Colloidal Ostwald ripening

The Kelvin equation has numerous applications, e.g. in the stability of colloids (Ostwald ripening, see below), supersaturation of vapours, atmospheric chemistry (fog and rain droplets in the atmosphere), condensation in capillaries, foam stability, enhanced oil recovery and in explaining nucleation phenomena (homo- and heterogeneous). The Kelvin (as well as the Gibbs equations, see Equation 4.7a) are also valid for solids/solid-liquid surfaces, and they can be used for estimating the surface tensions of solids. We discuss hereafter several applications of the Kelvin equation. 

Despite the limitations of (15.5) [Horanyi, 1985], it explains reasonably well the Ostwald ripening of Ag particles by the dissolution of small particles and the growth of large particles [Redmond et al., 2005 Schroeder et al., 2006]. The negative shift of the Cu +/Cu° redox potential for 3 nm colloidal Cu particles with respect to the value for bulk Cu has been confirmed, for example, in Savinova et al. [1988]. 

Redmond PL, Hallock AJ, Brus LE. 2005. Electrochemical Ostwald ripening of colloidal Ag particles on conductive substrates. Nano Lett 5 131-135. 

Stability" is a ubiqitous word as applied to colloidal dispersions it may apply to more than one physical process (e.g. flocculation, settling, Ostwald ripening) and may be used in either the kinetic or thermodynamic senses. We restrict discussion here to flocculation and consider first the thermodynamic aspects. 

An analogy may be drawn between the phase behavior of weakly attractive monodisperse dispersions and that of conventional molecular systems provided coalescence and Ostwald ripening do not occur. The similarity arises from the common form of the pair potential, whose dominant feature in both cases is the presence of a shallow minimum. The equilibrium statistical mechanics of such systems have been extensively explored. As previously explained, the primary difficulty in predicting equilibrium phase behavior lies in the many-body interactions intrinsic to any condensed phase. Fortunately, the synthesis of several methods (integral equation approaches, perturbation theories, virial expansions, and computer simulations) now provides accurate predictions of thermodynamic properties and phase behavior of dense molecular fluids or colloidal fluids [1]. 

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). 

In terms of the common features of colloids just listed, nanoparticles with a clean surface have tendency to amalgamate when placed on a TEM sample mesh that is evident in Figures 9.4.16 and 9.4.17. Furthermore, the particles often grow by collision (not by Ostwald ripening because metal ions are hard to dissolve in organic liquids) in the suspension state when the number density of particles in... 

However, there are a number of difficulties associated with the synthesis of colloidal semiconductor particles. The preparation of stable, monodispersed, well-characterized populations of nanosized, colloidal semiconductor particles is experimentally demanding and intellectually challenging. Small and uniform particles are needed to diminish non-productive electron-hole recombinations the mean distance by which the charge carriers need to diffuse to reach the particle surface from which they are released is necessarily reduced in small particles. Monodispersity is a requirement for the observation of many of the spectroscopic and electro-optical manifestations of size quantization in semiconductor particles. Small semiconductor particles are difficult to maintain in solution in the absence of stabilizers flocculations and Ostwald ripening... 

Interesting aging effects are frequently observed in these systems. If the precipitated particles are left in contact with the hydrolysis catalyst and water they appear to reorganize, so that their surfaces become better defined and their sizes become more uniform.15 The process seems quite analogous to the Ostwald ripening 33 much studied by colloid chemists. 

This difference is probably associated with the chain expansion and/or contraction relative to the interface between the blocks within the particular morphology. In analogy to solid inorganic materials whose solubility increases under external pressure and to colloid systems where the capillary pressure leads to an increase in the solubility (Ostwald ripening effect), one would intuitively expect larger swelling of... 

An important example of this phenomenon is to be found in the ageing of colloidal dispersions (often referred to as Ostwald ripening). In any dispersion there exists a dynamic equilibrium whereby the rates of dissolution and deposition of the dispersed phase balance in order that saturation solubility of the dispersed material in the dispersion medium be maintained. In a polydispersed sol the smaller particles will have a greater solubility than the larger particles and so will tend to dissolve, while the larger particles will tend to grow at their expense. In... 

Davis, S.S. Round, H.P. Purewal, T.S. Ostwald ripening and the stability of emulsion systems an explanation for the effect of an added third component. J. Colloid Interf. Sci. 1981, SO, 508-511. 

Kabalnov, A.S. Shchukin, E.D. Ostwald ripening theory applications to fluorocarbon emulsion stability. Adv. Colloid Interf. Sci. 1992, 38, 69-97. 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

Spadini L, Manceau A, Schindler PW, Charlet L (1994) Structure and Stability of Cd Surface Complexes on Ferric Oxides. 1. Results from EXAFS Spectroscopy. J Colloid Interface Sci 168 73-86 Steefel Cl, van Cappellen P (1990) A new kinetic approach to modehng water-rock interaction Role of nucleation, precursors, and Ostwald ripening. Geochim Cosmochim Acta 54 2657-2677 Stem LA, Durham WB, Kirby SH (1997) Grain-size-induced weakening of H2O ices I and II and associated anisotropic recrystallization. J Geophys Res-Solid Earth 102 5313-5325 Suzuki A, Kotera Y (1962) The kinetics of the transition of titanium dioxide. Bull Chem Soc Japan 35 1353-1357... 

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