Thermodynamics of C60 Solutes in Three Non-Aqueous Solvents

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1 Thermodynamics of C60 Solutes in Three Non-Aqueous Solvents William Steinsmith, MD Buckyball molecules (C60) forming solutes in any of three organic solvents have a eutectic-cusp triple-point at atmospheric pressure and near room temperature as shown in the phase diagram (below) abstracted from a 1993 NATURE paper (attachment 1): At this singular cusp point there coexist three binary phases a bi-saturated liquid solution phase and two crystalline solid phases, say, phase A and phase B. At any point along the left-branch of the cusp, there coexist two binary phases a saturated liquid phase and crystalline phase A. At any point along the rightbranch of the cusp, there coexist two binary phases a liquid solution phase and

2 crystalline phase B. When an experimentalist adds more C60 molecules to the saturated solution existing at any point along the left or right cusp branch, all the added C60 molecules aggregate to form, respectively, crystalline phase A or B. When C60 molecules are added at the cusp-triple- point, enlargement of both crystalline phases is conjectured, with the enlarging system remaining at the triple point. The dissolution of added C60 is apparently endothermic along the left branch and exothermic along the right branch, indicating that dissolution at the triple-point is adiabatic. If an experimentalist conducts the system up the left branch from 190K by integration of dk/dt he will come to a para-triple-point at which the liquid solution becomes saturated with both phase A and phase B, but, as yet, coexists with only phase A. From this state forward, experimental attempts to move the system down the right-branch curve via integration of dk/dt will be frustrated by a eutectic halt during which crystalline phase A is transformed into crystalline phase B, while all intensity variables hold constant and the system remains at the triple point (attachment 2). Only after the system reaches another para-triplepoint, at which the bi-saturated liquid coexists with only crystalline phase B, will it become experimentally feasible to move down the right-branch curve. ws ~ We have investigated the temperature-dependent solubility of C60 in hexane, toluene and CS2. We observe a solubility maximum near room temperature (around 280 K) for all three solvents We conclude that dissolution is endothermic below room temperature and exothermic above. We interpret this change as being due to a phase change in solid C60, presumably the phase change observed previously in the absence of a solvent A solubility maximum (or minimum) for organic compounds in non-electrolytes is highly unusual, and may be unprecedented The unusual temperature dependence is likely to be caused by the changes in the solid phase. We arrived at this hypothesis as a result of recognizing the nearly identical temperature dependence of k [ the mole-

3 fractionality of dissolved C60 in a saturated solution] in three different solvents The parameters derived are sensitive to the fact that the model requires a sharp maximum in contrast to the observed broad maximum This simple model explains only the salient features and does not fit the data perfectly. To understand the temperature dependence fully, we need, for example, to characterize the solid phases A and B, and to measure the heat of solution as a function of temperature. Anomalous Solubility Behavior of C60 (attachment 1) C60 In 1985, Professor Harry Kroto (UK) whilst working on the possible structures of interstellar carbon molecules approached Professors Curl and Smallery (US) to use their laser beam equipment so do lab simulations of carbon chain formation in star systems. The experiment carried out in September 1985 not only proved that carbon stars could produce the chains but revealed an amazing, serendipitous result - the totally unexpected existence of the C60 species Buckyball is a member of a class of carbon structures called fullerenes. Fullerenes are an allotrope (solid structure) of the element carbon - the best known being diamond and graphite. Fullerenes can be hollow spheres like buckyball, ellipsoid, or tubes (buckytubes)... (below)

4 click on the picture above to interact with the 3D model of the C60 structure (this will open a new browser window) C60 Buckyball is a member of a class of carbon structures called fullerenes. Fullerenes are an allotrope (solid structure) of the element carbon - the best known being diamond and graphite. Fullerenes can be hollow spheres like buckyball, ellipsoid, or tubes (buckytubes). Fullerenes are

5 similar in structure to graphite, which is composed of a sheet of linked hexagonal rings, but they contain pentagonal (or sometimes heptagonal) rings that prevent the sheet from being planar. The structure of C60 - buckminsterfullerene - is that of a truncated icosahedron, which resembles a round soccer ball of the type made of hexagons and pentagons, with a carbon atom at the corners of each hexagon and a bond along each edge. The molecule was named for Richard Buckminster Fuller, a noted architect who popularized the geodesic dome. In 1985, Professor Harry Kroto (UK) whilst working on the possible structures of interstellar carbon molecules approached Professors Curl and Smallery (US) to use their laser beam equipment so do lab simulations of carbon chain formation in star systems. The experiment carried out in September 1985 not only proved that carbon stars could produce the chains but revealed an amazing, serendipitous result - the totally unexpected existence of the C60 species. Kroto, Curl, and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of this new class of carbon compounds. Formal Chemical Name (IUPAC) References Update by Karl Harrison (Molecule of the Month for December 2005 )

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8 Phase Diagrams of Two-Phase Condensed Systems 1. Theory 1.1 Phase Rule and Equilibrium The phase rule, also known as the Gibbs phase rule, relates the number of components and the number of degrees of freedom in a system at equilibrium by the formula v = k f + 2 where v equals the number of degrees of freedom or the number of independent variables, k equals the number of components in a system in equilibrium and f equals the number of phases. The digit 2 stands for temperature and pressure. The number of degrees of freedom of a system is the number of variables that may be changed independently without causing the appearance of a new phase or disappearance of an existing phase. The number of chemical constituents that must be specified in order to describe the composition of each phase present. 1.2 Cooling Curves Phase diagrams for binary systems may be constructed by measuring cooling curves for different fixed compositions. This is accomplished by heating a sample of fixed composition above its melting point and measuring the temperature as a function of time while the mixture cools. Several cooling curves are shown in Fig. 1. for the phase diagram in Fig. 2. The first cooling curve is for the composition specified by the dashed line in Fig. 2. starting at b5 (f=1 v=2). As solid A begins to freeze out of the solution at b4, the rate of cooling decreases causing the slope of the cooling curve to change (f=2 v=1). When the point b2 is reached, the temperature remains constant until the complete mixture has solidified (f=3 v=0). The duration of solidification is called the eutectic halt. After solidification has occurred, the temperature resumes its steady decrease without further interruption. The second cooling curve in Fig. 1. is for a composition halfway between that for b5 and the eutectic composition. The third cooling curve is for the eutectic composition. Notice how the duration of the eutectic halt, which is maximum for the eutectic composition, increases as the eutectic composition is approached. 1

9 Fig. 1. Cooling curves for the phase diagram in Fig 2. Fig. 2. Temperature-composition diagram for a nearly immiscible binary mixture of solids and their completely miscible liquids 2. Measurement Temperature is measured using the iron-constantan (alloy of nickel and copper) thermocouple and plotted on the computer. The second end of the thermocouple is kept in ice water in a Dewards container. The sample is covered by carbon, whish protect it against oxidation. The melting furnace is with the crucible and the thermocouple turned on (switch 1 ). When the temperature in the melting furnace reach about 450 C, take out the crucible and put the 2

10 thermocouple into the melt. The thermocouple don t may contact the wall of the crucible. The melt is getting cold. The measurement is finished after the eutectic halt. The device is calibrated using pure tin. After third calibration, at least three measurements on tin-lead are performed. 2.1 Outline of Work 1. Insert ice into a Dewards container. 2. Turn on the computer. 3. Insert the thermocouple and the crucible with tin in the melting furnace. 4. Turn on the melting furnace (switch 1 ). 5. Melt the tin. 6. Take out the crucible and put the thermocouple into the Melt. 7. Measure the cooling curve. 8. Repeat three times. 9. Compute the correction (difference between freezing point of tin (231.8 C) and measured value). 10. Measure the cooling curve of tin-lead at least three times. The melt should be good mixed. 11. Turn off all devices. 2.2 Common Sources of Errors The melt is not mixed well. The thermocouple is contacting the wall of the crucible. The metal part of the crucible is standing out from the melt. 3