Thermodynamics of microemulsions: Combined effects of dispersion entropy of drops and bending energy of surfactant films

Abstract

so that the criterion for smau composition gradients becomes -B ' < < 1 (A61 According to ( A 2 ) the condition (A6) also insures small relative pressure gradients. To examine the conditions under which ( A 6 ) is valid, we take the following example: T = 800"K, R, = 50 pm, €4 = 0.015, weight loss = 20% in 10 s (half of which is gases and half is tar). Using molecular weights MG = 25, MT = 250 and coal particle density 1.2 g/cm3, we compute YG = 4.8 x gmols/cm%, YT = 0.48 x 10-4 gmols/ cm%, DGK = 2.74 a$/,, DTG = 0.48 cm2/s, f = 0.33, S = 0.1 so that = 0.015. On the other hand, taking T = 900°K the rates are larger by a factor 3-5 so thatB' -0.01-0.08. If, on the other hand, we take T = 900°K and R, = 100 pm, -0.06-0.11, -0.16-0.34, marking a departure from uniform conditions. Under conditions at which B' <$ 1, the use of an effective radius as defined by Equation ( 5 ) P. NEOGIA theory of dilute microemulsions is presented which includes for the first time both the entropy of dispersion of the drops and energy effects associated with bending the surfactant films at the drop interfaces. It yields expressions for drop size for (a) a dilute microemulsion in equilibrium with an excess bulk phase, e.g., an oil-in-water microemulsion in equilibrium with excess oil and (b) dilute oil-continuous and water-continuous microemulsions in equilibrium and containing equal amounts of surfactant. In the latter case, our theory indicates that existence of the two microemulsion phases sometimes is favored over a layered or lamellar phase, even though the "natural curvature" of the surfactant films is zero, corresponding to a perfectly flat f i l m . Department of Chemical EngineeringCarnegie-Mellon University Pittsburgh, Pennsylvania 15213 SCOPE "Microemulsions are dispersions of one liquid in another which have very small drops (< 0.1 pm) and are often thermodynamically stable. They have been used in such applications as cutting oils and pesticides and are currently of great interest in connection with processes for increasing recovery of petroleum from underground reser- voirs.An important property of a microemulsion is its drop size. Existing theory indicates that drop size has a large P. Neogi i c present11 at the St'itc University of New York, Buffalo,

Application of Theoretical Models to the Estimate of M1croemulsions Properties Part I Ternary Systems

“…So, the model can be used to explain qualitatively the alcohol effect on microemulsion stability, and the salt effect, in the extent of modifications of partition constants initiated by adding salt in water. Furthermore, it can be used as a starting point to describe the composition of pseudo phases that lead to stable dispersed systems (microemulsions) before working up stability theories already published [19][20][21] …”

Thermodynamic Approaches to Microemulsions

“…Two structures are compatible with a zero average curva- Equations [26] and [27] provide some information about g and C. The two are plotted in Figs. 1 and 2, which show that g is small.…”

Separation of Proteins by Using Reversed Micelles

“…In particular, it was demonstrated that the systems of interest for extraction, i.e. water in oil microemulsions in equilibrium with an excess phase, are governed by the bending stress of the interfacial film (64,65). In spite of their often being only theoretical, these approaches give a good understanding of the factors characterizing the reversed micellar phase, such as the size of the micelles.…”