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Here, we use multistep chronoamperometry to measure differential capacitance and charging dynamics of [CnC1Im]+[NTf2]\u2212 (n = 3, 4, 6, 8) and their binary mixtures. The neat ionic liquids exhibit chain-length-dependent capacitance profiles, while mixtures with increasing cation-size disparity develop localized capacitance maxima that are suppressed for closely sized ions. The magnitude of these features depends nonmonotonically on composition, indicating nonideal interfacial mixing. Crucially, capacitance maxima coincide with peaks in the slower charging time scale, demonstrating coupled electrostatic and dynamic changes. These results are consistent with a mechanism where ion-size heterogeneity promotes potential-driven reorganization between competing configurations, offering an extra degree of freedom to tune dense ionic-liquid interfaces. Direct molecular-level characterization remains a natural next step to confirm these microscopic transitions.<\/jats:p>","DOI":"10.1021\/acs.jpclett.6c02972","type":"journal-article","created":{"date-parts":[[2026,10,5]],"date-time":"2026-10-05T08:02:09Z","timestamp":1791187329000},"page":"11955-11962","source":"Crossref","is-referenced-by-count":0,"title":["Ion-Size Heterogeneity\nControls Interfacial Electrostatic\nResponse in Mixed Ionic Liquids"],"prefix":"10.1021","volume":"17","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-7691-5122","authenticated-orcid":true,"given":"Monchai","family":"Jitvisate","sequence":"first","affiliation":[{"name":"Institute of Science, Suranaree University of Technology , , 111 University Avenue, Suranaree , ,","place":["Muang, Nakhon Ratchasima, Thailand, 30000"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tharinda","family":"Kasemphong","sequence":"additional","affiliation":[{"name":"Institute of Science, Suranaree University of Technology , , 111 University Avenue, Suranaree , ,","place":["Muang, Nakhon Ratchasima, Thailand, 30000"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0007-3160-835X","authenticated-orcid":true,"given":"Michael","family":"Armstrong","sequence":"additional","affiliation":[{"name":"Chiang Mai University , , ,","place":["Chiang Mai, Thailand, 50200"]},{"name":"Chiang Mai University , , ,","place":["Chiang Mai, Thailand, 50200"]},{"name":"Chiang Mai University , , ,","place":["Chiang Mai, Thailand, 50200"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0328-5462","authenticated-orcid":true,"given":"Kamonwad","family":"Ngamchuea","sequence":"additional","affiliation":[{"name":"Institute of Science, Suranaree University of Technology , , 111 University Avenue, Suranaree , ,","place":["Muang, Nakhon Ratchasima, Thailand, 30000"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6364-504X","authenticated-orcid":true,"given":"Piyarat","family":"Nimmanpipug","sequence":"additional","affiliation":[{"name":"Chiang Mai University , , ,","place":["Chiang Mai, Thailand, 50200"]},{"name":"Chiang Mai University , , ,","place":["Chiang Mai, Thailand, 50200"]},{"name":"Chiang Mai University , , ,","place":["Chiang Mai, Thailand, 50200"]},{"name":"Ministry of Higher Education, Science, Research and Innovation , , ,","place":["Bangkok, Thailand, 10400"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sakhorn","family":"Rimjaem","sequence":"additional","affiliation":[{"name":"Chiang Mai University , , ,","place":["Chiang Mai, Thailand, 50200"]},{"name":"Chiang Mai University , , ,","place":["Chiang Mai, Thailand, 50200"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"316","published-online":{"date-parts":[[2026,10,5]]},"reference":[{"key":"2026101508222207500_cit1","doi-asserted-by":"publisher","first-page":"10821","DOI":"10.1021\/acs.chemrev.2c00097","article-title":"Understanding\nthe electric double-layer structure, capacitance, and charging dynamics","volume":"122","author":"Wu","year":"2022","journal-title":"Chem. 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