Wanda Barzyk

Novel results on the adsorption of ionic surfactants at the air/water interface — sodium-n-alkyl sulphates

Abstract Equilibrium surface tension vs. concentration isotherms of aqueous, surface-chemically pure solutions of the homologous series of sodium n-alkyl sulphates (from C7 to C14) have been evaluated using a new surface equation of state approach. All adsorption parameters (standard free energy of adsorption, interaction parameter, saturation adsorption) and the CMC reveal a pronounced even/odd effect. Surprisingly, it was found that the cross-sectional areas of the short-chain homologues decrease with increasing chain length, reaching a minimum value for C10 and C11. However the cross-sectional areas of the longer-chain homologues (Cn ≥ C12) increase again drastically with increasing chain length. This novel effect is attributed to increasing electrostatic repulsion within the adsorption layer caused by increasing Debye lengths of the stronger surface-active homologues. The good agreement between the data for the cross-sectional area of sodium-n-dodecyl sulphate, as obtained from the new surface equation of state approach, from the Gibbs equation and from crystal structure analysis, make us suggest that it is unreasonable to apply an additional factor in the Gibbs equation for ionic amphiphiles as long as the counterion is very small in comparison with the oppositely charged amphiphilic molecule.

The influence of surface oxidation of chalcocite on its floatability and ethyl xanthate sorption

Abstract The results obtained from measurements of potassium ethyl xanthate (EtXK) sorption by synthetic chalcocite (Cu 2 S) and the results of chalcocite floatability measurements are presented. Five chalcocite samples, denoted as Cu 2 S A, B, C, D and E, were used for the measurements. Chalcocite samples of 60–75 μm were prepared in the same manner but were kept under conditions which differed in their degree to prevent surface oxidation by atmospheric oxygen. Chalcocite surface oxidation has a strong effect both on xanthate sorption and on chalcocite floatability. The maximal amount of xanthate abstracted ( Q max ) by chalcocite samples from deoxygenated solutions after a long sorption period increases with increasing oxidation of the samples. The Q max values give information concerning the total amount of surface oxidation products. Chalcocite decreases its floatability with increasing surface oxidation. Most oxidized chalcocite samples required an about 100 times greater collector consumption to obtain the same flotation results than the least-oxidized samples studied.

Kinetic study of abstraction of ethyl xanthate ions by oxidized chalcocite surface

Abstract The results of potassium ethyl xanthate (EtXK) consumption measurements by synthetic chalcocite (Cu 2 S grain-sized classes of 60–75 μm and 120–200 μm, respectively) are presented. These measurements were done in a modified circulation apparatus in an argon or oxygen atmosphere at pH=9.5. The experimental results were compared with theoretical results predicted by a mathematical model based on the assumption that the EtX − ions were immobilized as a result of a chemical reaction (or reactions), e.g., of an ion-exchange type, taking place within the oxidation product layer (OPL), formed on the Cu 2 S surface. The experimental and theoretical results are in good agreement especially for small values of Q 0 ( Q 0 is the initial mass of the EtX − in solution per mass unit of Cu 2 S). In this case both the theoretical and experimental results show that the EtX − concentration in solution decays exponentially with time.

Influence of frothers on floatability. I. Flotation of single minerals (quartz and synthetic chalcocite)

Abstract The flotation and sorption properties of chalcocite and quartz in potassium ethyl xanthate (EtXK), frothers (α-terpineol and n-amyl alcohol), and (xanthate + frother) mixed solutions were investigated. Surface tension and frothing properties of the solutions were also measured. Floatability and sorption properties of the minerals investigated strongly depended on any controlled or uncontrolled change occurring at the mineral surface. Surface oxidation of chalcocite was shown to be an especially important parameter. The frothers studied showed some collecting properties in respect to chalcocite and quartz and influenced the xanthate sorption on chalcocite. Comparison of the flotation recoveries in frother solutions respectively with and without xanthate indicated an occurrence of noticeable collector-frother interactions during formation of the bubble/chalcocite grain aggregate. No such interactions were found for quartz. Influence of the frothers on floatability of the minerals was observed at different frother concentrations but at close values of surface pressure (π) and of retention time (rt).

Noble metal (Ag, Au) cementation on non-stoichiometric cuprous sulphide grains

Noble metal cementation on Cu2−xS grains was studied by measuring the concentration of Ag+ (or AuCl4−) ions in the solution circulating in a special apparatus containing sample of grains. The rate of the cementation process was determined as a function of the reaction extent, which was equivalent to the deposition of approximately 50 uniform atomic layers of the noble metal on the grains surfaces. Changes in the surface microstructure and composition of the samples were determined using X-ray diffraction (XRD), scanning electron microscopy (SEM/EDS) and atomic force microscopy (AFM). Different surface products: mixed copper–silver sulphides, gold sulphides and gold crystallites were found, depending on nature of the noble metal and the solution composition. The course of cementation of gold was greatly changed using chloride instead of sulphate ions. The notable change of the process rate is ascribed to the greatly different structure of gold cementation products on Cu2−xS, as found by the analysis techniques: XRD, SEM and AFM, respectively.

Transitional behaviour of amphiphiles at the air/water interface under equilibrium and dynamic conditions

According to an earlier hypothesis of one of us on the surface equation of state, extendedchain surfactants usually adsorb in (at least) two different surface configurations depending on the surface area available per adsorbate molecule. Evidence for the continuous transition between the different surface configurations in the equilibrium state is provided by thermodynamic (surface tension), rheological (surface elasticity as measured by surface laser light scattering) and electric (surface potential) surface properties of surface chemically pure amphiphiles. The transition between the two alternative surface configurations was also observed in surface potential kinetics of n-dodecanoic acid in 0.005 M HCl.

Influence of frothers on floatability. II. Flotation of chalcocite and quartz mixtures

Abstract Influence of frothers (α-terpineol or n-amyl alcohol) on the floatability of chalcocite and quartz mixtures, with potassium ethyl xanthate as a collector at pH 5.5–6, was investigated with a Hallimond tube. Activation of quartz by the presence of chalcocite was observed and interpreted as due to copper ionic species originating from chalcocite oxidation products. Best selectivity of separation between chalcocite and quartz was obtained when the frother concentration was kept sufficiently low. Collector—frother interactions during formation of three-phase mineral grain/solution/gas bubble contacts were observed.

Ideal (Langmuir-type) surface and electric properties of tert- isopropylphosphine oxide at the air/water interface

The surface pressure (π) and electric surface potential (ΔV) of tert-isopropylphosphine oxide (i-Pr3PO) were measured at the air/water interface. Solutions matching the criterion of “surface-chemical purity” were investigated. Measurable effects on π and ΔV are revealed at 1 x 10-4 mol/dm3 i-Pr3PO and extend throughout three concentration decades. The initial rise of the surface potential with the concentration is several times steeper compared to that of the surface tension. The dependence of the surface tension on the concentration of i-Pr3PO is well described by a Langmuir-type isotherm. Furthermore, the relation between ΔV and the surface concentration (ΓL) calculated using the Langmuir isotherm is linear. Such consistency with the Langmuir-type adsorption throughout the entire surface activity range is an uncommon feature among soluble surfactants. The ideal surface properties of i-Pr3PO are mainly due to the ellipsoidal hard-sphere-like structure of the molecule delimited by the isopropyl chain, which determines the closest packing density of the adsorbate. This is justified by the minimum cross-sectional area of about 80 A2/molecule adsorbed, which corresponds to a sphere delimited by the length of the fully extended isopropyl chain (5 A). The other reason for the ideal surface behavior is the compatibility of the dimensions of an i-Pr3PO molecule with an average water cluster (of 13 H2O). The slope (e0 dΔV/dΓL) determines the effective dipole moment (per molecule) μ⊥/es= 1.2 ± 0.1 D. This relatively high value of μ⊥/es is a result of the dipole moments of the highly polar ≡P = O head group and of six —CH3 terminal groups. Owing to the radial distribution of the terminal groups around the head group, the effective dipole moment is practically independent of the molecule’s conformation at the surface.

Penetration of Milk-Derived Antimicrobial Peptides into Phospholipid Monolayers as Model Biomembranes

Three antimicrobial peptides derived from bovine milk proteins were examined with regard to penetration into insoluble monolayers formed with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DPPG). Effects on surface pressure (Π) and electric surface potential (ΔV) were measured, Π with a platinum Wilhelmy plate and ΔV with a vibrating plate. The penetration measurements were performed under stationary diffusion conditions and upon the compression of the monolayers. The two type measurements showed greatly different effects of the peptide-lipid interactions. Results of the stationary penetration show that the peptide interactions with DPPC monolayer are weak, repulsive, and nonspecific while the interactions with DPPG monolayer are significant, attractive, and specific. These results are in accord with the fact that antimicrobial peptides disrupt bacteria membranes (negative) while no significant effect on the host membranes (neutral) is observed. No such discrimination was revealed from the compression isotherms. The latter indicate that squeezing the penetrant out of the monolayer upon compression does not allow for establishing the penetration equilibrium, so the monolayer remains supersaturated with the penetrant and shows an under-equilibrium orientation within the entire compression range, practically.