Solid particles at liquid interfaces

Solid Particles at Liquid Interfaces, Including Their Effects on Emulsion and Foam Stability... 

Only very recently, computer calculations were performed by Bresme and Quirke [44] to determine the magnitude of the line tension. Using molecular dynamics of small solid particles at the interface between a Lennard-Jones liquid and its vapor phase, they showed the line tension to be of the order N, which is consistent with the theoretical estimate given above. 

Denkov, N.D., Maiinova, K.G. Antifoam effects of solid particles, oil drops and oil-solid compounds in aqueous foams, in Colloidal Particles at Liquid Interfaces (Binks, B.P., Horozov, T., eds.), Cambridge University Press, Cambridge, UK 2006, p 383. Scheludko, A., Exerowa, D. Commun. Dept. Chem (Bulg. Acad. ScL), 7,123,1959. Scheludko, A. Adv. Colloid Interface ScL, 1, 391,1967. 

Phenomena at Liquid Interfaces. The area of contact between two phases is called the interface three phases can have only aline of contact, and only a point of mutual contact is possible between four or more phases. Combinations of phases encountered in surfactant systems are L—G, L—L—G, L—S—G, L—S—S—G, L—L, L—L—L, L—S—S, L—L—S—S—G, L—S, L—L—S, and L—L—S—G, where G = gas, L = liquid, and S = solid. An example of an L—L—S—G system is an aqueous surfactant solution containing an emulsified oil, suspended soHd, and entrained air (see Emulsions Foams). This embodies several conditions common to practical surfactant systems. First, because the surface area of a phase iacreases as particle size decreases, the emulsion, suspension, and entrained gas each have large areas of contact with the surfactant solution. Next, because iaterfaces can only exist between two phases, analysis of phenomena ia the L—L—S—G system breaks down iato a series of analyses, ie, surfactant solution to the emulsion, soHd, and gas. It is also apparent that the surfactant must be stabilizing the system by preventing contact between the emulsified oil and dispersed soHd. FiaaHy, the dispersed phases are ia equiUbrium with each other through their common equiUbrium with the surfactant solution. 

This can be accomplished by means of two different processes (1) an electrodeposition process in which z electrons (e) are provided by an external power supply, and (2) an electroless (autocatalytic) deposition process in which a reducing agent in the solution is the electron source (no external power supply is involved). These two processes, electrodeposition and electroless deposition, constitute the electrochemical deposition. In this book we treat both of these processes. In either case our interest is in a metal electrode in contact with an aqueous ionic solution. Deposition reaction presented by Eq. (1.1) is a reaction of charged particles at the interface between a solid metal electrode and a liquid solution. The two types of charged particles, a metal ion and an electron, can cross the interface. 

R. Aveyard and J. Clint, Liquid droplets and solid particles at surfactant solution interfaces, J. Chem. Soc., Faraday Trans. 91, 2681-2697 (1995). 

FIGURE 5.10. Sections of the inhomogeneous two-particle distribution function g(s, zi, Z2> of the Lennard-Jones fluid along the liquid/vapour interface for reduced temperature T = 1.15 at the distance z = Z = Z2 = -lOcr 0 -HOct from the interface, respectively in the vapour phase (thin solid line) at the interface (bold solid line) and in the liquid phase (dashed line). Predictions of the lOZ-KHM/LMBW theory. 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

The hydrodynamic resistance of dispersion medium in the gap between particles against flowing out is one of the kinetic stability factors. The decrease in thickness of fluid layer between the particles during coagulation is related to viscous flow of liquid out of a narrow gap between the particles. For solid particles the liquid flow velocity is zero at the interface and highest in the center of a gap. The rate with which the gap between two circular plane-parallel plates of radius r (Fig. VII-7) shrinks, dh/dt, is related to the volume of liquid that flows per second across the side surface of cylindrical gap, dV/dt, via the following relationship ... 

The particles at an interface between two phases, for example, those on the surface of a solid or liquid, are subject to different intermolecular forces than those inside a phase (Fig. 15.1). A particle inside a phase is attracted equally on all sides by identical neighboring particles. This means that intermolecular forces are in equilibrium and the net attractive pull on the particle in question equals zero. Particles at an interface such as between a solid and air or between a liquid and air are missing part of their neighbors. This leads to an imbalance of forces where (especially in the case of surfaces) a one-sided puU occurs toward the interior of the denser phase. Consequently, the neighboring particles in the interface will move... 

O Brien s theoretical analysis (8,10) is for a suspension of solid particles, but the evidence to date indicates that emulsion droplets behave in the same way as solid particles at the frequencies involved in the ESA effect. This is understandable on a number of counts. First, it is usually observed that surfactant-stabilized emulsion droplets in a flow field do not behave as though they were liquid. The presence of the stabilizing layer at the interface restricts the transfer of momentum across the phase boundary so that there is little or no internal motion in the drop. Also, the motions which are involved are extremely small (involving displacements of the order of fractions of a nanometer) so the perturbations are small compared to the size of the drop. Finally, O Brien has shown in some unpublished calculations that if the surface is unsaturated, so that the surfactant groups can move under the influence of the electric field, then the effect on the electroacoustic signal would depend on the quantity dy/dT, where y is the surface tension and T is the surface excess of the surfactant. We have not been able to find any evidence for such an effect, if it exists, so we will assume that the analysis for a solid particle holds also for emulsions. 

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