Jolanta Narkiewicz-Michałek

ADSORPTION OF CATIONIC SURFACTANTS ON HYDROPHILIC SILICA : EFFECTS OF SURFACE ENERGETIC HETEROGENEITY

Abstract The thermodynamics of adsorption of cationic surfactants on a hydrophilic silica surface is discussed. The adsorption isotherms of these surfactants on negatively charged silica surfaces exhibit a two-step character, as a rule. The heights of these steps and the concentration at which the second step begins depend strongly on the surface charge determined by the pH and indifferent electrolyte concentration in the equilibrium bulk solution. In our previous publication we have developed a theoretical description based on the model of the adsorbed phase being a mixture of monolayered and bilayered aggregates of different sizes. Only the “excluded area” interactions were taken into account. The potential energy per molecule was assumed to decrease linearly with the aggregate size. The theoretical expressions obtained for adsorption isotherms and heats of adsorption were next fitted to experimental data of cationic surfactants adsorption on precipitated silica. Although our model was able to reproduce well the two steps observed on the adsorption isotherms of these surfactants, it failed to predict the behavior of isosteric heats of adsorption at low surface coverages. In this paper our model is extended by taking into account the effects of surface heterogeneity on single monomer adsorption. This leads to a much better description of entropic effects accompanying surfactant adsorption in the low coverage region. It is therefore concluded that individual adsorption of surfactant ions, which is determined primarily by the specific solid–surfactant interactions, is strongly affected by the surface heterogeneity effects.

Use of a Dynamic Light Scattering Technique for SDS/Water/Pentanol Studies

The interpretation of micelle/aggregate size obtained by use of the DLS technique for SDS/water/pentanol systems was discussed by comparison of the results of measurement with theoretical data. For most of the studied systems, the apparent radii (R h,app ) did not satisfactorily characterize the size of the aggregates (R h,app < 1 nm). The use of a correction factor (f = 0.26) confirmed that the discrepancies were associated with the electrostatic intermicellar interactions. However, the fuzzy optical interface between dispersed and dispersing phases can also be the reason of such results. An increase of pentanol content caused a decrease of the droplet radius in w/o systems but in o/w systems the changes were negligible.

Theoretical modeling of cationic surfactants aggregation at the silica/aqueous solution interface: Effects of pH and ionic strength

A theory of ionic surfactant aggregation on oppositely charged surfaces is presented. In the proposed model the adsorbed phase is considered as a mixture of singly dispersed surfactant molecules, monolayered and bilayered aggregates of various sizes and the ions of simple electrolyte added to the aqueous solution. The electrostatic interactions between the adsorbed ions (surfactant ions and simple ions) and the charged surface are described by using the triple layer model (TLM) formalism. The calculated surfactant adsorption isotherms, surface charge isotherms, and electrophoretic mobilities of silica particles are compared with the experimental results obtained for two cationic surfactants (dodecylpyridinium chloride and cetylpyridinium chloride) adsorbed on Aerosil OX50 at two different values of pH. On the basis of the theoretical results the evolution of the adsorbed phase structure and the charge of silica particles with an increasing surface coverage is discussed.

Coverage dependence of aromatic hydrocarbon diffusion in silicalite: predictions of the three-site lattice model

The three-site lattice gas model (TSLG) was used to explain the experimentally observed changes in the transport diffusion coefficient of benzene adsorbed in silicalite. The influence of the surface coverage on the thermodynamic correction factor was studied and compared with the experimental data for the benzene/silicalite and also for the p-xylene/silicalite system at 303 K. In the case of benzene it was demonstrated that the maximum on the coverage-transport diffusion coefficient curve originates from a rapid rearrangement of the adsorbed molecules which takes place at the loading of ∼4 molecules per unit cell (m/uc). In particular, it was shown that the localization of the molecules in the channel intersections and the zigzag channels of silicalite causes the diffusivity to decrease at the loading exceeding 4 m/uc. This phenomenon, combined with the initial increase of the thermodynamic correction factor with the loading (up to 4 m/uc), was considered responsible for the presence of the maximum on the curve.

Adsorption from solutions onto solid surfaces. Effects of topography of heterogeneous surfaces on adsorption isotherms and heats of adsorption

A model investigation is presented, showing the effects of the topography of heterogeneous solid surfaces on adsorption from binary liquid mixtures onto solid surfaces. Equations are developed for excess adsorption isotherms and heats of immersion, for both patchwise and random surface topography, when the adsorbed solution is a quadratic mixture. Using the developed equations, numerical model calculations are carried out for some typical values of heterogeneity and interaction parameters.

Adsorption at the solid/liquid interface. An approach to multilayer adsorption from binary liquid mixtures on heterogeneous solid surfaces

The Rudzinski–Partyka and Rudzinski–Jagiello theoretical approaches to monolayer adsorption at slightly heterogeneous and strongly heterogeneous solid/solution interfaces are generalized to describe multilayer adsorption from binary liquid mixtures onto these solid surfaces. Concentration profiles in the vicinity of a solid surface, as well as the total excess isotherms, are calculated to show how various physical factors affect multilayer adsorption on the heterogeneous solid surfaces.

Adsorption of surfactants at air/solution interfaces: theoretical description of adsorption equilibrium based on a model of clustering of the surfactant molecules at the air/solution interface

On the basis of the scaled particle theory, a new theoretical approach is developed for adsorption of surfactants capable of forming two-dimensional aggregates at the air/solution interface. The adsorption layer is treated as a mixture of oblate aggregates of various dimensions, interacting via excluded area interactions. A five-parameter equation for the adsorption isotherm is developed, and then integrated to obtain the surface tension isotherm. Experimental surface tension data of a series of n-alkyldimethylphosphine oxides are used for comparison with the equation derived. The numerical best-fit analysis suggests the formation of small aggregates at the air/solution interface for these surfactants. The model parameters obtained while fitting best the model equation to the experimental data show a linear correlation with the alkyl chain length of the surfactant.

Effects of surface heterogeneity on liquid adsorption chromatography with mixed mobile phases. Analytical approximations for partition coefficients

The effects of the energetic heterogeneity of solid/solution interfaces on the behaviour of liquid–solid chromatography systems have been investigated theoretically and the existing theories have been re-examined. This has led to a more rigorous explanation of the theoretical background of some equations which were developed in a semi-intuitive manner. In other cases, e.g. Soczewinskis logarithmic relationship, a totally new theoretical interpretation has been obtained explaining why the tangent of this linear relationship is < 1 in many cases.

THEORETICAL MODELING OF IONIC SURFACTANT ADSORPTION ON MINERAL OXIDE SURFACES

Abstract A model for the adsorption of ionic surfactants on oppositely charged solid surfaces of uniform charge density is developed. The model is based on the assumption that, on the solid surface, adsorbed surfactant monomers, monolayered and bilayered surfactant aggregates of different sizes and specifically adsorbing ions of added electrolyte constitute a mixture of hard discs. It means that only excluded area interactions between the surface discs are taken into account. To avoid a rapid two-dimensional condensation of the adsorbed surfactant the potential energy per molecule in the surface aggregates, which is a sum of chemical and electrostatic interactions, is assumed to decrease linearly with the increasing aggregate size. The electrostatic interactions of ionic species with the charged solid surface are described in terms of the Guy-Chapman theory of the double layer formation. The appropriate equations for adsorption isotherms of surfactant and electrolyte ions are derived and used to predict the experimental adsorption isotherms of DTAB on the precipitated silica at two different salt concentrations in the aqueous solution, On the basis of the obtained results the evolution of the adsorbed phase structure and the charge of silica particles with an increasing surface coverage is discussed.

Co‐Adsorption of CTAB and Propyl Gallate on the Hydrophilic Silica Surface

Propyl gallate (PG) adsolubilization in the cationic surfactant micelles formed in the bulk solution and at the silica/solution interface has been investigated. It was found that in the absence of surfactant, propyl gallate does not adsorb on the silica surface from aqueous solution. However, in the presence of hexyltrimethylammonium bromide (CTAB), its uptake by silica strongly increases. The amount of PG in the surface layer depends on its concentration in the bulk solution and the amount of CTAB in the adsorbed layer.