Monolayer arrangement of fatty hydroxystearic acids on graphite: Influence of hydroxyl groups

• Published in: Thin solid films 2013-07, Vol.539, p.194-200
• Main Authors: Medina, S., Benítez, J.J., Castro, M.A., Cerrillos, C., Millán, C., Alba, M.D.
• Format: Article
• Language: English
• Subjects: Carboxylic acid; Cross-disciplinary physics: materials science; rheology; DSC; Exact sciences and technology; Fullerenes and related materials; diamonds, graphite; Graphite; Materials science; Monolayer; Physics; Solid–liquid interface; Specific materials; STM; VT-XRD
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/j.tsf.2013.05.053

Previous studies have indicated that long-chain linear carboxylic acids form commensurate packed crystalline monolayers on graphite even at temperatures above their melting point. This study examines the effect on the monolayer formation and structure of adding one or more secondary hydroxyl, functional groups to the stearic acid skeleton (namely, 12-hydroxystearic and 9,10-dihydroxystearic acid). Moreover, a comparative study of the monolayer formation on recompressed and monocrystalline graphite has been performed through X-ray diffraction (XRD) and Scanning Tunneling Microscopy (STM), respectively. The Differential Scanning Calorimetry (DSC) and XRD data were used to confirm the formation of solid monolayers and XRD data have provided a detailed structural analysis of the monolayers in good correspondence with obtained STM images. DSC and XRD have demonstrated that, in stearic acid and 12-hydroxystearic acid adsorbed onto graphite, the monolayer melted at a higher temperature than the bulk form of the carboxylic acid. However, no difference was observed between the melting point of the monolayer and the bulk form for 9,10-dihydroxystearic acid adsorbed onto graphite. STM results indicated that all acids on the surface have a rectangular p2 monolayer structure, whose lattice parameters were uniaxially commensurate on the a-axis. This structure does not correlate with the initial structure of the pure compounds after dissolving, but it is conditioned to favor a) hydrogen bond formation between the carboxylic groups and b) formation of hydrogen bonds between secondary hydroxyl groups, if spatially permissible. Therefore, the presence of hydroxyl functional groups affects the secondary structure and behavior of stearic acid in the monolayer. •Hydroxyl functional groups affect structure and behavior of acids in the monolayer.•Acids on the surface have a rectangular p2 monolayer structure.•Lattice parameters of acids are uniaxially commensurate on the a-axis.•Stearic and 12-hydroxystearic acids monolayers melt point is higher than bulk.•For 9,10-dihydroxystearic acid, melting point of monolayer and bulk are equal.