A. de Keizer

Temperature dependence of the electrical double layer on oxides: rutile and hematite

Abstract The surface charge σ0 on aqueous suspensions of hematite and rutile was measured as a function of pH in various KNO3 concentrations and at different temperatures. Two types of congruences were observed: (a) if σ0 is plotted as a function of the pH minus the pzc, curves on these two oxides coincide at the same KNO3 concentrations and (b) the same is observed for σ0 as a function of temperature. Thermodynamic analysis enabled us to separate double layer parameters into oxide-specific and generic contributions. Standard Gibbs energies, enthalpies, and entropies for proton adsorption were obtained.

Thermodynamics of micellization of n-alkylpyridinium chlorides

Abstract The effect of temperature on the thermodynamic parameters of micellization of a homologous series of n-alkylpyridinium chlorides has been studied by isothermal microcalorimetry. The decyl, dodecyl and tetradecyl compounds were studied both in water and in 0.1 M NaCl in the temperature range 6–60°C. Enthalpies and entropies of micellization are strongly temperature dependent whereas there exists only a weak temperature dependence for the standard Gibbs energies. With increasing temperature, the micellization process changes from endothermic to exothermic, indicating the importance of hydrophobic bonding. The transition temperature decreases with increasing chain length; this was interpreted in terms of a relationship between the temperature and the enthalpy per CH2 group. It was concluded by extrapolation that this enthalpy was zero at a temperature of about −12°C. Enthalpies of micelle formation calculated from the temperature dependence of the CMC compare satisfactorily with those obtained microcalorimetrically.

THE EFFECT OF CATIONIC SURFACTANTS ON WETTING, COLLOID STABILITY AND FLOTATION OF SILICA

Abstract Contact angle, colloid stability and flotation measurements have been carried out for silica as a function of the concentration of cationic surfactants at different salt concentrations. The results are compared with surfactant adsorption and surface charge isotherms. The surfactants used are dodecyl and cetyl pyridinium chloride, and dodecyl trimethyl-ammonium bromide. Colloid instability, contact angles and flotation recovery all pass through a maximum at about the charge compensation point that corresponds for the present system to the iso-electric point and the (near) common intersection point of surfactant isotherms measured at a given pH and different ionic strength values. Before the charge compensation point the surfactant progressively neutralises the negative surface charge by head-on adsorption and simultaneously the surfactant tail causes an increase in hydrophobicity. Beyond the charge compensation point admicelles are formed, leading to a net positive particle charge and an increase in hydrophilicity. In the charge compensation point the maximum of hydrophobicity occurs together with the minimal net particle charge. Therefore, the minimum in the stability and the maximum in the flotation recovery are related to both the low particle charge and the relatively large particle hydrophobicity around the charge compensation point.

Specific ion adsorption on oxides: Surface charge adjustment and proton stoichiometry.

In this paper a quantitative analysis of the coadsorption ratio for oxides, r OH , that is the number of OH − ions coadsorbing with each specifically adsorbing metal ion at given pH, is given. This ratio is an important double-layer parameter. It is shown that because of the high accumulation of charge on the surface and the effective compensation of this charge by countercharge in the Stern plane, r OH can be established with satisfactory precision using Gouy-Stern theory only. Computed r OH values depend primarily on the distance between the pH and the pzc and to a lesser extent on the indifferent electrolyte concentration. If the Stern layer capacitance is high, it is not a critical parameter. Agreement with data in the literature and new measurements for Cd 2+ adsorption on rutile and hematite is satisfactory.

Temperature dependence of cadmium adsorption on oxides. I: Experimental observations and model analysis

Abstract The influence of temperature on the adsorption of cadmium on rutile (TiO2) and hematite (α-Fe2O3) is measured at different values of the (initial) surface charge density and salt concentration. The formation of cadmium hydroxo-complexes in solution is simultaneously investigated. Experimental adsorption results are analyzed in terms of the Frumkin-Fowler-Guggenheim model, using a Gouy-Stern double layer picture. Thermodynamic analysis points to a great similarity between the binding of Cd2+ with hydroxyl groups on the surface and with OH− ions in solution. Both processes are entropically driven.

Mixed adsorption of poly(vinylpyrrolidone) and sodium dodecylbenzenesulfonate on kaolinite

The mixed adsorption of the nonionic polymer poly(vinylpyrrolidone) (PVP) and the anionic surfactant sodium dodecylbenzenesulfonate (SDBS) on kaolinite has been studied. Both components adsorb from their mixture onto the clay mineral. The overall adsorption process is sensitive to the pH, the electrolyte concentration, and the amounts of polymer and surfactant. Interpretation of the experimental data addresses also the patchwise heterogeneous nature of the clay surface. In the absence of PVP, SDBS adsorbs on kaolinite by electrostatic and hydrophobic interactions. However, when PVP is present, surfactant adsorption at 10(-2) M NaCl is mainly driven by charge compensation of the edges. The adsorption of PVP from the mixture shows similar behavior under different conditions. Three regions can be distinguished based on the changing charge of polymer-surfactant complexes in solutions with increasing SDBS concentration. At low surfactant content, PVP adsorbs by hydrogen bonding and hydrophobic interactions, whereas electrostatic interactions dominate at higher surfactant concentrations. Over the entire surfactant concentration range, polymer-surfactant aggregates are present at the edges. The composition of these surface complexes differs from that in solution and is controlled by the surface charge.

Surface and volume charge densities of monodisperse porous silicas

Abstract Potentiometric proton titrations of a series of monodisperse Stober silicas with different radii and of non-porous Aerosil OX50 are performed in the presence of different concentrations of alkali and tetraalkylammonium nitrates. The charge developed for the Stober silicas in the presence of alkali ions is extremely high as compared to that of the non-porous silica. In the presence of tetraalkylammonium ions, the charge density of Stober silica is much lower than that in the presence of alkali ions, whereas for the non-porous Aerosil, a small increase of the charge density is observed. The charge increase on the non-porous silica has been attributed to the effect of specific adsorption of the tetraalkylammonium ions, whereas the charge reduction for Stober silica must be a consequence of porosity. The degree of dissociation of hydroxyls inside the porous volume of oxidic materials depends on the ability of counterions to penetrate into the porous phase. This principle has been applied to characterise the porosity of silica. For monodisperse silicas, three regions of charge formation have been distinguished: the outer surface and two kinds of pores. On the outer surface, the charge is 10–18 times smaller than that inside the porous volume, but of the same order of magnitude as the surface charge of the non-porous Aerosil OX50. The large pores (“mesopores”) are accessible to both alkali and tetraalkylammonium ions. The small pores (“micropores”) are only accessible to the alkali ions. For the particle sizes investigated, the charge developed inside the mesopores is of the same order of magnitude as that on the outer surface. The distinction between the meso- and micropores is not necessarily determined by the dimensions of the pores only; also, the nature of the counterions is relevant.

Monodisperse, nonporous, spherical silica particles

Abstract Merck Monospher 250, which comprises monodisperse porous silica particles ≈250 nm in diameter, was subjected to acid-base titration at 20°C in 0.01 M KNO 3 and in 0.01 M Et 4 NNO 3 (with no K + present). The specific proton charge at pH 9 amounted to −55 C g −1 in 0.01 M KNO 3 , but only −8 C g −1 in 0.01 M Et 4 NNO 3 . This difference indicates significant microporosity. Samples of Monospher were heated in air at 800°C for 3–36 h. After heat treatment the particles were still spherical, but slightly smaller, as judged by scanning electron microscopy and dynamic light scattering. The specific charge of the heated samples determined by titration under the same conditions as before was −2 C g −1 for both counterions, which shows that the silica was no longer microporous. The change in the specific charge in 0.01 M Et 4 NNO 3 from −8 to −2 C g −1 suggests that the mesoporosity was also lost during the heat treatment. The specific charge of Aerosil OX-50 was not affected by heat treatment. The surface charge densities of Aerosil and heated Monospher were both between 0.12 and 0.17 C m −2 . For monosphere, a slow increase of the titrated charge took place after prolonged contact with water. After ageing for 2 months at pH 5, about 25% of the charge before heating was re-established; after ageing at pH 9, the proton charge increased to about 35% of the original charge. Nonporous, monodisperse spherical silica particles produced by this method have potential uses in theoretical studies and synthetic applications, at least for limited periods of contact with aqueous solutions.

Adsorption of cationic potato starch on microcrystalline cellulose

Abstract The adsorption of cationic potato starch on microcrystalline cellulose was investigated taking into account that starch is a mixture of amylose and amylopectin. The separate adsorption of both components could be calculated by determining starch concentrations twice using two methods having a different sensitivity for amylose and amylopectin. It was found that cationic amylose adsorbs preferentially. The preferential adsorption is probably related to the porous nature of microcrystalline cellulose. The smaller and linear amylose molecules are able to penetrate the pores, while the much larger and branched amylopectin molecules have only access to the exterior surface. The effects of monovalent and divalent electrolytes, pH and degree of substitution (DS) on the adsorption were investigated. The adsorption was found to decrease with increasing electrolyte concentration, the divalent cations being ten times as effective in suppressing the adsorption as the monovalent cations. Increasing the surface charge by increasing the pH led to higher adsorption. The effect of DS depended on the electrolyte concentration: at low electrolyte concentration the adsorbed amount was largest for the cationic starch with the lowest DS, whereas at higher electrolyte concentration starch with the highest DS adsorbed better. These effects can be explained with a recent theory on polyelectrolyte adsorption.

Electrophoresis of randomly oriented cylindrical particles

It has been derived, that the electrophoretic mobility of a randomly oriented charged cylinder is obtained by adding one-third of the mobility of a cylinder parallel to the field to two-thirds of its mobility perpendicular to the field, when the relaxation effect is neglected.