Electrokinetic phenomena in liquid foams and their applications in electrophoretic separations

The separation of biological proteins from complex samples is a key step in modern pharmaceutical research. This thesis is concerned with the development of a highly novel method of electrophoretic separation, whereby liquid foams are used to provide deformable networks of micro and nanochannels with high interfacial areas. Electrophoresis in nanoscale systems provides unique separation opportunities, but nanochannel systems are difficult and costly to produce. Deformable foam films may present a simple and easy to manufacture alternative. Interactions between the gas-liquid interface and suspended analytes can be used to modify separation operations based on the properties of the gas-liquid interface. The ease of production of liquid foams may provide a significant advantage over contemporary electrophoresis methods utilising solid wall channels, and the ability of liquid foams to flow provide an alternative to existing free flow electrophoresis methods. There are significant challenges with using liquid foams to accomplish electrophoretic separations, and this thesis lays the foundation for the establishment of foam electrophoresis techniques in four stages.

The first stage addresses issues regarding foam stability under electric field. The stability of horizontally oriented 2D liquid foams stabilised by anionic, cationic, non-ionic and zwitterionic surfactants under external electric field is investigated. By changing surfactant type, and through changing the solid material of the experimental device, the effects of the gas-liquid and solid-liquid interface are individually probed. A method for the quantification of foam stability under microgravity is devised, using time-lapse imaging to record the response of foam to external electric fields of varying strength. The presence of the electric field affected the stability of all foams stabilised by ionic surfactants, with the specific effect on foam stability varying depending on the surfactant type, electric field strength and the solid material used to construct the experimental device. Foam stabilised by cationic surfactant myristyltrimethylammonium bromide (MTAB) inside a glass cell exhibited increased foam stability under electric field, while all other cases trialled exhibited accelerated foam collapse with applied electric field. Numerical simulations using the Finite Element Method are used to gain insight into the velocity flow profiles inside the foam, and the effects of the gas-liquid and solid-liquid zeta potentials.

In the second stage, a foam electrophoresis device is designed and demonstrated in batch operation. Two fluorescent dyes, Rhodamine B and Fluorescein are used to demonstrate separation. The process is demonstrated using a range of anionic, cationic and non-ionic surfactants in order to demonstrate the effects of changing interfacial conditions on separation. Charged fluorescein exhibited visible migration, the direction changing depending on the charge of surfactant used and the local pH, while neutral rhodamine did not experience electrophoretic migration. The effects of varying electric field strength and initial pH were also investigated. Fluid motion created by electroosmosis and consequent backpressure, coupled with local pH changes created by electrochemical reactions resulted in complex effects on the dye separation. Separation effectiveness in this stage is analysed by visual inspection of the coloured dye regions.

In the third stage, the device developed in stage two is demonstrated using continuous foam flow. Liquid foam is generated using a mix of anionic and non-ionic surfactants, and continuously pumped through the device. Analytes suspended in the foam are continuously separated as the foam flows through the separation device. Here the ability of Free Flow Electrophoresis techniques to perform continuous electrophoretic separation is effectively combined with the ability to use charged interfaces to affect analyte dynamics, resulting in a highly novel device. Rhodamine B and Fluorescein are used to demonstrate the effectiveness of the device, and the concentration of analytes in the outlet is directly quantified. The device is demonstrated using a range of electric field strengths and volumetric flowrates. Fluorescein was concentrated up to 1.75 times its initial concentration, while Rhodamine B concentration was unaffected, demonstrating the potential for foam-based separations.

In the fourth stage, the methodology demonstrated in stage 3 is applied to the separation of two proteins, Immunoglobulin (IgG) and Wheat Germ Agglutinin (WGA). The method is demonstrated using multiple foams stabilized with a range of anionic, cationic and non-ionic surfactants, with the separation outcome heavily affected by the surfactant used.

The work presented in this thesis demonstrates the viability of liquid foams for electrophoretic separation operations, and opens up a very broad range of future possibilities to explore. While this work primarily deals with easily visible charged dyes, the methods described here have been demonstrated for the separation of proteins, with a wide range of possible configurations.