Single-chain collapse as a first-order transition: model for PEO in water

A simple model for polyethyleneoxide (PEO) is introduced and analyzed by Monte Carlo simulation. The occurrence of a solvation shell of PEO in water is modeled by two internal states of the monomers with an energy difference P . In the solvated state the monomer excluded volume increases due to the bound water and prevents the collapse of the poor-solvent backbone. As a function of P , which can be positive or negative, we find for P ≲0.7 k B T a first-order collapse transition, while for P ≳ 0.7 k B T the standard Θ-point is recovered. Thus, if the solvation shell is accompanied by a lower internal energy ( P < 0 as for PEO), the transition is always of first order.