The Effect of Surface on the Properties of Matter

Surface is a word that nearly everyone uses or hears multiple times every day. Yet when faced with the question "what is a surface?" most people struggle to provide an adequate definition. Most of the answers given are incomplete, even amusing: "The outside of something." "The visible part of materials." "The part of objects we can touch." "The top of objects, like the top of a table." "Things in contact with air." In reality, all these definitions are insufficient, and the definition of surface cannot be limited to the conditions mentioned above. Given the scope of this article, it is appropriate to provide a more comprehensive definition. A surface is the transition layer between two phases in contact that do not mix or mix only limitedly. These phases can be solid, liquid, gas, or even vacuum. Since gas-gas or vacuum-gas will mix with each other in any proportion, no surface can form between them. The remaining options consist of liquid-gas, solid-gas, solid-solid, solid-vacuum, and liquids that do not mix or mix only limitedly with each other (such as oil-water and butyl alcohol-water).

Liquids with very high boiling points (for example, above 350°C) can also be said to form a surface with vacuum due to their very low evaporation rates. For a surface, it is possible to make a simpler definition as the boundary or contact area between two immiscible phases.

Investigation of the surface properties of materials has become an increasingly important research topic, and the knowledge gained creates potential for more effective utilization of the relevant materials [1-11]. Due to the attractive force that occurs between molecules, their mutual attraction results in the formation of liquids and solids. Molecules found in the interior of materials are in equilibrium due to mutual attractions. However, in molecules in contact with another phase, there is a pull imbalance (Figure 1a). If one side is empty space, there is a net inward attraction. When the other side contains molecules of a different type, there is an attraction of different magnitude from that of molecules of its own type, resulting again in an imbalance. This imbalance decreases as one proceeds toward both phases and beyond a certain point drops to a negligible level, ending the surface—the transition layer. This imbalance produces positive or negative results in many industrial, even daily operations. For example, solid phase molecules in contact with a liquid or gas phase want to reduce their imbalance by retaining atoms, ions, or molecules from that phase on their surface. This surface retention process, called adsorption, has many useful applications. Air and water purification, recovery of useful compounds from solutions, sugar and oil purification in industry are just a few of these applications (Figure 1b).

a b

Figure 1. The pull imbalance between molecules in the first layer of the second phase decreases as one progresses inward, becoming negligible (a). If the second phase is solid and the first phase is a fluid—that is, liquid or gas—the topmost molecules want to reduce their imbalance by capturing atoms, molecules, or ions from the fluid phase. This surface retention process is called adsorption. Surface tension is defined as the force acting per unit length perpendicular to the surface and results from the pull imbalance between molecules at the surface. Particularly in liquid-liquid or liquid-gas systems, many benefits are obtained by controlling surface tension. Surfactants such as soaps and detergents are used in cleaning operations because they can convert water-repellent (hydrophobic) dirt into water-loving (hydrophilic) material. By reducing the surface tension between two liquids, it is possible to produce stable emulsions or, by raising it, to break (destabilize) emulsions. Through lubrication, wear of moving solids or friction force in machines can be reduced (Figure 2).

Figure 2. By lubricating between moving metal parts, the friction force and wear that would occur can be reduced.

Most substances acquire an electrical charge in a polar liquid, such as water. This charge, whose sign and magnitude depend on the properties of the relevant substance and liquid, is among the important parameters controlling the system, particularly in many industrial activities occurring in water. Adsorption, surface cleaning, flotation, flocculation, suspension stability control, and increasing settling rate in wastewater treatment are several examples of these applications (Figure 3).

a b c

Figure 3. Most substances acquire an electrical charge in a polar liquid, such as water. The sign and magnitude of this charge depend on the properties of the substance and the liquid.

Small solid particles placed in water, if they carry the same sign of charge, move away from each other due to repulsive forces between them, forming a stable suspension. If the particles acquire or are made to acquire charges of opposite sign, they unite, grow larger, and settle. In paint production, adjustments to increase stability—that is, shelf life—can again utilize similar principles. Surface preparation before protective coating is another area of surface treatment. Even the reactions of unprotected surfaces to threats can depend on the physical properties of these surfaces (Figure 4).

Figure 4. On rough metal surfaces, corrosion progresses faster than on smooth ones due to their larger surface area, at least in the initial stages [5].

Due to lack of knowledge and technology to examine the subject, before the twentieth century, surfaces were described as the world of unknown dimensions. Today's technological developments provide the possibility to examine surfaces in detail from physical and chemical perspectives down to nanometric scales. The methods and equipment names in these fields have reached the level of making long lists. These names have taken the form of abbreviations consisting of the initial letters of their long multi-word forms: SEM (scanning electron microscopy), TEM (transmission electron microscopy), STEM (scanning transmission electron microscopy), LEED (low energy electron diffraction), HEED, AES, ESCA, STM, TPD, AFM, FEM, and FIM. Better understanding of the physical and chemical properties of surfaces is gaining increasing importance not only for purposes such as surface protection and transformation into new identities but also due to its potential to serve research conducted with substances of very different fields and dimensions. Ömer Edip Kuzugüdenli Head, Industrial Chemistry Department Erciyes University Faculty of Science Chemistry Department  

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