Prof. Colin D. Bain
(email at firstname.lastname@example.org)
Chemistry of Surfaces and Interfaces
The research in my group can be loosely characterised as 'wet' surface chemistry. We are motivated by two general questions: What is the relationship between the microscopic structure of a thin film and the molecular structure of its constituent molecules? How do the microscopic properties of an interface determine the macroscopic behaviour of a system? Increasingly we are interested in kinetic processes on timescales from microseconds to seconds and in multi-component systems where interactions between different species lead to unexpected behaviour. Our focus is on fundamental physical chemistry, but the systems we study have potential applications in areas such as lubrication, detergency, printing, manufacturing engineering and oil recovery. Systems are chosen to be sufficiently complex to capture the essential behaviour of real applications, yet simple enough to permit a determination of structure and dynamics at an interface.
We use a wide range of experimental techniques, including evanescent wave Raman scattering, external reflection infrared spectroscopy, sum-frequency spectroscopy, neutron reflection, ellipsometry, tensiometry, optical tweezers and confocal microscopy. Flow cells, overflowing cylinders (right) and high-speed liquid jets are used to provide controlled hydrodynamics. The development of new methodology plays an important part in our research
Surfactants, lipids and polymers
Interactions between species in multi-component systems give rich and unexpected equilibrium and - especially - dynamic behaviour. The graph on the right shows the adsorption of a mixture of the anionic surfactant SDS and a cationic polymer at the expanding surface of an overflowing cylinder, measured by ellipsometry: the lower the reading, the more material is adsorbed at the surface. The complex behaviour arises from interactions between the polymer and surfactant aggregates that first give rise to surface-active complexes and then aggregates that diffuse too slowly to adsorb on the sub-second timescale of the overflowing cylinder.
Evanescent wave Raman scattering is a surprisingly sensitive technique for studying mixtures of surfactants or phospholipids at the solid-liquid interface. The plot on the left shows the displacement of the cationic surfactant CTAB from a silica surface by the non-ionic surfactant TX-100. A wall-jet cell and chemometric methods of data analysis permit quantitative measurements of kinetics in mixtures on 1-second timescales.
Liquid jets – micelles, Marangoni and manufacturing
Liquid jets provide a way of studying freshly created surfaces on millisecond and sub-millisecond timescales. Little is known about the adsorption kinetics of surfactants on such short timescales. One surprising observation is that surfactant micelles can adsorb directly to the nascent surface of a liquid jet. Near the jet surface, the solution can be very far from equilibrium with enormous consequences on the kinetics of breakdown of micelles. If surfactant adsorption is not uniform everywhere, surface tension gradients arise and cause flows in the bulk liquid adjacent to the surface. These Marangoni effects are important in stabilising foams, in spreading of droplets on solid surfaces, in the coalescence of droplets and in the breakup of jets. We are applying our understanding of liquid surfaces on short time scales to inkjet printing and to the manufacture of pharmaceuticals by liquid dosing technology.
To understand how lubricants work on a molecular level, we need ways of looking into the contact between two hard solids under pressures of thousands of atmospheres and at shear rates >105 s-1. Sum-frequency spectroscopy and Raman scattering have been exploited for this purpose: a circular contact is formed between a hemispherical prism and a sphere and lasers are focused into the small area of contact (the dark region in the centre of the Newton rings, left). Vibrational spectroscopy can tell us which organic molecules are in the contact and how they respond to pressure and shear.
Optical sculpting and binding
Light is an amazing tool for controlling matter on the micro and nanoscale. The scattering of light can induce attractive and repulsive interactions between particles leading the spontaneous organisation of sub-micron polymer spheres into organised arrays (such as the 640-nm diameter PVP spheres on the right).
Droplets of oil in water can be picked up and moved with tightly focussed laser beams (optical tweezers) but also deformed into other shapes, such as triangles or squares. When an emulsion droplet is divided into two smaller droplets, they remain connected by a thread of oil that is stable and can be used as a channel to pump liquid from one droplet to the other. The picture on the left shows a 'mother' drop of hexane connected by invisible oil threads to three smaller 'daughter' droplets about a micron in diameter.
- C. D. Bain: "Sum-Frequency Vibrational Spectroscopy of the Solid-Liquid Interface" J. Chem. Soc. Faraday Transactions, 1995, 91, 1281.
- Q. Lei, and C. D. Bain: "Surfactant-Induced Surface Freezing at the Alkane-Water Interface" Phys. Rev. Lett., 2004, 92, 176103.
- C. Lee and C. D. Bain: "Raman Spectroscopy of Planar Supported Lipid Bilayers" Biochim. Biophys. Acta, 2005, 1711, 50.
- D. M. Colegate and C. D. Bain: "Adsorption Kinetics in Micellar Solutions of Nonionic Surfactants" Phys. Rev. Lett. 2005, 95, 198302
- C. D. Mellor and C. D. Bain: "Array Formation in Evanescent Waves" ChemPhysChem, 2006, 7, 329.
- A. D. Ward, M. G. Berry, C. M. Mellor and C. D. Bain: "Optical Sculpture: Controlled Deformation of Emulsion Droplets with Ultralow Interfacial Tensions using Optical Tweezers" Chem. Comm., 2006, 4515.
- C. D. Bain: "The Overflowing Cylinder Sixty Years on" Adv. Colloid Interface Sci. 2008, 144, 4.
- E. C. Tyrode, M. W. Rutland and C. D. Bain: "Adsorption of CTAB on Hydrophilic Silica Studied by Linear and Nonlinear Optical Spectroscopy" J. Am. Chem. Soc. 2008, 130, 17434.