In a given medium, the most important nanoparticle characteristics that determine the surface properties are the material’s chemical composition,
surface functionalization, shape and angle of curvature, porosity and surface crystallinity, heterogeneity, roughness, and hydrophobicity or hydrophilicit. Other quantifiable properties, such as effective surface charge (zeta potential), particle aggregation, state of dispersion, stability/biodegradability, dissolution characteristics, hydration and valence of the surface layer, are determined by the characteristics of the suspending media, including the ionic strength, pH, temperature and the presence of large organic molecules (for example proteins) or detergents. The particle characteristics contribute actively to the interactions with the medium through: (i) promoting the adsorption of ions, proteins, natural organic materials and detergents; (ii) double-layer formation; (iii) dissolution; or (iv) minimizing free surface energy by surface restructuring. Many of these newly acquired particle properties or transformed states determine the forces that operate at the particle–medium interface with characteristic decay lengths. These forces include long-range forces arising from attractive van der Waals (VDW) and (generally) repulsive electrostatic double-layer interactions, plus short-range forces arising from charge, steric, depletion and solvent interactions. Media interactions (for example protein interactions) could also induce large-scale changes, such as nanoparticle dissolution, ion leaching, phase transformation and agglomeration.