Alex M Djerdjev

University of Sydney, Sydney, New South Wales, Australia

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Publications (21)71.99 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Journal of Hazardous Materials. 09/2014;
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    ABSTRACT: 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.
    Journal of Colloid and Interface Science 05/2014; 422:54-7. · 3.55 Impact Factor
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    ABSTRACT: HypothesisIt 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.Experiments and theoryAn 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.FindingsOur 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.
    Journal of Colloid and Interface Science 01/2014; 420:70–73. · 3.55 Impact Factor
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    ABSTRACT: 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 pH's below the isoelectric point, the surface is indeed positive, consistent with spectroscopic observations of the adsorption of hydrogen ions.
    The Journal of Physical Chemistry B 10/2009; 113(43):14146-50. · 3.61 Impact Factor
  • James K Beattie, Alex M Djerdjev, Gregory G Warr
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    ABSTRACT: 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.
    Faraday Discussions 01/2009; 141:31-9; discussion 81-98. · 3.82 Impact Factor
  • Alex M Djerdjev, James K Beattie
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    ABSTRACT: 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.
    Physical Chemistry Chemical Physics 09/2008; 10(32):4843-52. · 4.20 Impact Factor
  • Alex M Djerdjev, James K Beattie
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    ABSTRACT: 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.
    Langmuir 07/2008; 24(15):7711-7. · 4.38 Impact Factor
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    ABSTRACT: Droplet size distributions of concentrated, polydisperse oil-in-water emulsions have been measured using ultra small angle neutron scattering (USANS). The mean radii calculated by fitting a model for polydisperse hard spheres with excluded volume interactions to the USANS data were consistent with those derived from electroacoustics on diluted emulsions after correction for conductance behind the shear plane. The Porod radii measured by USANS were similarly consistent with the mean surface-area-weighted radii derived from electroacoustics, irrespective of the drop concentration or polydispersity.
    Langmuir 03/2007; 23(4):1694-700. · 4.38 Impact Factor
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    ABSTRACT: In Part I of this paper O'Brien presented a new theory that describes the dissipation of ultrasonic energy by porous colloidal particles in suspension. In this paper we present experimental measurements on several such suspensions and compare the resulting ultrasonic attenuation spectra with O'Brien's theory. We find that microporous colloids (e.g. zeolites) and mesoporous colloids (e.g. porous silicas) both show greater attenuation of ultrasound than would solid particles of the same size and effective density, as predicted by O'Brien. Fitting theoretical spectra to the experimental data provides information about the porosity and pore diameters in the particles. This makes the technique potentially suitable as a convenient method of characterizing nanoporous materials, especially those already in suspension.
    Physical Chemistry Chemical Physics 12/2006; 8(43):5124-30. · 4.20 Impact Factor
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    ABSTRACT: The formation of particle-stabilised emulsions by adding partially hydrophobised silica particles to surfactant-free oil-in-water emulsions (average drop diameter approximately 700 nm) stabilised by hydroxide ions adsorbed at the oil-water interface has been investigated. Nanoparticles (average particle diameter 18 nm) adsorbed onto the drops under alkaline conditions to produce particle-stabilised emulsions with the same drop size distribution as the surfactant-free emulsions. Unlike the surfactant-free emulsions, the particle-stabilised emulsions were stable even in acidic conditions. Strongly flocculated nanoparticles (average particle diameter 150 nm) adsorbed onto the drop surfaces under acidic conditions where the emulsions were destabilised, forming coarser particle-stabilised emulsions with micron-sized drops.
    Journal of Colloid and Interface Science 10/2006; 301(1):342-5. · 3.55 Impact Factor
  • Alex M Djerdjev, Robert J Hunter, James K Beattie
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    ABSTRACT: Oil-in-water emulsions of slightly soluble oils such as tetralin prepared by high-pressure homogenization and stabilized by sodium dodecyl sulfate undergo depletion flocculation induced by an initially polydisperse droplet size distribution. The smaller droplets flocculate the larger ones; the flocculation can be reversed by gentle sonication. After aging, the flocs disappear because the smaller droplets dissolve through Ostwald ripening. These effects were observed by electroacoustic measurements, supplemented by light scattering.
    Langmuir 02/2006; 22(1):84-7. · 4.38 Impact Factor
  • George V Franks, Alex M Djerdjev, James K Beattie
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    ABSTRACT: Surfactant-free 2 vol % hexadecane-in-water emulsions have been prepared at pH 9 in the presence of various alkali-metal salts. The surface charge and zeta potential of these emulsions are independent of the identities of the monovalent cations and anions up to 0.01 M electrolyte concentrations. The surface charge density of -5 microC cm(-2) is independent of the identity of the alkali-metal cation among Li, Na, and Cs. The zeta potentials decrease with the log of the salt concentration between 0.1 and 11 mM, independent of the identity of the anion of the sodium salts of iodide, bromide, chloride, fluoride, perchlorate, or iodate or of the identity of the cation of the chloride salts of Li, Na, or Cs. These results imply that neither hydration enthalpies nor ion dispersion potentials are significant in affecting the charge created by the hydroxide ion at the pristine oil/water interface at up to 0.01 M salt concentrations.
    Langmuir 10/2005; 21(19):8670-4. · 4.38 Impact Factor
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    ABSTRACT: Homogenization of hexadecane in water at pH 9 gives the same surface charge density in the presence of 0.2 mM thiocyanate or acetate anions as in the presence of chloride, indicating that these dipolar anions are not preferentially adsorbed at the oil/water interface. The decrease in the zeta potential of the emulsion droplets as the sodium salts of iodate, thiocyanate, or acetate are added from 0.1 to 10 mM is the same as that when sodium chloride is added, leading to the same conclusion. Increasing the sodium hydroxide concentration from pH 9 to 11.5 has a different effect on the zeta potential, consistent with the specific adsorption of hydroxide ion at the oil/water interface.
    The Journal of Physical Chemistry B 09/2005; 109(33):15675-6. · 3.61 Impact Factor
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    ABSTRACT: The coating of titania with a hydrous silica layer was investigated by electroacoustics, high-frequency dielectric response (DR), IR spectroscopy, and TEM microscopy. The shift in the isoelectric point measured by electroacoustics was the most sensitive indicator of silica coverage of the titania surface. Changes in the shape of the zeta potential pH dependence corresponding to formation of silica multilayers were accompanied by small changes in the IR spectra and TEM thickness. The coatings formed at 90 °C were dense and smooth and extended up to 10 nm from the titania surface. Changes in the dielectric constant of the coated pigment occurred over three domains and corresponded to changes in the nature of the adsorbing silica layer. At low concentrations, the relative permittivity of the coating layer was 5 but at higher concentrations it was 20, indicating greater hydration of the multilayer.
    Chemistry of Materials - CHEM MATER. 06/2005; 17(15).
  • James K. Beattie, Alex Djerdjev
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    ABSTRACT: The zeta potential (ζ) of 2-vol% and 10-vol% suspensions of a commercial α-alumina has been measured using the electroacoustic technique and found to be in good agreement with values reported previously from microelectrophoresis measurements. An increasingly negative ζ value was observed through semiautomatic addition of solutions of a ammonium salt of poly(methacrylic acid). The point of complete surface coverage was inferred from simultaneous conductivity measurements. The average particle size obtained from the electroacoustic dynamic mobility spectrum showed the effect of coagulation near the isoelectric point. The expected correlations among ζ, the viscosity, and the sedimentation volume were observed. The results illustrate the efficacy of the electroacoustic technique for examining colloidal dispersions.
    Journal of the American Ceramic Society 12/2004; 83(10):2360 - 2364. · 2.43 Impact Factor
  • James K Beattie, Alex M Djerdjev
    Angewandte Chemie International Edition 08/2004; 43(27):3568-71. · 11.34 Impact Factor
  • Alex M Djerdjev, James K Beattie, Robert J Hunter
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    ABSTRACT: 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.
    Journal of Colloid and Interface Science 09/2003; 265(1):56-64. · 3.55 Impact Factor
  • Alex M. Djerdjev, James K. Beattie, Robert J. Hunter
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    ABSTRACT: When sodium dodecyl sulfate-stabilized emulsions of n-eicosane are supercooled from about 40 °C, solidification of the droplets occurs below 24 °C; reheating the suspension causes the particles to melt at the normal melting point of 36−37 °C, and the emulsion returns to its initial state without any aggregation or coalescence observed. The crystallization changes the density of the particles and the volume fraction of the suspension. When this is properly considered, the dynamic mobility spectra obtained from electroacoustic measurements can be readily interpreted in terms of the particle size and ζ potential of the droplets with very low fit errors. The droplet size appears, however, to be significantly different at different salt concentrations. This anomalous result is an indication of stagnant layer conduction on the emulsion drops. When this effect is also taken into account, the droplet size becomes independent of the salt concentration. The results then become entirely self-consistent and indicate that the theory, which is used to describe the dynamic mobility, appropriately describes the effect for both solid particles and surfactant-stabilized oil droplets.
    Langmuir. 07/2003; 19(17).
  • James K Beattie, Alex M Djerdjev, Sophie E Gibb
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    ABSTRACT: 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.
    New Journal of Chemistry 01/2003; 27(10). · 2.97 Impact Factor
  • Alex M. Djerdjev, James K. Beattie, Robert J. Hunter
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    ABSTRACT: Previously reported zeta-potentials calculated from the electroacoustic behaviour of sodium dodecyl sulfate (SDS) stabilized hexadecane emulsion droplets show certain anomalies. These can be resolved when electrical conduction in the stagnant layer behind the shear plane is included in the analysis. If stagnant layer conduction is ignored the addition of salt causes the apparent droplet size to increase and the magnitude of the zeta-potential to show a maximum. When stagnant layer conduction is included the dynamic mobility spectra can be fitted to a constant size distribution independent of the salt concentration with zeta-potentials that decrease as expected with increasing electrolyte concentration. Increasing SDS concentration, before the homogenization process, causes a decrease in droplet size and an increase in the total surface conductance to a constant value corresponding to the saturation of the surface with SDS. It is shown that the surface conductance and particle size distribution of hexadecane at any given volume fraction are functions of the concentration of SDS and the oil volume fraction. The zeta-potential changes log-linearly with added electrolyte and is independent of the SDS concentration or oil volume fraction used during the emulsification process.
    Australian Journal of Chemistry - AUST J CHEM. 01/2003; 56(10).