Effectiveness in Surface Tension Reduction

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... 

As for other types of fluid particle, the internal circulation of water drops in air depends on the accumulation of surface-active impurities at the interface (H9). Observed internal velocities are of order 1% of the terminal velocity (G4, P5), too small to affect drag detectably. Ryan (R6) examined the effect of surface tension reduction by surface-active agents on falling water drops. 

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 effectiveness. This exists when the mixture of surfactants of its cmc reaches a lower surface tension than that obtained at the cmc of either component of the mixture by Itself. This is illustrated in Figure 5. 

Figure 5. Synergism in surface tension reduction effectiveness. (Ycmc 2 Y°cmc or Y°cmC2). (l) Pure surfactant 1 ...

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. 

Some of these factors affect Ym and the CMC/C2o ratio in parallel fashion (i.e., they increase both or decrease both) some in opposing fashion. When the effects are parallel, we can readily predict the resulting change in the effectiveness of surface tension reduction when they are opposed, it is difficult to do so. Thus, increase in the length of the hydrophobic group in ionic surfactants has little effect on either Ym or the CMC/C20 ratio, and we can therefore expect that an increase in the length of the hydrophobic group will have little effect on their effectiveness of surface tension reduction. 

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... 

It is apparent from condition 1 that synergism in surface tension reduction effectiveness can occur only when the attractive interaction between the two surfactants in the mixed monolayer at the aqueous solution-air interface is stronger than that in the mixed micelle in the solution phase. When the attraction between... 

Surfactants C and D of Problem 2 individually reduce the surface tension of an aqueous 0.1 M NaCl solution to 30 dyn/cm when their respective molar concentrations are 9.1 x 10 4 and 3.98 x 10-4. The mixture of them at a = 0.181 in Problem 2 has a surface tension value of 30 dyn/cm when the total molar surfactant concentration is 3.47 x 10-4. Will a mixture of surfactants C and D exhibit synergism or antagonism in surface tension reduction effectiveness ... 

C2 / Ci =0.35, the system will exhibit antagonism in surface tension reduction effectiveness. 

By their nature, the experiments on gas bubble nucleation, whether electrolytic, chemically generated, or by pressure release, all have high concentrations of gas in solution during the bubble nucleation process, and thus show the most pronoimced effects of surface tension reduction. Two aspects of this observation are particularly noteworthy first, that He of all the gases so far tested shows the smallest ratio of theoryjmeasured—and is also the only gas with a b coefficient of 0. Second, that organic liquids with N2 gas dissolved in them (including the case of diethyl ether) show relatively modest ratios theoryjmeasured. In the case of diethyl ether, the liquid is relatively close... 

Branching of the fluorocarbon chain decreases the efficiency of a fluorinated surfactant in surface tension reduction [57,58]. In analogy, a condensed (spread) monolayer of a perfluorinated n-alkanoic acid has a lower critical surface tension than its terminally branched isomer. Bemett and Zisman [59] attributed the effect of branching to different molecular packing and carbon chain adlineation. 

Anderson s group (Pino et al., 2009) studied the micellar properties of aqueous solutions of two ILs, l-hexadecyl-3-butylimidazolium bromide and 1,3-didodecylimidazolium bromide, in the presence of several organic solvents (methanol, 1-propanol, 1-butanol, l-p>entanol, and acetonitrile) by surface tensiometiy. For both ILs, increases in the cmc values and minimum surface area per surfactant molecule, decreases in the maximum surface excess concentration, adsorption efficiency and effectiveness of surface tension reduction were obtained when increasing the organic solvent content. 

As has already been mentioned, the effect of changes in the hydrophobic chain length on the effectiveness of surface tension reduction in nonionic surfactants is relatively minimal. Increases in the length of the polyoxyethylene chain, on the other hand, lead to significant reductions in the effectiveness of a given surfactant hydrophobic group. It appears, then, that the primary factor involved in the efficiency and effectiveness of nonionics in this application is the length of the... 

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

There is also the possibility of having surface tension affected directly by the presence of an electrostatic field. To some extent this will be a matter of definition since the outward pressure due to a surface charge could be defined as an apparent effect on surface tension. Hurd, Schmid, and Snavely (H15) measured the surface tension of water and water solutions when fields up to 0.7 V/micron were applied across the air-solution interface. The results showed a reduction in surface tension of less than 1 %. These data must not be considered conclusive, however, because insufficient details are reported to permit assessment of the exact nature of the electrostatic field applied or of the validity of a number of corrections that had to be applied but were reported to be very large and difficult to apply. 

---