Future work Our next steps will inves2gate the breakup of the water ﬁnger structure and further deﬁne this transfer event by carefully op2mizing a 2-­‐D free energy surface that simultaneously considers the ion transfer mechanism in both energe2c and spa2al terms. Background The transfer of a charged species across immiscible phases is a fundamental event that is studied and used in many ﬁelds and techniques. Molecular insight into ion transfer leads to a deeper understanding of many processes, including phase transfer catalysis and biological ion transport. The liquid-­‐liquid interface is buried, mobile, and constantly ﬂuctua2ng. Notable interfacial phenomena occur within only a few molecular diameters of the interface, making experimental study especially challenging. Techniques involving nonlinear spectroscopy, neutron or x-­‐ray scaIering, and scanning electrochemical microscopy con2nue to evolve but computer simula2ons, most notably molecular dynamics, contribute signiﬁcantly to our current theore2cal picture of ion transfer at the molecular scale. Ion transfer across a water-­‐nitrobenzene interface John Karnes and Ilan Benjamin Department of Chemistry and Biochemistry, University of California, Santa Cruz Free energy of transfer The water solva2on shell helps to shuIle the ion across the interface. We quan2fy this by calcula2ng the energe2cs of two ion transfer situa2ons: These preliminary results emphasize the importance of the solva2on water’s role in the transfer of a bare chloride ion. When the KillFinger rou2ne eﬀec2vely strips the ion of its solva2on shell, we see the required free energy nearly double! Exploring phase space Combining the energe2c and geometric coordinates deﬁnes a phase space through which we can track the ion transfer event. In the normal model, elonga2on of the water ﬁnger as the ion extends into the organic phase is seen as a subtle slope and break of the ﬁnger that occurs at sH2O ≈ 0.5. In the KillFinger model, the ion spends liIle 2me moving through solvent coordinate space. When the ion crosses the interface, the water ﬁnger breaks before it can ever truly form. This occurs at sH2O ≈ 0.1 when the ion is experiencing the last Coulombic aIrac2on with interfacial water as it moves into the organic phase. Quan2fying the water ﬁnger With these results, we consider the role of the water ﬁnger and may assign a geometric coordinate, w, to characterize.2 We deﬁne this coordinate as the largest step required to “hop” from the ion to the bulk water phase along the water molecules in the water ﬁnger. In the snapshots above, w is the water-­‐water distance indicated by the gold arrows. Literature cited 1. Benjamin, I. Science 1993, 261, 1558. 2. Schweighofer, K. J.; Benjamin, I. J. Phys. Chem. 1995, 99, 9974. 3. Kikkawa, N.; Wang, L.; Morita, A. J. Am. Chem. Soc. 2015, 137, 8022. This work is supported by the Na2onal Science Founda2on. Simula2on details Our model consists of two adjacent liquid slabs in a 31.3 Å × 31.3 Å × 300 Å rectangular box. The liquid-­‐liquid interface is perpendicular to the long axis, designated z. The aqueous phase contains 986 ﬂexible-­‐SPC water molecules; the organic phase is 252 nitrobenzene molecules. For nitrobenzene, we use an empirical 14-­‐site all-­‐atom model. System poten2als are calculated as the pairwise sum of Lennard-­‐Jones and Coulombic terms. The equa2ons of mo2on are integrated using the velocity Verlet algorithm with a 2me step of 0.5 fs. In our model system, we consider the transfer of a single chloride ion from the aqueous phase to an adjacent nitrobenzene phase. Representa2ve snapshots of this event are shown below. From lem to right, the ion is in the aqueous phase (a), at the interface (b), and moving into the organic phase (c). Note that in image (c) the ion carries with it the remnants of its water hydra2on shell and remains tethered to the aqueous phase by a “water ﬁnger.”1 In (d) the water ﬁnger has broken and the ion moves within the organic phas