity vs. SDS concentration plots, the critical aggregation concentration (CAC) and the polymer saturation point (PSP) were determined as a function of NaCl at several polymer concentrations. The PSP increases linearly with polymer concentration and the slope depends on the NaCl concentration. The plots of PSP against PEO converge at approximately 0.035 M. Below this PEO concentration, the second breakpoint (PSP) decreases, in contrast to that observed above this concentration. Based on conductivity titrations, plots of conductivity vs. SDS concentrations between the first and second breakpoints are linear only when the PEO concentrations are higher than 0.050 M. At low PEO concentrations, the absence of linearity above the CAC is interpreted by assuming that more than one type of SDS aggregate is developing.