Surface tension reduction efficiency

The conditions for synergism in surface tension reduction efficiency, mixed micelle formation, and Surface tension reduction effectiveness in aqueous solution have been derived mathematically together with the properties of the surfactant mixture at the point of maximum synergism. This treatment has been extended to liquid-liquid (aqueous solution/hydrocarbon) systems at low surfactant concentrations.) The effect of chemical structure and molecular environment on the value of B is demonstrated and discussed. 

Synergism in surface tension reduction efficiency. The efficiency of surface tension reduction by a surfactant is defined (9) as the solution phase concentration required to produce a given surface tension (reduction). Synergism in this respect is present in a binary mixture of surfactants when a given surface tension (reduction) can be attained at a total mixed surfactant concentration lower than that required of either surfactant by itself. This is illustrated in Figure 2. 

Figure 2. Synergism in surface tension reduction efficiency (Ci2 < C ° or 2°) or in mixed micelle formation...

Table I. Synergism in Surface Tension Reduction Efficiency ...

By mathematical treatment similar to that for synergism in surface tension reduction efficiency, we have found that the conditions for synergism in mixed micelle formation are ... 

From the relations upon which equations 11.1 and 11.2 are based and the definition for synergism or antagonism (negative synergism) of this type, it has been shown mathematically (Hua, 1982b, 1988) that the conditions for synergism or antagonism, in surface tension reduction efficiency to exist are ... 

Fig. 2 Synergism in surface tension reduction efficiency or in mixed micelle formation. Surfactant 1. Surfactant 2. 0 Mixture of surfactants 1 and 2 at mole fraction a in the solution phase. Synergism in surface tension reduction efficiency Cn < C , C . Synergism in mixed micelle formation < CT, Cf...

To obtain maximum synergism in surface tension reduction efficiency, the two surfactants should be used at a mole fraction, a (of surfactant 1 in the total surfactant in the solution phase), given by the expression... 

The log of the reciprocal of the bulk concentration of surfactant (C in mol/ L) necessary to produce a surface or interfacial pressure of 20 raN/m, log( 1 / On= 20 i e > a 20 mN/m reduction in the surface or interfacial tension, is considered a measure of the efficiency of a surfactant. The effectiveness of surface tension reduction is the maximum effect the surfactant can produce irrespective of concentration, (rccmc = [y]0 - y), where [y]0 is the surface tension of the pure solvent and y is the surface tension of the surfactant solution at its cmc. 

Although the efficiencies of surface tension reduction, pCao, for the betaines and their corresponding sulfobetaines are almost the same, the former appear to show greater effectiveness in surface tenion reduction, as indicated by the values. This may be due... 

For solutions of AEg with different distributions of hydrocarbon chain lengths, the Y log C curves appear to be different than mono-component system. The surface pressure at critical micelle concentration (iTcjic) AEg with a long hydrocarbon chain (C gEg) is Increased by adding the short AEg, but the effect is not significant if the hydrocarbon chain is in a wide distribution (i.g. coconut fatty radical) (Figure 2,3,4). As for the efficiency of surface tension reduction there is a synergestic effect for the mixed... 

A term to describe the aforementioned quotient is cohesive energy density (CED heat of vaporization/unit volume). To a first approximation, the lower the CED, the lower will be the surface tension and this is the source of the increased efficiency in surface tension reduction of fluorosurfactants versus hydrocarbon surfactants. Therefore, fluorosurfactants are often the choice for applications demanding ultimately low surface tension. Furthermore, fluorosurfactants are far less compatible with water than are hydrocarbon surfactants. This is the origin of the increased effectiveness compared to hydrocarbon surfactants. 

FIGURE 5-3 Surface tension-log C plot illustrating efficiency, —log C2o(pC2o), and effectiveness of surface tension reduction, IIcmc-... 

The replacement of the usual hydrocarbon-based hydrophobic group by a fluorocarbon-based hydrophobic group causes a very large increase in the efficiency of surface tension reduction (Shinoda, 1972), the C7 perfluorosulfonate showing greater efficiency than the corresponding C12 hydrocarbon-based sulfonate. 

The reduction of the tension at an interface by a surfactant in aqueous solution when a second liquid phase is present may be considerably more complex than when that second phase is absent, i.e., when the interface is a surface. If the second liquid phase is a nonpolar one in which the surfactant has almost no solubility, then adsorption of the surfactant at the aqueous solution-nonpolar liquid interface closely resembles that at the aqueous solution-air interface and those factors that determine the efficiency and effectiveness of surface tension reduction affect interfacial tension reduction in a similar manner (Chapter 2, Section IIIC,E). When the nonpolar liquid phase is a saturated hydrocarbon, both the efficiency and effectiveness of interfacial tension reduction by the surfactant at the aqueous solution-hydrocarbon interface are greater than at the aqueous solution-air interface, as measured by pC2o and IIcmc, respectively. The replacement of air as the second phase by a saturated hydrocarbon increases the tendency of the surfactant to adsorb at the interface, while the tendency to form micelles is not affected significantly. This results in an increase in the CMC/C2o ratio. Since the value of rm, the effectiveness of adsorption (Chapter 2, Section IIIC), is not affected significantly by the presence of the saturated hydrocarbon, the increase in the... 

Since surface tension reduction depends on the replacement of solvent molecules at the interface by surfactant molecules, the efficiency of a surfactant in reducing the surface tension should reflect the concentration of the surfactant at the interface (Cg) relative to that of the bulk liquid phase (Cb). Generally, when the surface tension is reduced to 20 dynes/cm, the surfactant adsorption at the interface is close to its maximum. Adsorption at... 

When discussing the performance of a surfactant in lowering the surface tension of a solution it is necessary to consider two aspects of the process (1) the concentration of surfactant in the bulk phase required to produce a given surface tension reduction and (2) the maximum reduction in surface tension that can be obtained, regardless of the concentration of surfactant present. The two effects may be somewhat arbitrarily defined as follows the surfactant efficiency is the bulk phase concentration necessary to reduce the surface tension by a predetermined amount, for example, 20 mN m. Its effectiveness is the maximum reduction in a that can be obtained by the addition of any quantity of surfactant. The typical shape of the surface tension-concentration curve for aqueous surfactants is shown in Figure 8.8. 

The efficiency of dynamic surface tension reductions within homoiogous series of surfactants in aqueous geiatin soiution... 

The second definition of efficiency used here is based on the surface tension reduction at a specific concentration, chosen to give maximum differentiation between homologous series. Taking all three surfactant classes into account, the obvious concentration to select for this purpose is 10 M. It adroitly avoids the region of limiting surface tension, the so-called plateau zone, where differentiation... 

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