Ostwald Ripening - Mechanism

Mechanism

This thermodynamically-driven spontaneous process occurs because larger particles are more energetically favored than smaller particles. This stems from the fact that molecules on the surface of a particle are energetically less stable than the ones in the interior.

Consider a cubic crystal of atoms: all the atoms inside are bonded to 6 neighbors and are quite stable, but atoms on the surface are only bonded to 5 neighbors or fewer, which makes these surface atoms less stable. Large particles are more energetically favorable since, continuing with this example, more atoms are bonded to 6 neighbors and fewer atoms are at the unfavorable surface. As the system tries to lower its overall energy, molecules on the surface of a small particle (energetically unfavorable, with only 3 or 4 or 5 bonded neighbors) will tend to detach from the particle, as per the Kelvin equation, and diffuse into the solution. When all small particles do this, it increases the concentration of free molecules in solution. When the free molecules in solution are supersaturated, the free molecules have a tendency to condense on the surface of larger particles. Therefore, all smaller particles shrink, while larger particles grow, and overall the average size will increase. As time tends to infinity, the entire population of particles becomes one large spherical particle to minimize the total surface area.

In 1961, Lifshitz and Slyozov performed a mathematical investigation of Ostwald ripening in the case where diffusion of material is the slowest process. They began by stating how a single particle grows in a solution. This equation describes where the boundary is between small, shrinking particles and large, growing particles. They finally conclude that the average radius of the particles ⟨R⟩, grows as follows:

where

Note that the quantity ⟨R⟩3 is different from ⟨R3⟩, and only the latter one can be used to calculate average volume, and that the statement that ⟨R⟩ goes as t1/3 relies on ⟨R⟩₀ being zero; but because nucleation is a separate process from growth, this places ⟨R⟩₀ outside the bounds of validity of the equation. In contexts where the actual value of ⟨R⟩₀ is irrelevant, an approach that respects the meanings of all terms is to take the time derivative of the equation to eliminate ⟨R⟩₀ and t. Another such approach is to change the ⟨R⟩₀ to ⟨R⟩_i with the initial time i having a positive value.

Also contained in the Lifshitz and Slyozov derivation is an equation for the size distribution function f(R, t) of particles. For convenience, the radius of particles is divided by the average radius to form a new variable, ρ = R(⟨R⟩)-1.

At the same time that Lifshitz and Slyozov published their findings, Carl Wagner performed his own mathematical investigation of Ostwald ripening, examining both systems where diffusion was slow and also where attachment and detachment at the particle surface was slow. Although his calculations and approach were different, Wagner came to the same conclusions as Lifshitz and Slyozov for slow-diffusion systems. This duplicate derivation went unnoticed for years because the two scientific papers were published on opposite sides of the Iron Curtain in 1961. It was not until 1975 that Kahlweit addressed the fact that the theories were identical and combined them into the Lifshitz-Slyozov-Wagner or LSW Theory of Ostwald ripening. Many experiments and simulations have shown LSW theory to be robust and accurate. Even some systems that undergo spinodal decomposition have been shown to quantitatively obey LSW theory after initial stages of growth.

Wagner derived that when attachment and detachment of molecules is slower than diffusion, then the growth rate becomes

where k_s is the reaction rate constant of attachment with units of length per time. Since the average radius is usually something that can be measured in experiments, it is fairly easy to tell if a system is obeying the slow-diffusion equation or the slow-attachment equation. If the experimental data obeys neither equation, then it is likely that another mechanism is taking place and Ostwald ripening is not occurring.

Although LSW theory and Ostwald ripening were intended for solids ripening in a fluid, Ostwald ripening is also observed in liquid-liquid systems, for example, in an oil-in-water emulsion polymerization. In this case, Ostwald ripening causes the diffusion of monomers (i.e. individual molecules or atoms) from smaller droplets to larger droplets due to greater solubility of the single monomer molecules in the larger monomer droplets. The rate of this diffusion process is linked to the solubility of the monomer in the continuous (water) phase of the emulsion. This can lead to the destabilization of emulsions (for example, by creaming and sedimentation).