Conductimetric, tensiometric and titrimetric studies of the Gelatin-DAC system

Abstract

A technique for the determination of the point of zero charge (p.z.c.) for proteins was investigated using fluorescence spectrophotometry. Different gelatin/pyrene solutions were prepared at different pH values. The ratio of the pyrene 11/b fluorescence peaks was determined and a plot of the ratio against pH yielded a minimum, corresponding to the p.z.c. of the gelatin type B as obtained by titration. The minimum has been attributed to the fact that at the p.z.c. the coiling up of gelatin molecules presents pyrene with more hydrophobic areas in which to reside. The interaction between gelatin and the cationic surfactant dodecylammonium chloride (DAC) was investigated by conductimetry, surface tension, pH measurements and back titration. Conductimetry indicated two critical points associated with the aggregation of gelatinbound surfactant critical aggregate concentration 1 (cac1) and the aggregation of free surfactant critical aggregate concentration 2 (cac2). The value of cac2 was found to be slightly lower than critical micelle concentration (cmc) for DAC when the gelatin concentration was lower than 0.7 wt %. This is unusual for polymer-surfactant systems and it is probably due to the fact that the interaction of DAC with gelatin is a weak one. Also values of cac1 were found to decrease with increasing gelatin concentration and those of cac2 were found to increase with gelatin concentration. This is in accordance with observations on other polymer-surfactant systems. Surface tension data showed similar trends to those observed by conductimetry and values of cac1 and cac2 were also found to agree with those obtained by conductimetry under the same conditions of pH and ionic strength. It was also observed that, the phosphate buffer previously used to keep a constant pH influenced surface tension of the gelatin surfactant mixtures. This is likely to be due to an interaction between the surfactant and the phosphate. The interaction between gelatin and DAC can also be characterised by a drop in pH on increasing surfactant concentration. The proton release is similar to that observed by the binding of cetyltrimethylammonium bromide to gelatin. The proton release was also studied by back titration and three observations were made. Firstly, the amount of protons released per gram gelatin decreases with increasing gelatin concentrations. Secondly, there is a threshold surfactant concentration at which protons are released, which in the absence of electrolyte is dependent on gelatin concentration. Thirdly, the threshold surfactant concentration becomes independent of gelatin concentration in the presence of 0.01M NaCI.