Size-dependent hydrophobic to hydrophilic transition for nanoparticles: a molecular dynamics study

C. C. Chiu, P. B. Moore, W. Shinoda, S. O. Nielsen

J. Chem. Phys. 131, 244706 (2009).

The physical properties of nanoscale materials often vary with their size, unlike the corresponding bulk material properties which can only be changed by modifying the material composition. In the case of particles composed of a bulk hydrophobic material, the solvation free energy is monotone increasing with particle size r, scaling like r3 for small size and r2 for large size. This crossover in solvation free energy as a function of size underlies the theory of protein folding and many other physical phenomena. However, the size-dependent solvation free energy for particles composed of a bulk hydrophilic material has not been studied. Here we show that the solvation free energy is not a monotonic function of particle size, but rather, changes sign from positive to negative as the particle size increases. In other words, the particle is hydrophobic at small size and hydrophilic at large size. This behavior arises from a purely geometrical effect caused by the curvature of the particle-water interface. We explore the consequences of this phenomenon on colloidal stability, and find that it dictates the shape of colloidal aggregates.