Investigation of the electrokinetic properties of paraffin suspension. 2. In cationic and anionic surfactant solutions

Electrical phenomena at nonionogenic hydrophobic surfaces (solid or liquid) in water, electrolyte, and/or surfactant solutions still attract research. In part 1 of this paper we described the electrokinetic behavior of paraffin wax suspension in water and electrolyte solutions (NaCl or LaCl3). On the basis of the latest data of water structure near hydrophobic surfaces it was concluded that immobilized water dipoles at the interface can play an essential role in the zeta potential formation. In this paper were investigated the zeta potentials of paraffin wax in cationic surfactants cetyltrimethylammonium bromide, C16H33(CH3)3NBr, and octadecyltrimethylammonium chloride, C18H37(CH3)3NCl, and anionic surfactant sodium dodecyl sulfate, C12H25SO4Na. Also changes in wettability of the paraffin surface due to the surfactant's adsorption were studied via wetting contact angle measurements and calculation of the surface free energy. It was concluded that at a low surfactant concentration (10(-6) M) the water dipole structure still contributes to the zeta potential, but at a higher one the zeta potential is determined by the surfactant molecules' adsorption. A special role of OH- ions is also clearly seen. Moreover, a functional relationship was found between the surface free energy of the surfactant-covered paraffin surface and the zeta potential.     

Adsorption of Sodium Dodecyl Sulfate to Colloidal Titanium Dioxide: An Electrophoretic Fingerprinting Investigation

The adsorption of sodium dodecyl sulfate to colloidal titanium dioxide was investigated using the electrophoretic fingerprinting approach. An electrophoretic fingerprint is a contour diagram of the observed electrophoretic mobility as a function of the bulk solution pH and plambda, the log of the bulk solution conductivity. Surfactant adsorption was observed to be strong under acidic conditions, as illustrated in the dramatic changes in the electrophoretic fingerprints. Electrokinetic data were compared with adsorption isotherm data obtained by a depletion method and good qualitative agreement was found. The observed pH changes associated with surfactant adsorption suggested ligand exchange as a possible mechanism of adsorption. Electrophoretic fingerprinting was shown to be a powerful means of examining surfactant adsorption to colloidal particles. Copyright 2000 Academic Press.     

Effect of temperature on n-tetradecane emulsion in the presence of phospholipid DPPC and enzyme lipase or phospholipase A2

Zeta potentials and effective diameters of n-tetradecane emulsions in 1 M ethanol were investigated in the presence of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) (1 mg/100 mL), Candida cylindracea lipase (CCL), and phospholipase PLA2 (1 mg/100 mL) at 20, 37, and 45 degrees C. The enzyme was added at the beginning of mechanical emulsion homogenization or 1 min before the end of stirring for 10 min at 10,000 rpm. It was found that DPPC decreases the negative zeta potentials at all three temperatures. The decrease was largest at 20 degrees C and smallest at 45 degrees C. The influence of the enzymes on the zeta potentials depended on the enzyme kind, time of its injection, and temperature. More negative values of the zeta potentials relative to n-C14H30/DPPC droplets were obtained if the lipase was present. Generally, the effective diameters correlate with the zeta potentials, i.e., lower zeta potential corresponds with bigger effective diameter. Possible reasons for the observed changes of the measured parameters are discussed.     

Effects of a static magnetic field on water and electrolyte solutions

Water and electrolyte solutions were exposed for 5 min to a weak static magnetic field (B = 15 mT). Their conductivity and the amount of evaporated water were then measured as a function of time. Simultaneously, these quantities were determined for magnetically untreated samples, as reference systems. It was found that a magnetic field influences these two parameters and their changes depend on the thermodynamic functions of hydration of these ions. A roughly linear change in conductivity versus 'scaled' functions was obtained. On this basis it was concluded that the magnetic field causes changes in the hydration shells of the ions.     

Influence of sodium dodecyl sulfate and static magnetic field on the properties of freshly precipitated calcium carbonate

Properties of calcium carbonate precipitated from aqueous solutions of CaCl(2) and Na(2)CO(3) in the presence of sodium dodecyl sulfate (SDS) and S-S 0.1 T magnetic field (MF) were studied. The nucleation and precipitation processes of CaCO(3) were investigated by pH and zeta potential measurements at 20 +/- 1 degrees C up to 2 h after mixing the solutions. Also the amounts of calcium carbonate deposited on the glass surfaces and its structure were examined. It was found that SDS influences the kinetics of precipitation, crystallographic forms, and crystal size of CaCO(3). The SDS effects are more pronounced in MF presence. A small amount of SDS accelerates transformation of vaterite into calcite, whereas increasing surfactant concentration moderates such a transformation. On the other hand, in all the systems, MF in the presence of SDS causes a slower transformation of vaterite into calcite. These effects are reflected in pH and zeta potential changes, although there is no clear dependence between the SDS amount present during the precipitation and changes of the parameters investigated. It seems that MF effect is most significant at a defined optimal SDS concentration. The results, however, do not allow suggestion of any detailed mechanism of the field interaction.     

Ice/Water Interface: Zeta Potential, Point of Zero Charge, and Hydrophobicity

The ice/water interface is a common and important part of many biological, environmental, and technological systems. In contrast to its importance, the system has not been extensively studied and is not well understood. Therefore, in this paper the properties of the H₂O ice/water and D₂O ice/water interfaces were investigated. Although the zeta potential vs pH data points were significantly scattered, it was determined that the isoelectric point (iep) of D₂O ice particles in water at 3.5°C containing 10⁻³ M NaCl occurs at about pH 3.0. The negative values of the zeta potential, calculated from the electrophoretic mobility, seem to decrease with decreasing content of NaCl, while the iep shifts to a higher pH. The point of zero charge (pzc) of D₂O ice and H₂O ice, determined by changes in pH of 10⁻⁴ M NaCl aqueous solution at 0.5°C after the ice particle addition, was found to be very different from the iep and equal to pH 7.0 ± 0.5. The shift of the iep with NaCl concentration and the difference in the positions of the iep and pzc on the pH scale point to complex specific adsorption of ions at the interface. Interestingly, similar values of iep and pzc were found for very different systems, such as hydrophilic ice and highly hydrophobic hexadecane droplets in water. A comparison of the zeta potential vs pH curves for hydrophilic ice and hydrophobic materials that do not possess dissociative functional groups at the interface (diamond, air bubbles, bacteria, and hexadecane) indicated that all of them have an iep near pH 3.5. These results indicate that the zeta potential and surface charge data alone cannot be used to delineate the electrochemical properties of a given water/moiety interface because similar electrical properties do not necessary mean a similar structure of the interfacial region. A good example is the aliphatic hydrocarbon/water interface in comparison to the ice/water interface. Although the experiments were carried out with care, both the zeta potential, measured with a precise ZetaPlus meter, and ΔpH values (a measure of surface charge) vs pH were significantly scattered, and the origin of dissemination of the data points was not established. Differently charged ice particles and not fully equilibrium conditions at the ice/water interface may have been responsible for the dissemination of the data.     

Study of the Leacril Dyeing Process by a Cationic Dye from an Emulsion System

Adsorption studies of a cationic dye, Rhodamine B, from an emulsion phase on Leacril fabric at different temperatures were conducted. The emulsion phase consisted of n-hexadecane emulsified by isopropyl alcohol (1 M) and stabilized by tannic acid. In the alcohol solution Rhodamine B was dissolved. The kinetics of its adsorption and desorption is discussed. The changes in Leacril surface free energy components in the dyeing process were also determined. The adsorption data show that the presence of an emulsion increases the dye adsorption at room temperature (293 K) and at 313 K, while at 333 K it is smaller than that from Rhodamine solution alone. However, Rhodamine desorbs more when adsorbed from the solution. Surface free energy components differ for the Leacril samples dyed at different temperatures, and the most hydrophobic surface was obtained for the samples dyed at 333 K, where the electron-donor component is the lowest one. In general, the work of water spreading is close to zero, except for the above sample for which it is relatively highly negative. Possible mechanisms of the dye adsorption are discussed. Copyright 2001 Academic Press.     

Investigation of the electrokinetic properties of paraffin suspension. 1. In inorganic electrolyte solutions

Although electrical properties of nonionogenic hydrophobic surface (solid or liquid) in water and/or electrolyte solutions have been studied for many decades, they are still not well recognized, especially as for the nature of the charge and potential origin. Similarly, water structure at such a surface is still extensively studied. One such system is paraffin wax/water (electrolyte). The zeta potentials and the particle diameters of this system were investigated in this paper. To obtain the suspension of paraffin in water or electrolyte solution (NaCl or LaCl3), the mixture was heated to ca. 70 degrees C and then stirred during cooling. For thus obtained suspensions, the zeta potential was determined as a function of time at 20 degrees C. Also the pH effect on the zeta potentials was investigated. The zeta potentials were calculated from Henry's equation. The results obtained by us are in agreement with those obtained earlier by others. They confirm that although H+/OH- are not surface charge creating ions, OH- ions to some extent are zeta potential determining for the paraffin surface. By use of the potentials and diameters, the electric charge for a spherical particle in the shear plane was calculated. These values are small in the range of 10(-3) C/m2. On the basis of the findings of water structure near hydrophobic surface and the calculated charges, it is concluded that in fact the potential may be created by immobilized and oriented water dipoles.     

Reversible charging of the ice-water interface. II. Estimation of equilibrium parameters

Equilibrium parameters characterizing the ice-water interface were obtained from electrokinetic and potentiometric data. The dependency of surface potential on pH was measured by the ice electrode. Electrokinetic zeta potentials were measured by electrophoresis. In the interpretation, the 1-pK and 2-pK concepts of the surface complexation model, developed for metal oxides, were used. The equilibrium parameters (constants of surface reactions) were determined. The results were confirmed by numerical simulations. Both 1-pK and 2-pK models fit the experimental data well.     

Surface free energy of sulfur--revisited II. Samples solidified against different solid surfaces

The apparent surface free energy of yellow and orange samples of sulfur crystallized at the surface of gold, silicon, and Teflon and in air was evaluated from the advancing and receding contact angles of water. The samples were prepared by casting melted rhombic mineralogical specimen sulfur onto the surface of the above-noted solids. Yellow samples were obtained when just melted sulfur (ca. 120 degrees C) was cast onto the surfaces, and the orange ones, when the melted sulfur was heated longer to a higher temperature (ca. 160 degrees C) and then cast onto the surfaces. The obtained results show that the apparent surface free energy depends on which surface it crystallized. The greatest value is for the sample crystallized at the gold surface, which is 35% higher than that crystallized in air. Generally, the surface free energy of orange samples is slightly higher than that of yellow ones. It is believed that the differences in the energy result from changes in the packing and structural orientation of sulfur atoms on the surfaces.