Polymorphism, ionic liquids

Some other studies showed that the combination of the three polymorphs with reduced crystallite size and high surface area can lead to the best photocatalysts for 4-chlorophenol degradation [37], or that particles in the dimension range 25-40 nm give the best performances [38]. Therefore, many elements contribute to the final photocatalytic activity and sometimes the increased contribution of one parameter can compensate for the decrease of another one. For example, better photocatalytic activity can be obtained even if the surface area decreases, with a concomitant increase in the crystallinity of the sample, which finally results in a higher number of electron-hole pairs formed on the surface by UV illumination and in their increased lifetime (slower recombination) [39]. Better crystallinity can be obtained with the use of ionic liquids during the synthesis [39], with a consequent increase of activity. 

The disordered nature of the a-AgI structure may be regarded as intermediate between that of a typical ionic solid in which every lattice site is occupied and a typical ionic liquid in which both anions and cations are disordered. This is borne out by entropy calculations based on heat capacity data (O Keeffe and Hyde, 1976). These show a large increase in entropy at the P (or y) to a transition and a similar increase in entropy on melting of the a polymorph at 557 °C ... 

Holbrey, J. D., Reichert, W. M., Nieuwenhuyzen, M., Johnston, S., Seddon, K. R., and Rogers, R. D., Crystal polymorphism in l-butyl-3-methylimidazolium halides Supporting ionic liquid formation by inhibition of crystallization, Chem. Commun., 1636-1637, 2003. 

Hayashi, S., Ozawa, R., and Hamaguchi, H., Raman spectra, crystal polymorphism, and structure of a prototype ionic-liquid [bmimJGl, Chem. Lett., 32, 498-499,2003. 

In terms of supramolecular chemistry, imidazolium type ionic liquids are interesting because they are excellent C-H hydrogen bond donors. The X-ray crystal structure of bmim Cl shown this property clearly with C-Cl 3.39 A, Figure 13.24. Interestingly, two different polymorphic forms of this material are known depending on whether it is crystallised from the molten ionic liquid for from solution,... 

Jayaraman S, Maginn EJ (2007) Computing the melting point and thermodynamic stabilty of the orthorhombic and monoclinic crystalline polymorphs of the ionic liquid l-n-butyl-3-methylimidazolium chloride. J Chem Phys 127 214504... 

Several new methods of crystal growth and polymorph search and form control were discussed at the recently held International Workshop on Crystal Growth of Organic Materials (CGOM). Crystallization in gels and ionic liquids and the mechanism for crystal growth in the presence of impurities/imposter molecules are some new methods for further reading. 

Investigations into the behaviour of ionic solids are important in the search for new battery and fuel cell electrolytes. Defects, plastic phases, disorder and polymorphism in crystalline solids (and their association with ionic conductivity) and associated solid-solid equilibria are thus of considerable interest. The basis of our understanding of these phenomena and their relationship to the characteristics of the con5)onent ions includes studies of materials that are ionic liquids. 

Among the earliest ionic liquids for which single-crystal characterisation of polymorphs has been reported are 1,3-dialkylimidazolium chlorides and bromides [230, 566]. Monoclinic and orthorhombic forms of [C4inim]Cl are associated with differences in butyl conformation, with a conputational study [567] suggesting that the free energy difference associated with a single... 

More interest has been shown in ionic liquids derived from the 5- and 6-membered saturated nitrogen heterocycles, pyrrolidine and piperidine. iV,lV-dialkyl salts from these secondary amines display polymorphism f Section 11.2.31 as well as a propensity to frustrate attenpts to crystallise them Related materials (with some ring unsaturation, two heteroatoms or wiA fused ring structures) are discussed in Section 11.19.S. 

Chesman, A.S.R., Yang, M., Mallick, B., Ross, T.M., Gass, LA., Deacon, G.B., Batten, S.R. and Mudring, A.V, Melting point suppression in new lanthanoid(III) ionic liquids by trapping of kinetic polymorphs An in situ synchrotron powder diffraction study, Chem. Commun. 48 (1), 124-126 (2012). 

Kawahata, M., Endo, T., Seki, H., Nishikawa, K. and Yamaguchi, K., Polymorphic properties of ionic liquid of l-isopropyl-3-methylimidazolium bromide, Chem. Lett. 38 (12), 1136-1137 (2009). 

Cherukuvada, S. and Nangia, A., Polymorphism in an API ionic liquid Ethambutol dibenzoate trimorphs, CrystEngComm 14 (23), 7840-7843 (2012). 

Saouane, S., Norman, S.E., Hardacre, C. and Fabbiani, F.P.A., Pinning down the solid-state polymorphism of the ionic liquid [bmim][PFgl, Chem. Sci. 4 (3), 1270-1280 (2013). 

Yoshimura, Y, Abe, H., Imai, Y, Takekiyo, T. and Hamaya, N., Deconpression-induced crystal polymorphism in a room-tenperature ionic liquid, JV,JV-diethyl-iV-methyl-iV-(2-methoxyethyl) ammonium tetrafluoroborate, J. Phys. Chem. B117 (11), 3264-3269 (2013). 

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