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Group Members: Philip Ash

Photo of Philip Ash

p.a.ash@durham.ac.uk

Bain Group History

Summer Research Project, University of Oxford, 2003
Part II Undergraduate Student, University of Oxford, 2004–2005
Ph.D. Student, Durham University, 2005–present

Project Title

Surface Freezing in Surfactant/Alkane/Water Systems

Research – Outline

Mixed monolayers of surfactant and alkane undergo first-order surface freezing transitions at the air–water interface. The behaviour exhibited is dependent upon the relative lengths of the surfactant tails and the alkane chains. So far, the main techniques used to study these systems have been ELLIPSOMETRY and SUM-FREQUENCY SPECTROSCOPY. Ellipsometry is a linear optical technique which is extremely sensitive to the structure of a monolayer, and hence is well suited to detecting the sudden changes which occur due to a phase transition. Sum Freqeuncy Spectroscopy gives information about the conformation of hydrocarbon chains at an interface.

Research – A Closer Look

The figure below shows a schematic illustration of the phenomenon of a) surface melting and b) surface freezing. The brick pattern denotes a crystalline bulk phase. Un-patterned substrate denotes liquid bulk.

Schematic illustration of a) surface melting and b) surface freezing.

Several questions arise from current research into surface freezing effects in mixed monolayers of surfactant and alkane, including:

  1. Why does alkane chain length affect the behaviour exhibited, and what is the nature of the different frozen phases formed?
  2. What structural variation is present in the frozen layers, particularly with respect to the end of the chains, as the alkane chain length is varied?

The first of these issues could be approached through reflectivity experiments, not least to give an independent measure of the thickness of the frozen layers. External reflection FTIR SPECTROSCOPY (ER-FTIRS) could also be used to assess whether the layers are solid, and if so whether they form crystalline or rotator phases. Structural variation in the ends of the chains could also be probed in this way, as could the composition of the mixed monolayers. Another question is whether the individual components in the surface frozen layers occupy fixed positions in the lattice or, as is perhaps more likely, they are randomly distributed amongst lattice sites. It could also be interesting to probe any change in the structure of the subphase as a result of surface freezing.

A consequence of surface freezing in some cases is the vanishing of interfacial tension. In these cases the interface is likely to be quite rough, so techniques such as X-RAY REFLECTIVITY could be used to study these systems (provided the interface is not too rough), in conjunction with spectroscopic techniques. In the regime of ultra-low surface tension it is the bending modulus of the frozen layer that will suppress surface fluctuations, and it might be interesting to see the magnitude of this effect. TENSIOMETRY could be used to probe how the interfacial tension decays upon freezing, and observe whether the interfacial tension becomes truly zero or simply ultra-low.

With these points in mind, below are listed some areas of initial work:

  1. X-ray reflectivity experiments to gain an independent measure of the thickness of the frozen layers.
  2. Ellipsometry to test the generality of surface freezing by extending the study to other classes of surfactant.
  3. Modification of the optical bench designed for an overflowing cylinder in order to use ER-FTIRS to study the systems.

Publications

R. A. Campbell, P. A. Ash, C. D. Bain "Dynamics of adsorption of an oppositely charged polymer-surfactant mixture at the air-water interface: poly(dimethyldiallylammonium chloride) and sodium dodecyl sulfate" Langmuir 2007, 23, 3242-3253 (DOI).

Note: you can check out other publications by Colin Bain SORTED BY YEAR or SORTED BY TOPIC.

Interests

Most of my spare time is devoted to music, and has been for many years. I have been playing the tuba in brass bands for as long as I care to remember, and am currently plying my trade with the many-named National Association of Schoolmasters Union of Women Teachers (NASUWT) Chester-le-Street Riverside Brass Band. Previous groups of note I have played with include Thoresby Colliery Band and the City of Sheffield Youth Orchestra. Although I haven't played with any orchestras for a few years now I am always open to offers, especially if any Mahler or Bruckner is involved (just in case anyone in need of a tuba player is reading this). Over the years music has taken me to a variety of glamorous locations all over Europe, but despite my best efforts I seem to end up in Blackpool on an almost yearly basis. Other interests include walking, food and watching rugby league.