Part II Undergraduate Student, University of Oxford, 1994–1995
D.Phil. Student, University of Oxford, 1995–1998
Phase Transitions in Surfactant Monolayers
Two-dimensional phase transition have been studied in surfactant monolayers at the air/water interface by SUM-FREQUENCY SPECTROSCOPY and ELLIPSOMETRY. In equilibrium monolayers of medium-chain alcohols CnH2n+1OH (n = 9–14) a transition from a two-dimensional crystalline phase to a liquid was observed at temperatures above the bulk melting point. The small population of gauche defects in the solid phase increased only slightly at the phase transition. A model of the hydrocarbon chains as freely rotating rigid rods allowed the area per molecule and chain tilt in the liquid phase to be determined. The area per molecule, chain tilt and density of the liquid phase all increased with increasing chain length, but for each chain length the density was higher than in a bulk liquid hydrocarbon. In a monolayer of decanol adsorbed at the air/water interface a transition from a two-dimensional liquid to a gas was observed. A clear discontinuity in the coefficient of ellipticity as a function of temperature showed that the transition is first-order. This result suggests that liquid–gas phase transitions in surfactant monolayers may be more widespread than once thought.
A solid–liquid phase transition has also been studied in mixed monolayers of dodecaonol with an anionic surfactant (sodium dodecyl sulphate) and with a homologous series of cationic surfactants (alkyltrimethylammonium bromides: CnTABs, n = 12, 14, 16). The composition and structure of the mixed monolayers was studied above and below the phase transition. At low temperatures the mixed monolayers were as densely packed as a monolayer of pure dodecanol in its solid phase. At a fixed temperature the monolayers underwent a first-order phase transition to form a phase that was less dense and more conformationally disordered. The proportion of ionic surfactant in the mixed monolayer was greatest in the high temperature phase. As the chain length of the CnTAB increased the number of conformational defects decreased in both phases. This change, along with shifts in the phase transition temperature and the frequency of the symmetric methyl stretch with chain length, can be understood in terms of the arrangement of the dodecanol molecules within the mixed monolayer.
B. D. Casson and C. D. Bain "Phase Transitions in Mixed Monolayers of Cationic Surfactants and Dodecanol at the Air/Water Interface" Journal of Physical Chemistry B 1999, 103, 4678–4686 (DOI).
B. D. Casson and C. D. Bain "Unequivocal Evidence for a Liquid–Gas Phase Transition in Monolayers of Decanol Adsorbed at the Air/Water Interface" Journal of the American Chemical Society 1999, 121, 2615–2616 (DOI).
R. Braun, B. D. Casson, C. D. Bain, E. W. M. van der Ham, Q. H. F. Vrehen, E. R. Eliel, A. M. Briggs and P. B. Davies "Sum-Frequency Generation from Thiophenol on Silver in the Mid and Far-IR" Journal of Chemical Physics 1999, 110, 4634–4640 (DOI).
B. D. Casson and C. D. Bain "Phase Transitions in Mixed Monolayers of Sodium Dodecyl Sulfate and Dodecanol at the Air/Water Interface" Journal of Physical Chemistry B 1998, 102, 7434–7441 (DOI).
B. D. Casson and C. D. Bain "Determination of the Optical Properties of Monolayers on Water" Langmuir 1997, 13, 5465–5469 (DOI).
B. D. Casson, R. Braun and C. D. Bain "Phase Transitions in Monolayers of Medium-chain Alcohols on Water Studied by Sum-Frequency Spectroscopy and Ellipsometry" Faraday Discussions, 1996, 104, 209–229 (DOI).
R. Braun, B. D. Casson and C. D. Bain "A Sum-Frequency Study of the Two-Dimensional Phase Transition in a Monolayer of Undecanol on Water" Chemical Physics Letters 1995, 245, 326–334 (DOI).