Microemulsions versus nanoemulsions: A comparative overview of features, formulation, and pharmaceutical applications.

Microemulsions and nanoemulsions are two prominent types of colloidal dispersions utilized in the pharmaceutical sector for the delivery of hydrophobic drugs. These systems typically contain small droplets (d < 200 nm), which enhances their stability and improves the bioavailability of the encapsulated molecules. Besides pharmaceuticals, they find diverse applications across various fields, including nutraceuticals, food, cosmetics, biomedicine, enhanced oil recovery, and material synthesis, among numerous others. Despite their widespread use, confusion persists regarding the distinctions between these two systems, often leading to the interchangeable use of the terms 'microemulsion' and 'nanoemulsion'. This misnomer is increasingly common in the scientific literature, resulting in confusion and inaccurate reporting. Microemulsions are thermodynamically stable, while nanoemulsions are only kinetically stable. Consequently, microemulsions form spontaneously upon mixing the components and should remain stable indefinitely. In contrast, nanoemulsions require the input of external energy to form them and tend to break down over time. This review aims to compare microemulsions and nanoemulsions across various aspects to clarify misconceptions and the terminology associated with these colloidal dispersions. It includes an elucidation of their types, droplet characteristics, stability, formulation aspects (composition and fabrication methods), pharmaceutical applications, regulatory overview, and recent advancements. Practical approaches for distinguishing microemulsions and nanoemulsions, including evaluating their long-term stability, assessing the reversibility of structural changes upon temperature variation, and examining the order of component mixing, are presented. This review provides valuable insights into the similarities and differences between microemulsions and nanoemulsions, which have direct implications for effective drug delivery.