DEFINITION AND PROPERTIES
Microemulsions are especially well suited to active ingredients used at low application rates, low active content formulations, wood treatment or when a clear solution is required at the application rate.
A microemulsion is a clear, thermodynamically stable mixture of at least three components:
a hydrophobic component (non water-soluble oil) which could be a liquid, a low melting solid (less than +50 °C), or a solid dissolved in an organic solvent.
a surfactant system
A cosurfactant (cosolvent) is often added to the mixture to increase the solubilising power of the surfactant system.
Different substances can be used as cosurfactants, mainly alcohols, amines or ether-alcohols. Differently to an emulsion, the characteristic size of the hydrophobic or the hydrophilic domains is typically 10 nm. In fact, microemulsions are a particular type of colloidal system. The typical dimension of the local structure explains why microemulsions are transparent.
Phase diagrams are very useful to describe such systems.
This simplified ternary representation describes the different phases obtained when oil, surfactants and water are blended.
Each corner of the phase diagram represents a pure compound. Each side represents the different compositions of a blend of two components and a point inside the diagram represents the composition of a blend of the three components. These diagrams are often complex.
When the water content of a microemulsion is low (near the oil corner of the phase diagram) the local structure consists of swollen inverse micelles. The oil phase is continuous and water micellar droplets are the dispersed phase. Surfactant and
co-surfactant molecules are disposed on the interface between water and the hydropho­bic phase (oil). As the
water content increases, the shape and the volume of the hydrophilic core of inverse micelles expands. At a given water content, there exists a continuous water path; the water phase becomes continuous. The water and the oil phases make an interpenetrated bi-continuous network. Near the water comer, the oil content is low and then the microemulsion looks like a direct micellar solution. The structure of these different systems can be studied or characterized by low angle X rays of neutron scattering, rheological or conductivity measurements.
GENERAL METHOD OF PREPARATION
The composition of a microemulsion is usually:
Active ingredient(s) (+ solvents); 100 to 700 g/l
Surfactants 100 to 300 g/l
Cosurfactants 0 to 200 g/l
Antifoam, salts, antifreeze or buffer 0 to 100 g/l
Water up to 1000 ml
Antifoams, salts, antifreezes or a buffer could also be added.
Defining a suitable active ingredient
Microemulsions are suitable for liquid active ingredients. A low melting point or solid active ingredient must first be solubilised
in an appropriate solvent. Active ingredients have to be chemically stable in water.
The criteria for the choice of a solvent are the following:
high flash point and not toxic
high solubilising properties for the active ingredient to avoid crystallizations during storage
Effect of surfactants
The process to obtain microemulsions (ME) is simpler than those used to prepare emulsions (EW). Since a microemulsion is a thermodynamically stable phase, the result is absolutely independent of the way of preparation.
Microemulsification is spon­taneous. Water in oil (w/o) microemulsions, (near the oil-surfactant border of the phase diagram) are easier to formulate than oil in water (o/w) microemulsions. This difference arises from the energy difference between solubilising water in oil and solubilising oil in water. The water in oil process is thermodynamically more favourable.
Surfactants are basic components in microemulsions. Nevertheless, it is somehow difficult to find the appropriate surfactants and cosurfactants (surfactant system) to obtain stable formulations over a large temperature range (-10 °C to +54 °C). Microemulsions require quite high concentrations of surfactants (typically 2% to 30%). This high amount is due to the small size of the oil and water domains and so to the large area of the interface. Nevertheless this amount can be reduced by optimizing the choice of the surfactant and co-surfactant. There are no real definitive guide rules that one may use, but we recommend to use a blend of nonionic and anionic surfactants to guarantee the physical stability vs temperature.
This effect is illustrated by the following diagram. It represents the affinity of a surfactant (nonionic or anionic) with oil or water at different temperatures.
For example at high temperature a nonionic surfactant has a better affinity with oil (area b), at a lower temperature it has a better affinity with water (area a). It is the contrary for an anionic surfactant.
By blending nonionic and anionic surfactants it is possible to combine these effects and obtain microemulsion stable over a large range of temperature. The ratio between these emulsifiers must be optimized in order to obtain the best thermal stability. The phase diagram below shows the influence of temperature on an optimized surfactant system for Cypermethrin.
Surfactants to make a microemulsion must be chosen according to the nature of the active ingredients.
Selection of a cosurfactant
A cosurfactant is often added to the mixture to increase the solubilizing power of the surfactant system. The best cosurfactants are, generally speaking, small molecules which have a great affinity for the Oil/Water interface. They have to be chosen according to the nature of surfactants and to the nature of the oil to be microemulsioned. They can also prevent
the formation of a viscous phase (lamellar phase etc.). The ratio R = Cosurfactant/Surfactants normally varies from 0.3 to 0.8.
This ratio and the nature of the cosurfactant have to be optimized in order to lower the surfactants content, to obtain
the best thermal stability and to avoid forming viscous phases.
This representation of a quaternary water-oil-alcohol-surfactants system in a tetrahedric phase diagram illustrates the influence of the cosurfactant on the nature of the different phases. The ratio R = 0.5 could be considered as the optimum ratio for this system.
The three following diagrams are the two-dimensional representation of ternary water-oil-surfactants systems for ratios
R = Cosurfactant/Surfactant of 0.25 and 0.5.
Different molecules (alcohols, amines, ether-alcohols etc.) can be used as cosurfactants. Light alcohols like isobutanol, propanol, butanol are suited for micro-emulsions. Nevertheless, their flash points are too low to be used in plant protection
LABORATORY FORMULATION AND INDUSTRIAL PRODUCTION
Microemulsions are thermodynamically stable formulations, the result is independent from the way of preparation. This point makes microemulsions very attractive from a manufacturing point of view. However we recommend to blend the organic phase (active ingredient with solvent if necessary) with surfactants and cosurfactant and then to add water mixing slowly.
The commercial formulations should be stable for at least 2 years. Microemulsions are thermodynamically stable:
it means that in a determinate range of temperature their physical stability is not affected by time if no chemical degradation occurs. Some accelerated aging tests give an indication of the long term stability:
tropical test: 2 weeks at +54 °C (CIPAC 1 -MT 46.1.3)
cold stability test: 1 week at 0 °C (CIPAC 1 -MT 39)
stability at high temperature for two months at +45 °C
stability to thermal shocks: samples in sealed opaque glass bottles are submitted to temperature cycles
(24 hours at -5 °C and 24 hours at +45 °C) for a one or two month period.
Microemulsions should remain clear. Chemical degradation of the active ingredient should be evaluated especially for low active content formulations.