Objectives The aim of this review is to highlight relevant considerations when implementing a rational strategy for the development of lipid and surfactant based drug delivery system and to discuss shortcomings and challenges to the current classification of these delivery systems. We also aim to offer suggestions for an improved classification system that will accommodate lipid based formulations that are not currently accommodated in the lipid formulation classification system. Key findings When categorising lipid and surfactant based drug delivery systems, the current Lipid Formulations Classifications System is a useful tool. However, it does not apply to all marketed lipid and surfactant systems or those reported in research papers. A more profound understanding of the functionalities of lipids and surfactants and their role in emulsion formation will enable a rational development strategy and will create the basis for a revised classification system encompassing all employed lipid and surfactant drug delivery systems. Summary The ever-increasing number of poorly soluble compounds in drug discovery and development calls for the serious need for effective and affordable drug delivery strategies that will enhance bioavailability and decrease variability. Lipid and surfactant based drug delivery systems offer these advantages; however, the development of these systems requires proper understanding of the physicochemical nature of the compound as well as the lipid excipients and gastrointestinal digestion. One major challenge of lipid excipients and delivery systems is the varying range of compounds they contain. This has contributed to the challenge of proper characterisation and evaluation of these delivery systems, their stability, classification and regulatory issues, which consequently have affected the number of these formulations that eventually reach the market. Suggestions as to proper classification of these delivery systems based on their main lipid component and recommended use are put forward. The prospect of these delivery systems looks promising.

Room-temperature ionic liquids (ILs) have been demonstrated to absorb SO(2) efficiently. However, after absorbing a large amount of SO(2), the viscosity, the conductivity, and the density of the ILs have not been studied systematically, and the mechanism of the interaction between SO(2) and ILs is still being disputed. In this work, two kinds of ILs (task-specific ILs and normal ILs) have been studied to absorb pure SO(2) at atmospheric pressure. It is found that the viscosity, the conductivity, and the density show different behaviors between task-specific ILs and normal ILs. For the task-specific ILs to absorb SO(2), before a 0.5 mol ratio of SO(2) to IL, the viscosity and density increase, and the conductivity decreases with an increase of the mole ratio of SO(2) to IL. After that, the conductivity and density increase, and the viscosity decreases with further increasing the mole ratio of SO(2) to IL. However, for the normal ILs, the conductivity and density increase and the viscosity decreases with an increase of the mole ratio of SO(2) to IL. A new mechanism of ILs absorbing SO(2) has been proposed. Task-specific ILs can chemically absorb SO(2) when the mole ratio of SO(2) to IL is not more than 0.5, and they can physically absorb SO(2) when the mole ratio is more than 0.5. The normal ILs can only physically absorb SO(2).

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