Thermodynamics of Adsorption of L-Amino Acids at Oil–Water Interfaces

Lowering of the interfacial tension of heptane–water, benzene–water, and nitrobenzene–water interfaces due to addition of 20 different amino acids to the aqueous phase has been measured. From the plot of surface pressure against molar concentration of amino acids, the initial slope and the surface excess ₂ ¹ for different amino acids have been calculated using the Gibbs adsorption equation. ₂ ¹ for most amino acids at benzene–water and heptane–water interfaces was found to be positive, with only a few being negative. At the nitrobenzene–water interface, both positive and negative ₂ ¹ values were observed. The area per adsorbed molecule at surface saturation A _m was found to vary widely, indicating different orientations of amino acid molecules at the interfaces. Using the integrated form of the Gibbs adsorption equation, the standard Gibbs energy change G ^o in kJ-m² of the adsorbed surface have been calculated for various interfaces. G ^o was found to vary linearly with the ₂ ¹ of different amino acids and the slope of the line, designated as –G _B ⁰ was found to be 22 kJ-mol^–1 for heptane–water, 23.2 kJ-mol^–1 for benzene–water, and 19.3 kJ-mol^–1 for nitrobenzene–water interfaces, irrespective of the nature of the amino acid. The origin of the linear scale of the Gibbs energy for heptane–water, benzene–water and nitrobenzene–water interfaces has been discussed in terms of hydrophobic and other interactions.     

Electron transport properties of Co71−xFexCr7Si8B14 (x=0, 2, 3.2, 4, 6,8 and 12 at%) amorphous alloys during devitrification

The investigation addresses the electron transport properties of Co_71−xFe_xCr₇Si₈B₁₄ (x=0, 2, 3.2, 4, 6, 8 and 12 at%) amorphous alloys. The variation in electrical resistivity of as-cast amorphous materials with thermal scanning from room temperature to 1000 K was measured. The CoFe-based alloys revealed an initial decrease in temperature coefficient of resistivity (TCR), a characteristic of spin-wave phenomena in glassy metallic systems. This behaviour in the present alloys was in a sharp contrast to the Co-based amorphous materials that indicate the drop in resistivity much below room temperature. In the studied alloys, the variation in initial TCR values and the full-width at half-maxima determined from X-ray diffraction of as-quenched materials exhibited a similar trend with increasing Fe content, indicating the compositional effect of near neighbouring atoms. After the initial decrease in resistivity, all the alloys indicated a subsequent increase at T_min. The Curie temperature (T_C), which was measured from thermal variation of ac susceptibility showed non-monotonic change with Fe content. In the temperature range between T_min and T_C the relative scattering by electron-magnon and electron-phonon resulted in the non-monotonic change in Curie temperature. At crystallization onset (T_X1) all the alloys except there with X=6, showed a sharp decrease in electrical resistivity which was attributed to ordering phenomena. In contrast to this resistivity decrease, X=6 alloy exhibited a drastic increase in resistivity around T_X1 observed during amorphous to nanocrystalline transformation. Such nanocrystalline state was observed by Transmission electron microscopy.