Hydrophobicity and surface activities of sodium salts of N-dodecanoyl amino acids

Abstract

Surface tensions of N-dodecanoyl amino acids were measured in 100 m M NaOH solution at 45.0°C. The dependence of the surface tension γ on the surfactant concentration ( C A − ) was expressed by the equation γ = γ 0 - RTΓ mln(1 + KC A ). The saturated adsorbed amount of surfactant at the air-water interface Γ m varied somewhat with different amino acid side chains. The hydrophobicity of an amino acid was defined by using the critical micelle concentration (CMC( X)) of its N-dodecanoyl derivative as Δ g = ln CMC( X) − ln CMC(glycinate). A progressive increase in hydrophobicity produced an increase in K and in the surface pressure at CMC (π CMC). A linear relationship existed between Δ g and efficiency in surface tension reduction, pC 20, i.e., the logarithm of reciprocal of the concentration when γ 0 − γ is 20 mN/m. It was found that the amino acid side chains in the surfactants did not have a hydrophobic interaction with the dodecanoyl grup.