| A Quantum Orientational Ensemble for Understanding the Point of Charge at the Pt(111)/Pure-Water Interface |
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Byoung-Yong Chang1, Christopher H. Hendon2 |
1Department of Chemistry, Pukyong National University, 45 Yongso-ro, Nam-gu, Busan 48513, Korea
2Department of Chemistry and Biochemistry, University of Oregon, Eugene, OR 97403, USA |
Correspondence:
Byoung-Yong Chang,
Email: bychang@pknu.ac.kr
Christopher H. Hendon,
Email: chendon@uoregon.edu
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Received: 22 April 2026 • Accepted: 26 May 2026 |
| Abstract |
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The microscopic meaning of the point of zero charge at the Pt(111)/pure-water interface remains unresolved because interfacial water has often been described through selected structural descriptions rather than through a statistical-mechanical distribution. Here, we examine whether the point of zero charge is associated with the emergence of a distinct new interfacial-water structure or with redistribution within an orientational distribution that remains occupied over the applied-potential range. To address this question, we analyzed the Pt(111)/pure-water interface by constant-potential quantum mechanics/molecular mechanics calculations using an interfacial-water orientation coordinate and the corresponding reduced orientational distribution. Across an applied-potential range from -0.50 to +0.50 V with two replicas at each potential, the same broad low-angle and high-angle orientational regions remained occupied, while the mean induced charge crossed zero between -0.04 and -0.02 V. The distribution was further expressed in reduced free-energy and orientational-entropy forms defined on the same reduced partition after accounting for the geometric degeneracy of the polar-angle coordinate. In the pure-water limit examined here, the differential capacitance remained comparatively featureless. These results show that the point of zero charge is interpreted as a balance point within an orientational distribution of interfacial water rather than as the appearance of a distinct new structure. The small decrease in the mean orientation near this interval is consistent with a slight shift toward lower-angle orientations within the pre-existing distribution. This work also shows that a reduced-ensemble description can provide a practical microscopic route for relating interfacial structure to macroscopic electrochemical response. |
| Keywords:
Pt(111), pure-water interface, potential of zero charge, orientational distribution, quantum mechanics/molecular mechanics, ensemble, differential capacitance |
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