Contrasting effects of hydrophobic and hydrophilic soft surfaces on ice nucleation

Polyvinyl alcohol (PVA), a widely used synthetic polymer, is known to effectively promote ice nucleation, but the underlying microscopic mechanism has long been controversial. On the basis of experimental evidence that PVA forms nanoscale aggregates in aqueous solution, we propose that its ice nucleation activity originates from heterogeneous nucleation on the surfaces of these aggregates. To validate this hypothesis, we performed large-scale molecular dynamics simulations using two idealized surface models representing the hydrophobic carbon backbone and the hydrophilic hydroxyl group surface of PVA aggregates. Our simulations demonstrate that the hydrophobic surface exhibits potent ice nucleation activity via a heterogeneous mechanism, whereas the hydrophilic surface shows a negligible effect on nucleation. To elucidate the physical mechanism, we employed a theoretical framework combining classical nucleation theory with a non-homogeneous Poisson process model. The analysis revealed that the hydrophobic surface promotes nucleation by substantially lowering the thermodynamic nucleation free energy barrier, an effect that overwhelms the concurrent slowing of interfacial water dynamics. Microstructural analysis further revealed that the hydrophobic surface induces the formation of an ordered, ice-like 6-membered ring structure in the interfacial water layer prior to the nucleation. This pre-ordered interfacial water layer serves as a template for ice crystal growth while also leading to slowed interfacial dynamics. This work provides a molecular explanation for the ice nucleation activity of PVA and highlights the critical role of polymer surface hydrophobicity and interfacial water structuring in heterogeneous ice nucleation.