Solid-liquid interface three-phase

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. 

For the solid-liquid system changes of the state of interface on formation of surfactant adsorption layers are of special importance with respect to application aspects. When a liquid is in contact with a solid and surfactant is added, the solid-liquid interface tension will be reduced by the formation of a new solid-liquid interface created by adsorption of surfactant. This influences the wetting as demonstrated by the change of the contact angle between the liquid and the solid surface. The equilibrium at the three-phase contact solid-liquid-air or oil is described by the Young equation ... 

FIG. 20-39 Normalized free-energy difference between distributed (II) and nondistributed (I) states of the solid particles versus three-phase contact angle (collection at the interface is not considered). A negative free-energy difference implies that the distributed state is preferred over the nondistributed state. Note especially the significant effect of n, the ratio of the liquid droplet to solid-particle radius. [From Jacques, Hovarongkura, and Henry, Am. Inst. Chem. Eng. J., 23(1), 160 (1979).]... 

An interface is the area which separates two phases from each other. If we consider the solid, liquid, and gas phase we immediately get three combinations of interfaces the solid-liquid, the solid-gas, and the liquid-gas interface. These interfaces are also called surfaces. Interface is, however, a more general term than surface. Interfaces can also separate two immiscible liquids such as water and oil. These are called liquid-liquid interfaces. Solid-solid interfaces separate two solid phases. They are important for the mechanical behavior of solid materials. Gas-gas interfaces do not exist because gases mix. 

In this chapter we discuss preparative routes for inorganic materials in three basic types of systems involving the presence of a distinct solid-liquid interface those in which the liquid and solid phases are of the same chemical identity (solidification and vitrification processes), those in which the liquid and solid phases are not of the same chemical identity (crystallization, precipitation), and the special case in which the liquid phase is a pure ionic liquid or molten salt. Ionic liquids can serve as the solvent as well as a templating agent, and the liquid components may or may not become incorporated into the final solid product. We also discuss two areas where the distinct solid-liquid interface becomes somewhat blurred namely, sol-gel and solvothermal processes. 

Contact angle — The contact angle is the angle of contact between a droplet of liquid and a flat rigid solid, measured within the liquid and perpendicular to the contact line where three phases (liquid, solid, vapor) meet. The simplest theoretical model of contact angle assumes thermodynamic equilibrium between three pure phases at constant temperature and pressure [i, ii]. Also, the droplet is assumed to be so small that the force of gravity does not distort its shape. If we denote the - interfacial tension of the solid-vapor interface as ysv. the interfacial tension of the solid-liquid interface as ySL and the interfacial tension of the liquid-vapor interface as yLV, then by a horizontal balance of mechanical forces (9 < 90°)... 

The Young equation contains the surface tension of the liquid yi - which can easily be measured, and the difference of the surface tensions of the solid-vapor ysv and the solid-liquid interface ysL That the surface tension enters the Young equation is not beyond doubt. Linford I6 inserted the generalized intensive surface parameter ys, arguing that at the three-phase contact line elastic deformations take place. In accordance with Rusanov [I7 we use the surface tension, because the spreading of a liquid on a surface is a process similar to immersion or adsorption. Immersion is usually considered to effect the surface tension since no extension or contraction of the surface occurs. 

From pairing two-phases of gas, liquid and solid, nine different pairs can be obtained, which include three homo-pairs of gas-gas, Uquid-liquid and solid-solid pairs. The gas-gas interface, however, is practically no use under the ordinary conditions. Among the interfaces produced by the pairing, the liquid-liquid interface is most slippery and difficult to be studied experimentally in comparison with the gas-liquid and solid-liquid interfaces, as the liquid-liquid interface is flexible, thin and buried between bulk liquid phases. Therefore, in order to study the liquid-liquid interface, the invention of innovative measurement methods has a primary importance. 

In reactions between a solid and a gas or a solution, three steps are distinguished transport of the reactant in the gas phase or the solution to the surface of the solid reaction at the surface of the solid and transport of the reaction products into the solid, the gas phase or the solution, respectively. Reaction at the solid/gas or solid/ liquid interface comprises the following steps (a) adsorption of the reactant, (b) reaction and (c) desorption of the products, These steps are considered in more detail in the following paragraphs. 

Many theories have been advanced concerning the mechanisms involved in surfacing the mineral particles so as to create a hydrophobic hydrocarbon film on the mineral surface, and many investigations have been carried out to define these mechanisms. When firoth flotation is used in an aqueous medium that carries the solids to be separated (together with dispersed air bubbles and possibly an organic liquid) a three- or possibly a four-phase system must be considered. In most froth flotation processes, the solid particles are initially completely water-wetted, and the solid-liquid interface must be replaced by... 

Even though the flotation process involves three phases and three interfaces, most research work has been solely on the behavior of the solid-liquid interface. This is in spite of the fact that ndsorption at the solid-liquid interface, as shown In Fig. 16,3-10, is of a considerably smaller megnitede than diet at the solid-gas or liquid-gas interface. It is to be noted that excellent correlation has been obtained recently between surfactant ndsorption at the liquid-gas interface and flotation for the hemadte-oleate system (see Fig. 16.3-11). It is also important to note thel the migration of the surf at rent at the liquid-ges interface is faster than its diffusion from bulk to the interface, as least for this system.w Such a migration at the interface can help toward (aster attainment of required surfactant ndsorption density at the solid-gas interface upon the contact of the bubble with the panicle. 

This equation is simply the thermodynamic expression of the fact that the reversible work of separating the liquid and solid phases must be equal to the change in the free energy of the system. Therefore, a correct derivation implies that the three terms on the right of Equation 2 are the nature of free energies per unit surface area of the solid-vapor, liquid-vapor, and solid-liquid interfaces, respectively. 

Wetting and Spreading, Fig. 1 Force balance at the three-phase contact line where the solid-liquid and vapor phases meet. The outward unit normals protruding from the solid and liquid at the contact line are shown. The angle 8 subtending these unit normal vectors or that subtending the interface between the drop and the solid substrate is known as the contact angle... 

The microstructure is defined as a network of one or more crystalline phases with various types of imperfection. The crystalline phase, which covers the greater part of the whole system, is the origin of the intrinsic properties of a specific material. Imperfections are defects to which most of the processing-dependent properties are attributed. Typical examples of imperfections include solid/vapor and solid/liquid interfaces, grain boundaries, phase boundaries, pores, secondary phases, and so on. Among those listed, the first three defects - that is, the solid/vapor and solid/liquid... 

When a small liquid drop (diameter < Kc) is placed on an ideal (i.e. chemically and topographically homogeneous) solid surface, its shape is now changed due to the introduction of a new interface — three-phase contact line (Fig. 1). In this situation, each of the soUd-liquid, liquid-gas and solid-gas interfaces draws the contact Une so as to minimize the corresponding total surface Iree energy. Balancing these... 

At any point along the equilibrium line between the contacting phases, there are three surface tension forces aij <71,2 between the liquid drop and the solution, at the contact angle 9 with respect to the surface ai 3 between the solid and the solution, directed along the plane of the surface, and 0-2,3 due to the solid liquid interface, also along the plane but opposing 01,3. To maintain equilibrium, the solid reacts with an additional force perpendicular to the surface and directed towards the bulk of the solid. The equilibrium condition is expressed by the Young equation... 

For an aqueous droplet supported on a solid (Pt or C) electrode surface, the potential difference at the solid/liquid interface is fixed because the concentration ratio of the redox couples in the droplet is constant and it can be used as a pseudo-reference electrode. It forms a W/0 interface when the assembly is immersed in an organic solution. The W/O interface formed between the aqneons droplet and the organic phase can be studied with a three-electrode system. The FT process at the solid/liquid interface and the charge transfer processes at the liquid/liquid interface are coupled as reactions in series. The disadvantage of this setup is the cations associated with the redox couples usually limit the potential window and very few redox couples can be chosen (103). The droplet supported on Ag/AgCl or Ag/AgX (X is a big anion) can function similarly and only needs Cl (or X ) to be present in the aqueous (organic) droplet to fix the potential difference at the solid/liquid interface. In this way one can obtain a wider potential window and study the effect of the phase volume ratio on charge transfer reactions at a L/L interface. 

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