Supercritical C02 (SCC02) has attracted increasing attention as a promising alternative for volatile organic solvent s, due to its abundance, non-toxic, non-flammable, and non-hazardous properties which make C02 a strong candidate for use as a green solvent. However, SCC02 is generally a very poor solvent, especially for polar and high molecular weight solutes, which has largely limited the applications of this supercritical liquid as a processing medium. On the other hand, emulsions and microemulsions have been widely accepted as effective media to stabilize immiscible components, such as water and oil. Studies have been carried out extensively for water-in-scC02 (w/c) microemulsions over the last two decades, in particular, Small-Angle Neutron Scattering (SANS) has been recognised as a powerful technique characterise the self-assembly structures, and frequently applied to investigate such systems. In this thesis, the validity of contrast variation SANS (CV-SANS) has been examined in w/c microemulsions by employing mixtures of D20 and H20 as contrast agents. With this method, the core-shell structure of w/c microemulsions has been highlighted, which allows behaviour of the microemulsions and in particular, the properties of surfactant films to be studied in detail. In addition, a systematic study has also been conducted on the effects of amphiphilic additives known as 'hydrotropes' on surfactant films in both water-in-oil and water-in -C02 microemulsions. Based on these observations, a novel surfactant has been synthesised and applied to w/c microemulsions as an additive, which effectively triggers elongation of microemulsion droplets. This work takes significant step in both exploring the fundamental aspect of the self-assembly structures in C02, and also for the optimization of the physicochemical properties of such systems for potential applications.