Irreversible structural change of a dry ionic liquid under nanoconfinementElectronic supplementary information (ESI) available: Complementary force-distance isotherms, refractive index, dynamics of film-thickness transitions, model for elastic forces, Gaussian fit to the low-q WAXS reflection, AFM force measurements with a sharp tip, XPS spectrum, schematics of the main features of the eSFA and optimized geometry of [HMIM] EtSO4. See DOI: 10.1039/c4cp05592f

Studies of 1-hexyl-3-methyl-imidazolium ethylsulfate ([HMIM] EtSO 4 ) using an extended surface forces apparatus show, for the first time, an ordered structure within the nanoconfined ionic liquid (IL) between mica surfaces that extends up to ∼60 nm from the surface. Our measurements show the growth of this ordered IL-film upon successive nanoconfinements-the structural changes being irreversible upon removal of the confinement-and the response of the structure to shear. The compressibility of this system is lower than that typically measured for ILs, while creep takes place during shear, both findings supporting a long-range liquid-to-solid transition. AFM (sharp-tip) studies of [HMIM] EtSO 4 on mica only reveal ∼2 surface IL-layers, with order extending only ∼3 nm from the surface, indicating that confinement is required for the long-range IL-solidification to occur. WAXS studies of the bulk IL show a more pronounced ordered structure than is the case for [HMIM] with bis(trifluoromethylsulfonyl)imide as anion, but no long-range order is detected, consistent with the results obtained with the sharp AFM tip. These are the first force measurements of nanoconfinement-induced long-range solidification of an IL. Repeatedly applied nanoconfinement of [HMIM] EtSO 4 between mica surfaces induces a long-range liquid-to-solid transition that remains after confinement has been removed.