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Physicochemical Studies on the Interaction of Gelatin with Cationic Surfactants Alkyltrimethylammonium Bromides (ATABs) with Special Focus on the Behavior of the Hexadecyl Homologue
journal contribution
posted on 2008-05-29, 00:00 authored by Debolina Mitra, Subhas C. Bhattacharya, Satya P. MoulikThe 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.
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Keywords
octadecyltrimethylammonium bromidedenatured proteinCationic Surfactants Alkyltrimethylammonium Bromidesalkyl chain lengthschain lengthaggregation concentrationconcentration rangesurfactant aggregatesstrength μinteractional profileHexadecyl HomologueThe interactionCTABSpecial Focustransition pointsdenatured interfaciallycounterion bindingtemperature variation studycationic amphiphilemicellar solutionmicellar concentrationcationic surfactantstrimethylammonium bromideATAB micellizationinteraction featuresalkyltrimethylammonium bromideinteraction modelbinding interactionsPhysicochemical StudiesGSmB formation
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