Soft Report - Dipartimento di Fisica - Sapienza

We found an increasing rigidity of micelles rising thetemperature over the percolation line.assigned to the existence of αCD/4MP complexes.Based on the crucial presence of a small amount ofwater, the transition is assigned to a rearrangementof hydrogen bonds, leading from a solvated αCD in4MP at lower temperatures to a regular network ofαCD molecules linked by water molecules at highertemperatures. The formed crystalline gel iscomposed of a fairly open regular structure of the4MP-αCD mixtureFig. 5. Here we show the light scattering data on AOTmicroemulsion, obtained by Sopra monochromator inbackscattering configuration (7.1 GHz), with theappropriate fitting function.signal HD-TG (a.u.)pure 4MP solventsound velocity (km/s)1,91,81,71,61,51,44MPαCD-4MPliquidphase1,3-40 -20 0 20 40 60Temperature (°C)solidphaseReferences[1] C.Ziparo, R.Eramo, I.Malfanti, C.M.C.Gambi andR.Torre. “Acoustic phenomena in AOT microemulsionsby frequency and time-resolved opticalspectroscopy”, in preparation.AuthorsR.Eramo (a,b), C.Ziparo (a,d), I.Malfanti (b,c),C.M.C.Gambi (a,c) and R.Torre (a,b,c). (a) INFM-CRS-Soft (CNR), Univ. la Sapienza. (b) LENS, Univ.di Firenze. (c) Dip. di Fisica, Univ. di Firenze.Freezing on heatingUnder the title “Freezing on heating” was recentlyreported the unexpected solidification upon heatingof a solution of a-cyclodextrine(αCD), 4methylpyridine (4MP) and water [1]. The solution ishomogeneous and transparent at ambienttemperature, and forms a milky white solid whenheated to temperatures between 45° and 75°. Thisprocess is fully reversible, on cooling the solid meltsand the original homogeneous solution is recovered.The role of water is essential, as demonstrated bythe incomplete solidification of samples withoutaddition of water (ca. 10mg/ml). A major argumentfor the explanation of the phase transition is thenegative solubility of the αCD in 4MP, that dropsnearly vertically around 75°C, or at lowertemperature when more water is added to thesolution [2]. Diverse experimental techniques havebeen used to characterize the liquid and solid phasesof the mixture, and the mechanisms driving thisunusual phase transition. X-Ray diffraction showedthat the solid phase is crystalline, and undergoesseveral phase transitions between 60 and 100°C.The lowest temperature phase is metastable, but 4other are stable and reversible with large hysteresisin temperature. Two of the transitions are alsoobserved in DSC measurements. Quasi elasticneutron scattering indicates that the dynamics of thecyclodextrine in the solid phase slows down (> 500ps) while a large part of the solvent stays liquid overthe whole temperature range, moving oncharacteristic timescales of ps. In the liquid phase isalso detected a motion slower than the solvent itself,1E-8 1E-7 1E-6 1E-5time (seconds)Fig. 6. HD-TG signal of the pure solvent and of themixture, at room temperature. The right part of thefigure shows the variation of the sound velocity inthe αCD/4MP solution, that tends to join the 4MPbehavior when increases the temperature,extrapolated (solid line) from experimental points(square).αCD molecules, the remaining space being filled with4MP molecules, which remain partially mobile. In atypical solution of 300 mg/ml of αCD in 4MP, withca.10mg/ml of water added, the solid phase isformed of crystallites of 5 to 30 micron length,surrounded by liquid solvent. Acoustic studies,through Transient Grating spectroscopy, probedirectly the viscoelastic properties of the solution.We therefore started to investigate structuralchanges in the liquid and pre transitional effectsusing this technique. The reorganization of thecomplex αCD/4MP could be responsible for thenegative solubility, by undergoing a change in itsstructure to which TG measurements will besensitive. A first comparison of the propagation ofthe acoustic wave in the pure solvent and αCD/4MPmixture exhibit very different behavior of bothsamples at room temperature (see on figure 6),difference that increases when the temperaturedecreases. Using the property that the solid phaseforms slowly (ca. 30 min), pre-transitional effectswere investigated at fixed temperature (following atemperature jump) by measurements at variousincreasing times: no change was observed in thedynamics of the liquid approaching the solidification.The system transforms directly from the solvatedαCD in 4MP into the crystalline phase. Preliminarymeasurements have been performed in the hightemperature phase (mixed liquid and solid). Onfigure 6 is shown the behavior of the sound velocityin the pure 4MP and the αCD/4MP solution: the liquidpart of the sample tends to join the pure 4MPbehavior. Surprisingly, the sound wave is notaffected by the solid phase, how can be expectedfrom this media presenting heterogeneities on themicrometer scale.83SOFT Scientific Report 2004-06