Name : Fianza Panji Fahmi PraditaNIM : 21100113120002 Geological engineering class B

Surfaces and Interfaces: General Concepts

For purposes of terminology, it is common practice to refer to that nebulous region as a ‘‘surface’’ or an ‘‘interface.’’, In general, however, one usually finds that the term ‘‘surface’’ is applied to the region between a condensed phase (liquid or solid) and a gas phase or vacuum, while ‘‘interface’’ is normally applied to systems involving two condensed phases. There are several types of interfaces that are of great practical importance and that will be discussed in turn. These general classifications include, solid– vacuum, liquid–vacuum, solid–gas, liquid–gas, solid–liquid, liquid–liquid, and

solid–solid. A list of commonly encountered examples of these interfaces is given in a table below

Interface Type Occurrence or ApplicationSolid–vapor Adsorption, catalysis,

contamination, gas–liquid chromatography

Solid–liquid Cleaning and detergency, adhesion, lubrication, colloids

Liquid–vapor Coating, wetting, foamsLiquid–liquid Emulsions, detergency, tertiary

oil recovery

TABLE 2.1. Common Interfaces of Vital Natural and Technological Importance

In order for two phases to exist in contact, there must be a region through which the intensive properties of the system change from those of one phase to those of the other, as for example in the boundary between a solid and a liquid. In order for such a boundary to be stable it must possess an interfacial free energy such that work must be done to extend or enlarge the boundary or interface.

In order to define an interface and show in chemical and physical term that it exists, it is necessary to think in terms of energy, nature will always act so as to attain a situation of minimum total free energy. In the case of a two-phase system, if the presence of the interface results in a higher (positive) free energy, the interface will spontaneously be reduced to a minimum—the two phases will tend to separate to the greatest extent possible within the constraints imposed by the container, gravitational forces, mechanical motion, and other factors. Overall, the interfacial energy will still be positive, but the changes caused by the alteration may prolong the ‘‘life’’ of any ‘‘excess’’ interfacial area. Such an effect may be beneficial, as in the case of a cosmetic emulsion, or detrimental, as in a petroleum–seawater emulsion. Although thermodynamics is almost always working to reduce interfacial area, we have access to tools that allow us to control, to some extent, the rate at which area changes occur.

The concept of the interfacial region will be presented from a molecular (or atomic) perspective and from the viewpoint of the thermodynamics involved. In this way one can obtain an idea of the

situations and events occurring at interfaces and have at hand a set of basic mathematical tools for understanding the processes involved and to aid in manipulating the events to best advantage.

As will be seen throughout, the unique characters of interfaces and interfacial phenomena arise from the fact that atoms and molecules at interfaces, because of their special environment, often possess energies and reactivities significantly different from

those of the same species in a bulk or solution situation. If one visualizes a unit (an atom or molecule) of a substance in a bulk phase, it can be seen that, on average, the unit experiences a uniform force field due to its interaction with neighboring units (Fig. 2.1a). If the bulk phase is cleaved in vacuum, isothermally and reversibly, along a plane that just touches the unit in question (Fig. 2.1b), and the two new faces are separated by a distanceH, it can be seen that the forces acting on the unit are no longer uniform.

The net increase in free energy of the system as a whole resulting from the new situation will be proportional to the area,A,of new surface formed and the density (i.e., number) of interfacial units. The actual change in system free energy will also depend on the distance of separation, since unit interactions will generally fall off by some inverse power law. When the term ‘‘specific’’ excess surface free energy is used it refers to energy per unit area, usually in mJ m-2. It should be remembered that the excess free energy is not equal to the total free energy of the system, but only that part resulting from the units location at the surface.

It should be intuitively clear that atoms or molecules at a surface will experience a net positive inward (i.e., into the bulk phase) attraction normal to the surface, the resultant of which will be a state of lateral tension along the surface, giving rise to the concept of ‘‘surface tension.’’ For a flat surface, the surface tension may be defined as a force acting parallel to the surface and perpendicular to a line of unit length anywhere in the surface (Fig. 2.2). The definition for a curved surface is somewhat more complex, but the difference becomes significant only for a surface of very small radius of curvature.

The specific thermodynamic definition of surface tension for a pure liquid is given by

Where AH is the Helmholtz free energy of the system, Wis the amount of reversible work necessary to overcome the attractive forces between the units at the new surface, andA is the area of new surface formed. The proportionality constant σ, termed the ‘‘surface tension,’’ is numerically equal to the specific excess surface free energy for pure liquids at equilibrium; that is, when no adsorption of a different material occurs at the surface. The SI (International System of Units) units of surface tension are mN m -1, which can be interpreted as a two-dimensional analog of pressure (mN m-2 ). As a concept, then, surface (and interfacial) tension may be viewed as a two-dimensional negative pressure acting along the surface as opposed to the usual positive pressures encountered in

our normal experience. The work of cohesion,Wc, is defined as the reversible work required to separate two surfaces of unit area of a single material with surface tension σ (Fig. 2.3a).

Based on the distinction between solid and liquid surfaces the definition applies strictly to liquid surfaces, although the concept is useful for solid surfaces as well.

the work of cohesion is simply

Related toWc is the work of adhesion, Wa(12), defined as the reversible work required to separate unit area of interface between two different materials (1 and 2) to leave two ‘‘bare’’ surfaces of unit area (Fig. 2.3b). The work is given by

Surface Activity and Surfactant Structures

Surface-active materials (surfactants) possess a characteristic chemical structure that consists of (1) molecular components that will have little attraction for one surrounding (i.e., the solvent) phase, normally called the lyophobic group, and (2) chemical units that have a strong attraction for that phase—the lyophilic group

(Fig. 3.1).

In an aqueous surfactant solution, for example, such a distortion (in this case ordering) of the water structure by the hydrophobic group decreases the overall entropy of the system (Fig. 3.2).

The amphiphilic structure of surfactant molecules not only results in the adsorption of surfactant molecules at interfaces and the consequent alteration of the corresponding interfacial energies, but it will often result in the preferential orientation of the adsorbed

molecules such that the lyophobic groups are directed away from the bulk solvent phase (Fig. 3.3).

The chemical structures having suitable solubility properties for surfactant activity vary with the nature of the solvent system to be employed and the conditions of use. In water, the hydrophobic group (the ‘‘tail’’) may be, for example, a hydrocarbon, fluorocarbon, or siloxane chain of sufficient length to produce the desired solubility characteristics when bound to a suitable hydrophilic group. The hydrophilic (or ‘‘head’’) group will be ionic or highly polar, so that it can act as a solubilizing functionality.

The chemical reactions that produce most surfactants are rather simple, understandable to anyone surviving the first year of organic chemistry. The challenge to the producer lies in the implementation of those reactions on a scale of thousands of kilograms, reproducibly, with high yield and high purity (or at least known levels and types of impurity), and at the lowest cost possible.

Surfactants may be classified in several ways, depending on the intentions and preferences of the interested party (e.g., the author). One of the more common schemes relies on classification by the application under consideration, so that surfactants may be classified as emulsifiers, foaming agents, wetting agents, dispersants, or similar.

Surfactants may also be generally classified according to some physical characteristic such as it degree of water or oil solubility, or its stability in harsh environments. Alternatively, some specific aspect of the chemical structure of the materials in question may serve as the primary basis for classification; an

example would be the type of linking group (oxygen, nitrogen, amide, etc.) between the hydrophile and the hydrophobe. The four general groups of surfactants are defined as follows:

Synthetic surfactants and the natural fatty acid soaps are amphiphilic materials that tend to exhibit some solubility in water as well as some affinity for nonaqueous solvents. As an illustration, consider the simple, straight-chain hydrocarbon dodecane,

CH3(CH2)10CH

a material that is, for all practical purposes, insoluble in water. If a terminal hydrogen in dodecane is exchanged for a hydroxyl group (-OH), the new material,n-dodecanol,

CH3(CH2)10CH2OH

still has very low solubility in water, but the tendency toward solubility has been increased substantially and the material begins to exhibit characteristics of surface activity. If the alcohol functionality is placed internally on the dodecane chain, as in 3-dodecanol, the resulting material will be similar to the primary alcohol but will have slightly different solubility characteristics (slightly more soluble in water).

If the original dodecanol is oxidized to dodecanoic acid (lauric acid) is CH3(CH2)10COOH the the compound still has limited solubility in water; however, when the acid is neutralized with alkali it becomes water soluble—a classic soap. The alkali carboxylate will be a reasonably good surfactant.

The solubilizing groups of modern surfactants fall into two general categories: those that ionize in aqueous solution (or highly polar solvents) and those that do not. Obviously, the definition of what part of a molecule is the solubilizing group depends on the solvent system being employed. For example, in water the solubility will be determined by the presence of an ionic or highly polar group, while in

organic systems the active group (in terms of solubility) will be the organic ‘‘tail.’’ It is important, therefore, to define the complete system under consideration before discussing surfactant types.

The functionality of ionic hydrophiles derives from a strongly acidic or basic character, which, when neutralized, leads to the formation of true, highly ionizing salts. The most common hydrophilic groups encountered in surfactants today are illustrated in Table 3.1, where R designates some suitable hydrophobic

By far the most common hydrophobic group used in surfactants is the hydrocarbon radical having a total of 8–20 carbon atoms. Commercially there are two main sources for such materials that are both inexpensive enough and available in sufficient

quantity to be economically feasible: biological sources such as agriculture and fishing, and the petroleum industry (which is, of course, ultimately biological). Listed below and illustrated structurally in Figure 3.4 are the most important commercial sources of hydrophobic groups, along with some relevant comments about each.