title: Effect of the Methylation and N-H Acidic Group on the Physicochemical Properties of Imidazolium-Based Ionic Liquids.
abstract: This work presents and highlights the differentiation of the physicochemical properties of the [C1Him][NTf2], [C2Him][NTf2], [(1)C1(2)C1Him][NTf2], and [(1)C4(2)C1(3)C1im][NTf2] that is related with the strong bulk interaction potential, which highlights the differentiation on the physicochemical arising from the presence of the acidic group (N-H) as well as the methylation in position 2, C(2), of the imidazolium ring. Densities, viscosities, refractive indices, and surface tensions in a wide range of temperatures, as well as isobaric heat capacities at 298.15 K, for this IL series are presented and discussed. It was found that the volumetric properties are barely affected by the geometric and structural isomerization, following a quite regular trend. A linear correlation between the glass transition temperature, Tg, and the alkyl chain size was found; however, ILs with the acidic N-H group present a significant higher Tg than the [(1)CN-1(3)C1im][NTf2] and [(1)CN(3)CNim][NTf2] series. It was found that the most viscous ILs, ([(1)C1Him][NTf2], [(1)C2Him][NTf2], and [(1)C1(2)C1Him][NTf2]) have an acidic N-H group in the imidazolium ring in agreement with the observed increase of energy barrier of flow. The methylation in position 2, C(2), as well as the N-H acidic group in the imidazolium ring contribute to a significant variation in the cation-anion interactions and their dynamics, which is reflected in their charge distribution and polarizability leading to a significant differentiation of the refractive indices, surface tension, and heat capacities. The observed differentiation of the physicochemical properties of the [(1)C1Him][NTf2], [(1)C2Him][NTf2], [(1)C1(2)C1Him][NTf2], and [(1)C4(2)C1(3)C1im][NTf2] are an indication of the stronger bulk interaction potential, which highlights the effect that arises from the presence of the acidic group (N-H) as well as the methylation in position 2 of the imidazolium ring.
authors.affiliation: †Centro de Investigação em Química, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, R. Campo Alegre 687, P-4169-007 Porto, Portugal.
authors.forename: Ana S M C
authors.initials: AS
authors.lastname: Rodrigues
authors.affiliation: †Centro de Investigação em Química, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, R. Campo Alegre 687, P-4169-007 Porto, Portugal.
authors.forename: Marisa A A
authors.initials: MA
authors.lastname: Rocha
authors.affiliation: ‡CICECO - Aveiro Institute of Materials, Chemistry Department, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal.
authors.forename: Hugo F D
authors.initials: HF
authors.lastname: Almeida
authors.affiliation: ‡CICECO - Aveiro Institute of Materials, Chemistry Department, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal.
authors.forename: Catarina M S S
authors.initials: CM
authors.lastname: Neves
authors.affiliation: §QOPNA Unit, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
authors.forename: José A
authors.initials: JA
authors.lastname: Lopes-da-Silva
authors.affiliation: ‡CICECO - Aveiro Institute of Materials, Chemistry Department, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal.
authors.forename: Mara G
authors.initials: MG
authors.lastname: Freire
authors.affiliation: ‡CICECO - Aveiro Institute of Materials, Chemistry Department, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal.
authors.forename: João A P
authors.initials: JA
authors.lastname: Coutinho
authors.affiliation: †Centro de Investigação em Química, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, R. Campo Alegre 687, P-4169-007 Porto, Portugal.
authors.forename: Luís M N B F
authors.initials: LM
authors.lastname: Santos