The PMF quantifies the free energy difference (including solvent effects) upon varying the distance between a sodium and chloride ion. For bulk NaCl in water, the PMF exhibits a characteristic shape consisting of two clear minima. The first corresponds to a contact ion pair (CIP), in which the sodium and chloride ions are directly coordinated, and the second corresponds to a solvent-separated ion pair (SSIP), where a water molecule separates the two ions. The largest slit heights studied here exhibit very similar PMFs to the bulk electrolyte, down to the H = 10.05 Å slit where there are only two layers of water. In contrast, the PMF changes significantly in the most confined electrolyte, where H = 6.70 Å. Here, we observe an approximately 0.5 k B T (T = 300 K) decrease in the stability of the CIP relative to the free-ion limit, and an increase in the barrier along the PMF from CIP to SSIP of approximately 1.7 k B T compared to the other confined systems. While the ionic separation is not an adequate reaction coordinate for NaCl dissociation, 49 the barrier height of the PMF should nevertheless correlate with the ion pair dissociation barrier. Indeed, we observe an approximate doubling of the average lifetime of the CIP from the H = 6.70 Å to the H = 10.05 Å system from 9 to 22 ps (see Fig. ). We further observe an increase in long-range order in the H = 6.70 Å system, with a significantly more stable SSIP and the emergence of a well-defined third minimum of the PMF corresponding to ions separated by two layers of water molecules. These changes in ion pairing are further quantified in Fig. , where we plot the fraction of sodium ions existing as CIPs, SSIPs, or free ions (those with more than one water molecule separating them). Consistent with the trends shown by the PMFs directly, we observe an 11% decrease in the fraction of CIPs for the H = 6.70 Å system, along with a 6% increase in SSIPs.