title: Microdomains in mixed monolayers of oleanolic and stearic acids: thermodynamic study and BAM observation at the air-water interface and AFM and FTIR analysis of LB monolayers.
abstract: Monolayers of oleanolic acid (OLA) mixed with stearic acid (SA) were studied at the air-water interface. The surface pressure-area (pi-A) isotherms, measured over the whole composition range, and BAM observations were used to investigate the phase behaviour and self-organization of these components in a two-dimensional structure. Pure OLA forms a very compressible monolayer, and BAM observation revealed the coexistence of large and irregular solid domains of different thickness dispersed in a gas matrix, compatible with the two most probable orientations of the OLA molecule at the interface. Mixtures of OLA/SA form condensed monolayers from low surface pressures and the thermodynamic analysis indicates that OLA molecules, in the presence of the long-chain SA, orient with the major axis almost perpendicular to the interface. Langmuir-Blodgett (LB) monolayers of pure SA and mixtures were further characterized by atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR). AFM images of LB mixed monolayers evidenced microphase separation, not observable by BAM. The SA rich domains are 4-6A thicker than those rich in OLA. The FTIR spectra of mixed LB films on CaF2 substrates showed that OLA does not perturb the all-trans conformation of the SA long alkyl chains, up to a mole fraction of 0.4. The carbonyl-stretching band of OLA suggests that the carboxylic groups of neighbour OLA molecules are involved in hydrogen bonds, forming dimers, as in pure solid phase OLA. These interactions seem to prevail over the OLA-water hydrogen bonds.
authors.affiliation: Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal.
authors.forename: Ana C T
authors.initials: AC
authors.lastname: Teixeira
authors.forename: Anabela C
authors.initials: AC
authors.lastname: Fernandes
authors.forename: Ana R
authors.initials: AR
authors.lastname: Garcia
authors.forename: Laura M
authors.initials: LM
authors.lastname: Ilharco
authors.forename: Pedro
authors.initials: P
authors.lastname: Brogueira
authors.forename: Amélia M P S
authors.initials: AM
authors.lastname: Gonçalves da Silva
mesh_terms: D000388:Air; D000475:Alkenes / Q000737:chemistry*; D019281:Dimerization; D006860:Hydrogen Bonding; D021962:Membrane Microdomains / Q000737:chemistry*; D018625:Microscopy, Atomic Force; D044367:Phase Transition; D017550:Spectroscopy, Fourier Transform Infrared; D013229:Stearic Acids / Q000737:chemistry*; D013501:Surface-Active Agents / Q000737:chemistry; D013816:Thermodynamics; D014867:Water
chemical_list: D000475:Alkenes; D013229:Stearic Acids; D013501:Surface-Active Agents; D014867:Water; C031183:stearic acid