D.Phil. Student, University of Oxford, 1995–1998
The Dynamics of Surfactant Adsorption
The study of the behaviour of surfactant solutions away from equilibrium is of considerable interest and numerous examples exist in both natural and industrial processes. This thesis describes the dynamics of adsorption of surfactants at the surface of an overflowing cylinder (OFC), which was used to create a continually expanding surface.
Where non-equilibrium conditions occur the surface excess and hence the surface tension depart from their equilibrium values. A surface tension gradient may be created at the interface, which can accelerate the surface flow: this is one example of the Marangoni effect. To improve our understanding of Marangoni flow and the dynamics of adsorption, we developed several experimental techniques to measure dynamic surface properties. Measurements of the dynamic surface tension were performed on the expanding surface of the OFC by three independent methods. Direct measurements were made with the Wilhelmy plate (invasive) and surface light scattering (non-invasive). The coefficient of ellipticity, was also measured by ELLIPSOMETRY and concerted to surface tension by a simple method of calibration. Ellipsometric measurements could be made from the centre of the cylinder to the outer rim at 1 mm intervals. The dynamic surface excess was measured by NEUTRON REFLECTION. LASER DOPPLER VELOCIMETRY was developed to measure both the surface velocity and to profile the horizontal velocity component below the surface. These techniques together provided very powerful tools to probe the expanding surface. Direct measurements of the surface tension (surface light scattering) and surface excess used in combination with ellipsometric data enables the analysis of the variation of the surface properties over the surface of the OFC.
A model is presented for the mass transport of surfactants to the expanding surface of the overflowing cylinder. The model is used to determine the sub-surface concentrations of the different surfactants from the experimental measurements of surface excess and surface tension. The experimental results are compared to the calculated values of the sub-surface concentration. Good agreement was found for both non-ionics studied and the cationic in excess electrolyte, the calculated values lay above the experimental values of the sub-surface concentration, suggesting some form of barrier to adsorption.
The last section of the work described the measurement of the dynamic ellipticity at the surface of a liquid jet. The jet operates on a faster time-scale than the OFC, 1 to 20 milliseconds compared to 0.1 to 1 seconds, respectively.
Finally, future directions for this field of research are discussed.
C. D. Bain, S. Manning-Benson and R. C. Darton "Rates of Mass Transfer and Adsorption of Hexadecyltrimethylammonium Bromide at an Expanding Air–Water Interface" Journal of Colloid and Interface Science 2000, 229, 247–256 (DOI).
J. Hutchison, D. Klenerman, S. Manning-Benson and C. Bain "Measurements of the Adsorption Kinetics of a Cationic Surfactant in a Liquid Jet by Ellipsometry" Langmuir 1999, 15, 7530–7533 (DOI).
S. Manning-Benson, S. R. W. Parker, C. D. Bain and J. Penfold "Measurement of the Dynamic Surface Excess in an Overflowing Cylinder by Neutron Reflection" Langmuir 1998, 14, 990–996 (DOI).
G. R. Bell, S. Manning-Benson and C. D. Bain "Effect of Chain Length on the Structure of Monolayers of Alkyltrimethylammonium Bromides (CnTABs) at the Air–Water Interface" Journal of Physical Chemistry B 1998, 102, 218–222 (DOI).
S. Manning-Benson, C. D. Bain, R. C. Darton, D. Sharpe, J. Eastoe and P. Reynolds "Invasive and Noninvasive Measurements of Dynamic Surface Tensions" Langmuir 1997, 13, 5808–5810 (DOI).
S. Manning-Benson, C. D. Bain and R. D. Darton "Measurement of Dynamic Interfacial Properties in an Overflowing Cylinder by Ellipsometry" Journal of Colloid and Interface Science 1997, 189, 109–116 (DOI).