Understanding the interactions between vesicles within a vesicle dispersion is key to understanding the stability of the dispersion. If the interactions between the vesicles are sufficiently repulsive the system will be stable. Whilst this behaviour is relatively easy to predict for simple colloid and vesicle dispersions. In complex systems, containing various additives such as salts, polymers, and polyelectrolytes, the interactions effecting the stability of the dispersion are more complex.

In this study methodologies for directly measuring the interactions between two supported cationic surfactant bilayers using the colloidal probe atomic force microscopy technique were successfully developed.

The surfactant vesicle system of interest in this work was the double tailed, cationic surfactant DIPEDMAMS, often used in commercial fabric softeners. The system was observed to form two co-existing lamellar phases which melt to forma a single fluid lamellar phase at around $~$\SI{40}{\degreeCelsius}. The addition of different alkyl alcohols was used to modify the transition temperature to create supported bilayers for the colloidal probe technique.

Electrolytes were observed to decrease the repulsive interactions between the bilayers with the identity of the salt determining the strength of the effect. Polyelectrolytes whilst having a similar impact as to the salts the depletion attraction was also observed and shown to be dependent on the molecular weight of the polymer. The interactions with uncharged polymers were complex due to the absorption of the polymer to the bilayers as observed by AFM imaging. The addition of fatty acids to simulate the breakdown of the surfactant molecules within the bilayers exhibited complex behaviour dependent on the nature of the fatty acid in question. All these measurements were supported by AFM imaging and x-ray scattering to decouple changes to the supported bilayers and the interactions between them. Finally, direct force measurements were compared to the behaviour of the bulk vesicle dispersions.