Unbalanced Forces

Every surface molecule of a liquid or every surface ion/molecule of a solid crystal is in a state of unsaturated or unbalanced force, because its bonding or attractive interactions are only satisfied on the side facing into the bulk material -- there are no matching neighbours on the side facing the surrounding gas, liquid or vacuum. A molecule deep inside the bulk, by contrast, is surrounded on every side by identical neighbours, so the pulls in every direction cancel and it experiences no net force. This asymmetry at the surface is the fundamental reason solid and liquid surfaces are 'sticky' toward whatever they contact: the unsatisfied attractive capacity left over at the surface reaches out and pulls in nearby gas molecules or dissolved substances, and once pulled in, those substances accumulate specifically at the surface of the solid or liquid. This is the structural basis for adsorption as a phenomenon, and it explains wh …