Group Energetic Characteristics Of Non-ionic Surfactants Adsorption Process
Group energetic characteristics of non-ionic surfactants adsorption process
M.R. Maksymiuk, V.Ya. Poberezhny, V.V. Goncharuk
Institute of Colloid Chemistry and Chemistry of Water, Ukrainian National Academy of Sciences, Vernadsky avenue 42, 03680 Kyiv, Ukraine
The scope of the definite experimental evidence regarding studies of adsorption and micellation of surfactants, and, in particular, the regularities of surface activity and critical micellation concentration in the surfactant homologic series had resulted in the development of various approaches to the prediction of surface-active properties and technologic characteristics of surfactants. Among these, the approaches based on the concepts of the hydrophobic-lyophobic balance of surfactants and the adsorption work increments should be emphasised. These concepts, while being interrelated with the theoretical models of surfactant adsorption and micellation, are nevertheless essentially empirical. This fact implies the existence of numerous contrarieties in the theory of adsorption of surfactants in general, and the adsorption of ionic surfactants, in particular.
It is quite interesting therefore to consider the surfactants adsorption and micellation consistently, on the basis of the well-defined statistical model of these processes.
The theory is based primarily on the obvious difference in the molecular masses of the solvent (water) and dissolved species (surfactant). This fact enables one to differentiate in the system studied (surfactant solution – air) between two statistical subsystems: the fast subsystem (solvent molecules) and slow subsystem (surfactant molecules), with the energy exchange between these subsystems being accomplished via the averaged potential jS, with different values in the bulk phase and in the surface layer.
Introducing some additional assumptions which correspond to different model concepts, various adsorption and surface tension isotherms could be derived.
Here the most interesting is the model of non-localised adsorption of rigid oriented cylinders, with the intermolecular forces approximated by square
well potential. The adsorption and surface pressure isotherms in this case are expressed via the set of transcendental equations known as the Hill-De Boer adsorption isotherm and the two-dimensional Van der Waals equation:
(1)
(2)
where s and s0 is the surface tension of the solution and the pure solvent, respectively; is the relative adsorption, is the adsorption in the extremely saturated monolayer, b2 is the adsorbate-adsorbate interaction constant, W is the adsorption work, d is the adsorption monolayer thickness, ca is the surfactant concentration.
It follows from the analysis that in the limiting cases this set of equations can be reduced to the equations well-known in the surfactant adsorption theory, namely the Langmuir, Buttler, Frumkin and empiric von Szyszkowski equations.
The integral energetic characteristics W, b2 (and also ) could be readily calculated from the experimental surface tension isotherm of the solution. Also, the value can be determined in the explicit independent experiment, and calculated from the surfactant molecule cross-section. This enables one to judge about the consistency of the accepted surfactant adsorption model. The adsorption monolayer thickness can be taken equal to the surfactant molecule length.
The approach summarised above was employed for the processing of the surface tension adsorption isotherms measured in lower alcohol solutions. The most nameworthy result is the fact that the area per one molecule calculated in this way was almost equal to the values obtained by Langmuir balance method for the insoluble monolayers of higher alcohols, and the values calculated from the Van der Waalsian radii, valence angles and interatomic distances of the alcohol molecules.
The variations in the adsorption work W and the adsorbate-adsorbate interaction constant b2 along the homologic series of the alcohols studied comply with the increments rule, the fact which is quite natural because of the additivity of the dispersion forces of intermolecular attraction.
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