Theoretical And Experimental Studies
theoretical and experimental studies
of electrosurface phenomena in the processes
of adsorption and micellation in ionic surfactants
V.Ya. Poberezhny
Ecology, Technology, Science Ltd., Podlesnaya st. 1, Kyiv, Ukraine
e-mail: tovetna@ucr.net
The explicit experimental studies of the electrosurface phenomena at ionic surfactant/air interface, however scarce, enable one to come to unambiguous conclusions:
– the formation of Double Electric Layer (DEL) takes place at the interface, with the potential corresponding to the polarity of surface active ion;
– even in the very low ionic surfactant concentration range, very high values of the electrokinetic potential are observed, and the electric work ey required for the ion transfer from the adsorption monolayer to the solution bulk amounts to 3-6kT, which is comparable to the surfactant adsorption work.
These facts, being considered together, enable one to make two conclusions which are important from the fundamental point of view:
– the characteristic space region in which the physicochemical phenomena in DEL are taking place is much larger than the adsorption monolayer; also, the difference between the concentrations of ions in this region and their bulk values is much larger than the range of the solvent concentration variation from the bulk value in the liquid phase to the value in the gas phase;
– the isothermic work of the unit interface area formation (the surface tension of the solution) is necessarily related with the electric work of the formation of the corresponding part of the DEL;
The formal treatment of the ionic surfactants adsorption and micellation process, in contrast to the non-ionic surfactants, is based on the fact that the adsorption isotherm for each ion present in the system, in its most general form, is expressed by the equality between the electrochemical potentials throughout the transition region and in the solution bulk.
In the model of the transition region considered here, which is, in fact, quite similar to that proposed by Davies in 1950s, two essentially different areas are distinguished: the adsorption monolayer of the potential-defining ions, which consists mostly of the surface-active ions (surface, two-dimensional structure), and the diffusion portion of the DEL ("bulk", three-dimensional structure), which is separated form the monolayer by the proximity plane. In this case, the non-electric constituent of the chemical potential, corresponding to the ions localised in the adsorption monolayer plane, is calculated from the monolayer non-localised adsorption model, while the structure of the diffuse part of the DEL is considered in the framework of the self-consistent field approximation.
This model, being consistently implemented, leads to the set of the equations for adsorption and surface pressure, which are essentially the generalised versions of the Hill-De Boer equation and the two-dimensional Van der Waals equation (for the adsorption monolayer), while the adsorption isotherms in the diffuse part of the DEL are identical to the uniparticle distribution functions for ions, and correspond to the relations developed in the modified Goüy-Chapman theory.
In this case, the work of adsorption for ions in the adsorption monolayer (similarly to the neutral surfactant molecules) is determined only by the non-electric constituent of the work associated with the transfer of the isolated ion from the adsorption monolayer into the solution bulk. The non-additive electrical constituent (which is dependent also on the solution concentration) should be accounted for independently. To calculate the work of adsorption of surface-active ions, defined in this way, the increments method can be applied.
The theoretical results summarised above enable one to explain consistently all fixed experimental facts related to the adsorption and micellation processes in ionic surfactants, in particular, to explain the applicability restraints of the Gibbs’ adsorption equations for ionic surfactants, to introduce an unambiguous definition of the amount of coupled counterions, to explain apparent increase of the surface activity of ionic surfactant related to the addition of the indifferent inorganic electrolyte to the solution and resulting essentially from the decrease of the electrostatic repulsion between surface-active ions in the adsorption monolayer (for constant adsorption work, i.e., constant surface activity), and to describe quantitatively the synergism feature of anionic and cationic surfactants.
Related articles::