PHASE-TRANSITION IN A HETEROPOLYMER CHAIN AT A SELECTIVE INTERFACE

We consider the localization transition of a heteropolymer chain witho ut volume interactions consisting of two types of links - ''hydrophobi c'' (A) and ''hydrophilic'' (B) at a selective oil-water interface: th e A links are repelled from the water into the oil whereas B links pre fer to be located in the water. We study the behavior of the free ener gy near the point of transition from a delocalized (Gaussian) to a loc alized (adsorbed) regime for two basic models: chains with periodic (A -B-A-B... ) and ''annealed random'' sequences of links. We show that t he transition is sensitive to the type of direct repulsive interaction of the links with the interface, and can be of ''fourth'' Order in pa rticular cases.