Phenol release from pNIPAM hydrogels: scaling molecular dynamics simulations with dynamical density functional theory.

Abstract

We employed molecular dynamic simulations (MD) and the Bennett's acceptance ratio method to compute the free energy of transfer, Δ<i>G</i>_trans, of phenol, methane, and 5-fluorouracil (5-FU), between bulk water and water-pNIPAM mixtures of different polymer volume fractions, <i>ϕ</i>_p. For this purpose, we first calculate the solvation free energies in both media to obtain Δ<i>G</i>_trans. Phenol and 5-FU (a medication used to treat cancer) attach to the pNIPAM surface so that they show negative values of Δ<i>G</i>_trans irrespective of temperature (above or below the lower critical solution temperature of pNIPAM, <i>T</i>_c). Conversely, methane switches the Δ<i>G</i>_trans sign when considering temperatures below (positive) and above (negative) <i>T</i>_c. In all cases, and contrasting with some theoretical predictions, Δ<i>G</i>_trans maintains a linear behavior with the pNIPAM concentration up to large polymer densities. We have also employed MD to compute the diffusion coefficient, <i>D</i>, of phenol in water-pNIPAM mixtures as a function of <i>ϕ</i>_p in the diluted limit. Both Δ<i>G</i>_trans and <i>D</i> as a function of <i>ϕ</i>_p are required inputs to obtain the release halftime of hollow pNIPAM microgels through Dynamic Density Functional Theory (DDFT). Our scaling strategy captures the experimental value of 2200 s for 50 μm radius microgels with no cavity, for <i>ϕ</i>_p ≃ 0.83 at 315 K.