Temperature evolution of the bulk nano-structure in a homologous series of room temperature ionic liquids

Nanoscale Structure in Short‐Chain Ionic Liquids

“…No T-dependent shift is discernible by eye in the position of peak I of C 6 , in agreement with our previous measurements. [22] We note that for n � 8 this peak upshifts with increasing T, [22,29] indicating contraction rather than the expected expansion. The strong overlap of peaks II and III precludes confident extraction by eye of T-and n-dependent trends in the raw curves in Figure 1, and inferring structural conclusions therefrom.…”

“…For C 4 this feature appears only as a very weak, but still discernible, shallow and broad convex bump, upshifted in q closer to peak II. [21,22,29] For C 1 -C 2 no corresponding bumps are observed. For C 3 a clear increase in intensity, relative to C 1 and C 2 , is observed in the 0:2 � q � 0:7 Å À 1 region, albeit having a concave shape rather than the convex peak-like shapes of C 4 and C 6 .…”

“…The measured I(q) for n ¼ 1 À 3 are plotted for the q-range relevant to the RTILs' nanoscale structure in Figure 1(a,b) at selected temperatures. For comparison, our published [22] curves for n ¼ 4; 6 are also plotted. The common notation [26,29,35,40] of the lowest diffraction peaks in the range q ≲ 2Å À 1 as I, II and III are shown in (b).…”

“…Peak I arises from the segregation of the apolar cationic alkyl tails from the charged moieties, the cationic imidazolium ring and the anion, by locally forming alternating layers of headgroups and chains. [22,27,29,41,42] Peak II is due to the same-charge moieties' spacing (i. e. cation-cation and anion-anion). [2,11,31,43] Peak III, known as the "adjacency peak", corresponds to the spacing of contacting atoms or small atom groups, [11] e. g. the cation-anion contacts, [26,31] possible cation or anion contact with the alkyl chains, the methylene-methylene contacts of the chains, etc.…”

“…In experimental studies occasional surprises occur as well, such as the recently-discovered charge underscreening, the origins of which are presently not understood, [16][17][18][19] or the surface freezing of long-chain RTILs at the liquid-air interface. [20] In recent publications [21,22] we have studied experimentally the evolution with cationic chain length and temperature (T) of the nanoscale structure observed in a broad homologous series of a model ionic liquid [C n mim] + [NTf 2 ] À , n being the number of carbons in the cationic alkyl chain. The nano-layered structure observed is a dominant feature in numerous ionic liquids, [23][24][25] appearing, however, only for n � n c , where the critical n c is usually a small number.…”