Interfacial tension miniemulsions

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

A third type of emulsion process is the so-called microemulsion [123]. In microemulsions, the polymerization starts in droplets as well. However, these are thermodynamically stable and, in contrast to miniemulsions, they form spontaneously by gentle stirring. They consist of large amounts of surfactants or mixtures of them, and they possess an interfacial tension close to zero at the water/oil interface, with droplet sizes usually ranging between 5 and 50 nm. In... 

Super-swelhng was postulated as a cause of instabihty in controlled free radical miniemulsion polymerization. Asua [312] thought that the interfacial tension used in the simulations was too high and the droplet size was too smaU. However, it is turned out that the question about droplet size is due to a misunderstanding. It is weh accepted that a well-performed miniemulsion has... 

Mixed emulsifiers are commonly used in combination with electrolytes to attain oil/water interfacial tensions substantially less than 1 dyne/cm, eg. 10 1 to 10 dynes/cm (31). The stability of the resulting microemulsions is usually attributed to the formation of an interfacial film (32,33). Even though the mechanism of stabilization has not yet been resolved, the excess surfactant used in microemulsions usually assures good stability. However, due to the very low mixed emulsifier concentrations used in miniemulsions, an understanding of the interactions between mixed emulsifier molecules at oil/water interfaces should greatly facilitate the development of miniemulsion and mini-latex formulations to achieve good stability. 

Earlier conductivity measurements have indicated that the most stable miniemulsions are produced with mixed emulsifier molar ratios between 1 1 and 1 3 (22,23). This correlation agrees with a theoretical analysis of mixed emulsifier adsorption onto oil droplets by Lucassen-Reynders (35), who have determined the optimum stability to occur at molar ratios near 1 1. However, the maximum interfacial tensions at these molar ratios were unexpected because, minimum interfacial tensions are usually associated with maximum emulsion stability. In fact, minima values substantially less than 1 dyne/cm have been reported for several oil/mixed emulsifier systems (31,33, 36,37). 

The interfacial tension values increase from A.l dynes/cm for SLS/ decanol to 8.3 dynes/cm for SLS/octadecanol. Conductometric titration results have indicated that all of these mixed emulsifier systems, except the one with decanol, should give a relatively stable emulsion (22,23). Interestingly, the SLS/decanol mixed emulsifier solution was the only case in which the presence of the fatty alcohol reduced the interfacial tension with styrene to below the value measured for SLS alone. Studies are in progress to investigate this phenomenon and to determine the effect of alcohol chain length on miniemulsion stability. 

These authors concluded that the differences in the hydrophobicity of the oil and the polymer turned out to be the driving force for the formation of nanocapsules. Due to the pronounced difference of polarity of PMMA and hexadecane, the system was very well suited for the formation of nanocapsules. With more hydrophobic monomers such as styrene, however, it was more difQcult to create nanocapsules as the cohesion energy density of the polymer phase was close to that of the oil, and adjustment of parameters to influence the interfacial tensions and spreading coefficients became critical in order to form nanocapsules. The parameters studied were monomer concentration, type and amount of surfactant and initiators, and the addition of functional comonomers. For example, addition of 10 wt% acrylic acid as a comonomer in the miniemulsion leads to an increase in the number of close-to-perfect nanocapsules. 

Although the considerations give a general idea of the resulting morphology, surfactants that alter the interfacial tensions of the components are employed in miniemulsions, and at least one component is not a liquid but a high molecular polymer. Thus, more elaborate models have to be used for more accurate prediction of the equilibrium morphologies. These models are discussed in detail elsewhere [64]. 

Titanium dioxide was dispersed by sonification in a cyclohexane oil phase with polybutene-succinimide diethyl triamine stabiUser and hexadecane as cosurfactant A miniemulsion was formed, the droplets being characterised by surface tension and interfacial tension measurements. The inclusion of the titanium dioxide particles inside the miniemulsion droplets limited the size of the droplets, which were stabilised by and ionic surfactant at the oil-water interface. 8 refs. 

Until now, the formahon of capsules of size >1 im has predominantly been described, though for many applicahons - especially in medicine and high-resolution electronic inks - smaller capsules of 50 to 300 nm attract much more interest. The approach to synthesizing nanocapsules as described below is based on the principle of miniemulsion using the differences of interfacial tension and the phase separation process during polymerization to obtain a nanocapsule morphology. 

A similar situation occurs with larger miniemulsions, where the droplet deformability could be due to the surface extension only [i.e. to the interfacial tension. 

Figure 10.13 3D plot of the osmotic pressure for microemulsion (a) and miniemulsion (b) as a function of the volume faction and the bending elasticity constant (a) or interfacial tension (b), respectively... 

As shown in the simulation of the morphology of hybrid monomer-clay miniemulsion droplets (Figures 10.2 and 10.4), the encapsulation of clay platelets is possible provided that the clay/water interfacial tension is very high (superhydrophobic clay) and that the clay/monomer interfacial tension is low (high compatibility between clay and monomer). Another aspect that the simulations show is that the size of the clay platelets might also play a role in the encapsulation of the clay in the monomer droplets (polymer particles). 

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