Steady-state miniemulsion

Also in the inverse case, the droplet size throughout the miniemulsification process runs into an equilibrium state (steady-state miniemulsion) which is characterized by a dynamic rate equilibrium between fusion and fission of the... 

The majority of the recipes described in the literature are based on the anionic sodium dodecylsulfate (SDS) as a model system. The possibility of using cationic surfactants such as octadecyl pyridinium bromide for the preparation of miniemulsions was first exploited in 1976. However, the emulsions were prepared by stirring and the resulting emulsions showed broadly distributed droplet sizes [2,39,50]. Recent work on steady-state miniemulsions showed that cationic and nonionic surfactants form well-defined miniemulsions for further miniemulsion polymerization processes, resulting in narrow size distributed stable cationic and nonionic latex particles [51]. Similar molecular amounts of the simple cationic surfactant, cetyltrimethylammonium bromide or chloride... 

The Laplace pressure pLapiace and the osmotic pressure can be calculated as shown in [20]. For a typical hexadecane concentration of 2 mol% relative to styrene, this results in an osmotic pressure of 4.5 bar, which is usually well below the Laplace pressure of 12 bar in a typical miniemulsion system (1.7% SDS relative to styrene, ca. 100 nm diameter). This means that right after steady-state miniemulsification the droplet size is not given by an effective zero droplet pressure, i.e., pLaPiace-A>sm=0> which would represent a real thermodynamic... 

Steady-state dispersed miniemulsions are stable against diffusional degradation, but critically stabilized with respect to colloidal stability. 

The formation of a miniemulsion requires high mechanical agitation to reach a steady state given by a rate equilibrium of droplet fission and fusion. 

In steady-state or mechanically equilibrated miniemulsions, the droplet size can be easily varied by variation of the amount of surfactant. Depending on the droplet size of the miniemulsions, we obtained calorimetric curves with various kinetic features which are shown in Fig. 9 [66]. Disregarding the complexity of the kinetics and the existence of the three intervals, the reaction time to reach 95% conversion depends as a rule of thumb about linearly on the particle size and thus varies between 20 and 120 min. 

Samer also demonstrates the existence of multiple steady states in isothermal miniemulsion polymerization in a CSTR. This is not surprising, since multipH-city is a function of gel or Trommsdorf effect, and not of nucleation mechanism. 

The polymerisation of styrene in miniemnlsions stabilised with anionic sodium dodecyl sulphate or nonionic Lntensol AT50 results in stable polymer dispersions with particle diameters between 30 and 480 nm and narrow particle size distributions. Steady-state mini-emulsification results in a system with critical stability , i.e. the droplet size is the prodnct of a rate equation of fission by ultrasound and fusion by collisions, and the mini-droplets are as small as possible for the timescales involved. The droplet growth by monomer exchange, or the T1 mechanism, is effectively suppressed by addition of a very hydrophobic material, whereas droplet growth by collisions, or the T2 mechanism, is subject to the critical conditions. The growth of the critically stabilised miniemulsion droplets is usually slower than the polymerisation time therefore, in ideal cases, a 1 1 copy of droplets to particles is obtained, and the critically stabilised state is frozen. 6 refs. 

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