title: Self assembly of pH-sensitive cationic lysine based surfactants.
abstract: Three cationic surfactants of the type N(ε)-acyl lysine methyl ester hydrochloride have been studied with respect to solution behavior and adsorption on the air/water interface, as well as the thermolyotropic behavior. The self-assembly of these surfactants, which have the cationic charge on amine protonated groups, was assessed by different physicochemical methods. Depending on the pH value, these surfactants can dissociate in aqueous solutions, losing the cationic charge. Therefore, knowledge of the pK(a) of these compounds is essential to explain their behavior in aqueous solutions. The bulk techniques, conductivity, and nuclear magnetic resonance diffusion (NMR) obtained similar critical micellar concentration (CMC) values, which were well above those obtained from surface tension. Surface tension measurements were strongly dependent on the technique used, namely, Wilhelmy plate and pendant drop. The phase behavior at medium to high concentrations has been studied by optical polarizing microscopy and small angle x-ray scattering (SAXS). The X-ray studies showed that the lysine-based surfactants at low hydration have rich thermotropic liquid crystalline behavior. The results are discussed in terms of the structure of the compounds and the cationic charge of the molecule. We will show how apparently small changes in molecule structure have a large influence on phase behavior.
authors.affiliation: Departament de Tecnologia Química i de Tensioactius, Institut de Química Avançada de Catalunya, IQAC-CSIC, c/Jordi Girona 18-26, 08034 Barcelona, Spain.
authors.forename: Amalia
authors.initials: A
authors.lastname: Mezei
authors.forename: Lourdes
authors.initials: L
authors.lastname: Pérez
authors.forename: Aurora
authors.initials: A
authors.lastname: Pinazo
authors.forename: Francesc
authors.initials: F
authors.lastname: Comelles
authors.forename: Maria Rosa
authors.initials: MR
authors.lastname: Infante
authors.forename: Ramon
authors.initials: R
authors.lastname: Pons
mesh_terms: D000327:Adsorption; D000388:Air; D002412:Cations / Q000737:chemistry*; D004553:Electric Conductivity; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008239:Lysine / Q000031:analogs & derivatives*; D009682:Magnetic Resonance Spectroscopy; D008823:Micelles; D015394:Molecular Structure; D044367:Phase Transition; D053838:Scattering, Small Angle; D055672:Static Electricity; D013500:Surface Tension; D013501:Surface-Active Agents / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D002412:Cations; D008823:Micelles; D013501:Surface-Active Agents; D014867:Water; D008239:Lysine