  The collapse and swelling behavior of a generic homopolymer is studied using
implicit-solvent, explicit-cosolvent Langevin dynamics computer simulations for
varying interaction strengths. The systematic investigation reveals that
polymer swelling is maximal if both monomer-monomer and monomer-cosolute
interactions are weakly attractive. In the most swollen state the cosolute
density inside the coil is remarkably bulk-like and homogenous. Highly
attractive monomer-cosolute interactions, however, are found to induce a
collapse of the chain which, in contrast to the collapsed case induced by
purely repulsive cosolvents, exhibits a considerably enhanced cosolute density
within the globule. Thus, collapsed states, although appearing similar on a
first glance, may result from very different mechanisms with distinct final
structural and thermodynamics properties. Two theoretical models, one based on
an effective one-component description where the cosolutes have been integrated
out, and a fully two-component Flory -- de Gennes like model, support the
simulation findings above and serve for interpretation. In particular, the
picture is supported that collapse in highly attractive cosolvents is driven by
crosslinking-like bridging effects, while the ratio of attraction width to
cosolute size plays a critical role behind this mechanism. Only if
polymer-cosolute interactions are not too short-ranged swelling effects should
be observable. Our findings may be important for the interpretation of the
effects of cosolutes on polymer and protein conformational structure, in
particular for highly attractive interaction combinations, such as provided by
urea, GdmCl, NaI, or NaClO4 near peptide-like moieties.
