Finally, to probe the energetics of these various degradation mechanisms, we carried out well-tempered metadynamics simulations to calculate free energy activation barriers for C- bond dissociation is 3 times larger for solvated PFOS than PFOA, resulting in a slower defluorination of PFOS. However, the free energy activation barrier for both PFOS and PFOA falls below the diffusion-controlled limit, and, therefore, the degradation process essentially becomes independent of the overall chemical rate constant. The relatively low free energy activation barriers obtained from our AIMD calculations provide atomistic details in this process to rationalize these previous experimental observations. Moreover, the trends in our computed activation barriers also support previous experiments that found PFOS to be more resistant to degradation than PFOA (although both degradations are still diffusion controlled).