Surfactant are surface active agents that adhere to the fluid interface and affect the interfacial tension. Reduction of surface or interfacial tension is one of the most commonly measured properties of surfactants in solution. Since it depends directly on the replacement of molecules of solvent at the interface by molecules of surfactant, and therefore on the surface (or interfacial) excess concentration of the surfactant, as shown by the Gibbs equation
dσ = −X
i
σidµi,
it is also one of the most fundamental of interfacial phenomena.
As we know, the molecules at the surface of a liquid have potential ener-gies greater than those of similar molecules in the interior of the liquid. (This is because attractive interactions of molecules at the surface with those in the interior of the liquid are greater than those with the widely separated molecules in the gas phase.) Because the potential energies of molecules at the surface are greater than those in the interior of the phase, an amount of work equal to this difference in potential energy must be expended to bring a molecule from the interior to the surface. The surface free energy per unit area, or surface tension, is a measure of this work; it is the minimum amount of work required to bring sufficient molecules to the surface from the interior to expand it by unit area. Although more correctly thought of as a surface free energy per unit area, surface tension is often conceptualized as a force per unit length at a right angle to the force required to pull apart the surface molecules in order to permit expansion of the surface by movement into it of molecules from the phase underneath it.
At the interface between two condensed, phases, the dissimilar molecules
Figure 1.4: Simplified diagram of the interface between two condensed phases a and b.
in the adjacent layers facing each other across the interface (Fig. 1.4) also have potential energies different from those in their respective phases. Each molecule at the interface has a potential energy greater than that of a similar molecule in the interior of its bulk phase by an amount equal to its interaction energy with the molecules in the interior of its bulk phase minus its interac-tion energy with the molecules in the bulk phase across the interface. For most purposes, however, only interactions with adjacent molecules need be
taken into account. If we consider an interface between two pure liquid phases a and b (Fig. 1.4), then the increased potential energy of the a molecules at the interface over those in the interior of that phase is Aaa and Aab, where Aaa symbolizes the molecular interaction energy between a molecules at the interface and similar molecules in the interior of the bulk phase and Aab sym-bolizes the molecular interaction energy between a molecules at the interface and b molecules across the interface. Similarly, the increased potential of b molecules at the interface over those in the interior is Abb−Aab. The increased potential energy of all the molecules at the interface over those in the interior of the bulk phases, the interfacial free energy, is then (Aaa−Aab)+(Abb−Aab) or Aaa + Abb− 2Aab, and this is the minimum work required to create the interface. The interfacial free energy per unit area of interface, the interfacial tension σI is then given by the expression
σI = σa+ σb − 2σab, (1.1) where σa and σb are the surface free energies per unit area (the surface ten-sions) of the pure liquids a and b, respectively, and σabis the a − b interaction energy per unit area across the interface.
The value of the interaction energy per unit area across the interface σab is large when molecules a and b are similar in nature to each other (e.g., water and short-chain alcohols). When σab is large, we can see from equation (1.1) that the interfacial tension σI will be small; when σab is small, σI is large.
The value of the interfacial tension is therefore a measure of the dissimilarity of the two types of molecules facing each other across the interface.
In the case where one of the phases is a gas (the interface is a surface), the molecules in that phase are so far apart relative to those in the condensed phase that tensions produced by molecular interaction in that phase can be disregarded. Thus if phase a is a gas, σa and σab can be disregarded and σI ≈ σb, the surface tension of the condensed phase b.
When the two phases are immiscible liquids, σa and σb, their respective surface tensions, are experimentally determinable, pennitting the evaluation of σab, at least in some cases. If one of the phases is solid, on the other hand, experimental evaluation of σab is difficult, if not impossible. However here, too, the greater the similarity between a and b in structure or in the nature of their inkrmolecular forces, the greater the interaction between them (i.e., the greater the value of σab) and the smaller the resulting interfacial tension between the two phases. When 2σab becomes equal to σa+ σb, the interfacial region disappears and the two phases spontaneously merge to form a single one.
If we now add to a system of two immiscible phases (e.g., heptane and
Figure 1.5: Diagrammatic representation of heptane-water interface with adsorbed surfactant.
water), a surface-active agent that is adsorbed at the interface between them, it will orient itself there, mainly with the hydrophilic group toward the wa-ter and the hydrophobic group toward the heptane (Fig. 1.5). When the surfactant molecules replace water and/or heptane molecules of the original interface, the interaction across the interface is now between the hydrophilic group of the surfactant and water molecules on one side of the interface and between the hydrophobic group of the surfactant and heptane on the other side of the interface. Since these interactions are now much stronger than the original interaction between the highly dissimilar heptane and water molecules, the tension across the interface is significantly reduced by the pres-ence there of the surfactant. Since air consists of molecules that are mainly non-polar, surface tension reduction by surfactants at the air-aqueous solu-tion interface is similar in many respects to interfacial tension reducsolu-tion at the heptaneVaqueous solution interface.
We can see from this simple model why a necessary but not sufficient condition for surface or interfacial tension reduction is the presence in the surfactant molecule of both lyophobic and lyophilic portions. The lyophobic portion has two functions: (1) to produce spontaneous adsorption of the sur-factant molecule at the interface and (2) to increase interaction across the interface between the adsorbed surfactant molecules there and the molecules in the adjacent phase. The function of the lyophilic group is to provide strong interaction between the molecules of surfactant at the interface and the molecules of solvent. If any of these functions is not performed, then the marked reduction of interfacial tension characteristic of surfactants will
probably not occur. Thus, we would not expect ionic surfactants containing hydrocarbon chains to reduce the surface tensions of hydrocarbon solvents, in spite of the distortion of the solvent structure by the ionic groups in the surfactant molecules. Adsorption of such molecules at the airhydrocarbon interface with the ionic groups oriented toward the predominantly non-polar air molecules would result in decreased interaction across the interface, com-pared to that with their hydrophobic groups oriented toward the air.
For significant surface activity, a proper balance between lyophilic and lyophobic character in the surfactant is essential. Since the lyophilic (or lyophobic) character of a particular structural group in the molecule varies with the chemical nature of the solvent and such conditions of the system as temperature and the concentrations of electrolyte and/or organic additives, the lyophilic-lyophobic balance of a particular surfactant varies with the sys-tem and the conditions of use. In general, good surface or interfacial tension reduction is shown only by those surfactants that have an appreciable, but limited, solubility in the system under the conditions of use. Thus surfac-tants which may show good surface tension reduction in aqueous systems may show no significant surface tension reduction in slightly polar solvents such as ethanol and polypropylene glycol in which they may have high solubility.