N. Mucic

Dynamics of interfacial layers-Experimental feasibilities of adsorption kinetics and dilational rheology

Each experimental method has a certain range of application, and so do the instruments for measuring dynamic interfacial tension and dilational rheology. While the capillary pressure tensiometry provides data for the shortest adsorption times starting from milliseconds at liquid/gas and tens of milliseconds at liquid/liquid interfaces, the drop profile tensiometry allows measurements in a time window from seconds to many hours. Although both methods together cover a time range of about eight orders of magnitude (10(-3) s to 10(5) s), not all surfactants can be investigated with these techniques in the required concentration range. The same is true for studies of the dilational rheology. While drop profile tensiometry allows oscillations between 10(-3) Hz and 0.2 Hz, which can be complemented by measurements with capillary pressure oscillating drops and the capillary wave damping method (up to 10(3) Hz) these six orders of magnitude in frequency are often insufficient for a complete characterization of interfacial dilational relaxations of surfactant adsorption layers. The presented analysis provides a guide to select the most suitable experimental method for a given surfactant to be studied. The analysis is based on a diffusion controlled adsorption kinetics and a Langmuir adsorption model.

Adsorption of Alkyltrimethylammonium Bromides at Water/Alkane Interfaces: Competitive Adsorption of Alkanes and Surfactants

The adsorption of members of the homologous series of alkyl trimethylammonium bromides (C(n)TAB) is studied at water/alkane interfaces by drop profile analysis tensiometry. The results are discussed in terms of a competitive adsorption process of alkane and surfactant molecules. A thermodynamic model, derived originally for the adsorption of surfactant mixtures, is adapted such that it describes a competitive adsorption of the surfactant molecules from the aqueous phase and alkane molecules from the oil phase. This new model involves the interspecies attraction coefficient, which mutually increases the adsorption activities of the alkane and C(n)TAB. The effects of the alkyl chain length n of C(n)TABs and the influence of the number of C atoms in the alkane chain are discussed, and the physical quantities are compared to those determined at the aqueous solution/air interface. The new theoretical model for aqueous solution/oil interfaces is also compared to a theory that does not consider the adsorption of alkane. The proposed new model demonstrates good agreement with the experimental data.

Thermodynamics of adsorption of ionic surfactants at water/alkane interfaces

On the basis of experimental data for the homologous series of alkyltrimethylammonium bromides (CnTAB) the equilibrium surface tension isotherms at three types of liquid-fluid interfaces are discussed: solution/air, solution/alkane vapor and solution/liquid alkane interfaces. It is shown that the adsorption characteristics can be described at all three interfaces by the same thermodynamic approach. In the presence of alkane molecules (in the liquid alkane phase or in the alkane vapor phase) the CnTAB adsorption layers can be best described by a co-adsorption of the alkane molecules.

Thermodynamic Models for the Adsorption of Alkyl Trimethyl Ammonium Bromides at the Water/Hexane Interface

Based on surface/interfacial tension isotherms measured for the homologous series of alkyl trimethyl ammonium bromides (CnTAB) using the drop profile analysis tensiometry the adsorption behavior at three different liquid-fluid interfaces is discussed: solution/air, solution/hexane vapor and solution/hexane bulk liquid. The adsorption behavior can be described by different models. In the presence of hexane molecules (as a bulk liquid or as vapor in the air phase) the adsorption of the CnTAB molecules can be best described by a competitive adsorption with hexane molecules. This competitive thermodynamic model can be applied successfully to all three interfaces.

Adsorption of equimolar aqueous sodium dodecyl sulphate/dodecyl trimethylammonium bromide mixtures at solution/air and solution/oil interfaces

Two oppositely charged surfactants in an aqueous solution interact with each other via electrostatic interactions and produce surfactant complexes—catanionics. However, their quantitative influence on the interfacial tension at the solution/air and solution/oil interfaces is still unknown. We have measured the interfacial tension of sodium dodecyl sulphate (SDS) and dodecyltrimethylammonium bromide (DoTAB) aqueous mixtures using the du Nouy ring tensiometer and drop profile analysis tensiometry (PAT-1). For the oil phase, we used hexane. The obtained kinetic and equilibrium experimental results were fitted with the common theoretical Frumkin adsorption model. We found that SDS + DoTAB complexes show much higher surface activity than the single surfactants. At solution/air and solution/oil interfaces, SDS and DoTAB produce ion pairs with low water affinity and relatively low oil solubility. SDS + DoTAB partition coefficient between hexane and water phases is 0.32. Such surfactant complexes with advanced surface characteristics may be beneficial in different industrial branches nowadays.