Relationship of structure to properties in surfactants: II. Efficiency in surface or interfacial tension reduction

The concept of a quantitative measure for the efficiency with which surfactants depress surface or interfacial tension is introduced. It is shown that the quantity, log (1/C)_π=20, where C is the bulk concentration or surfactant required to reduce the surface or interfacial tension by 20 dynes/cm (surface pressure, π=20), is a suitable measure of the efficiency. This quantity is a linear function of the free energy of transfer of the surfactant molecule from the interior of the bulk phase to the interface. The effect upon the efficiency of various structural groupings in both the hydrophobic and hydrophilic portions of the molecule is calculated and the results discussed. From surface and interfacial tension data, it is shown that the free energy decrease involved in the transfer of a −CH₂-group at π=20 to the aqueous solution-air interface is 450–500 cal mole⁻¹ at 25 C; for the aqueous solution-heptane interface, the free energy decrease/−CH₂-group transferred is greater than 427 cal mole⁻¹ at 50 C. Presented at the AOCS Meeting, Philadelphia, September 1974.