Wiesław Wójcik

Wettability of polytetrafluoroethylene by aqueous solutions of two anionic surfactant mixtures

Advancing contact angle (theta) measurements were carried out on mixtures of aqueous solutions of sodium dodecyl sulfate (SDDS) and sodium hexadecyl sulfonate (SHDSs) on polytetrafluoroethylene (PTFE). The obtained results indicate that there were only small contact angle changes over the range of surfactant concentrations in the solution, corresponding to the unsaturated surfactant layer at the aqueous solution-air interface. However, when saturation of the surfactant layer was achieved a considerable decrease in the contact angle (increase in costheta) as a function of concentration was observed. The dependence of costheta on the monomer mole fraction of SHDSs in the mixture of the surfactants (alpha) for aqueous solutions of mixtures at concentrations corresponding to the critical micelle concentration (CMC) had a maximum at alpha=0.2. From the results of these measurements and application of the Gibbs and Young equations the ratio of the excess concentration of surfactants at the solid-aqueous solution interface to the excess of their concentration at the aqueous solution-air interface was calculated. On the basis of the measurements and calculations it was found that there was a straight linear relationship between the adhesion tension and surface tension of aqueous solutions of surfactant mixtures at a given alpha, and that the slope of the obtained straight lines was equal to -1, which suggests that the surface excess of the surfactant concentrations at the PTFE-solution interface is the same as that at the solution-air interface for a given bulk concentration of the surfactant mixtures. The dependence of the surface concentration excess at the PTFE-solution interface on the monomer mole fraction of SHDSs in the mixture of the surfactants for the concentration region of the mixture of aqueous solutions, corresponding to a saturated monolayer, had a maximum at alpha=0.4, probably resulting from increased degree of binding between adsorbed surface-active ions and counterions. It was also found that the critical surface tension of PTFE wetting by aqueous solutions of two anionic surfactant mixtures was equal to 23.35 mN/m, which was higher than the critical surface tension of wetting determined from straight linear dependence between costheta and surface tension of alkanes (19.94 mN/m). This suggests that polar interaction at PTFE-aqueous solutions of surfactant mixtures can probably take place.

Changes in zeta potential and surface free energy of calcium carbonate due to exposure to radiofrequency electric field

Abstract A calcium carbonate (Fluka; more than 99.0% pure) suspension or a thin layer of its powder deposited on a glass slide was exposed to an r.f. electric field (44 MHz; 60 V peak-to-peak no-load amplitude) for 5–30 min. The zeta potential of the suspension in water (natural pH, 9.9 ± 0.1) or in 10−2 M NaCl (natural pH, 9.6 ± 0.1) was then measured electrophoretically. The surface free energy components, i.e. non-polar Lifshitz—van der Waals (γLWS) and polar (acid—base) electron donor (γ−S) and electron acceptor (γ+S) were determined by the “thin-layer wicking technique”. It was found that a negative zeta potential of CaCO3 in water measured immediately (within 2–3 min) after the exposure to the r.f field changed in an oscillatory fashion depending on the exposure time. For the sample treated for 15 min with the r.f field the zeta potential decreased by 5–7 m V in relation to an untreated sample. Residual changes occurred also in an oscillatory fashion during the 40 min after field removal. A similar behavior was found for the suspension in 10−2 M NaCl. The observed zeta potential changes were accompanied by changes in the electron donor component of the surface free energy and the appearance of a small electron acceptor component, which resulted from the presence of hydrogen bonding on the calcium carbonate surface. The r.f. field treatment led to a decrease in γ−S from 83 mJ m−2 (untreated sample) to 40 mJ m−2 (15-min-treated sample) and 64 mJ m−2 (25-min-treated sample) and caused the appearance of γ+S components of 1.34 and 1.77 mJ m−2. The observed changes in the zeta potential and the surface free energy components are believed to result from the changes in the surface charge of calcium carbonate and the structure of the water film at the surface.

The surface tension components of aqueous alcohol solutions

Abstract Measurements of the contact angle were carried out in paraffin-drop of the aqueous alcohol solution-air systems for methanol, ethanol and propanol solutions in the whole range of their concentrations with special consideration of the low concentration range. On the basis of the results obtained the dispersion components of the surface tension of the solutions studied were determined. The dispersion components of the solution surface tension calculated from the contact angle were found to differ distinctly from those calculated from the interfacial tension of the alcohol solution-n-dodecane as well as from those calculated from Eqn (15). Using literature data concerning the properties of alcohols (methanol, ethanol and propanol) and n-dodecane molecules, the interaction parameters Φ were calculated, as well as the dispersion components of the surface tension of the alcohols mentioned above. These values of the dispersion component are in perfect agreement with those calculated from the alcohol-n-dodecane interfacial tension. It has been found that using the geometric-mean rule harmonic-mean rule and the parameter Φ of interfacial interactions it is possible to calculate the dispersion components of the alcohol surface tension, because the results obtained are, in principle, in good agreement. Knowing the interaction parameter Φ one may calculate the dispersion components of the surface tension of the solutions of two polar liquids from the equation derived by us earlier.

Components of the surface free energy of low rank coals in the presence of n-alkanes

Abstract Measurements of contact angles for water, glycerol, formamide and diiodomethane on the surfaces of four low rank coals (31.1, 31.2, 32.1 and 32.2) covered with n-alkanes (n-hexane, n-undecane and n-hexadecane) were made. Using the approaches of van Oss et al., the geometric mean of the interfacial free energy, and Youngs equation, the components of the surface free energy of coals and coal precovered with n-alkane films were calculated. Also the total surface free energy was determined from the so-called equation of state. Using the values of the components of surface free energy or total free energy of the coal, the spreading of the n-alkanes at the coal/water interface and the free energy of interaction between coal particles in the water phase were determined. On the basis of these results we have stated that, on the coal surface in air, a very stable, n-alkane film, resulting from a wetting process by spreading is formed, which first of all reduces the apolar component of the surface free energy of the coal and can partially block the polar component. However, it was found that n-alkanes cannot completely spread over a coal surface immersed in water, and the contact angle depends on the type of n-alkane. It was also found that the presence of an n-alkane film on a coal surface decreased the attractive interactions between the coal particles in the water phase, owing to Lifshitz-van der Waals forces and increased interactions resulting from acid-base forces in comparison to ‘pure’ coal.

Investigation of the surface free energy components of thermally treated controlled porosity glasses by inverse gas chromatography

Adsorption isotherms of n-octane and toluene were measured on thermally treated controlled porosity glasses (CPG) and on boron-coated silica gel (SG). From the isotherms the film pressure (π) for both adsorbates was determined. Based on extrapolated πmax values the dispersion (γdS) and the non-dispersion (γnS) components of the surface free energy for all the investigated materials were calculated. It was found that the changes in γdS caused by the surface modification were relatively small (∼ 4mJ/m2), while those of γnS were significant. The boron compounds decrease the γnS value from 145–160 to 37–55 mJ/m2. The dependences between δπ/δa and the adsorbed amount can give some suggestion about the adsorption mechanism and surface homogeneity. The presence of Na on the surface of both CPG and SG seems to be a more important factor for adsorption than that of boron. The significant changes in non-dispersion interactions were attributed to surface dehydroxylation, which may result from both the thermal treatment and the presence of boron on the surface.

The influence of an apolar collector on the contact angle, detachment force and work of adhesion to the coal surface in agglomeration flotation of a low rank coal

Abstract Tests on agglomeration/flotation of coal rank 32.1 for different size fractions (−0.15, −0.25, −0.385 and −0.5 mm) were made using kerosene as the collector. The overall flotation results were correlated with the contact angle, the detachment force and the work of adhesion of water to the coal surface in the system coal/n-undecane film-air bubble — water. It was found that the mass of concentrate and the amount of coal in the concentrate increased as the dosage of kerosene increased. This increase depended on the size fraction of the flotation feed. These flotation indices were improved by increasing the contact angle and the detachment force of an air bubble from the coal surface, by increasing the thickness of the apolar liquid film on the coal surface. This increase of the film thickness also caused a decrease in the work of adhesion to the coal/film surface. The efficiency of coal beneficiation in agglomeration/flotation processes was strongly influenced by contact angle, detachment force and work of adhesion of water to the coal surface.

The enrichment of fine particles of coal by aggregation part II. The use of the selective aggreagation method

Abstract Tests on aggregation enrichement of fine particles of coal are presented. The selective aggregation process, named ‘shear flocculation’ by Warren [7, 8], was conducted in a cylindrical beaker. The aggregation of the coal grains results from the action of shear forces while the feed is mixing with the collector (diesel oil). The aggregates formed were then separated from gangue by aeration in a flotation machine. This allows for almost complete recovery of coal. Preliminary aggregation of coal grains made it possible to eliminate the frothing agents usually needed in such a process.

A Gas-Adsorption study of the influence of a flotation collector on the polar/nonpolar surface properties of barite

Abstract A modified elution gas chromatography method was used to determine the adsorption isotherms of n-octane and n-propanol on a barite surface covered with oleic acid. The extrapolated film pressure ☎ max values of both adsorbates were determined from the adsorption isotherms, and then the dispersion and polar free surface energy values were calculated as a function of the surface coverage with oleic acid. The calculations were performed on the assumption that the ☎ max value equals the work of spreading or the work of immersion. The energetic changes induced by the collector were then correlated with the flotability of barite. It was found that the collector completely reduced the polar interactions of the mineral surface and this was the main reason of the high flotability of this polar mineral.

An estimation of surface free energy of galena

Abstract Adsorption isotherms of n-octane and n-propanol were determined on galena samples pretreated in various ways. Based on the values of maximal extrapolated film pressure π max for both adsorbates (obtained from the isotherms), the dispersion and non-dispersion components of surface free energy were calculated and discussed. The spreading adsorption films seemed the most probable on the galena surface. For the non-oxidized surface of galena the dispersion component was 70.2 mJ m −2 , while the non-dispersion component was only 3.7 mJ m −2 . The surface oxidation resulted in a decrease of the dispersion component to 50−-55 mJ m −2 and in an increase of the non-dispersion component to 13−-14 mJ m −2 . Certain changes of the surface free energy of galena were also stated, depending on the method of sample pretreatment. They probably resulted from a partial elimination of the oxidized layer or from other changes in the structure of this layer (e.g. recrystallization).

Surface free energy of some lead compounds compared to galena

Abstract Measurement of contact angles for water, formamide and diiodomethane with lead compounds of sulfate, thiosulfate, oxide and hydroxycarbonate were made on both dried and prewetted galena surfaces. Using the values obtained, Lifshitz-van der Waals and Lewis acid-base (electron acceptor and electron donor) components of the surface free energy of the solids were determined, and compared to the values of the surface free energy of nonoxidized and oxidized galena. It was found that the free energy of the galena surface originated from apolar and polar interactions, and that polar interactions were strongly dependent on the surface hydration. Moreover, it may be concluded that the wettability of galena depends on the lead compounds formed on its surface as a result of the oxidation process, and on the degree of their hydration.