parallel to the surface that counteracts the stretching; otherwise the balloon would pop. The internal pressure of a balloon must be larger than the external pressure since the gas is not only counteracting the external pressure but also stretching the balloon. This force is analogous to the surface tension of a liquid, which arises due to the rubber molecules' attraction or the liquid's molecular attraction with each other.

In general, the smaller the interfacial tension, the less energy is required to create an emulsion between the phases. As a rule of thumb, the interfacial tension between water and edible oil should be below 10 dyne/cm to facilitate emulsification of the two phases (2). To meet this requirement, surface-active agents, or surfactants, are often added to reduce the interfacial tensions between the two phases. Surfactants in small quantities are known to reduce interfacial tensions. However, there are other solutes, such as inorganic salts and compounds with a large number of hydroxyl groups, such as sugars, when added to water, the surface tension of the solution increases slightly as the concentration of the solute increases. The molecular-level explanations for such distinctive phenomena are the following. Surface-active chemicals exhibit positive adsorption phenomena that lead to a higher concentration of the added chemicals at the surface (or interface) than in the bulk. For sugars and salts that show a negative adsorption phenomenon in water, the concentration in the bulk is higher than that in the surface (or interface). Not only are their effects in opposite directions, but the effects of surface-active agents on the surface (or interfacial) tension are much more drastic than those caused by sugars and salts. Consequently, small amounts of surfactants are normally used. At the molecular level, surfactant molecules are oriented at the surface (or interface) by pointing their polar (hydrophilic) groups toward the aqueous phase and their apolar (hydrophobic) groups toward the gaseous (nonaqueous) phase. In this process, the surface free energy is minimized. The surfactant molecules will form a monolayer or a film on the surface until the surface is completely covered and will form micelles with organized structures in the bulk solution as the concentration of surfactant exceeds the critical micelle concentration, which is in the range 0.004 to 0.15 mol/L for many surfactants (2).

Surfactants are either natural or synthetic, nonionic, or ionic. Food-grade surfactants are generally nonionic and are preferably natural. Soaps and detergents act as surfactants in order to lower the surface tension, which helps to remove oily dirt particles from solid surfaces (including skin). This enables these products to do what they do best—clean! Many proteins are good surfactants. Protein molecules can be adsorbed on the interface to form either a monolayer or multiple layers, depending on the individual protein. This is related to the fact that different proteins have different abilities in reducing the surface tension of water. Table 3 shows the equilibrium surface tension of four different protein solutions at different concentrations.

Similarly, Table 4 shows the influence of several milk proteins on the interfacial tension between water and butter oil. The values of surface and interfacial tension shown in Table 3 and 4 are both equilibrium values that were obtained after a transient period of surface (or interfacial) tension reduction after the addition of the protein. An equilibrium is reached when the surface tension measured stays at a constant value over an arbitrarily selected time period, such as 30 min.

Table 3. The Equilibrium Surface Tension of Protein Solutions at Various Concentrations
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