Emulsions Ostwald ripening

N. Hedin, I. Furo 2001, (Ostwald ripening of an emulsion monitored by PGSE NMR), Langmuir 17, 4746. 

A.S. Kabalnov, A.V Pertzov, and E.D. Shchukin Ostwald Ripening in Emulsions 1. Direct Observations of Ostwald Ripening in Emulsions. J. Colloid Interface Sci. 118, 590 (1987). 

J. Weiss, N. Herrmatm, and D.J. McClements Ostwald Ripening of Hydrocarbon Emulsion Droplets in Surfactant Solutions. Langmuir 15, 6652 (1999). 

N. Hedin and I. Euro Ostwald Ripening of an Emulsion Monitored by PGSE NMR. Langmuir 17, 4746 (2001). 

Kizling and coworker [21] suggested that salts in the aqueous phase stabilised w/o HIPEs by two means. First, the Ostwald ripening process is inhibited due to the decreased solubility of the aqueous solution in the continuous oil phase. Secondly, the attractive forces between adjacent aqueous droplets are lowered, as a result of the increase in refractive index of the aqueous phase towards that of the oil phase. When the refractive indices of the two phases are matched, the attractive forces are at a minimum and highly stable, transparent emulsions are formed. The attractive force, A, is given by ... 

A logarithmic time interval may be chosen if the emulsion consists of an oil that has a substantial solubility in the aqueous phase (e.g., aromatic or flavor oils see Background Information, discussion of Ostwald ripening). 

Surfactants are used for stabilization of emulsions and suspensions against flocculation, Ostwald ripening, and coalescence. Flocculation of emulsions and suspensions may occur as a result of van der Waals attraction, unless a repulsive energy is created to prevent the close approach of droplets or particles. The van der Waals attraction Ga between two spherical droplets or particles with radius R and surface-to-surface separation h is given by the Hamaker equation,... 

Figure 3.27 Breakdown mechanisms of emulsions (from top to bottom creaming, coalescence, flocculation and Ostwald ripening).

Emulsion Stability Against Ostwald Ripening, Collisions,... 

Emulsions are understood as dispersed systems with liquid droplets (dispersed phase) in another, non-miscible liquid (continuous phase). Either molecular diffusion degradation (Ostwald ripening) or coalescence may lead to destabilization and breaking of emulsions. In order to create a stable emulsion of very small droplets, which is, for historical reasons, called a miniemulsion (as proposed by Chou et al. [2]), the droplets must be stabilized against molecular diffusion degradation (Ostwald ripening, a unimolecular process or r, mechanism) and... 

In 1962, Higuchi and Misra examined the quantitative aspects of the rate of growth of the large droplets and the rate of dissolution of the small droplets in emulsion for the case in which the process is diffusion controlled in the continuous phase [4]. It was proposed that unstable emulsions may be stabilized with respect to the Ostwald ripening process by the addition of small amounts of a third component, which must distribute preferentially in the dispersed phase [4]. The obtained stability in miniemulsions is said in the literature to be metastable or fully stable. The stabilization effect by adding a third component was recently theoretically described by Webster and Cates [5]. The authors considered an emulsion whose droplets contain a trapped species, which is insoluble in the continuous phase, and studied the emulsion s stability via the Lifshitz-Slyozov dynamics (Ostwald ripening). 

The rate of Ostwald ripening depends on the size, the polydispersity, and the solubility of the dispersed phase in the continuous phase. This means that a hydrophobic oil dispersed as small droplets with a low polydispersity already shows slow net mass exchange, but by adding an ultrahydrophobe , the stability can still be increased by additionally building up a counteracting osmotic pressure. This was shown for fluorocarbon emulsions, which were based on perfluo-rodecaline droplets stabilized by lecithin. By adding a still less soluble species, e.g., perfluorodimorphinopropane, the droplets stability was increased and could be introduced as stable blood substitutes [6,7]. 

Welin-Berger, K., and Bergenstahl, B. (2000), Inhibition of Ostwald ripening in local anesthetic emulsions—By using hydrophobic excipients in the disperse phase, Int. J. Pharm., 200-202, 249-260. 

Stability of emulsions refers to the resistance to the formation of two separate phases [13,14]. Coalescence of the droplets is responsible for the phase separation. Ostwald ripening constitutes an additional mechanism by which the large droplets grow in size at the expense of the smaller ones, which decrease in size. 

Mouran et al. [105] polymerized miniemulsions of methyl methacrylate with sodium lauryl sulfate as the surfactant and dodecyl mercaptan (DDM) as the costabilizer. The emulsions were of a droplet size range common to miniemulsions and exhibited long-term stability (of greater than three months). Results indicate that DDM retards Ostwald ripening and allows the production of stable miniemulsions. When these emulsions were initiated, particle formation occurred predominantly via monomer droplet nucleation. The rate of polymerization, monomer droplet size, polymer particle size, molecular weight of the polymer, and the effect of initiator concentration on the number of particles all varied systematically in ways that indicated predominant droplet nucleation. 

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