The influence of the addition of the nonionic surfactants dodecyl dimethyl phosphine oxide (C12DMPO), tetradecyl dimethyl phosphine oxide (C14DMPO), decyl alcohol (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 surface tension is studied. It is shown that a significant decrease of the water/air surface tension occurs for all the surfactants studied at very low concentrations (10(-5)-10(-3) mmol/L). All measurements were performed with the buoyant bubble profile method. The dynamics of the surface tension was simulated using the Fick and Ward-Tordai equations. The calculation results agree well with the experimental data, indicating that the equilibration times in the system studied do not exceed 30 000 s, while the time required to attain the equilibrium on a plane surface is by one order of magnitude higher. To achieve agreement between theory and experiment for the mixtures, a supposition was made about the influence of the concentration of nonionic surfactant on the adsorption activity of the protein. The adsorption isotherm equation of the protein was modified accordingly, and this corrected model agrees well with all experimental data.

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Published data and the author's own data on the surface energy of hydrophilic oxides, silicates, and hydrophobic adsorbents based on them are reviewed. The prospects of using the combined Gibbs-Helmholtz-Young equation to obtain data on the surface pressure, heat of wetting, and wetting contact angle of hydrophilic and hydrophobic adsorbents are demonstrated. These data are used to estimate the thermodynamic characteristics of the surface and interfacial regions at the boundary between the materials and water. It is shown that the boundary layers of water close to the hydrophobic surfaces are more ordered while those close to the hydrophobic surfaces are less ordered than with liquid water.

Colloid chemical properties of humic acids in aqueous solutions have been studied as functions of their concentration, medium pH, and counterion nature. It has been established that, at pH < 4, Na form humic acids exist in aqueous solutions as 30-120 nm aggregates. At pH > 4, the state of humic acids depends on the solution concentration. At low concentrations, they exist as individual molecules; as the concentration is increased, they associate to form supramolecular structures at 5 mg/dm 3 and micelles at 8 g/dm 3 . An increase in the counterion charge has been shown to decrease the critical concentration of supramolecular structuring. Supramolecular structures have increased adsorbability on clay minerals and promote the mani festation of solubilizing properties of humic acids in solutions even at a concentration of 5 mg/dm 3 , which is three orders of magnitude lower than the critical micelle concentration (8 g/dm 3 ).

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ABSTRACT:Values for the surface tension, p, of 85 mJ/m 2 and for the contact angle of wetting by water as its saturated vapour, q V , of 75º are proposed as criteria for distinguishing between hydrophilic and hydrophobic solid surfaces. It is shown that the water boundary layers at hydrophilic surfaces are more ordered while those at hydrophobic surfaces are less ordered relative to the situation in the bulk water structure. The localised model can be used to describe the state of motion of water molecules at a hydrophilic surface, while the motion of water molecules near a hydrophobic surface conforms to the non-localised model.

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