Reduction of Surface and Interfacial Tension by Surfactants

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 

Surfactants and Interfacial Phenomena, Third Edition. Milton J. Rosen ISBN 0-471-47818-0 2004 John Wiley Sons, Inc. 

FIGURE 5-1 Simplified diagram of the interface between two condensed phases a and b. 

The value of the interaction energy per unit area across the interface yllb is large when molecules a and b are similar in nature to each other (e.g., water and short-chain alcohols). When yab is large, we can see from equation 5.1 that the interfacial tension y will be small when yab is small, y, is large. The value of the interfacial tension is therefore a measure of the disimilarity 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, ya and yab can be disregarded and yb k, yb, the surface tension of the condensed phase b. 

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The concentration at which this phenomenon occurs is called the critical micelle concentration (CMC). Similar breaks in almost every measurable physical property that depends on size or number of particles in solution, including micellar solubilization of solvent-insoluble material (Chapter 4) and reduction of surface or interfacial tension (Chapter 5), are shown by all types of surfactants—nonionic, anionic, cationic, and zwitterionic in aquecus media. 

For the purpose of comparing the performance of surfactants in reducing surface or interfacial tension, as in adsorption, it is necessary to distinguish between the efficiency of the surfactant (i.e., the bulk phase concentration of surfactant required to reduce the surface or interfacial tension by some significant amount) and its effectiveness, the maximum reduction in tension that can be obtained, regardless of bulk phase concentration of surfactant. These two parameters do not necessarily mn parallel to each other and sometimes even run counter to each other. 

The solubilities of micelle-forming surfactants show a strong increase above a certain temperature, termed the Krafft point (Tk). This increased solubility is explained by the fact that the single surfactant molecule has limited solubility, whereas the micelles are very soluble. As shown in Figure 6, at temperatures below the Krafft point, the solubility of the surfactant is too low for micellization, and solubility alone determines the surfactant—monomer concentration. As temperature increases, the solubility increases until at Tk the CMC is reached. At this temperature, a relatively large amount of surfactant can be dispersed in micelles, and solubility increases greatly. Above the Krafft point, maximum reduction in surface or interfacial tension occurs at the CMC, because now the CMC determines the surfactant—monomer concentration. Krafft points for a number of surfactants are listed in reference 11. 

Surfactant efficiency can be expressed as the surfactant concentration needed to reduce the surface or interfacial tension by 20 mN m from the value of the pure solvent(s). Surfactant effectiveness, on the other hand, refers to the maximum reduction in surface or interfacial tension achievable by a surfactant (corresponding to saturation of the surface or interface). To give some sense of the extent to which surfactants can lower surface and interfacial tension, many hydrocarbon surfactants, at high concentrations (above the critical micelle concentration (cmc) see Section 3.5.3) can lower the surface tension of water at 20 "C from 72.8 to about 28 mN m . Polysiloxane surfactants can reduce it further, to about 20 mN m , and perfluoroalkyl surfactants can reduce it still further, to about 15 mN m . Similarly, hydrocarbon surfactants can reduce the interfacial tension of water - mineral oil from about 40 mN m down to about 3 mN m . ... 

This transition may j-.e. reducing the specific surface energy, f. The reduction of f to sufficiently small values was accounted for by Ruckenstein (15) in terms of the so called dilution effect". Accumulation of surfactant and cosurfactant at the interface not only causes significant reduction in the interfacial tension, but also results in reduction of the chemical potential of surfactant and cosurfactant in bulk solution. The latter reduction may exceed the positive free energy caused by the total interfacial tension and hence the overall Ag of the system may become negative. Further analysis by Ruckenstein and Krishnan (16) have showed that micelle formation encountered with water soluble surfactants reduces the dilution effect as a result of the association of the the surfactants molecules. However, if a cosurfactant is added, it can reduce the interfacial tension by further adsorption and introduces a dilution effect. The treatment of Ruckenstein and Krishnan (16) also highlighted the role of interfacial tension in the formation of microemulsions. When the contribution of surfactant and cosurfactant adsorption is taken into account, the entropy of the drops becomes negligible and the interfacial tension does not need to attain ultralow values before stable microemulsions form. 

For most of the conventional amphiphiles it was demonstrated by Rosen [141] that at a surface pressure H = 20 mN/m the surface excess concentration reaches 84-100 % of its saturation value. Then, the (l/c)n=2o value can be related to the change in free energy of adsorption at infinite dilution AG , the saturation adsorption F and temperature T using the Langmuir and von Szyszkowski equations. The negative logarithm of the amphiphile concentration in the bulk phase required for a 20 mN/m reduction in the surface or interfacial tension can be used as a measure of the efficiency of the adsorbed surfactant ... 

Gradients in surface (or interfacial) tension can accelerate the spreading of fluids, enhance the stability of surfactant-laden films of liquid, emulsions, and foams, and increase rates of mass transport across interfaces. The motion of fluid driven by a gradient in surface tension is referred to as a Marangoni flow . We have demonstrated that electrochemical reduction of IF to IF at an electrode that... 

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