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Previous Group Members: Thomas Curwen

Photo of Thomas Curwen

thomas.curwen@chem.ox.ac.uk

Bain Group History

D.Phil. Student, University of Oxford, 2002–present

Project Title

Kinetics of Adsorption of Surfactants Measured Under Well-Defined Hydrodynamic Conditions

Research – Outline

The initial aim of my research was to combine channel flow cell technology with ELLIPSOMETRY to enable the study of the adsorption of surfactants at the solid–solution interface under controlled hydrodynamic conditions. This aim is motivated by a desire to be able to decouple the adsorption kinetics from the bulk mass transport. Having developed the technique I have applied it to carry out a thorough investigation of the adsorption kinetics of Cetylpyridinium Chloride to a hydrophilic silica interface. I am now working to synthesise a pair of model hydrophobic interfaces for use in the flow cell. I hope that this work will yield information on how sensitive surfactant adsorption kinetics are to changes in surface chemistry.

Research – A Closer Look

Surfactants are often deployed under non-equilibrium conditions where the kinetics of adsorption are at least as important as the equilibrium adsorption properties: for example, the dispersion of fine powders during the milling of agrochemical or pharmaceutical ingredients, lubricant adsorption on a clean metal surface exposed by wear in a sheared contact, penetration of a fluid into a porous medium (e.g. rock, textile).

The relevant timescale for the adsorption process is frequently seconds – or less. One difficulty in studying adsorption kinetics on these timescales is that microscopic processes such as adsorption and micellar breakdown are invariably convoluted with mass transport. Unless the mass transport rate is known, any attempt to interpret dynamic adsorption measurements in terms of detailed adsorption mechanisms is doomed. While methods do exist for creating well-defined hydrodynamics there is no established mechanism for initiating surfactant adsorption in a controlled fashion.

My research has focussed on overcoming this problem by using a dual inlet channel flow cell to deliver a controllable and calculable flux of surfactant to a solid–solution interface. The adsorption at the interface is followed by ellipsometry. Here is a schematic of the dual-inlet channel flow cell.

Schematic cross-section of the flow cell showing formation of the calculable steady-state and subsequent diffusion of the surfactant to the interface

Concentrated surfactant solution is injected through a narrow slit upstream of the sample. A cross-flow of water sweeps the surfactant away before it is able to diffuse across the channel to the sample. A finite-difference algorithm is used to compute accurately the steady-state surfactant distribution in the channel.

Both flows are then switched off. The surfactant then diffuses across the channel and adsorbs to the sample. This process of diffusion and adsorption can be modelled by a time-dependent finite-difference simulation using the steady-state as the initial conditions. The modelling contains no variable parameters, the adsorption kinetic parameters can be determined from equilibrium and desorption experiments, which makes the methodology a severe test of the model used.

Results / Outstanding Aims

I have carried out a thorough study of the adsorption kinetics of Cetylpyridinium Chloride (CPC) to a hydrophilic silica interface both in the presence and absence of excess background electrolyte. This study has served to validate the flow cell technology as well as yielding detailed information on the adsorption kinetics CPC.

I am now working to synthesise a pair of model hydrophobic interfaces for use in the flow cell. I have chosen to approach this by using the chemistry developed in the last 15 years to functionalise Si(111) interfaces with covalently tethered alkyl monolayers. I hope that this work will yield information on how sensitive surfactant adsorption kinetics are to changes in surface chemistry.

Project Funding

This project is sponsored by the EPSRC in conjunction with an industrial CASE award from Syngenta (Huddersfield).

Publications

T. D. Curwen, C. D. Bain, J. K. Eve "Adsorption Kinetics in a Dual-Inlet Channel Flow Cell: II. Cetyl Pyridinium Chloride on Methyl and Methyl Ether Surfaces" Journal of Physical Chemistry C 2007, 111, 12305-12314 (DOI).

T. D. Curwen, J. A. Warner, C. D. Bain, R. G. Compton, J. K. Eve "Adsorption Kinetics in a Dual-Inlet Channel Flow Cell: I. Cetyl Pyridinium Chloride on Hydrophilic Silica" Journal of Physical Chemistry C 2007, 111, 12289-12304 (DOI).

T. D. Curwen and C. D. Bain "Kinetics of Adsorption of Cetylpyridinium Chloride at the Silica–Water Interface Measured Under Well-Defined Hydrodynamic Conditions" Poster Presentation, 19th ECIS Conference, Geilo, Norway, 2005.

Interests

My main interest outside the lab at the moment is teaching 1st year undergraduate Chemistry students Maths, which I do at Wadham and Magdalen Colleges in Oxford in order to help fund my research habit. When I’ve got a bit of spare time on my hands I like to get out on my road bike and disappear off to the Chiltern or Cotswold hills and reminisce about the days when I had enough time to train hard enough to be almost competitive in races.