Nanostructured surfaces
Formation of nanolayers upon adsorption
of cationic surfactants on solid surfaces
 
Streltsova E.A., Dzhyga G.M., Mazuryk A.A., Bondar A.D.
Department of Physical and Colloid Chemistry, Odessa I.I. Mechnikov National University, Dvoryanska Str.,65082Odessa,Ukraine 
E-mail: Al_bondar_dm@ukr.net
A significant number of technological processes are based on adsorption from solutions of surfactants on solid surfaces. The starting point for calculating and optimizing such processes is information on the adsorption isotherm and the structure of adsorption nanolayers on a solid surface. Despite of the large number of adsorption equations available in literature, it is not clear whether they can be used to describe the adsorption isotherms of surfactants from aqueous solutions on energetically homogeneous adsorbents and adsorbents of natural origin, and which of them are preferable for obtaining information on the structure of adsorption layers.
The used equations of adsorption isotherms (Langmuir, Temkin, Freundlich, Dubinin-Radushkevich, Hill-de Boer, as well as the Hildebrand equation obtained in the framework of the lattice model) made it possible to determine the concentration regions that are most informative for obtaining the main parameters of the adsorption process of the studied dodecylammonium chloride, dodecylpyridinium chloride and hexadecylpyridinium perchlorate on the surface of paraffin and natural bentonite. The data show that Langmuir and Dubinin-Radushkevich models have the best linearity with the data than other model. The adsorption of surfactants on a low-energy solid surface of paraffin is due to the action of dispersion forces, as evidenced by the values​of the parameters of the Temkin equation (α = 2.2), Freundlich (for DDPC and DDAC n> 1). As the adsorption layer is filled (when θ →1), hydrophobic interactions between hydrocarbon radicals of surfactant molecules adsorbed on a solid surface and located in the bulk of the solution begin to play a significant role. These interactions lead to the formation of surfactant nanoassociates on a solid surface (Langmuir equation A∞ = (2.0 - 21.3) 10-6 mol/g, Hill - de Boer equation (K2 = 4.5 - 13.7)).The interaction between adsorbed surfactants and bentonite depends on several possible mechanisms (electrostatic forces, van der Waals forces, ion exchange and hydrophobic interactions).