The entropies of hydrating molecular and macroscopic non-polar surfaces differ in sign at room temperature, suggesting a size scale at which hydration transitions between these behaviors. Following a curvature expansion for the surface tension, we derive an expression for the radius of a cavity-like solute at which the solvation entropy is zero. This expression accurately predicts the crossover radii observed in a simulation/scaled-particle theory analysis of cavity solvation in water and n-hexane along their saturation curves. Our results indicate differences between water and organic solvents are encoded in the temperature dependence of the surface tension and its leading curvature correction. (c) 2009 Elsevier B.V. All rights reserved.