D.Phil. Student, University of Oxford, 2000–2004
Phase Transitions in Monolayers of Soluble Surfactants
Ellipsometric studies of the air/solution interface of decanol, octanoic acid, nonanoic acid, decanoic acid and dibucaine hydrochloride (DC.HCl) solutions provide no evidence of first-order gas/liquid phase transitions. The ellipsometric data are consistent with continuous changes in monolayer structure during adsorption. Sum-frequency spectra of decanol, octanoic acid and nonanoic acid solutions suggest continuous changes in structure between a disordered monolayer with a high fraction of gauche defects at low surface excesses to an ordered monolayer at saturation. Further insights into the changes of monolayer structure during adsorption are provided by modelling of the ellipsometric data for decanol.
Ellipsometric studies of the wetting behaviour of tetradecyltrimethylammonium bromide (C14TAB), dodecyltrimethylammonium bromide (C12TAB) and DC.HCl solutions by alkanes show first-order transitions between partial wetting and pseudo-partial wetting regimes with increasing surfactant concentration. In the pseudo-partial wetting regime, floating lenses of alkane exist in equilibrium with thin films (~1 nm thick). Pseudo-partial wetting of DC.HCl solutions by hexadecane and C12TAB solutions by squalane demonstrates that wetting transitions are not limited to surfactants and oils with linear chains. The effect of length of the alkane and nature of the surfactant on the wetting behaviour is discussed.
Mixed monolayers of CnTABs with alkane (C16TAB+Cm with m = 11–20 and C14TAB+C12) at the air/solution interface are shown by ellipsometry to undergo first-order liquid/solid phase transitions. Sum-frequency spectra of specifically deuterated monolayers of C16TAB+C14 show that both the surfactant and alkane chains became more ordered on freezing. Ellipsometry shows that monolayers of C16TAB+Cm, m = 18–20 (and monolayers of C12TAB+C16) exhibit a different type of freezing behaviour from that of C16TAB+Cm, m = 11–17. Possible structures of the solid layers are proposed.
H. Matsubara, M. Aratono, K. M. Wilkinson and C. D. Bain "Lattice Model for the Wetting Transition of Alkanes on Surfactant Solutions" Langmuir, in press.
K. M. Wilkinson, Q. Lei and C. D. Bain "Freezing Transitions in Mixed Surfactant/Alkane Monolayers at the Air–Solution Interface" Soft Matter 2006, 2, 66–76 (DOI).
K. M. Wilkinson, C. D. Bain, H. Matsubara and M. Aratono "Wetting of Surfactant Solutions by Alkanes" ChemPhysChem 2005, 6, 547–555 (DOI).
D. Valkovska, K. M. Wilkinson, R. A. Campbell, C. D. Bain, R. Wat and J. Eastoe "Measurement of the Dynamic Surface Excess of the Nonionic Surfactant C8E4OMe by Neutron Reflection and Ellipsometry" Langmuir 2003, 19, 5960–5962 (DOI).