New insights on the interaction between hydroxypropylmethyl cellulose and sodium dodecyl sulfate

The effect of hydroxypropylmethyl cellulose (HPMC) concentration on the critical association concentration (cac) of the surfactant and on the polymer saturation points (psp) was analyzed using an electrical conductivity technique, which allowed establishing association and micellization thermodynamic parameters at different polymer concentrations. The effect of temperature on these parameters was also investigated. It is seen that the free energy change in the association process is constant up to 0.5% HPMC, but increases after this point. A similar behavior is found for the bulk micellization process. It is also seen that the association process becomes more and more favorable as the temperature increases. Rheological measurements were performed to investigate the influence of surfactant concentration in the viscosity of HPMC hydrogels at room temperature. Three regimes were found: an initial decrease, up to the cac, followed by a drastic increase and a further decrease starting close to the psp. Studies with temperature ramps have been also conducted to assess the effect of SDS on the thermal gelation of HPMC, showing that SDS delays or even inhibits gelation. Optical transmittance studies were carried out in order to evaluate the behavior of the polymer chains (extension and compaction) as a function of SDS concentration. A model for HPMC-SDS interaction has been proposed, rationalizing the development of the system at different SDS and HPMC concentrations.