  Protein adsorption at the solid-liquid interface is an important phenomenon
that often can be observed as a first step in biological processes. Despite its
inherent importance, still relatively little is known about the underlying
microscopic mechanisms. Here, using multivalent ions, we demonstrate the
control of the interactions and the corresponding adsorption of net-negatively
charged proteins (bovine serum albumin) at a solid-liquid interface. This is
demonstrated by ellipsometry and corroborated by neutron reflectivity and
quartz-crystal microbalance experiments. We show that the reentrant
condensation observed within the rich bulk phase behavior of the system
featuring a nonmonotonic dependence of the second virial cofficient on salt
concentration c_s is reflected in an intriguing way in the protein adsorption
d(c_s) at the interface. Our findings are successfully described and understood
by a model of ion-activated patchy interactions within the framework of
classical density functional theory. In addition to the general challenge of
connecting bulk and interface behavior, our work has implications for, inter
alia, nucleation at interfaces.
