Physicochemical Studies on the Interaction of Gelatin with Cationic Surfactants Alkyltrimethylammonium Bromides (ATABs) with Special Focus on the Behavior of the Hexadecyl Homologue

The interaction of a denatured interfacially active protein, gelatin (G) (at pH 9, above its isoelectric pH 4.84, and ionic strength μ = 0.005), with a cationic amphiphile, hexadecyl (or cetyl) trimethylammonium bromide, CTAB, has been elaborately studied using a variety of techniques. Two types of protein−surfactant complexes at a concentration below the normal critical micellar concentration (cmc) were formed in solution. The first, G−CTAB (monomer) combined complex (GSnI) adsorbed at the air/solution interface, followed by its gradual transformation to the poor interfacially active second G−CTAB (aggregate) complex (GSmB) at a critical aggregation concentration (cac) of the interacting oppositely charged surfactant. In the higher concentration range, upon completion of GSmB formation, coacervation (association of GSmB) led to add turbidity. With increasing addition of CTAB, the coacervates became disintegrated and ultimately remained dissolved in the free micellar solution of CTAB. The above features were studied using the techniques of tensiometry, conductometry, turbidimetry, fluorimetry, and microcalorimetry. The interaction features were prominent at [G] ≥ 0.05 g %, and several of these were either marginal or absent at [G] < 0.05 g %. The denatured protein was found to form viscous as well as gel-forming consistencies at higher [G] and at lower temperature. A temperature variation study on the interaction of G with CTAB has revealed that enhanced interaction takes place at higher temperature. The effect of [G] on its interaction with cationic surfactants of varying chain length in the alkyltrimethylammonium bromide (ATAB) series has been also studied; a similar interactional profile as that of CTAB has been exhibited by octadecyltrimethylammonium bromide; however, the lower homologues (dodecyl- and tetradecyl-) of ATAB have offered different profiles. It has been found that the ATABs with higher alkyl chain lengths were more interactive with negatively charged G than their lower homologues. Quantification of the results in terms of different transition points, counterion binding of the protein-bound surfactant aggregates and free micelles, the enthalpy of binding interactions and energetics of ATAB micellization, and so forth have been studied. The results have been rationalized in terms of an interaction model.