E. V. Aksenenko

Thermodynamics of the clusterization process of cis isomers of unsaturated fatty acids at the air/water interface.

In the framework of the semiempirical PM3 method, the thermodynamic parameters of cis isomers of unsaturated carboxylic acids at the air/water interface are studied. The model systems used are unsaturated cis fatty acid of the composition Delta = 12-15 and omega = 6-11, where Delta and omega refer to the number of carbon atoms between the functional group and double bond, and that between the double bond and methyl group, respectively. For dimers, trimers, and tetramers of the four acid series, the thermodynamic parameters of clusterization are calculated. It is shown that the position of the double bond does not significantly affect the values of thermodynamic parameters of formation and clusterization of carboxylic acids for equal chain lengths (n = Delta + omega). The calculated results show that for cis unsaturated fatty acid with odd Delta values the spontaneous clusterization threshold corresponds to n = 17-18 carbon atoms in the alkyl chain, while for monounsaturated acids with even Delta values this threshold corresponds to n = 18-19 carbon atoms in the alkyl chain. These differences in the clusterization threshold between the acids with even and odd Delta values are attributed to the formation of additional intermolecular hydrogen bonds between the ketonic oxygen atom of one monomer and the hydrogen atom linked to the alpha-carbon atom of the second monomer for the acids with odd Delta values or between the hydroxyl oxygen atom of one monomer and hydrogen atom linked to the alpha-carbon atom of the second monomer for the acids with even Delta values. The results obtained in the study agree satisfactorily with our experimental data for cis unsaturated nervonic (Delta15, omega9) and erucic acids (Delta13, omega9), and published data for some fatty acids, namely cis-16-heptadecenoic (Delta16, omega1), cis-9-hexadecenoic (Delta7, omega9), cis-11-eicosenoic (Delta11, omega9) and cis-9-octadecenoic acid (Delta9, omega9).

Adsorption layer characteristics of mixed oxyethylated surfactant solutions.

In the presented work, bubble profile analysis tensiometry is used to study the equilibrium surface tensions and the rheological behavior of solutions of mixed oxyethylated alcohols (C(12)EO(5)/C(14)EO(8)) and their mixtures with polyethylene glycol octylphenyl ethers (Triton X45/Triton X165). For the analysis of the experimental data, a new theoretical model for mixtures of two nonionic oxyethylated surfactants was employed which assumes two states of surfactant molecules with different molar areas in the surface layer and an intrinsic compressibility of the molecules in the state of closest packing. The theoretical models allow an accurate description of the experimental equilibrium surface tensions for all studied mixed solutions. For the analysis of the rheological behavior of the studied mixed surfactant solutions, the theory for a diffusional adsorption mechanism was applied, and a satisfactory agreement between the experimental data and the calculated viscoelasticity modulus and phase angle was achieved.

Adsorption of Proteins at the Solution/Air Interface Influenced by Added Nonionic Surfactants at Very Low Concentrations for Both Components. 3. Dilational Surface Rheology

The influence of the addition of the nonionic surfactants C12DMPO, C14DMPO, C10OH, and C10EO5 at concentrations between 10(-5) and 10(-1) mmol/L to solutions of β-casein (BCS) and β-lactoglobulin (BLG) at a fixed concentration of 10(-5) mmol/L on the dilational surface rheology is studied. A maximum in the viscoelasticity modulus |E| occurs at very low surfactant concentrations (10(-4) to 10(-3) mmol/L) for mixtures of BCS with C12DMPO and C14DMPO and for mixtures of BLG with C10EO5, while for mixture of BCS with C10EO5 the value of |E| only slightly increased. The |E| values calculated with a recently developed model, which assumes changes in the interfacial molar area of the protein molecules due to the interaction with the surfactants, are in satisfactory agreement with experimental data. A linear dependence exists between the ratio of the maximum modulus for the mixture to the modulus of the single protein solution and the coefficient reflecting the influence of the surfactants on the adsorption activity of the protein.