Preparation methods

As said, NEs are heterogeneous system consisting of at least two immiscible liquid phases. Consequently, they do not form spontaneously and their properties depend not only on the thermodynamic conditions such as composition, temperature or pressure, but also on the preparation method and the order of addition of components. The structure of an emulsion generally consists of droplets of the dispersed phase (or internal) in a continuous phase (or external), stabilized by a surfactant, as illustrated in Figure 2.1. This structure decisively determines the physical properties and, thus, the functionality and quality of the emulsion. In the preparation of emulsions, from both fundamental and technological point of view, it is fundamental to obtain a desired droplet size and a narrow size distribution [73].

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Figure 2.1. Typical structure of an emulsion.

Being non-equilibrium systems of structured liquids, NEs preparation typically involves the input of a large amount of either energy or surfactants [66].

The high-energy method utilizes mechanical equipments, such as ultrasonicators, microfluidizer and high pressure homogenisers, to create intensely disruptive forces which break up the oil and water phases to form nano-sized droplets. Final particle size here will depend on the type of instruments employed and their operating conditions like time and temperature along with sample properties and composition. This method allows for a great control of particle size and a large choice of composition, which in turn controls the stability, rheology and colour of the emulsion.

NEs can also be prepared by a low-energy emulsification method, which has been recently developed according to the phase behavior and properties of the constituents, to promote the formation of ultra-small droplets. These low-energy techniques include phase inversion temperature (PIT) technique, solvent displacement methods and phase inversion composition (PIC) method. This emulsification can be brought about by changing the parameters which would affect the hydrophilic lipophilic balance of the system like temperature (in the case of PIT), composition, etc.. The low energy method is interesting because it utilizes the stored energy of the system to form small droplets. Moreover, in the case of solvent displacement method and PIC, emulsification conditions are quite gentle making possible the use of thermolabile drugs such as retinoids and macromolecules, including proteins, enzymes and nucleic acids. However, also in the high energy process temperature can be controlled.

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2.1.2.1. Low energy emulsification

PIT method employs temperature dependent solubility of non ionic surfactants like polyethoxylated, which become hydrophobic above certain temperature allowing therefore a formation of water in oil emulsion due to the acquired solubility of the surfactant in the oil. However, it typically uses high temperature to create the phase inversion.

In the case of solvent displacement, the oily phase is dissolved in water-miscible organic solvents, such as acetone, ethanol and ethyl methyl ketone and it is poured into an aqueous phase containing surfactant to yield spontaneous NE by rapid diffusion of organic solvent. The organic solvent is removed from the NE by a suitable means, such as vacuum evaporation. Spontaneous nanoemulsification were also reported when solution of organic solvents containing a small percentage of oil is poured into aqueous phase without any surfactant. A major drawback of this method is the use of organic solvents, such as acetone, which require additional inputs for their removal from NE. In particular, a high ratio of solvent to oil is required to obtain NEs with a desirable droplet size.

PIC method has drawn a great deal of attention from scientists in various fields (including pharmaceutical sciences) as it generates NEs at room temperature without use of any organic solvent. Kinetically stable nanoemulsions with small droplet size (~50 nm) were generated by the stepwise addition of water into solution of surfactant in oil, with gentle stirring and at constant temperature. The spontaneous nanoemulsification was related to the phase inversion.

The main limitation of low energy techniques is that they pose several difficulties during scale- up. It has been demonstrated this capability in the case of PIC even though more experimentation should be done at higher scales in order to prove it [74].

2.1.2.2. High energy emulsification

One technique for producing NEs is based on the use of an immersion sonicator. The emulsification power of ultrasound has been known and applied for long. The ultrasonic power is adjustable and can be adapted to particular products and emulsification requirements. Highly intensive ultrasound provided by a vibrating metal probe supplies the power needed to disperse a liquid phase in small droplets in a second phase continuous phase. In the dispersing zone, imploding cavitation bubbles cause intensive shock waves in the surrounding liquid and result in

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the formation of liquid jets of high liquid velocity. The most important aspect is to stabilize the newly formed droplets of the dispersed phase against, for example, coalescence in order to maintain the final droplet size distribution at a level that is equal to the distribution immediately after the droplet disruption in the ultrasonic dispersing zone (coalescence of the droplets after disruption influences the final droplet size distribution). Studies on emulsions have shown correlation between the energy density and droplet size. There is a clear tendency for smaller droplet size at increasing energy density. At appropriate energy density levels, ultrasound can well achieve a mean droplet size below micrometer scale. However, this method presents several limitations mainly due to the polydispersion, the difficulty to scale up to large volumes and also in terms of minimum size of the dispersion which can be reached. To make possible NE preparation at industrial scale high pressure homogenization techniques are needed.

High pressure homogenization allows producing extremely low particle sizes due to several forces, such as hydraulic shear, intense turbulence and cavitation [66]. The resultant product can be re-subjected to high-pressure homogenization until nanoemulsion with desired droplet size and polydispersity index is obtained. The smaller droplet size the higher the energy thus the pressure required. The emulsion is preferably prepared at high volume fraction of the disperse phase and diluted afterwards. However, very high phase volume ratios may result in coalescence during emulsification, even though more surfactant could be added to create a smaller reduction in effective surface tension and possibly diminishing recoalescence. If possible the surfactant is dissolved in the disperse phase rather than the continuous phase; this often leads to smaller droplets. It may be useful to emulsify in steps of increasing intensity, particularly with emulsions having highly viscous disperse phase.

To the field of high pressure homogenizer it belongs also a patented mixing technology called microfluidization, which makes use of a device called microfluidizer. This device uses a high- pressure positive displacement pump (500 - 20,000 psi), which forces the product through the interaction chamber, consisting of small channels called “microchannels”. The principle of the technology consists of a liquid that is divided into two microchannels and then is recombined later in a reacting chamber where the jets of liquid collide together. This allows for reducing of the size of suspended particles, resulting in stable, uniform and consistent mixtures of product. The emulsion can be passed through the interaction chamber of the microfluidizer repeatedly until the desired particle size is obtained.