Soft Report - Dipartimento di Fisica - Sapienza

Phase Diagram of a Solution Undergoing Inverse MeltingDifferent theoretical models have been recentlyproposed to describe the counterintuitive phenomenaof inverse melting and inverse freezing [1-4]. Theseinverse transitions happen when a liquid heated atconstant pressure undergoes a reversible liquid solidtransition generating a solid with entropy higherwith respect to its liquid counterpart. A new liquidsystem showing this kind of phenomenology atnormal pressure and in conditions easily reachableexperimentally has been recently found [5]. It is asolution of α-cyclodextrin (αCD) (C 36H 60O 30), waterand 4-methyl-piridyne (4MP) (C 6H 7N) which in propermolar ratio give rise to the phenomenon of inversemelting. Differential scanning calorimetric (DSC)measurements have been performed with a DiamondPerkin-Elmer calorimeter to characterize the inversetransition of these solutions from an energetic pointof view. The thermograms, i.e. the heat flow (dH/dt)as a function of the temperature, reported in Figure1 have been obtained at a heating rate r = 10 K/min.Depending on the concentration, one, two or threepeaks of endothermic nature are observed. The peakand onset temperatures associated to each transitionare reported in Figure 2. At high concentrations ofαCD (right side of the phase diagram of Figure 2)three endothermic peaks are present: the firstcorresponds to a liquid solid phase transition typicalof those systems undergoing inverse melting; theintermediate one has been attributed to a solid solidphase transition and the third one is associated to asolid liquid transition as also observed by naked eye.The DSC data show a perfect agreement with theliquid solid transition temperatures determined withelastic and quasielastic neutron scatteringheat flow dH/dt (mW) Endo up1.00.50.00.01.00.50.50.00.20.01:6:951.01:6:800.51:6:701:6:501:6:40310 320 330 340 350 360 370 380T(K)Fig. 1: DSC thermograms of solutions of αCD,water and 4MP at different concentrations withmolar ratio 1:6:x respectively (40