Structural and kinetic studies of surfactant and phospholipid monolayers at the air/liquid interface using sum-frequency generation spectroscopy

Change log
Goussous, Sami 

In this work, the non-linear optical technique of sum-frequency spectroscopy (SFG) has been used to investigate a variety of surfactant and phospholipid systems at different interfaces. Firstly, the effects of temperature and ionic strength on the adsorption of cationic and non-ionic surfactants were investigated. The binary surfactant system used here is a simplified hair conditioner model, consisting of a fatty alcohol and a long chain quaternary ammonium salt, self-assembled from solution onto a silica surface (which serves as a very approximate model of the hair surface). SFG was used to investigate and the effects of temperature and ionic strength on the conformational ordering of the adsorbed monolayer.

The structure of a tethered bilayer lipid membrane (tBLM) produced by different methods was investigated. The tBLM is a bilayer of phospholipids, which are tethered to a gold substrate by a self-assembled monolayer (SAM). The SAM is a mixture of disulfide tether molecules and disulfide spacer molecules, which form covalent bonds to the gold surface. Ideally, the tether molecules should inter-digitate into the phospholipid membrane and hold it to the gold surface. This type of system is often used as a robust model for a cell membrane. Here, SFG was used to investigate the structure of the SAM, and of the membrane when formed either by rapid solvent exchange, or by a combination of Langmuir-Blodgett and Langmuir-Schaefer deposition. Significant differences in the resulting spectra of the tBLM were observed, as a result of the fabrication technique. SFG was also used to confirm that the polar orientation of the phospholipid in the tethered membrane systems was as expected for a bilayer.

Two versions of the quaternary ammonium surfactant used previously were also investigated at the air/water interface. The only difference between the two surfactants was their counterion. The slow collapse of the compressed monolayer was monitored using SFG and by surface pressure measurements. Langmuir-Blodgett deposition onto glass and mica was used at different stages of the monolayer collapse; the resulting air/solid systems were investigated using SFG and atomic force microscopy. Two different methods of obtaining sum-frequency intensity data over time were evaluated, and the effect of the counterions on the kinetics of the slow collapse mechanism was discussed.

The structure of surfactin, a surface-active lipopeptide, was investigated at the air/water interface. Deuterated analogues of the bio-surfactant were produced from \textit{Bacillus Subtilis} by feeding the bacteria with deuterated glucose and/or deuterated leucine. These analogues were used to confirm spectroscopic assignments and make conclusions of the structure of surfactin at the air/water interface. The effect of the surfactin concentration, the pH of the subphase, and the addition of calcium ions to the subphase was also investigated.

The acquired knowledge of the structure of surfactin was then used to investigate the interaction of surfactin with a phospholipid monolayer. The surface pressure and sum-frequency spectra were monitored over time after the injection of surfactin into the subphase beneath a phospholipid monolayer compressed at the air/water interface. The surface pressure rose, indicating that surfactin penetrated into the monolayer, but it was found that sum-frequency signals arising from surfactin disappear in the final equilibrium system. Mechanisms and possible structures which are compatible with the data are discussed.

Davies, Paul
Chemistry, Surfactant, Sum frequency spectroscopy, SFG, Langmuir monolayer, Surfactin, Phospholipid, Quaternary ammonium surfactants, quats, Langmuir-Blodgett, Spectroscopy, monolayer, cell membrane model, surfactin phospholipid interaction, phospholipid monolayer
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
EPSRC Industrial Case award: jointly funded by Unilever and the Engineering and Physical Sciences Research Council.