In this paper, results of physicochemical studies on the interaction of bovine serum albumin (BSA) with alkyltrimethylammonium bromide (ATAB), pentaethylene glycol mono-n-dodecyl ether (C12E5), and sodium dodecyl sulfate (SDS) under the experimental conditions of phosphate buffer at pH 7 in the presence of 10 mM sodium bromide (NaBr), maintaining the ionic strength of the overall solution at micro = 0.015 M, have been presented and discussed. Here, BSA-ATAB corresponds to a polyion-surfactant system bearing opposite charges. BSA precipitated out of the solution on addition of ATAB solution over a certain range of ATAB concentration, the concentration range being dependent on the particular member of the ATAB family. In our earlier reports on the precipitation of oppositely charged polymer-surfactant, the tensiometric profile for surfactant addition in polymer solution differed significantly from that expected from addition of surfactant in the dispersion medium. In the present study, the precipitation process could hardly affect the smoothness of the tensiometric profile. This indicates the interaction process is operative in bulk solution. Microcalorimetric profiles also evidenced an extra hump in the interaction profile at lower surfactant concentrations, without much affecting the dilution enthalpograms beyond micellization. This interaction appeared unimodal and the extent of interaction increased with increasing tail length of ATAB, evidencing the hydrophobic effect to be an important factor. Addition of salt (NaBr) also affected the nature of interaction: at lower concentration of NaBr, the interaction was mildly assisted, whereas 50 mM NaBr fairly assisted the interaction. The nonionic surfactant C12E5 modestly interacted with BSA. The anionic amphiphile SDS, on the other hand, interacted with BSA in two distinctly different stages, as evidenced from the tensiometric profile. The complexity of the BSA-SDS tensiometric isotherm compared to that of BSA-ATAB arose from the presence of cationic binding sites adjacent to hydrophobic patches of BSA in its native state, so that electrostatic and hydrophobic interactions can cooperatively operate side by side. The interfacial saturation occurred at a lower concentration in the presence of BSA compared to the normal cmc of SDS under identical solution conditions in the absence of BSA, which was slightly delayed for nonionic C12E5. The multitechnique approach evidenced that different experimental techniques probe different physicochemical phenomena and an attempt to show the concurrence of the break points in different techniques is only diluting the essence of this area.

Mixed micelles formed with cetyl pyridinium chloride (CPC), cetyl trimethylammonium bromide (CTAB), and polyoxyethylene (10) cetyl ether (Brij-56) mixed in different combinations in aqueous medium have been studied in detail by tensiometric, conductometric, calorimetric, spectrophotometric, and fluorimetric techniques. Different physicochemical properties such as critical micellar concentration (cmc), micellar dissociation, energetic parameters (free energy, enthalpy, and entropy) of micellization, interfacial adsorption, and micellar aggregation number have been determined. The results have been analyzed in terms of the equations of Clint, Motomura, Rosen, Rubingh, Blankschtein et al., and Rubingh and Holland for justification of the experimental cmc, determination of micellar composition parameters, quantification of interaction among the mixed micelle components, and estimation of their activity coefficients.

Interaction between polymer and surfactant bearing opposite charges is much more complex from a physicochemical point of view as compared to interaction between ionic surfactant and nonionic polymer. Electrostatic and hydrophobic interactions interplay in the former, whereas the hydrophobic effect is the prevailing factor in the latter. We have studied the interaction between a water-soluble polyanion, sodium salt of carboxymethylcellulose (NaCMC), with a cationic amphiphile, CTAB, in aqueous medium. There were manifold discrepancies with the reported works in NaCMC-alkyltrimethylammonium bromide, which is assumed to be an effect of difference in degree of substitution, which in turn affects the charge density of the polymer chain. We have noticed that the bulk complexation and interfacial interaction driven by electrostatic forces operate side by side. Thereafter, there is a wrapping process by the polyanion to the polymer-induced smaller surfactant aggregates driven by increase in entropy of the solution as a result of expulsion of the counterions from the ionic atmosphere around the surfactant aggregate. Because of the electrostatic interaction, hydrophobicity of the polymer-surfactant complex increases, leading to coacervation, and again solubilization in the hydrophobic core of the self-aggregated structure provided by the added excess CTAB. The tensiometric, conductometric, microcalorimetric, and turbidimetric techniques have been applied to address these problems.

The interaction between pepsin and CTAB has been elaborately studied with a number of techniques. The enzyme-induced interaction produced complexes, aggregates, and micelles of CTAB with distinct physicochemical features. It was found that at very low surfactant concentration (much below the critical micellar concentration (cmc) of pure CTAB), the surfactant got adsorbed both in monomeric and lower aggregated forms to the high-energy sites of the native biopolymer, leading to enhanced hydrophobicity of the combine, and hence, lowering of the interfacial (air/solution) tension. This was followed by the formation of a faintly turbid solution of the polymer-surfactant coacervate. The CTAB interacted unfolded pepsin along with the surfactant monomer remained adsorbed at the interface to decrease the interfacial tension (gamma) to a low level to produce a break in the gamma vs log [CTAB] plot prior to the normally observed extended cmc (cmce) in presence of polymers. The cac-like aggregation (as observed in tensiometry and viscometry) was not found in conductometry and microcalorimetry, whereas microcalorimetry evidenced the formation of the cmce of CTAB in the presence of the biopolymer. The CTAB influenced structural features of the pepsin were assessed from spectral, viscometric, and circular dichroism measurements.

No abstract

Single crystalline Fe-doped hexagonal BaTiO 3 samples with varying oxygen content are created by specifically intended post-growth annealing treatments, in order to check the influence of defects on the unusual high temperature ferromagnetism observed in this system. The various defects have been shown to play a crucial role in dilute magnetic systems and therefore, it is important to carry out this check for the Fe-doped BaTiO 3 system also, in which unusual ferromagnetism was reported even in its bulk single crystalline form. The x-ray diffraction and dielectric studies carried out here have confirmed that the Fe doping of Ti is intrinsic, while the high resolution transmission electron microscopy (HRTEM) and x-ray photoemission spectroscopy (XPS) studies proved the absence of unwanted magnetic metal clusters in the sample. The transport studies show that the oxygen concentrations could be varied substantially by the thermal treatments. Finally, magnetization measurements on the samples demonstrated that ferromagnetism is stronger in samples with higher oxygen deficiency, which could interestingly be retreated under high oxygen atmosphere and reversibly be taken back to a lower magnetic state. The vacancy-induced ferromagnetism is further confirmed by EPR measurements, which is consistent with earlier studies and, consequently, put the doped BaTiO 3 in the list of true dilute magnetic oxide (DMO) systems.

The micellization of anionic gemini surfactant, N,N'-ethylene(bis(sodium N-dodecanoyl-beta-alaninate)) (212), and its monomer, N-dodecanoyl-N-methyl alaninate (SDMA), and polyethoxylated nonionic surfactants, C(12)E(5) and C(12)E(8), has been studied tensiometrically in pure and mixed states in an aqueous solution of 0.1 M NaCl at pH 11 to determine physicochemical properties such as critical micellar concentration (cmc), surface tension at the cmc (gamma(cmc)), maximum surface excess (Gamma(max)) and minimum area per surfactant molecule at the air/water interface (A(min)). The theories of Rosen, Rubingh, Motomura, Maeda, and Nagarajan have been applied to investigate the interaction between those surfactants at the interface and in the micellar solution, the composition of the aggregates formed, the theoretical cmc in pure and mixed states, and the structural parameters as proposed by Tanford and Israelachvili. Various thermodynamic parameters (free energy of micellization and interfacial adsorption) have been calculated with the help of regular solution theory and the pseudophase model for micellization.

No abstract