Physico-Chemical nanomaterials science
The association processes and physical-chemical properties of casein–benzethonium cloride aqueous systems.
A.V. Sachko1, Yu.R. Lukan1, V.P.Zakordonskiy2
1 Biology, Chemistry and Bioresourses Institute, Yuriy Fedkovych Chernivtsi National University, Kotsyubynsky 2, 58012, Chernivtsi, Ukraine.
E-mail: an.sachko@chnu.edu.ua
2 Chemistry Faculty, Ivan Franko National University of Lviv, Universytetska St, 1, 79000, L'viv, Lviv, Ukraine E-mail: vzakordonskiy@ukr.net

The aggregation between benzethonium cloride (BTC, M.m.=448.1, Aldrich Chemical Co) and casein in aqueous solutions has been studied by different experimental techniques: surface tension, conductivity, viscosity measurements and optical density investigation. The solubilization of the pyrene in mixtures of BTC-casein was investigated. It was shown that mixing of BTC and casein solutions leads to irreversible changes in the whole complex of physico-chemical properties of the systems. On the basis of the obtained results a four-stage scheme of BTC–casein interaction was proposed.
The first stage occurs at concentrations of BTC <5×10-4 mol/l. This is a region of transparent solutions, in which there are no significant changes in the properties of systems. All BTC molecules bind to casein macromolecules due to the electrostatic interaction of positively charged cations of benzethonium with negatively charged amino acids fragments of casein.
The second stage (CBTC = (5-8)×10-4 mol/l) is the formation of compact insoluble aggregates. At this stage we observed strongly marked features of phase separation of the system; appearances of bends on the surface tension isotherms due to the binding of all BTC. Both the viscosity and solubilization ability of the systems decrease under these BTC concentrations.
The last two stages occure when CBTC≥ 8×10–4 mol/l. Here the formation of hydrophilic highly structured systems takes place. Surface tension of the investigated mixtures is weakly depend on the BTC concentration for this interval. This indicates the possibility of coexistence of BTC-casein complexes with free BTC micelles. At the same time, both the viscosity and specific conductivity of binary mixtures are rapidly increase. Such increasing could be connected with the releasing of a large number of counterions due to the binding of BT-cations to casein macromolecules.
It is interesting that complexes formation is not accompanied by the protein denaturation. The mechanism of association is very similar to the association of BTC with polymethacrylic acid, which was described in our previous works.