Why boundary molecules have more energy? In a system of a solid or a liquid, why do the molecules at the interface have more energy, compared to the bulk of solid or liquid?
 A: Surface effects!  The molecules, atoms, nucleons, etc at the edge of a bulk material are exposed to a different environment than those in the interior.  Since the bulk material is bound, the interior constituents have a higher binding energy than those at the edge.  This makes those constituents near the edge more weakly bound which is equivalent to having a higher energy.
A: I am assuming you are asking this question with in relation to surface tension at interfaces. Molecules at interfaces don't have 'more' or 'less' energy by definition it's just that the net interaction force on molecules in the bulk is zero because the molecule is surrounded by molecules of the same type everywhere around it. Contrary to the bulk, at the interface, a molecule is on one side surrounded by molecules of the same type and on the other side of another type. Therefore the net interaction force is no longer zero and there exist a surface tension force because the molecule at the interface is being pulled into one phase more than another.
