--- res: bibo_abstract: - Interfaces of room temperature ionic liquids (RTILs) are important for both applications and basic science and are therefore intensely studied. However, the evolution of their interface structure with the cation’s alkyl chain length n from Coulomb to van der Waals interaction domination has not yet been studied for even a single broad homologous RTIL series. We present here such a study of the liquid–air interface for n=2to22, using angstrom-resolution X-ray methods. For n<6, a typical “simple liquid” monotonic surface-normal electron density profile ρe(z) is obtained, like those of water and organic solvents. For n>6, increasingly more pronounced nanoscale self-segregation of the molecules’ charged moieties and apolar chains yields surface layering with alternating regions of headgroups and chains. The layering decays into the bulk over a few, to a few tens, of nanometers. The layering periods and decay lengths, their linear n dependence, and slopes are discussed within two models, one with partial-chain interdigitation and the other with liquid-like chains. No surface-parallel long-range order is found within the surface layer. For n=22, a different surface phase is observed above melting. Our results also impact general liquid-phase issues like supramolecular self-aggregation and bulk–surface structure relations.@eng bibo_authorlist: - foaf_Person: foaf_givenName: Julia foaf_name: Haddad, Julia foaf_surname: Haddad - foaf_Person: foaf_givenName: Diego foaf_name: Pontoni, Diego foaf_surname: Pontoni - foaf_Person: foaf_givenName: Bridget M. foaf_name: Murphy, Bridget M. foaf_surname: Murphy - foaf_Person: foaf_givenName: Sven foaf_name: Festersen, Sven foaf_surname: Festersen - foaf_Person: foaf_givenName: Benjamin foaf_name: Runge, Benjamin foaf_surname: Runge - foaf_Person: foaf_givenName: Olaf M. foaf_name: Magnussen, Olaf M. foaf_surname: Magnussen - foaf_Person: foaf_givenName: Hans-Georg foaf_name: Steinrück, Hans-Georg foaf_surname: Steinrück foaf_workInfoHomepage: http://www.librecat.org/personId=84268 orcid: 0000-0001-6373-0877 - foaf_Person: foaf_givenName: Harald foaf_name: Reichert, Harald foaf_surname: Reichert - foaf_Person: foaf_givenName: Benjamin M. foaf_name: Ocko, Benjamin M. foaf_surname: Ocko - foaf_Person: foaf_givenName: Moshe foaf_name: Deutsch, Moshe foaf_surname: Deutsch bibo_doi: 10.1073/pnas.1716418115 bibo_volume: 115 dct_date: 2018^xs_gYear dct_isPartOf: - http://id.crossref.org/issn/0027-8424 - http://id.crossref.org/issn/1091-6490 dct_language: eng dct_title: Surface structure evolution in a homologous series of ionic liquids@ ...