Physicochemical Studies of Super-cooled LiquidsCyclic Carbonates and cc,P-Unsaturated AldehydesBY A. K. M. MASOOD AND R. A. PETHRICK*Department of Pure and Applied Chemistry, Thomas Graham Building,University of Strathclyde, Glasgow G1 1 XL, ScotlandANDF. L. SWINTONSchool of Physical Sciences, New University of Ulster,Coleraine, Northern IrelandReceived 3 1st January, 1975A range of measurements are reported on a series of substituted cyclic carbonates and a$-unsatur-ated aldehydes. The properties studied include the zero shear viscosity, thermal pressure coefficient,adiabatic compressibility, specific heat at constant pressure and the density. Certain of the liquidsstudied show a marked tendency to supercool and to exhibit pre-freezing phenomena. The magni-tude and temperature dependence of the properties studied are discussed in terms of cluster formationoccurring above the melting point.The source of the molecular interactions responsible for thecluster formation is discussed.Cluster formation or restriction of free isotropic rotation ’- has been suggestedas a prerequisite for the observation of pre-freezing phenomena in bulky covalentmolecular systems. The a priori prediction of the occurrence of pre-freezing pheno-mena requires a detailed understanding of the importance of the intermolecularpotential, the molecular anisotropy and of the chemical structure on local mobilityin the liquid state. The present study was undertaken in an attempt to identify theorigins of the observed variations of the physicochemical properties of a series ofsimple liquids.Previous acoustic attenuation studies of the cyclic carbonates andof the a,P-unsaturated aldehydes suggested that these systems exhibit very largetemperature coefficients of viscosity and a marked tendency to supercool.The cyclic carbonates form an ideal group for this type of study since they arerigid molecules of essentially similar size.6 Within the group studied the directionand magnitude of the dipole moment varies significantly. The a,a-unsaturated alde-hydes were studied for comparison purposes since they have a dipole of similarmagnitude but are flexible structures and possess significantly different rotationalvolumes. Studies of these two closely related groups of organic compounds allowsthe effects of size, flexibility and intramolecular potential to be identified.EXPERIMENTALFour cyclic carbonates were studied : Q-rnethyl-l,3-dioxolan-2-one, (propylene carbonate)obtained from B.D.H., 4-chloromethyl-l,3-dioxolan-2-one, (4-chloromethyl carbonate)obtained from Ralph N.Enianuel and Fluka A. G. (both samples were found to have identi-cal physical properties), 4-chIoro-1,3-dioxolan-2-one and 4,5-dichloro-1,3-dioxolan-2-oneobtained from Fluka. The three aldehydes, cinnamaldehyde, a-methylcinnamaldehydeand a-n-hexylcinnamaldehyde were obtained respectively from May and Baker, Kodak and2A . R. M. MASOOD, R . A . PETHRICK AND F . L . SWINTON 21K & K Laboratories. All samples were redistilled under reduced pressure and dried overmolecular sieves before use.In the case of 4-chloro- and 4,5-dichloro-l,3-dioxolan-2-onedecomposition is accompanied by darkening of the sample. Precautions were taken toavoid decomposition influencing the experimental observations. No detectable chemicalinstability was found in the remaining compounds studied. The physical properties, boilingpoints and refractive indices agree well with literature data.7*The following physical properties were measured on the cyclic carbonates and thea,B-unsaturated aldehydes.(i) Thermal expansion coefficient (ap) and density (p) were measured using a variety ofdilatometers calibrated with standard fluids and capable of being used below room tem-pera t ure .(ii) Thermal pressure coefficient at constant volume (yU) was obtained using an apparatuswhich is a modification of that originally described by Gee et aL9(iii) The adiabatic compressibility (&) was obtained from sound velocity measurementsas described elsewhere.' O(iv) The specific heat at constant pressure (C') was obtained using a Perkin Elmer DSCdifferential scanning calorimeter calibrated against indium and n-octane.l ' The highestinstrumental sensitivity and a scan rate of 8 K per minute were used. These data werecollected over a limited temperature range of 20 K per scan and are assumed to have anaccuracy of better than & 5 %. It should be appreciated, as will be discussed later, that thecurves obtained are sensitive to the precise thermal history of the sample.(v) The zero shear viscosity (Q) was obtained using a series of calibrated Ubbelohde sus-pended level viscometers (BS.IP.SL(S)71).12 All the liquids were passed through a sinterglass filter to trap solid particles.The temperature precision of all the experiments was better than +O.l K except in thedetermination of thermal pressure coefficient and the adiabatic compressibility where it waskO.003 K and *O.Ol K respectively.The pressure was measured in the thermal pressurecoefficient experiment to better than kO.0068 bar.RESULTS AND DISCUSSIONThe cyclic carbonates form an ideal system for the study of possible correlationsbetween the nature of molecular interactions and the observed bulk time volumeaveraged thermodynamic properties.indicated that slight puckering of the ring may occur in the case of the highly sub-stituted dichloro compound whilst the remainder possess slightly skewed planarstructures.The a,B-unsaturated aldehydes possess a flexible structure due to internalrotation of the carbonyl group and hence significantly different spatial extensionsand intermolecular interactions.Spectroscopic studies of the cyclic carbonatesVISCOSITY DATAThe cyclic carbonates and the unsaturated aldehydes were studied over a tempera-ture range from just above their melting points to approximately 353 K and theresults are shown in fig. 1. Most simple liquids exhibit linear plots of bog (q) againstreciprocal temperat~re,~ however exceptions to this ideal behaviour have been ob-served in the region of the melting point in certain systems.1 If these deviations werelarge the liquids showed a marked tendency to form glasses.Earlier studies of anumber of rather complex organic molecules such as the phenylbenzenes l5 and theterphenyls, indicated that the observation of so-called pre-freezing phenomena maybe associated with either cluster formation or with a restriction of the free isotropicrotatiofl of the molecules in the system. Both 4-methyl-l,3-dioxolan-2-one and4,s-dichloro- 1,3-dioxolan-2-0ne show only slight tendencies to supercool, exhibitalmost ideal Arrhenius behaviour and form glassy solids. The 4-~hloromethyl-l,3-dioxolan-2-one and 4-chloro- 1,3-dioxdan-2-0ne both show significant departun22 CYCLIC CARBONATES AND OI,~-UNSATURATED ALDEHYDESfrom ideal Arrhenius behaviour and possess a marked tendency to supercool.Bothliquids can be supercooled up to twenty degrees below their melting points and yetstill form apparently stable liquids. The three aldehydes show deviations from theideal Arrhenius behaviour similar to those observed in the cyclic carbonates.103 KITFIG. 1 .-Experimental viscosity against temperature relationships for the cyclic carbonates and theunsaturated aldehydes. A, 4-chloromethyl-l,3-dioxolan-2-one (m.p. 270 K) ; B, 4-chloro-1,3-dioxolan-2-one (m.p. 240 K) ; C, 4,5-dichloro-l,3-dioxolan-2-one (m.p. 273 K) ; D, 4-methyI-1,3-dioxolan-Zone (m.p. 232 K) ; E, a-n-hexyldnnamaldehyde (m.p. 282 K) ; F, cinnamaldehyde (m.p.275 K) ; G, cc-methyl cinnamaldehyde (m.p.265 K). Melting points indicated by arrows.The enhanced viscosity of the liquid as the melting point is approached may beconsidered in terms of the occurrence of cluster formation. If a volume fraction ofthe liquid consists of regions that move together like colloidal particles, the actualviscosity ve will be enhanced relative to that of the hypothetical supporting mono-molecular fluid tfm according to the equationwhere 9 is the volume fraction of clusters and the above equation can be shown tohold approximately provided that 9 does not exceed 0.3.l Applying this treatmentto the above systems and assuming that the viscosity of the monomolecular fluid canbe obtained by extrapolation of the linear high temperature region leads to fig.2.The plots are approximately linear as expected from the simple theory. If it is assumedthat cluster formation is controlled by a simple distribution equation then a plot ofy,/vm = 1+2.54+7$2 (1A . K . M. MASOOD, R. A . PETHRICK AND F . L . SWINTON 23log [( 1 - 4)/4] against 1 /T yields the mean enthalpy of formation for a cluster. Thedata obtained from this analysis are summarized in tabk 1. It is clear from the datathat the stabilization energy is larger in the cyclic carbonates than in the unsaturatedaldehydes. The errors in the determination of the enthalpies shown in table 1 arelarge although it would appear that it is a balance between the enthalpy and entropywhich is primarily responsible for cluster formation in these systems."t3.5 4.0 4.5103 KITFIG.2.-Plots of log 4 against reciprocal temperature. I$ is the volume fraction of clusters calcu-lated from eqn (1) (key as fig. 1).TABLE 1 .-EQUILIBRIUM DATA FOR CLUSTER FORMATION DERIVED FROM VISCOSITY MEASURE-MENTScompound4-chloromethyl-l,3-dioxolan-2-one4-methyl-l,3-dioxolan-2-one4-chloro-l,3 -dioxolan-2-one4,5-dichloro-l,3-dioxolan-2-onea-methylcinnamaldehydea-n-hexy lcinnamaldehy decinnamaldehydetemperaturerange/K278-303237-247243-293283-270293-270320-277305-270enthalp y I entropy /kJ mol-1 kJ mol-1 K-121.9 0.75420.2 0.84514.9 0.62712.7 0.5854.23 0.2807.03 0.3558.50 0.426THERMAL PRESSURE COEFFICIENTSThe thermal pressure coefficients were measured over a comparable temperaturerange to that used in the study of the viscosity for the cyclic carbonates and are shownin fig.3. These molecules behave abnormally in that their thermal pressure coeffi-cients are typically one and a half to two times larger than those found in comparableorganic liquids and also exhibit a tendency towards temperature independence astheir melting points are approached. Normal liquids exhibit thermal pressurecoefficients which fall continuously with increasing temperature.l The therma24 CYCLIC CARBONATES AND CL,#~-UNSATURATED ALDEHYDESpressure coefficient reflects the combined effects of " free " volume and the strengthof the intermolecular forces betwea the constituent molecules. The low values ofthe " compressibility " observed in the cyclic carbonates would suggest that themolecules are relatively closely packed but sufficiently disordered not to form solids.DENSITIESThe density data presented in table 2 indicate that 4-methyl-l,3-dioxolan-2-onehas a significantly higher molar volume than that of 4-chloro-l,3-dioxolan-2-one atcomparable temperatures.These molecules have similar " molecular " volumes andthe difference in their molar volumes indicates large differences in the magnitude ofthe " free " volume of these systems. A lowering in the free volume appears to beconsistent with the occurrence of deviations from ideal behaviour accompanying an12 6 0 3 0 0 3 4 0TIKFIG, 3.--y,,, the thermal pressure coefficients for the cyclic carbonates (key as fig. 1).increase in the cohesive energy.If it is assumed that deviations in the transportproperties are associated with the effects of free volume it may be expected that adiscontinuity in the thermal expansion coefficient similar to that observed in polymersin the vicinity of the glass transition temperature should be observed. No such dis-continuities were observed within the precision of these experiments. It may thcre-fore be suggested that the deviations from ideal behaviour must arise as a consequenceof enthalpic and entropic effects as much as from free volume limitations.ADIABATIC COMPRESSIBILITYContrary to the observations of the thermal pressure coefficients the adiabaticcompressibilities of the cyclic carbonates and unsaturated aldehydes shown in Q.4exhibit no significant deviations from apparently linear behaviour. Note, however,that those liquids with high values of the adiabatic compressibility also show the leasttendency to depart from ideal behaviourA . K. M, MASOOD, R. A . PETHRICK AND F . L . SWINTON 25SPECISIC HEAT AT CONSTANT PRESSUREThe specific heat at constant pressure for the cyclic carbonates was measured overan extended temperature range covering the region of interest in the viscosity studies,fig. 5. The plots obtained are similar to those previously reported for terphenyl l7TABLE 2.-DENSITIES OF CYCLIC CARBONATES AND UNSATURATED ALDEHYDESp/g ~ r n - ~ = a+b(T/K-273.2)a4-methyl-l , 3-dioxdan-2-one 1.241 154-chloromethyl- 1,3-dioxolan-Z-one 1.482 034,5-dichloro-l,3-dioxolan-2~one 1.619 16Cchloro-1,3-dioxolan-2-one 1.552 34cinnamaldehyde 1,089 5a-me t hylcinnamaldeh yde 1.058 2a-n-hexylcinnamaldehyde 1.049 5b x 1030.621 70.955 81.099 71.400 10.8610.877 50.860and the other glass-forming solids.The specific heat plots shown in fig. 5 representthe average of a number of measurements. In practice the exact curve is a functionof the thermal history of the sample. In the case of the study on terphenyl l7 it waspossible to obtain the specific heat data for a crystal of the material. In the presentcase it proved impossible to seed a crystal and the corresponding data are not available.263 2 83 3 0 3 323T KFIG. 4.-&, the adiabatic compressibility (key as fig. 1).It was found that for the glassy material the general shape of the curve changed littlewith thermal cycling and the uncertainty in the data reported is approximately ;fi: 5 %.Comparison of the traces with the previous thermodynamic observations Suggeststhat a correlation exists between the upturn at the high temperature end of the plateauregion and the onset of the ideal behaviour in these systems.A detailed investigationof the traces indicated that certain of the molecules possess exo- and endo-thermi26 CYCLIC CARBONATES AND C@UNSATURATED ALDEHYDESproperties which are modified by thermal cycling. No melting transition wasobserved in 4-methyl-l,3-dioxolan-2-one ; however, 4,5-dichloro- 1,3-dioxolan-2-0neshowed semblances of such a process at 269 K. In the latter system the amplitudeof the transition was very much a function of the thermal history of the sample.Transitions were observed with variable amplitude in both 4-chloro- and 4-chloro-methyl-l,3-dioxolan-2-one at respectively 264 and 285 K.It is clear that the exo-therms are not associated with “ melting ” transitions which should occur 10-20 Klower than these temperatures and it is more reasonable to suggest that these observa-tions may in some way be attributed to the onset of ‘‘ free ” isotropic rotation of thewhole molecule. In all cases the magnitude of the transitions were small and difficultto quantify and have therefore not been included in fig. 5. A correlation betweenthe observed specific heat and free rotation has been proposed for anisotropic mole-cules and is compatible with the above data.2* The glass transition temperatureshave not been satisfactorily established for these materials but may be expected tocorrelate with low temperature change in slope of the specific heat curve.The sen-sitivity of the curves and transitions to the thermal history of the sample would suggestthat a free rotational model should be favoured but does not preclude these effectsarising from the break-up of local structure in the 1iq~ids.l~25T/KFIG. 5.-Cp, the heat capacity at constant pressure (key as fig. 1).CALCULATION OF THE PAIR-WISE INTERACTION ENERGYConsideration of the basic structures of the cyclic carbonates suggests that signifi-cant dipolar interactions may occur via the carbonyl group.Calculations wereperformed using van der Waals and dipolar interactions, the former being calculatedon the basis of standard Lennard-Jones 6-12 potentials as described elsewhere.2oThe pair of molecules were assumed to line-up with their carbonyl groups parallelbut in opposite directions. In this configuration the planes of the rings will beparallel and the separation between the rings is taken as a variable. The carbonyldipole moment is assumed to have a value of 10.7 x C m and the van der WaalA . K . M. MASOOD, R . A . PETHRICK AND F . L . SWINTON 27interactions up to 600 pm are considered to be significant. The interaction potentialwas calculated at a series of separations and minimum energy condition obtained byan iterative process. The minimum in the potential (fig.6 ) occurs at approximately320 pm and has a stabilization energy of 4.57 kJ mol-I. It is apparent from this cal-culation that the carbonyl interactions do in fact form a source of significant inter-action which may be considered as the driving force for the formation of clusters inthe liquid but do not totally explain the observed effects. A similar dipolar inter-action may be expected to occur between the carbonyl groups of the unsaturatedaldehydes.32I- 2- 3- 4\ 300 400 500 600FIG. 6.-Potential energy diagram for the pair-wise interaction of cyclic carbonate rings.CONCLUSIONThe measurements reported on the cyclic carbonates and unsaturated aldehydessuggest that supercooling is associated with the occurrence of a large configurationalentropy l9 accompanied by a significant enthalpy of association arising from pairwiseintermolecular interactions and often a limitation on the free volume of the system.The cyclic carbonates exhibit deviations from ideal behaviour as a result of cluster-ing favoured by dipolar interactions involving the ring carbonyl groups.This cluster-ing may involve a significant number of molecules forming dynamic cylindrical struc-tures. The non-ideal nature of the clusters which may be associated with the aniso-tropy of the molecules adds to the configurational entropy of the liquids which inturn leads to an increase in the viscosity. It is perhaps significant that the occurrenceof a dipole component perpendicular to the carbonate ring assists the cluster formationand leads to the marked deviations from ideal behaviour.The perpendicular com-ponent will assist the alignment of the rings and stabilise the pairwise interactionfurther. It is clear that a more detailed understanding of the mechanism responsiblefor the observed premelting phenomena requires characterization of the detailedmotion of the constituent molecules28In the unsaturated aldehydes the pendant group appears to produce cooperativeinteractions and leads to the observed deviations from ideality. It is also clear thatin the latter series the strengths of the interactions are lower than in the cyclic carbon-ates (table 1) and it must be assumed that configurational effects possibly involvingthe flexible end-groups play an important role in the observed behaviour.CYCLIC CARBONATES AND CC,/?-UNSATURATED ALDEHYDESA.R. Ubbelohde, Melting and Crystal Structure (Clarendon, Oxford, 1965).D. B. Davis and A. J. Matheson, Disc. Furuday Soc., 1967, 43, 216,D. B. Davis and A. J. Matheson, Trans. Fnraday Sue., 1967, 63, 596.R. A. Pethrick, E. Wyn-Jones, P. C. Harnblin and R. E. M. White, J. Chem. Soc. A , 1969,1852.C. J. Brown, Acta Crysi., 1954,7, 92.N. F. Grishchenko and V. N. Pokorskii, Nefteppererab Nefiekhim., 1966, 33 (Chem. Abs. 1966,54960~).P. C. Hamblin, Thesis (University of London, 1968).G. Allen, G. Gee, D. Mangaraj, D. Sims and G. J. Wilson, Polymer, 1960, 1,467.Perkin Elmer manual for DSC-1.A. R. Ubbelohde, J. Inst. Petroleum, 1973, 23, 427.’ R. A. Pethrick and E. 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