Colloidal systems are the most attention demanding system across the globe. They are known as biphasic system which is oriented in such a manner that is not visible to differentiate visually. Some liquid-liquid colloidal systems which are in general called as “Emulsion” are very much difficult to identify as biphasic system due to their stability. Their stability decrease with the time and are converted to the biphasic system from monophasic system. They are defined as the two phase system comprising of one liquid dispersed into the other liquid in which both the liquid systems are immiscible with each other. One phase is called as dispersed phase which resides into the other liquid in form of tiny droplets whose size ranges from few micrometers to nanometers.
Emulsion
Apart from all the various colloidal systems, liquid-liquid colloidal systems are mostly studied. These systems are often termed as “Emulsion” systems which are dispersion of oils in aqueous phase. Various reliable sources define emulsion as following:
According to the Merriam-webster dictionary, emulsion is “The system which consists of liquid dispersed with or without an emulsifier in an immiscible liquid usually in the droplets larger than the colloidal size.”
According to the Cambridge online dictionary, emulsion is “A mixture that results when one liquid is added to another and is mixed with it but does not dissolve into it.”
According to the Oxford dictionary, emulsion is “A fine dispersion of minute droplets of one liquid in another in which it is not soluble or miscible.”
Scientifically, emulsion is just a mixture of two immiscible phase in which dispersed phase is in form of tiny droplets in continuous phase. They are highly unstable systems as they have very high surface tension and droplets have high affinity for agglomeration and coalescence. As a result there is very lesser chance of emulsion remaining stable for longer period of time. They get immediately phase separated due higher surface tension. They are considered to be the most ancient in liquid-liquid colloidal systems. Best example of emulsion is milk which is mixture of fats and aqueous system.
Microemulsion
The microemulsion is the similar system as that of emulsion with exception of instability of emulsion system. To eradicate the instability of emulsion systems and make them stable for long period of time, system of surfactant and co-surfactant were selected to incorporate into the emulsion, making the whole system completely stabilized. A surfactant system forms the flexible film on the interface between oil and water thereby decreasing the interfacial tension to a very lower value between both the surfaces. As a consequence, the system as a whole becomes very stable and can be stored for longer period of time. These are called as microemulsion system.
According to the Merriam-webster dictionary, microemulsion is defined as “An emulsion in which the dispersed phase is in the form of very small droplets usually produced and maintained with the aid of surfactants and having diameters of from 50 to 500 angstroms.”
According to Oxford dictionary, microemulsion is defined as “An emulsion in which the droplet size is very small or relatively small.”
According to the IUPAC, microemulsion is defined as “Dispersion made of water, oil, and surfactant(s) that is an isotropic and thermodynamically stable system with dispersed domain diameter varying approximately from 1 to 100 nm, usually 10 to 50 nm.”
According to the Danielsson and his research team (1981), microemulsion is defined as “A system of water, oil and amphiphile which is a single isotropic and thermodynamically stable liquid solution.” The team is considered to be one of the initial researchers is the field of microemulsion. Their experience and understanding of the microemulsion system led to a definition of system as mentioned above.
Understanding of microemulsion from various sources indicates that it is the transparent, homogenous system with single phase comprising of oil, water and emulsifiers as stabilizers. Generally droplets size range is below 100 nm in case of microemulsion. According to Hoar and Schulman (1943), microemulsion should get spontaneously generated due to considerable amount to stabilizers in form of emulsifiers. In fact, any transparent homogenous single phase emulsion system which can be prepared spontaneously can be called as microemulsion as no shear is required to generate the nano droplets.
Nanoemulsion
As the world of science moves ahead, more and more innovations and newer developments are engulfing the current technologies in each and every sector. This era of 21st Century is of nanotechnology and various sectors of industries are marching towards nanotechnological advancement. Pharmaceutical & healthcare sector is one of the largest consumers of nanotechnology called as nanomedicine. Specifically drug delivery technologies are emerging with a latest advancement at each stage. Nanoemulsion is also a part of such innovation for liquid-liquid colloidal system. It can also be called as advancement of microemulsion based systems.
The structural configuration, some physical properties and basic platform of formulation is similar in both the systems which are the mainstay for creating the confusion between both the systems. Nanoemulsion is long lasting efforts of formulation scientists for making the microemulsion better and restricts its limitations. The nanoemulsion is formulated with the similar components as of microemulsion except co-surfactant. It uses the application of external energy for generation of nano droplets rather than large amount of surfactant. Microemulsion uses extreme levels of surfactant altogether with co-surfactant that nanoemulsion does not. In fact, nanoemulsion should be formulated without surfactant, if possible or with minimum surfactant which leads to less toxicity to the human biological membrane. Formation of nanoemulsion uses the application of high energy using mechanical devices namely high pressure homogenizers, microfluidizers, ultrasonicators etcetera. These systems have kinetic stability due to brownian motion of the nano droplets therein. Their appearance is transparent, preferably translucent with bluish tint. However, their overall stability is compromised due to reduced surfactant concentration which understandable.
However, there are many terms which are almost similar to the nanoemulsion. The concepts behind this terms which reflect the ideas of the scientist at source are nearer to similarity. Often nanoemulsion was termed as submicron emulsion in 1990s. Further, it is also called as ultrafine emulsion, mini emulsions. The meaning of all the terminologies are one and the same reflecting nanoemulsion based drug delivery systems. Old literatures of nanoemulsion are the evidence of overlapping terminologies. Looking at the technical depth of the old research papers encompassing the terms “ultrafine” or ”submicron” or “mini” emulsions one can find the research done in that period was exclusively for the system, which is now called as “nanoemulsion”.
Conclusion
This article covers information related to the colloidal system specifically liquid-liquid systems. For a formulation scientist, it is much important to learn about the similarities and differences present between this all overlapping terminologies namely emulsion, microemulsion, and nanoemulsion. Innovation is not only developing the newer things all the times but also to understand the limitations of currently running technologies and overcoming those in future times. Hence, there are very minute differences between the innovations of similar categories like that of emulsion, microemulsion and nanoemulsion which needs to be cleared in the minds of formulation scientist to get best understanding of technology gaps colloidal systems and move forward with advancement in the current technology of liquid-liquid colloidal systems.
(Author is ex-research fellow, A. R. College of Pharmacy and G. H. Patel Institute of Pharmacy, Mota Bazaar, Vallabh Vidyanagar, Gujarat 388 120)