Suzana Žalac

Chapter 6 – Thermodynamics of the solid/liquid interface - its application to adsorption and colloid stability

This chapter discusses thermodynamics of the solid/liquid interface and its application to adsorption and colloid stability. Charging of solid surfaces in liquid medium is a consequence of interactions of ions from the bulk of the solution with surface groups. Surface charge determines the distribution of ions in the liquid layer near the surface. Electrostatic interactions between charged particles are responsible for the stability of colloid dispersions—that is, the rate of particle aggregation depends on the equilibrium in the electrical interfacial layer (EIL) located between solid and liquid medium. It is shown that the colloid stability caused by electrostatic repulsion among particles is determined by the electrostatic potential at the onset of the diffuse layer and the distribution of ions within the diffuse layer. To understand the colloid stability phenomena, one should consider the equilibrium at solid/liquid interface. Interfacial reactions are considered by surface complexation model (SCM) and corresponding interfacial equilibrium constants. There are different theoretical models describing the structure of EIL differing in the quantitative relationships between electrostatic potentials and surface charge densities. Theoretical interpretation of experimental data depends on the choice of the employed model. The most frequently investigated aqueous dispersions are those the surface charge of which depends on pH, such as metal oxides and hydroxides. In the derivation of the activity coefficients of interfacial species, it will be assumed that the major contribution to the non-ideality is because of the electrostatic effects. The reason for such a simplification is that electrostatics plays a major role.

Characterization of micellar systems by the conductivity method; sodium salt of perfluoropolyether carboxylic acid

A new method for the interpretation of the conductivity of ionic surfactants is proposed. The model considers the contribution of ions and of charged micelles, and is based on the approximation of constant concentration of surfactant chains above the critical micellization concentration (CMC), and on the equilibrium constant of counterion bindings to the micelles (K). These two parameters evaluated for aqueous solutions of sodium salt of perfluoropolyether carboxylic acid are as follows:K=35800 dm3 mol−1; CMC=1.19×10−3 moldm−3. The third quantity obtained by this procedure is the product of the aggregation number (N) and the size parameter (f=e2L/6πηr):Nf=76.5 S cm2 mol−1.

Equilibrium of counterion association in micellar systems: cetyltrimethylammoniumbromide

Equilibrium of counterion association in micellar systems: cetyltrimethylammoniumbromide Surface Complexation Model was applied for the analysis of counterion association in the micellar systems. The equilibrium in cetyltrimethylammoniumbromide (CtaBr) solution was examined. Thermodynamic equilibrium constant was calculated using experimental data obtained with ion-selective electrodes (silver/silver bromide and surfactant electrode).