Molecular dynamics (MD) simulations are used to predict the partitioning of per- and polyfluoroalkyl substances (PFASs) to smectite clay, a high surface area adsorbent ubiquitous in temperate soils. Simulated systems model a stack of flexible smectite lamellae in contact with a bulk aqueous reservoir containing PFAS molecules. Perfluorobutanesulfonic acid (PFBS), perfluorohexanesulfonic acid (PFHxS), and perfluorooctanesulfonic acid (PFOS) are simulated at various aqueous chemistry conditions to examine the effect of PFAS size, salinity, and coordinating cation type (K+, Na+, and Ca2+) on adsorption. The metadynamics technique is employed to facilitate the exploration of the simulation cell and to reconstruct the underlying free energy landscape. Adsorption is favorable on the hydrophobic domains of the external basal surfaces with the fluorinated chain adopting a flat orientation on the surface. Analysis of the adsorption energetics reveals large favorable entropic contributions to adsorption. The enthalpy of adsorption is unfavorable, though much less so in the presence of Ca2+ due to stabilizing 'lateral cation bridging' interactions between divalent cations and PFAS sulfonate head groups. Overall, this research advances the mechanistic understanding of PFAS-smectite interactions and provides new insights that could help inform fate and transport models and the development of adsorbents and remediation techniques. (C) 2020 The Authors. Published by Elsevier Inc.