The Collaborative International Dictionary of English v.0.48:

adsorb \ad*sorb"\ v. t.
   to attract and bind (molecules of a substance in a fluid) so
   as to form a thin layer on the surface, by non-electrostatic
   forces; to bind by adsorption. Distinguished from absorb,
   in which the foreign substance penetrates the body of the
   absorbing material.

   Syn: surface assimilation
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

adsorb
    v 1: accumulate (liquids or gases) on the surface