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10. P. Hubert and O. G. Vitzthum, "Fluid Extraction of Hops, Spices, and Tobacco with Supercritical Gases," in G. M. Schneider, E. Stahl, and G. Wilke, eds., Extraction with Supercritical Gases, Verlag Chemie, Deerfield Beach Fla., 1980.

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M. O. Balaban University of Florida Gainesville, Florida M. A. A. Meireles

State University of Campinas—UNICAMP Sao Paulo, Brazil

SURFACE (INTERFACIAL) TENSION

Surface tension can be referred to as free energy per unit area or force per unit length. Customary units are either ergs per square centimeter or dynes per centimeter. In SI (International System of Units) units, they are joules per square meter or newtons per meter (J/m2, N/m).

The mathematical notation used for surface tension is usually y, and in performing dimensional analysis, y is represented by M/t2, where AT is mass and t is time. The methods available for the measurement of surface tension include the capillary rise method, the capillary wave method, the ring method, and the tensiometer method (1).

Surface energy (tension) at the boundary of two phases, such as solid-liquid in skim milk, liquid-liquid in salad dressing, gas-liquid in meringue, gas-solid in foam candy, and solid-gas in smoke, is an important parameter for studying the manufacturing and the shelf-life stability of food systems (2). Furthermore, many chemical and physical processes can occur at the boundary; for example, dissolution and crystallization, heterogeneous catalysis, and phenomena related to the colloidal state. Surface energy at the boundary often determines whether the process will occur.

London, van der Waals, or cohesive forces act among the molecules of all substances irrespective of their state of aggregation. Such forces are significant only when the molecules are rather close to each other, say, only several nanometers apart. For polar compounds, such as water, hydrogen bonding is also a significant cohesive force. In a single bulk phase, intermolecular cohesive forces are balanced. This is because molecules in the interior of a phase are attracted equally in all directions to the other molecules in their vicinity. However, in either solids, liquids, or gases, if the atoms, ions, or molecules exist in the interface, they are exposed to the action of unbalanced forces due to both phases. In other words, those at the interface are not surrounded completely by other entities of the same type or same physical state. Thus, the surface molecules or atoms are in an energy state different from that of those in the bulk phase. This additional energy, generally called surface energy, imparts to the surface region distinct features that are unique to the region. Surface tension acts parallel to the substance's surface, opposing any attempt to expand the surface area. A component of surface energy causes forces to act normally at the phase boundary, resulting in an inward attraction, which, in turn, tends to reduce the number of molecules at the interface and to reduce the interfacial area to a minimum. A liquid, for example, has a tendency to obtain a spherical shape, the shape with the smallest ratio of surface to volume, if it is not contained. On the other hand, if the liquid is contained, it does not alter to the spherical shape because gravity imposes an added requirement. The surface must be uppermost and parallel to the plane of the earth, as it takes the shape of its container. Work or energy is required to increase the area of a surface or of an interface. This requires molecules to be moved from the body of the substance to the surface (interface). The total surface energy required to expand a surface by 1 cm2 is the sum of the work required to overcome the surface tension and the amount of heat that must be supplied to maintain the expanding surface at a constant temperature. If the surface of water is at 20°C, the thermal energy required is 47.7 ergs/cm2, and since surface tension, y = 72.8 ergs/cm2, the total energy is 120.5 ergs/cm2 (2). With an increase in temperature, the kinetic energy of the molecules increases and the cohesive forces among them decreases, which leads to a decreased surface tension. The influence of temperature on the surface tension of water and vegetable oils is shown in Table 1 (3). The surface tensions shown in Table 1 are those between the liquid (water or oil) phase and the gaseous phase (air). For making stable food emulsions, the interfacial tensions between edible oils and water, as shown in Table 2 (4,5), are important factors. An important consequence of surface tension (and its existence) is that there is a pressure difference on the two sides of a surface. This can be visualized by considering a gas-filled balloon stretched by the internal pressure of the gas. It experiences an elastic force

Table 1. Influence of Temperature on the Surface Tension (dyne/cm) of Water and Vegetable Oil

Temperature,

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