resumo
The heat capacity and thermal behavior trend along the 1-alkyl-3-methylimidazolium hexafluorophosphate [C(n)C(1)im][PF6] (with n = 2-10,12) ionic liquids series, is used to explore the effect of the alkyl chain length in the nanostructuration. The heat capacities of the studied ILs were measured with an uncertainty better than +/- 0.15% and are in excellent agreement with the available data in the literature. An odd-even effect for the specific and volumic heat capacities of the [C(n)C(1)im][PF6] series was found. The observed odd-even effect in the liquid heat capacity was rationalized considering the preferential orientation of the terminal -CH3 group. The higher specific/volumic heat capacities shown for the [C(n)C(1)im][PF6] and [C(8)C(1)im][PF6] are an indication of an additional conformational disorder increase in the liquid phase that could be related with a weaker alkyl chain interdigitation capability of the even number chain ILs. The melting temperatures and consequent Delta H-1(s)m(0) and Delta S-1(s)m(0) trend along the alkyl series present a V-shape profile that is explained based on the analysis of the balance between the initial decrease of the electrostatic interaction potential and the increase of the van der Waals interactions with the increasing size of the alkyl side chain of the cation, The inhibition of crystallization for intermediate alkyl chain size (from [C(n)C(1)im][PF6] to [C(8)C(1)im][PF6]) seems to arise from the overlapping of the hypothetical cold crystallization temperature by the melting temperature. Above the critical alkyl size, CAS, a regular increase in the entropy and enthalpy profiles presents a similar shape than the observed in other alkane series and is a strong support of the intensification of the ILs nanostructuration. (C) 2017 Elsevier B.V. All rights reserved.
palavras-chave
TEMPERATURE IONIC LIQUIDS; ALKYL-CHAIN-LENGTH; PHYSICOCHEMICAL PROPERTIES; THERMODYNAMIC PROPERTIES; MELTING-POINTS; THERMOPHYSICAL PROPERTIES; HOMOLOGOUS SERIES; CATION SYMMETRY; 1-BUTYL-3-METHYLIMIDAZOLIUM HEXAFLUOROPHOSPHATE; IMIDAZOLIUM HEXAFLUOROPHOSPHATE
categoria
Chemistry; Physics
autores
Serra, PBP; Ribeiro, FMS; Rocha, MAA; Fulem, M; Ruzicka, K; Coutinho, JAP; Santos, LMNBF
nossos autores
Projectos
agradecimentos
Thanks are due to Fundacao para a Ciencia e Tecnologia (FCT through the project SFRH/BD/94211/2013), Lisbon, Portugal and to FEDER for financial support to Centro de Investigacao em Quimica, University of Porto through the project Pest-C/QUI/UI0081/2013 and NORTE-01-0145-FEDER-000028, Sustainable Advanced Materials (SAM). CICECO, University of Aveiro, through the project Pest-C/CTM/LA0011/2013. Paulo B.P. Serra acknowledges the financial support from specific university research (MSMT No. 20/2014). Filipe M.S. Ribeiro acknowledges the financial support from FCT and the European Social Fund (ESF) under the Community Support Framework (CSF) for the award of his Research Grants, SFRH/BD/94211/2013. The authors gratefully acknowledge a partial financial contribution from the COST (project CM1206 on "Exchange on Ionic Liquids").