Nanosegregation and Structuring in the Bulk and at the Surface of Ionic-Liquid Mixtures

TY - JOUR

T1 - Nanosegregation and Structuring in the Bulk and at the Surface of Ionic-Liquid Mixtures

AU - Slattery, John Martin

AU - Bruce, Duncan Watson

AU - Cabry, Christopher Patrick

AU - Canongia Lopes, Jose Nuno

AU - Costen, Matthew L.

AU - D'Andrea, Lucia

AU - Grillo, Isabelle

AU - Marshall, Brooks C.

AU - McKendrick, Kenneth G.

AU - Minton, Timothy K.

AU - Purcell, Simon M.

AU - Rogers, Sarah

AU - Shimizu, Karina

AU - Smoll, Eric J.

AU - Tesa-Serrate, Maria A.

N1 - © American Chemical Society. This is an author-produced version of the published paper. Uploaded in accordance with the publisher’s self-archiving policy. Further copying may not be permitted; contact the publisher for details

PY - 2017/6/22

Y1 - 2017/6/22

N2 - Ionic-liquid (IL) mixtures hold great promise, as they allow liquids with a wide range of properties to be formed by mixing two common components, rather than by synthesizing a large array of pure ILs with different chemical structures. In addition, these mixtures can exhibit a range of properties and structural organization that depend on their composition, which opens up new possibilities for the composition-dependent control of IL properties for particular applications. However, the fundamental properties, structure and dynamics of IL mixtures are currently poorly understood, which limits their more widespread application. This paper presents the first comprehensive investigation into the bulk and surface properties of IL mixtures formed from two commonly encountered ILs: 1-ethyl-3-methylimidazolium and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][Tf2N] and [C12mim][Tf2N]). Physical property measurements (viscosity, conductivity and density) find that these IL mixtures are not well described by simple mixing laws, suggesting that their structure and dynamics are strongly composition-dependent. Small-angle X-ray and neutron scattering (SAXS and SANS) measurements, alongside molecular dynamics (MD) simulations, show that at low mole fractions of [C12mim][Tf2N], the bulk of the IL is composed of small aggregates of [C12mim]+ ions in a [C2mim][Tf2N] matrix, which is driven by nano-segregation of the long alkyl chains and the polar parts of the IL. As the proportion of [C12mim][Tf2N] in the mixtures increases, the size and number of aggregates increases until the C12 alkyl chains percolate through the system and a bicontinuous network of polar and non-polar domains is formed. Reactive atom scattering-laser-induced fluorescence (RAS-LIF) experiments, also supported by MD simulations, have been used to probe the surface structure of these mixtures. It is found that the vacuum-IL interface is enriched significantly in C12 alkyl chains, even in mixtures low in the long-chain component. These data show, contrary to previous suggestions, that the [C12mim]+ ion is surface active in this binary IL mixture. However, the surface does not become saturated in C12 chains as its proportion in the mixtures increases and remains unsaturated in pure [C12mim][Tf2N].

AB - Ionic-liquid (IL) mixtures hold great promise, as they allow liquids with a wide range of properties to be formed by mixing two common components, rather than by synthesizing a large array of pure ILs with different chemical structures. In addition, these mixtures can exhibit a range of properties and structural organization that depend on their composition, which opens up new possibilities for the composition-dependent control of IL properties for particular applications. However, the fundamental properties, structure and dynamics of IL mixtures are currently poorly understood, which limits their more widespread application. This paper presents the first comprehensive investigation into the bulk and surface properties of IL mixtures formed from two commonly encountered ILs: 1-ethyl-3-methylimidazolium and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][Tf2N] and [C12mim][Tf2N]). Physical property measurements (viscosity, conductivity and density) find that these IL mixtures are not well described by simple mixing laws, suggesting that their structure and dynamics are strongly composition-dependent. Small-angle X-ray and neutron scattering (SAXS and SANS) measurements, alongside molecular dynamics (MD) simulations, show that at low mole fractions of [C12mim][Tf2N], the bulk of the IL is composed of small aggregates of [C12mim]+ ions in a [C2mim][Tf2N] matrix, which is driven by nano-segregation of the long alkyl chains and the polar parts of the IL. As the proportion of [C12mim][Tf2N] in the mixtures increases, the size and number of aggregates increases until the C12 alkyl chains percolate through the system and a bicontinuous network of polar and non-polar domains is formed. Reactive atom scattering-laser-induced fluorescence (RAS-LIF) experiments, also supported by MD simulations, have been used to probe the surface structure of these mixtures. It is found that the vacuum-IL interface is enriched significantly in C12 alkyl chains, even in mixtures low in the long-chain component. These data show, contrary to previous suggestions, that the [C12mim]+ ion is surface active in this binary IL mixture. However, the surface does not become saturated in C12 chains as its proportion in the mixtures increases and remains unsaturated in pure [C12mim][Tf2N].

UR - http://www.scopus.com/inward/record.url?scp=85021634266&partnerID=8YFLogxK

U2 - 10.1021/acs.jpcb.7b01654

DO - 10.1021/acs.jpcb.7b01654

M3 - Article

SN - 1520-6106

VL - 121

SP - 6002

EP - 6020

JO - Journal of Physical Chemistry B

JF - Journal of Physical Chemistry B

IS - 24

ER -