Solution behavior of aqueous solutions of hydrophobically modified polyvinylamine and dodecyltrimethylammonium bromide
The polymer-surfactant interactions of polyvinylamine, PVAm, and hydrophobically modified polyvinylamine with 0.4 and 0.8% of the amine units modified with a dodecyloxybenzoic acid moiety, HMPVAm-0.4% and HMPVAm-0.8%, respectively, with dodecyltrimethylammonium bromide, DTAB, were studied. The polyelectrolytes and surfactant studied are all positively charged. The charge density and hydrophobe content of the polyelectrolyte was altered to study the effect of the ratio of charge density to hydrophobe content on the polymer-surfactant interactions of a like charge system. The polymer-surfactant interactions were studied as a function of concentration, pH, and hydrophobe content. PVAm lowered the surface tension and critical micelle concentration, CMC, of DTAB, increased the area per molecule at the air water interface in surface tension measurements, experienced little to no change in viscosity behavior upon addition of DTAB, and an induced a hexagonal phase at lower DTAB concentrations. The interactions described can be attributed to ionic effects and micro-phase separation. HMPVAm-0.4% and HMPVAm-0.8% increase the CMC of DTAB, show significant increases in viscosity upon addition of DTAB, and have little effect on the phase behavior of DTAB. The observed effects were all diminished as the pH was increased from 3 to 7. The above results lead to the conclusion that the hydrophobically modified polymers and DTAB are able to form mixed micelles in solution. These interactions were found to be reduced as the pH was increased due to conformational changes in the polymer. The polymer is highly charged at lower pH and is extended in solution, while at higher pH it is less charged and can adopt a more collapsed conformation. It is believed that the collapsed conformation involves unimolecular micelles with the hydrophobes hidden inside which would limit the accessibility of the hydrophobes to the surfactant and ultimately limit the interactions. HMPVAm-0.8% was the only polymer to exhibit a plateau region in the surface tension curves of DTAB. The increases in CMC as well as viscosity were greatest for this polymer. PVAm exhibited the least interactions with DTAB and, therefore, it is concluded that an increase in hydrophobe content of the polyelectrolyte appears to enhance the interactions of the polymer-surfactant system, while increases in pH decrease the interactions.