Emulsions

Details Emulsions Emulsions

2001-06-05

2004-03-23

Lovering, Richard D. (Department: 1712)

Colloid systems and wetting agents; subcombinations thereof; pro

Continuous liquid or supercritical phase: colloid systems;...

Aqueous continuous liquid phase and discontinuous phase...

C504S363000, C504S364000, C504S365000, C514S941000, C514S942000, C516S053000, C516S067000, C516S069000, C516S071000

Reexamination Certificate

active

06710092

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to emulsions of the nano and micron size and to a process for their production. More particularly, this invention relates to emulsified droplets of a liquid material which is substantially insoluble in water, and where the stabilising interface has surface active agents incorporated therein, thereby forming an interface having a number of advantages. Further, this invention relates to the processes for the production of such emulsions and methods for their use.
BACKGROUND OF THE INVENTION
The use of emulsions is well known in the chemical art, including the pharmaceutical, specialty chemical and agricultural industries. In agriculture, emulsions provide appropriate formulations vehicles for delivery of herbicides, insecticides, fungicides, bactericides and fertilizers. Non-agricultural uses have included formulation of dyes, inks, pharmaceuticals, flavoring agents and fragrances.
Surfactants are required to aid in the emulsification process of oil into water (and vice versa) and to stabilise the thus formed emulsion against physical degradation processes.
A surfactant can adsorb to (and be desorbed from) an interface relatively easily. This process can lead to destabilisation of an emulsion; moreover, micellisation effects will also lead to redistribution of components throughout the system, often leading to Ostwald ripening and other undesirable interactions. All the many approaches to stabilisation of emulsions rely on one or more of the above described effects with typical surfactant adsorption as the stabilising mechanism.
Oil-in-Water emulsions (EW's) consist of a dispersion of oil droplets in a continuous aqueous medium. Such products are widely employed and encountered in various industries e.g., food (e.g. mayonnaise), detergency (e.g. removal of oil deposits), pharmaceuticals (e.g. drug administration), cosmetics (e.g. skin creams) and agricultural products (both as concentrates and diluted into water for application).
EW's are important in agriculture as a means of formulating oil-based systems in a more environmentally attractive form than the conventional Emulsifiable Concentrate (EC) where less solvent is sometimes possible per unit active ingredient and also as a precursor to Suspension-Emulsions (SE's) or Suspo-Emulsions which consist of a mixture of an oil-in-water and a suspension concentrate (SC). Such EW or SE products tend to have lower skin and eye toxicity ratings than the corresponding EC products as well as higher flash points and better compatibility with HDPE containers.
EW's, unlike EC's in the undiluted state, are only stable in the kinetic sense. This is because the system is inherently thermodynamically unstable and can only be formed non-spontaneously. This can be understood if one considers a large drop of oil, say with a volume of 1 ml, which is emulsified into many droplets each containing on average 0.001 ml. The interfacial area is greatly increased as a result of the sub-division of the bulk oil into much smaller units. This large interfacial energy is accompanied by a large surface energy that is given by the product of interfacial tension and increase in surface area &Dgr;A (where &Dgr;A=A
2
−A
1
with A
2
being the total area of the subdivided droplets and A
1
that of the bulk oil). In the absence of any absorbed molecules at the interface the interfacial tension &mgr;
SL
is relatively large and hence the interfacial free energy for creating the interface &Dgr;A&mgr;
SL
can be quite large. Thus the interfacial free energy opposes the process of emulsification.
It should be mentioned, however, that in an emulsification process, a large number of small droplets are formed and this is accompanied by an increase in the total entropy of the system. This increase in entropy facilitated the emulsification process although its value is relatively small compared to the interfacial free energy. From the second law of thermodynamics, the free energy of emulsification is given by the expression
&Dgr;G
form
=&Dgr;A&mgr;
SL
−T&Dgr;S
Conf
where &Dgr;S
Conf
is the configurational entropy term
In most dispersion processes &Dgr;A&mgr;
SL
>>−T&Dgr;S
Conf
and therefore &Dgr;G
form
is large and positive. Thus the process of emulsification is non-spontaneous and hence with time the droplets tend to aggregate and/or coalesce to reduce the total energy of the system.
To prevent flocculation and/or coalescence one needs to create an energy barrier between the droplets to prevent their close approach. This energy barrier is the result of the creation of a repulsive force that overcomes the ever present van der Waals' attraction. The balance between repulsive and attractive forces determines the stability of the system against flocculation and coalescence. Assuming one can arrange to achieve a sufficient barrier to prevent close approach between the particles (i.e. stability in the colloid sense) what other factors play a role in keeping the droplets uniformly suspended in the continuous phase? One of the most important factors is gravity, which can cause separation of the droplets into a compact layer of cream depending on the density difference between the droplets and the medium, and their size.
There are basically five ways in which the structure of a dispersion of liquid droplets in a continuous medium can change. These are summarised as follows:
1. No change in droplet size (or droplet size distribution), but build-up of an equilibrium droplet concentration gradient within the emulsion. In limiting cases, the result is a close packed array (usually random) of droplets at one end of the system with the remainder of the volume occupied by the continuous phase liquid. This phenomenon results from external force fields, usually gravitational, centrifugal or electrostatic, acting on the system. “Creaming” is the special case in which the droplets collect in a concentrated layer at the top of an emulsion. A parallel effect may be seen when the oil phase has a density greater than 1.00 such that the cream sediments on the bottom of the container rather than rising to the top of the container when the density of the oil phase is less than 1.00.
2. Again, no change in basic droplet size or distribution but the build-up of aggregates of droplets within the emulsion. The individual droplets retain their identity. This process of flocculation results from the existence of attractive forces between the droplets.
3. In which flocculated droplets in an aggregate in the bulk of the emulsion, or alternatively, droplets within a close-packed array resulting from sedimentation or creaming, coalesce to form larger droplets. This results in a change in the initial droplet size distribution. The limiting state here is the complete separation of the emulsion into the two immiscible bulk liquids. Coalescence thus involves the elimination of the thin liquid film (of continuous phase) which separates two droplets in contact in an aggregate or a close-packed array. The forces to be considered here are therefore the forces acting within thin-liquid films in general. These can be complex and varied. In mixed emulsion systems, for example, in which there are droplets of liquid 1 and also liquid 2 dispersed in a continuous phase of liquid 3, coalescence between liquid 1 and liquid 2 droplets only occurs if liquids 1 and 2 are miscible with each other. If they are immiscible, then either droplet adhesion or droplet engulfment occurs. In either case, the thin liquid film between the contacting droplets is eliminated. When two droplets approach each other, surface fluctuations result since the interface is deformable. The amplitude of these fluctuations may grow to a considerable extent such that droplet coalescence occurs. However, this growth is opposed by the interfacial tension gradients that result in film expansion during growth of fluctuation. As a result of film expansion, regions of relatively higher interfacial tension than the rest of the film are create

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