Lennard-Jones parameters for sodium were used for the buffers in these simulations. In the other set of simulations, we computed the free energy difference between introducing a neutral buffer in water (BUF 2 ) and inside the hydrophobic region of a POPC bilayer system (MEMB 2 , Table ), for various values of the Lennard-Jones parameters of the buffer. In these simulations, the Lennard-Jones interactions between the buffer and the rest of the system were increased from non-interacting at λ = 0 to fully interacting (λ = 1) in 10 discrete steps.