14×11 inch ink drawing; Art marker, metallic paint and pigmented ink felt tip pen (micron) on acid free Bristol smooth paper.
The surface of a condensed phase has special properties that are different from the interior. If a molecule thermodynamically “likes” being surrounded by other molecules on all sides, at the surface that molecule is exposed and thermodynamically “unhappy”. If the atoms in a material have a three dimensional preferred bonding pattern, at the surface some of those bonds are unsatisfied.
Sometimes a molecule will segregate to a surface or interface. Usually these surface enriched molecules are pushed out of the bulk (thermodynamically – no maxwell’s demons here!). In this case, the molecules at the surface are usually different from the ones in the bulk interior. Since something has to go to the surface, thermodynamics favors keeping the best conditions for the majority of molecules in the bulk, and the less common chemistries are enriched at the surface.
In the case of polymers, even a subtle difference like deuteration can cause noticeable surface enrichment. Polymers are long floppy molecules with low mixing entropy, so a number of effects are magnified.
This artist’s creative and somewhat surreal rendition of surface enrichment uses archival Prismacolor art markers and Windsor and Newton archival pigment markers with Sakura pigment ink fine line pens and metallic ink pens on acid free archival RendR brand opaque paper.