Alex M. Djerdjev

The surface of neat water is basic

Theoretical studies which conclude that the surface of neat water is acidic (with a pH < or = 4.8), due to the preferential adsorption of hydronium ions, are contrary to the available experimental evidence. Air bubbles in water have a negative charge, as do hydrophobic oil drops in water, and streaming potential measurements on inert surfaces such as Teflon in water show a similar negative surface charge. In each case the pH dependence of the zeta potential has an isoelectric point between pH 2-4. An isoelectric point of pH 4 implies a preference for hydroxide over protons of 10(6), the opposite of what was inferred from the theoretical simulations. Water behaves similarly at all inert hydrophobic interfaces with the preferential adsorption of hydroxide ions to give a negatively charged surface at neutral pH. The surface-charge density at the oil/water interface in mM salt solutions is -5 to -7 microC cm(-2), which corresponds to one hydroxide ion on every 3 nm2 of the surface. The homogenisation of an inert oil such as hexadecane in water in the absence of any salt or base still leads to formation of an emulsion. The hydroxide adsorbed on the large surface area of the emulsion greatly exceeds that present at 10(-7) M in neutral water; it is created by the increased autolysis of water, driven by the strong adsorption of hydroxide ions at the oil/water interface. These surfactant-free, salt-free emulsions are stable for some hours, with protons as the only counterions to the negative hydroxide surface.

Strong Specific Hydroxide Ion Binding at the Pristine Oil/Water and Air/Water Interfaces

Despite claims, based largely on molecular dynamics simulations, that the surface of water at the air/water interface is acidic, with a positive charge, there is compelling experimental evidence that it is in fact basic, with a negative charge due to the specific adsorption of hydroxide ions. The oil/water interface behaves similarly. The pH dependence of the zeta potentials of oil drops has been measured by two very different techniques: on a single drop in a rotating electrophoresis cell and on about 10(14) submicrometer drops in a 2 vol % emulsion by an electroacoustic method to give similar results with a sigmoidal pH dependence characterized by an isoelectric point at pH 2-3 and a half adsorption point about pH 5.5, or at 10(-8.5) M hydroxide ion. This indicates that hydroxide ion is absorbed much more strongly than other anions. The pH dependence of a single N(2) bubble has also been measured and has the same pH dependence, independently of whether HCl or HI is used to adjust the pH. These similarities between the pH dependences of the zeta potentials of air bubbles and oil drops, as well as those reported from streaming potentials on solid inert surfaces such as Teflon, indicate that water behaves similarly, with only subtle differences, at each of these low dielectric hydrophobic surfaces, with an isoelectric point of pH 2-4. In acidic solutions at pHs below the isoelectric point, the surface is indeed positive, consistent with spectroscopic observations of the adsorption of hydrogen ions.

pH and the surface tension of water.

Despite the strong adsorption of hydroxide ions, the surface tension of water is almost independent of pH between pH 1 and 13 when the pH is adjusted by addition of HCl or NaOH. This is consistent with the Gibbs adsorption isotherm which measures the surface excess of all species in the double layer, if hydronium ions and hydroxide ions are adsorbed and sodium and chloride ions are not. The surface tension becomes pH dependent around pH 7 in millimolar NaCl or KCl solutions, for now sodium ions can replace hydronium ions as counterions to the adsorbed hydroxide ions.

Electroacoustic and ultrasonic attenuation measurements of droplet size and ζ-potential of alkane-in-water emulsions: effects of oil solubility and composition

The effects of oil solubility and composition on the zeta potential and drop size of oil-in-water emulsions stabilised by sodium dodecyl sulfate (SDS) were studied by electroacoustics and ultrasonic attenuation. The zeta-potentials of toluene and alkane emulsions were found to decrease (be less negative) as the water solubility of the dispersed oil phase increased. The zeta-potentials also depended on the composition of mixed oils, becoming more negative with increasing mole fraction of an insoluble oil (hexadecane). As the water solubility of the dispersed oil phase increased, the conductance within the Stern layer relative to the diffuse layer (K/K) increased, which is interpreted as due to the displacement of the shear plane further into the diffuse layer. The shear plane was calculated to increase from approximately 0.50 nm at the insoluble oil-water interface (hexadecane) to approximately 2.5 nm at a soluble oil-water interface of toluene. The lowering of the zeta-potentials of the soluble oils is ascribed to the shift of the shear plane into the diffuse layer, resulting in a more diffuse interface. The total surface conductance of the mixed oils was related to the log of the oil solubility and decreased from approximately 7 x 10(-9) Omega(-1) to 3 x 10(-9) Omega(-1) with increasing oil solubility from hexadecane to toluene, respectively. The lower surface conductance at the soluble oil-water interface is attributed to a reduction in the dielectric constant of the water inside of the shear plane, caused by the presence of the soluble oil.

Enhancement of Ostwald ripening by depletion flocculation.

The time dependence of the dynamic mobility and the ultrasonic attenuation of octane and decane oil-in-water emulsions stabilized by sodium dodecyl sulfate (SDS) was measured. The emulsions grew to larger droplets due to Ostwald ripening. The growth rate measured by attenuation depends on the surfactant concentration and the polydispersity of the emulsion. At surfactant concentrations below the critical micelle concentration (cmc) of SDS, the growth was linear with time and the rate was dependent on the polydispersity of the drops; the rate was several times faster than that predicted on the basis of a diffusion growth mechanism. Above the cmc, however, as the droplets grew in size there was a point at which the rate of growth increased, which corresponds to the droplet size at which depletion forces due to the surfactant micelles become significant. Under these conditions both the electroacoustic dynamic mobility and the acoustic attenuation spectra displayed characteristics of flocs: a large decrease in the phase lag at higher frequencies in the dynamic mobility spectrum and a decrease in the attenuation coefficient at low-megahertz frequencies with an increase at higher frequencies. This depletion flocculation enhancement in ripening rates in the presence of SDS micelles provides another, alternative, and self-consistent mechanism for the effect of surfactant micelles on Ostwald ripening.

The electrophoretic mobility of an uncharged particle.

HYPOTHESISnIt has been claimed that uncharged particles can have negative electrophoretic mobilities, and so a negative mobility need not imply a negative particle charge. We show that although a steady electrophoresis may be possible for the uncharged infinite slabs studied in Molecular Dynamics simulations, it is not possible for a finite particle.nnnEXPERIMENTS AND THEORYnAn uncharged particle may initially move when the field is turned on, but our analysis shows that this motion ceases as charges of opposite sign build up on the front and back of the particle. Uncharged particles may move in alternating electric fields, but their mobility is predicted to increase with electrolyte conductivity. Experimentally, however, the mobility of hexadecane oil drops in water at pH 9 decreases with increasing NaCl concentrations.nnnFINDINGSnOur results are consistent with the usual compression of the double layer with added salt, and with the traditional interpretation, that hydrophobic particles have negative mobilities because they are negatively charged. Uncharged particles may have a transient mobility but it is quickly quenched by polarisation of the double layer.

Premature detonation of an NH4NO3 emulsion in reactive ground

When NH4NO3 emulsions are used in blast holes containing pyrite, they can exothermally react with pyrite, causing the emulsion to intensively heat and detonate prematurely. Such premature detonations can inflict fatal and very costly damages. The mechanism of heating of the emulsions is not well understood though such an understanding is essential for designing safe blasting. In this study the heating of an emulsion in model blast holes was simulated by solving the heat equation. The physical factors contributing to the heating phenomenon were studied using microscopic and calorimetric methods. Microscopic studies revealed the continuous formation of a large number of gas bubbles as the reaction progressed at the emulsion-pyrite interface, which made the reacting emulsion porous. Calculations show that the increase in porosity causes the thermal conductivity of a reacting region of an emulsion column in a blast hole to decrease exponentially. This large reduction in the thermal conductivity retards heat dissipation from the reacting region causing its temperature to rise. The rise in temperature accelerates the exothermic reaction producing more heat. Simulations predict a migration of the hottest spot of the emulsion column, which could dangerously heat the primers and boosters located in the blast hole.

Electroacoustic and dielectric response of suspensions of zeolites A and Y

The dynamic electrokinetic mobility spectra and dielectric response of suspensions of zeolites A and Y have been measured. The dynamic mobility data cannot be fit with the surface conductivity calculated from the zeta potential, but require the inclusion of a large stagnant layer conduction, which is attributed to ionic conduction within the micropores. With the constraint of a constant particle size the electroacoustic spectra give surface conductivity parameters which agree with those measured independently from the dielectric response. Negative zeta potentials that vary with salt concentration are then obtained from the electroacoustic spectra.

An electroacoustic and high-frequency dielectric response study of stagnant layer conduction in emulsion systems

Stagnant layer conduction (or anomalous surface conduction) in perfluoromethyldecalin (PFMD) and n-hexadecane emulsions has been measured by electroacoustics and verified by high-frequency dielectric response experiments. The electroacoustic technique can detect the presence of stagnant layer conduction from the salt dependence of the dynamic mobility. As the indifferent electrolyte concentration is increased from low values (<5 mM), the zeta-potential and droplet size, estimated from the dynamic mobility by the normal procedures, gradually increase in magnitude until the size plateaus and the zeta-potential begins to decrease with added salt in the usual fashion. When stagnant layer conduction is taken into account, the dynamic mobility can be fitted to a constant size distribution and more realistic zeta-potential values with varying electrolyte concentration. High-frequency dielectric response has been used to measure the total conduction in a PFMD emulsion system. Very good agreement between these two independent techniques verifies the existence of conduction behind the shear plane and demonstrates that electroacoustics alone can detect and quantify its extent. This is possible because of the unique character of the AcoustoSizer procedure, which estimates both particle size and zeta-potential from the same signal.

Research ArticlePremature detonation of an NH4NO3 emulsion in reactive ground

When NH4NO3 emulsions are used in blast holes containing pyrite, they can exothermally react with pyrite, causing the emulsion to intensively heat and detonate prematurely. Such premature detonations can inflict fatal and very costly damages. The mechanism of heating of the emulsions is not well understood though such an understanding is essential for designing safe blasting. In this study the heating of an emulsion in model blast holes was simulated by solving the heat equation. The physical factors contributing to the heating phenomenon were studied using microscopic and calorimetric methods. Microscopic studies revealed the continuous formation of a large number of gas bubbles as the reaction progressed at the emulsion-pyrite interface, which made the reacting emulsion porous. Calculations show that the increase in porosity causes the thermal conductivity of a reacting region of an emulsion column in a blast hole to decrease exponentially. This large reduction in the thermal conductivity retards heat dissipation from the reacting region causing its temperature to rise. The rise in temperature accelerates the exothermic reaction producing more heat. Simulations predict a migration of the hottest spot of the emulsion column, which could dangerously heat the primers and boosters located in the blast hole.