SURFACE CHEMICAL CHARACTERIZATION OF MALEIC-ACID MONO[2-(4-ALKYLPIPERAZINYL)ETHYL ESTERS] .1. THE COMPLEX ADSORPTION BEHAVIOR OF AN AMPHOLYTIC SURFACTANT

Maleic acid mono[2-(4-akylpiperazinyl)ethyl esters] (C-8-C-13) and the ir precursors, 1-alkyl-4-(2-hydroxyethyl)piperazines, are synthesized and the pK(a) values of the surfactants are determined by potentiometr ic titration. The different ionic forms of the surfactant and their co ntent in the solution at definite pH values are determined within the range pH 0.5-10. At pH = 6.2 the predominant component in the solution has a betaine-like structure. As this form is expected to have the hi ghest surface activity and the concentrations of other ionic species a re low in the bulk phase, this pH is selected to determine. the adsorp tion parameters of the homologous series. A special method to determin e equilibrium values of the surface tension is used in cases of low su rfactant concentrations. To determine the adsorption parameters from s igma-log C curves of the aqueous surfactant solutions a Frumkin-Damask in isotherm is used. The cmc (critical micelle concentration), the int erfacial activity, and the interaction parameter show a significant ev en/odd alternation with the number of alkyl groups of these surfactant s. in accordance with the alternation of the melting points.