Efficiency in Surface Tension Reduction

Since surface or interfacial tension reduction depends on the replacement of solvent molecules at the interface by surfactant molecules, the efficiency of a surfactant in reducing surface tension should reflect the concentration of the surfactant at the interface relative to that in the bulk liquid phase. A suitable measure for the efficiency with which a surfactant performs this function would therefore be the ratio of the concentration of surfactant at the surface to that in the bulk liquid phase at equilibrium, both concentrations being expressed in the same units, e.g., [Cj]/Ci, where both concentrations are in moles/liter. 

The surface concentration of surfactant [C, in moles per liter, is related to its surface excess concentration T], in moles per cm2, by the relation Oj] = (1000ryd) + Ci, where d = the thickness of the inter-facial region, in centimeters. For surfactants, F] is in the range 1-5 x 10 10 mol/cm2, while d = 50x 10 8 cm or less and C = 0.01 M or less. Thus, [Cj] = lOOOTi/ii without significant error, and [Cj]/Ci = 1000r i / Cd. 

When the tension has been reduced by 20 mN/m (dyn/cm), the value of V is close to its maximum value, Tm, as shown in Chapter 2, and most surfactant 

as discussed there, it is more useful and convenient to use the quantity C20 in the form of its negative logarithm pC20, since the latter quantity is related (equations 2.30, 2.31, 2.35, 2.36) to standard free energies of adsorption. The factors that determine the value of pC2o have been discussed in Chapter 2, Section HIE. 

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. 

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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. 

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