The curvature dependence of the symmetric interface between two immiscible polymer solutions in a common monomeric solvent is analyzed using a self-consistent field theory. Contrary to symmetry arguments we find that the surface tension depends in first order on a nonzero Tolman length. These interfaces further have a negative mean and a positive Gaussian bending modulus. The finite spontaneous curvature is attributed to the adsorption of the solvent at the interface. Mesoscopic length scales

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... soft condensed matter are intimately linked to curved interfaces. The interfacial tension of a fluid interface in general depends on the curvature. In the context of lipid bilayers Helfrich [1] pointed out that Here J 1 R 1 1 R 2 is the mean curvature and K 1=R 1 1=R 2 the Gaussian curvature, wherein R 1 and R 2 as the principle radii of curvature of the surface, k c is the mean and k the Gaussian bending modulus. Typically, as, for example, in bilayers, the mean bending modulus is positive. Then J 0 is the optimum curvature of a cylindrically curved interface, and therefore J 0 is known as the spontaneous curvature. Molecularly realistic modeling of selfassembly shows that for lipid bilayers the ground state is flat, i.e., J 0 0, even for multicomponent systems [2]. One may argue that the system of mutually incompatible amphiphilic components in the bilayer is a possible exception: a lateral interface between the lipid domains can be avoided by assigning each phase to its own membrane face. However, in this case already the flat state (J 0) is symmetry broken and the natural consequence is that J 0 Þ 0. Equation (1) may also be used for interfaces between separated liquids. The coefficient in front of the linear term in J [after normalization with 0; 0] is often referred to as the Tolman length. In this case it is more natural to expect a nonzero J 0 and thus a nonzero Tolman length, in particular, for interfaces with an intrinsic asymmetry, such as in emulsion systems. In 1893 van der Waals [3] introduced a molecular model for the interface between two immiscible (monomeric) species. From a mean-field analysis it follows that not far from the critical point the interface has a hyperbolic tangent profile and the interfacial tension is given by / 3=2 . Here ÿ 2 / T c ÿ T is a measure for the distance to the critical point (reached at the critical interaction parameter c 2 or equivalently at the critical temperature T T c ). Within this mean-field model Blokhuis and Bedeaux [4] analyzed the bending moduli and found a negative mean bending modulus k c / ÿ 1=2 , a positive Gaussian bending modulus k / 1=2 , and J 0 0. (Here and below all energy units are normalized by the thermal energy k B T.) The absence of a finite Tolman length was attributed to the (component A)-(component B) symmetry, according to which the phases are transformed, one into the other by changing A to B and vice versa. In such a symmetric case it is irrelevant which direction the positive curvature is defined, and therefore the surface tension at J 0 should be an optimum. When k c is negative (as in this case), the interfacial tension has a maximum at J; K 0; 0 and J 0 is the worst curvature. Nevertheless, we will continue referring to J 0 as the spontaneous curvature. In this Letter we extend the binary van der Waals model to a ternary system and consider again a fully symmetric case: two equally long polymeric components A N and B N that have a solubility gap are mixed with a common monomeric (good) solvent S, such that there is only one nonzero Flory-Huggins interaction parameter AB that expresses the strength of the nearest-neighbor interactions. By increasing the amount of solvent, it is possible to bring this system toward the critical point, i.e., cr 2='N, where ' ' A ' B is the overall polymer volume fraction [5] . If the solvent would distribute homogeneously, we return essentially to the two-component system. However, the solvent accumulates at the interface, simply because the adsorption of solvent screens some of the unfavorable A-B contacts. This adsorption changes the problem in a fundamental way. Below we will demonstrate that in such a symmetric three-component interface J 0 does not vanish. Our conclusion is that the symmetry argument used repeatedly above is flawed. Dextran and gelatin, both water-soluble biomacromolecules, have a miscibility gap where two aqueous phases separate. Emulsions of these components find applications in food systems. With increasing water content the interfacial tension can become very low and there are speculations for this system about the mechanical parameters [6],