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

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "adsorb":
   absorb, assimilate, blot, blot up, chemisorb, chemosorb, digest,
   drink, drink in, drink up, engross, filter in, imbibe, infiltrate,
   osmose, percolate in, seep in, slurp up, soak in, soak up, sorb,
   sponge, swill up, take in, take up