Aqueous Phase Effect on Nanoemulsions Properties – Choose Your Media Wisely!

December 2019 | By Dr. Natalia Sannikova, Senior Formulation Scientist at Ascension Sciences

Cannabinoids (CBD, THC, etc.) are hydrophobic oily substances and have a very low solubility in water (2.8 mg/L at 23°C) and are well soluble in most organic solvents. They can, however, be formulated to be water compatible and appear water soluble. One type of such a formulation is a nanoemulsion.

An emulsion is a mixture of two or more liquids that are normally immiscible; dispersions of oil in water (o/w) or water in oil (w/o). Nanoemulsions are thermodynamically unstable transparent or translucent nanosized emulsions stabilized by surfactants having the droplet size 10–100 nm. ‘Thermodynamically unstable’ refers to the fact that liquids in the mixture will become separated eventually. Nanoemulsion formulations can be beneficial in several ways: not only do they provide water compatibility but also increase bioavailability of cannabinoids while being biocompatible and biodegradable. They are also easier to prepare compared to other nanoparticle formulations (liposomes and polymer nanoparticles).

One of the important factors to consider when preparing nanoemulsions is aqueous phase composition. How does it affect nanoemulsion properties? Salt concentration, pH of the solution and presence of other additives can exhibit different modes of action including changing the viscosity of the continuous phase and coating of individual oil droplets, hence reducing coalescence and prolonging shelf life of the product. Or, otherwise, promoting flocculation and reducing stability and storage time of emulsion systems.

Aqueous media choice can be based on intended application (pharmaceutical, nutraceutical, cosmetics, agriculture, food, …), route of administration of the final product (oral, buccal, pulmonary, nasal, transdermal, ….) and other criteria. Furthermore, for in vivo applications it’s important to maintain physiological osmolarity.

Based upon these considerations we have chosen three different variants of media: deionized water, phosphate buffer saline pH = 7.4 and isotonic sucrose solution.

Materials and Methods

Emulsions were prepared with Tween 80 and Span 80 at 7:3 ratio (HLB = 11.79) and hemp oil. Surfactants and oil have been dissolved in absolute ethanol (1:1 ratio, 10 mg/mL). Appropriate aqueous phase and organic phase were rapidly mixed using the NanoAssemblr Benchtop microfluidic instrument at flow rate ratio 4:1 (aqueous : organic) and total flow rate of 4 mL/min. Ethanol has been removed by dialysis. Particle size and integrity were measured using dynamic light scattering (Zetasizer, Malvern Instruments, UK). Formulations have been prepared in triplicate; size and PDI represented as the mean of 3 samples, error bars represent standard deviation of the mean.

With the same formulation parameters, droplet size differs drastically depending on the aqueous phase. The PBS emulsion has the largest droplets of 147 nm after preparation and is the least stable where we observed complete aggregation after 1 week. Particles formed in water and sucrose solution are smaller and emulsions are more stable.

The effect of PBS buffer can be explained by interactions of ions (sodium and potassium cations and phosphate anions) with non-ionic surfactants (Tween 80 and Span 80). Many simple electrolytes, such as sodium or potassium salts, decrease the solubility of non-ionic surfactants (‘salting-out’) and decrease the stability of emulsions stabilized by non-ionic surfactants. In the case of non-ionic surfactants, the stabilization force between emulsion drops is assumed to be steric. Steric forces are composed of an osmotic (mixing) contribution and an elastic one. Distilled water and isotonic sucrose solution have different osmolarity which can lead to the particle size differences. Non-ionic surfactant distribution can also depend on the nature of the aqueous phase and hence cause formation of the different size droplets depending on the media used.

Choose your media wisely!