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Efficient determination of crystallisation and melting points at low cooling

and heating rates with novel computer controlled equipment

Philipp Wachter a, Hans-Georg Schweiger a,b, Franz Wudy a, Heiner J. Gores a,*

a Institut fr Physikalische und Theoretische Chemie der Universitt Regensburg, Universittsstrae 31, D-93040 Regensburg, Germanyb Continental Automotive Systems Division, Sickingenstrae 29-38, D-10553 Berlin, Germany

a r t i c l e i n f o

Article history:Received 17 February 2008

Received in revised form 17 May 2008

Accepted 19 May 2008

Available online 28 May 2008

Keywords:

Phase transition points

Organic solvents

Ionic liquids

Crystallisation aids

Fast computer-controlled equipment

a b s t r a c t

We studied melting and solidification points of 14 pure solvents and two ionic liquids with a recentlyconstructed automatic computer-controlled equipment, which is able to record simultaneously temper-

aturetime functions of up to 30 samples at very low heating and cooling rates down to 1.5 K h1. The

effects of viscosity of the studied samples and of carbon fibres as an added crystallisation aid were also

investigated. Equilibrium temperatures for the solidliquid phase transition are in accordance with liter-

ature for materials that were often checked, such as acetonitrile, showing the quality of our new equip-

ment, whereas the value of the transition temperature of some other materials differed from published

results. It is shown that both the viscosity of the material and carbon fibres as crystallisation aids have an

effect on supercooling. The value given for the equilibrium point of the ionic liquid trioctylmethylammo-

nium trifluorocetate Ttr= (285.62 0.1) K is new.

2008 Elsevier Ltd. All rights reserved.

1. Introduction

Phase transition temperatures are not only important proper-

ties for the characterisation of materials but they also play a vital

role in technical applications. For example, the operating range of

several electrochemical devices for energy storage and transforma-

tion, such as lithium-ion-batteries, double layer capacitors, and

photo-electrochemical or dye-sensitised solar cells is limited by

the freezing temperature of the applied electrolyte. Therefore the

knowledge of precise liquid to solid phase transition temperatures

plays an essential role in the formulation of these electrolytes.

The main problem of all discussed methods is the determina-

tion of an equilibrium property (a freezing point) by heating or

cooling, whereas the system does not reside in the equilibrium

state. The determination of invariant points (degree of freedom

f = 0) in single component systems (triple points) can be readily

performed with high accuracy, often better than 5 mK. Therefore

values of the reference temperature on the international tempera-

ture scale (ITS) are based on these points, such as the triple point

temperature of water at 273.160 K. It is commonly known, that

univariant points (f= 1) can be measured often with high precision.

Recently, differential thermal analysis (DTA) and differential

scanning calorimetry (DSC) were applied mainly to investigate of

melting and freezing points[13]. These methods are fast and re-

quire only small sample volumes but are also less accurate [4]. Dueto the small sample volumes, they are subject to systematic errors.

Additionally, the applied cooling rates may enhance effects of

supercooling on the substances studied.

The effects of supercooling can be a severe source of errors, if li-

quid to solid phase transitions are observed. For common solvents,

supercooling can reach levels of 15 K and more. If phase transition

temperatures of ionic liquids are measured, the supercooling effect

is even worse and can provoke errors up to 200 K[5], making the

determination of phase transition temperatures almost impossible.

A further disadvantage of these methods is the use of very small

sample masses in the range of (1 to 100) mg. As a result of these

small sample masses, the effects that are caused by the wall of

the sample vessel become strongly apparent. In addition, even

marginal amounts of impurities play a major role, because they

strongly falsify the true phase transition temperatures.

A good homogenisation of the sample is an absolute must, if

phase transitions in mixtures are examined, so that the composi-

tion of the solution remains constant and corresponds to the equi-

librium composition. Conventional DTA/DSC devices have no

homogenisation equipment at all, restricting the ability to investi-

gate multi-component systems.

As an alternative to the above-mentioned methods, the investi-

gation of cooling curves may be appropriate as presented by An-

drewet al. [6]; other examples of this approach are shown in the

literature[4,7]. This method requires the transfer of a sample into

a stirred container and observation of the change of the tempera-

ture within the sample, while the sample is being very slowly

0021-9614/$ - see front matter 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.jct.2008.05.012

* Corresponding author. Tel.: +49 941 943 4746/9402 8040; fax: +49 9402 8035.

E-mail addresses: [email protected], w.heitzer_h.j.gores

@t-online.de (H.J. Gores).

J. Chem. Thermodynamics 40 (2008) 15421547

Contents lists available at ScienceDirect

J. Chem. Thermodynamics

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j c t
mailto:[email protected]:[email protected]:w.heitzer_h.j.gores%[email protected]:w.heitzer_h.j.gores%[email protected]://www.sciencedirect.com/science/journal/00219614http://www.elsevier.com/locate/jcthttp://www.elsevier.com/locate/jcthttp://www.sciencedirect.com/science/journal/00219614mailto:w.heitzer_h.j.gores%[email protected]:w.heitzer_h.j.gores%[email protected]:[email protected]


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cooled down or heated up. Applying this procedure, a noticeable

increase of the accuracy occurs, because larger sample volumes

can be used. As shown by Rossini et al. [4], the melting point of

pure substances showing no supercooling effects can be measured

with an uncertainty of 10 mK. By this method, it is possible to

determine the melting point of acetonitrile with an uncertainty

of (13) mK [8,9]. To reach this level of accuracy, a huge effort must

be made to eliminate all systematic errors, such as heat conduction

via the thermometer, the Joule heating of the thermometer, insuf-

ficient equilibration, and pressure effects. Due to the depression of

the freezing point caused by impurities, the required purity level

of the investigated substances is tremendous. For example, Rich-

ards observed a freezing point depression of 30 mK caused by dis-

solved air in benzene[10].

The observation of melting processesby this method is also diffi-

cult, just like their determination using DTA/DSC, unless the sample

is mixed sufficiently [11]. A solid, which is in the sample container,

melts beginning from an edge of the sample container towards the

centre, where the temperature sensor is usually attached. This sen-

sor is then still surrounded by the solid, despite the melting process

has already started. Therefore, the melting points determined with

this method are usually strongly affected by errors that can be only

reduced by an effectivemixingdevice. Large coolingor heating rates

areleadingto T(t) curves that canbe interpretedeasily but areprone

to errors, e.g. dueto supercooling. If lowrates arechosen,the thermal

effect of the phase change is blurred by the heat transfer to the sur-

roundings thereby making interpretation of the curves impossible.

This results in increased errors in the range from 0.1 K [4] u p t o 1 K

[1214] and they can reach upto 10 K [4].

Another similar non-equilibrium method is described by Schr-

dleet al. [15], making use of a photo detector to determine the

phase transitions. However, this method is limited to transparent

samples and gives no information if supercooling has occurred.

In this article, we present a new apparatus based on the mea-

surement of cooling and heating curves. With this apparatus, the

rapid and simultaneous investigation of up to 30 samples is possi-

ble, therefore enabling the investigation of the influence of variousparameters, such as viscosity, cooling and heating rate, and crystal-

lisation aids on phase transition points or supercooling in adequate

times.

2. Theory

The test sample is treated by a temperature profile linear in

time for all methods presented above, while the change of the sam-

ples temperature is recorded over time. If a phase transition occurs

in a single component system, a halt is observed due to the phase

transition enthalpy. Because in this experiment, retarded crystalli-

sation or supercooling can occur, a curve similar to the one shown

in figure 1 is commonly observed. Therefore, it may be necessary to

determine the crystallisation point by extrapolation of the horizon-

tal or quasi-horizontal parts of the curves, also illustrated in figure

1. The intersection point of the two curves determines the crystal-

lisation point. Of course, supercooling effects cannot be noticed in

heating curves. But these experiments point out homogenisation of

the samples to be the crucial factor. This problem cannot be solved

easily, because samples containing large amounts of solid phase

have to be mixed.

3. Experimental

3.1. Apparatus and measurements

The temperature of the liquids was measured with a home-builtfast multi-channel precision thermometer, which was recently de-

scribed, along with its calibration[16]. This device measures tem-

peratures of 30 temperature sensors, the so called thermistors

(BetaTHERM Betacurve 30K6A1) over the temperature range of

(193 to 313) K. Over the temperature range from (223 to 283) K,

it provides accuracy better than 30 mK. Temperatures are recorded

with a constant sampling rate of 1 data point per second. The ther-

mometer is controlled by a MS Windows computer and home-built

FIGURE 1. Plot of temperature against time to show the cooling curve of

c-butyrolactone (GBL) with added carbon fibres recorded at a cooling rate of 30K h1 (), extrapolation of the linear parts (- --) for determination of the

crystallisation temperature.

FIGURE 2. Measuring cell: Inner Pyrex tube (A), magnetic stirrer (B), glass tube

with thermistor (C), SQ 18 closure with Teflon coated rubber seal (D), thermalinsulation (E), outer Pyrex tube (F), connector for the thermistor (G).

P. Wachter et al. / J. Chem. Thermodynamics 40 (2008) 15421547 1543


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software, enabling long term data recording of the temperature

measurements.

Infigure 2, the measuring cell forT(t)-measurements is shown.

About 2 cm3 of the sample is placed within the inner Pyrex tube

(A infigure 2). A Teflon coated magnetic stirrer bar (B) is used for

mixing the liquid. Temperature is measured with the thermistor

(C), which is located inside a thin glass tube. The inner glass tube

(A) is closed with a SQ 18 screw cap and a Teflon coated rubber seal

(D). The inner tube is centred in the outer tube (F) with o-rings,

leaving airspace (E) for thermal insulation. The cell is connected

using shielded twisted pair cable and a shielded connector to the

thermometer. The planetary stirring device shown infigure 3has

a capacity of 21 measuring cells. A similar stirring device based

on alternating magnetic field and with a capacity of 30 measuring

cells was constructed additionally.

The magnetic stirring bar is driven by a cobalt samarium mag-

net whose holder is connected with a gear wheel. The 21 gear

wheels are arranged in two concentric circles. The gear wheels in

the inner circle drive the gears in the outer circle. The inner wheels

are driven by an auxiliary wheel, which in turn is driven by the

centre drive wheel itself. An electrical motor drives this wheel

via a drive shaft. The measuring cells were filled and sealed tightly

in a glove box under an inert argon atmosphere. To investigate the

possibility of reducing effects of supercooling through application

of crystallisation aids, as suggested by Ding et al. [17], each sub-

stance was examined with and without the addition of carbon fi-

bres used in double layer capacitors. Furthermore, the effect of

these carbon fibres on crystallisation and melting points was

studied.

The stirring device with the measuring cells is placed inside the

bath of a thermostat. This thermostat, described by Barthel et al.

[18,19], consists of an insulated bath filled with approximately

60 dm3 silicone oil (Baysilon M5, Bayer) in which a mechanical

stirrer, a heat exchanger, a source of heat, and a platinum resis-

tance thermometer are immersed. Via the heat exchanger, a cryo-

stat (Unistat 390w, Huber, Germany), which acts as cold bath, is

coupled to the measurement thermostat. A thorough mixing of

the bath is ensured by the mechanical stirrer. Measurements at

FIGURE 3. Planetary stirring device for 21 sample tubes. Magnetic stirring bar (A), cobalt samarium magnet (B), gear wheel (C), electrical engine (D), and drive shaft (E).

1544 P. Wachter et al. / J. Chem. Thermodynamics 40 (2008) 15421547


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temperature gradients are enabled by the applied cryostat. The

cryostat is able to carry out heating- and cooling-experiments with

variably adjustable rates, respectively. The temperature range is

limited to a minimum of 183 K and a maximum of 323 K.

As mentioned above, the supercooling effects for a given mate-

rial relate to the magnitude of the applied cooling rates. To verify

this dependence and to examine its extent, every sample wasinvestigated at three different cooling rates. Furthermore the

influence of different cooling and heating rates on crystallisation

and melting points was studied, by application of (1.5, 10, and

30) K h1 as cooling rates and (1.5, 10, and 30) K h1 as heating

rates. The measurements were conducted within a temperature

range between 193 K up to (293 or 313) K, if necessary. In order

to reduce the influence of statistical outliers, every sample was

cooled down and heated up at least 10 times up to a maximum

of 62 times. As a result, crystallisation and melting points and their

errors could be determined with satisfying accuracy.

3.2. Materials

All materials were used as received and stored in a glove box

under inert argon atmosphere. Water contents of the investigated

substances that were liquid at room temperature were determined

via KarlFischer titration using a DL 18 titrator from Mettler (Gies-

sen, Germany) and are shown in table 1.

4. Results

The determined crystallisation and melting points are com-

pared intable 2to literature values. The results show good accor-

dance with those from the literature; as well they show small

values of standard deviations in a range between (0.03 and 1) K.

This demonstrates the precision of the measuring method men-

tioned above. The magnitude of the standard deviations correlates

strongly with the differences between the literature values for

given materials. The melting points obtained by heating curves

are normally slightly lower than the crystallisation points obtained

by cooling curves, except for the studied ionic liquids due to their

addiction to strong supercooling. Therefore if it is not possible to

obtain reliable crystallisation points, melting points offer a poten-

tial alternative.

Decreasing the cooling rate results in most cases in a reduction

of supercooling effects, as shown in figure 4for EMIOTf and GBL,

but there is no clear effect on the crystallisation points by this

reduction of supercooling. In general, there is no distinct conse-

quence on the location of crystallisation and melting points by var-

iation of cooling and heating rates, apart from arising problems in

evaluating cooling and heating curves for very low cooling and

heating rates, due to diffuse transitions.

Application of carbon fibres as an aid to crystallisation leads to a

strong decrease of supercooling. For instance the decrease from

10.73 K down to 2.84 K for GBL is shown in figure 4, except for sub-

TABLE 1

List of studied materials in with corresponding acronym, mass fraction water,

supplier and purity grade

Acronym Material 106w Supplier/purity

grade

AN Acetonitrile 38.5 Merckselectipur

GBL c-Butyrolactone 42 Merckselectipur

DEC Diethyl carbonate 17 Merckselectipur

DME Dimethoxyethane 72 MerckselectipurDMC Dimethyl carbonate 45 Merckselectipur

EC Ethylene carbonate Merckselectipur

EMC Ethyl methyl carbonate 26 Merckselectipur

EP Ethylpropionate 160 Merckselectipur

DO 1,4-Dioxane 30 Merckseccosolv

DMSO Dimethylsulfoxide 50 Merckseccosolv

NM Nitromethane 196 Merck P98%

BC Butylene carbonate 69 Merck

EMIOTf 3-Ethyl-1-methylimidazolium

trifluoromethanesulfonate

10 Merck

TOMATFA Trioctylmethylammonium

trifluoroacetate

48 Merck

MPN Methoxypropionitrile 275 Fluka P99%

DMSO-d Dimethylsulfoxide-d6 DeuteroP 99.50%

w= mass fraction of water as determined by KarlFischer titration.

TABLE 2

Crystallisation points Tc and melting points Tm for the investigated substances

compared with their literature values

Material Tc/K Tm/K

This work Literature This work Literature

AN 229.27 0.03 229.30 0.05[14],

229.30 0.003[20],

229.312[21]

229.07 0.2

BC 220.34 0.6 220[22]

GBL 229.44 0.2 228.44 0.1 229.62[23],

230.1[24]

DEC 195.49 0.1 195.86 0.3 198.85[3],

230.2[25],

199.0[24]

DMEa 204.06 0.07 203.41 0.1 215[23],204[21]

DMC 277.61 0 .06 2 77.76 0.1[11] 277.06 0.3 277.8[26]

DMSO 291.19 0.4 290.95 1 291.690 0.005

[27],

291.70 0.02

[28]

DMSO-d 293.72 0.04 293.40 0.1 293.97 0.02

[29]

DO 284.85 0.02 284.48 0.2 284.939 0.036

[30],

284.93 0.01

[31]

EC 308.83 0 .2 309.50 0 .1[11], 308.53 0.1 309.75[24]

EMC 218.80 0.2 218.14 0.2 220[2],

216.7[24]

EMIOTf 259.11 0.8 262.23 0.3 264[32]

EP

a

199.71 0.03 199.3[33]MPN 210.85 0.9 209.53 0.2 216[22]

NM 244.45 0.1 243.36 0.2 244.60[34],

244.6[25],

243.11[35]

TOMATFA 285.62 0.1

a From measurements with added carbon fibres.

FIGURE 4. Plot of the amount of supercooling dTagainst the cooling rate v: (d) GBL,

(s) GBL with added carbon fibres, (j) EMIOTf, and () EMIOTf with added carbonfibres.



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stances with little tendency to supercooling, such as AN or DO. As

found for variation of cooling and heating rates, there is no clear

trend observable by addition of carbon fibres either on the location

of crystallisation and melting points or on their standard devia-

tions. In most cases there is a small increase in temperature of

the crystallisation points by addition of carbon fibres, but overall

the difference between the values obtained with or without addi-

tion of carbon fibres remains very small, except for the crystallisa-

tion point of EMIOTf where a difference of more than 2 K was

observed, as shown intable 3.The viscosities gatT 298 K of the investigated substances are

listed in table 4, since it was expected that the viscosity has an

influence on supercooling and on reproducibility of crystallisation

and melting points determined.

The influence of the viscosity on the magnitude of calculated

standard deviations for crystallisation and melting points is shown

infigure 5. As expected, the standard deviations for crystallisation

points increase slightly with increasing viscosity whereas the stan-

dard deviations for melting points stay nearly constant, apart from

one outlier, due the fact that the melting starts from a solid.

As noted above, the viscosity also has an important influence on

supercooling, due to decreasing transport processes (in this case

the heat-transport) with increasing viscosity, which cannot be

completely equalised by mixing the sample. A second importantfactor for the degree of supercooling is the different ability to form

crystals for different substances that depends strongly on molecu-

lar geometry,e.g. organic carbonates with a strong increase in the

magnitude of supercooling with a loss in symmetry from thesymmetric DMC via EMC to the asymmetric PC. These twodifferent

TABLE 3

Crystallisation points Tc and melting points Tm for the investigated substances with and without added carbon fibres

Substance Tc/K Tm/K

With carbon fibres Without carbon fibres With carbon fibres Without carbon fibres

AN 229.27 0.03 229.00 0.2 229.07 0.2 228.40 0.8

BC 220.34 0.6

GBL 229.44 0.2 229.68 0.07 228.44 0.1 228.87 0.2

DEC 195.49 0.1 195.9 0.3

DME 204.06 0.07 203.41 0.1

DMC 277.61 0.06 277.59 0.05 277.06 0.3 277.21 0.4

DMSO 291.19 0.4 291.33 0.3 289.95 1 290.54 0.6

DMSO-d 293.72 0.04 293.755 0.007 293.40 0.1 293.32 0.1

DO 284.85 0.02 284.50 0.2 284.48 0.2 283.75 0.3

EC 308.83 0.2 309.12 0.09 308.53 0.1 308.69 0.1

EMC 218.80 0.2 218.64 0.1 218.14 0.2 218.03 0.09

EMIOTf 259.11 0.8 261.24 0.2 262.23 0.3 262.59 0.2

EP 199.71 0.03

MPN 210.85 0.9 209.53 0.2

NM 244.45 0.1 244.54 0.06 243.36 0.2 243.57 0.2

TOMATFA 285.62 0.1 285.48 0.08

TABLE 4

Values of viscosities g at T 298 K for the investigated substances

Substance g/(mPa s)

AN 0.341[23]

BC 3.1419[25]

GBL 1.7315[23]

DEC 0.753[25]

DME 0.407[23]

DMC 0.5902[26]

DMSO 1.991[36]

DMSO-d

DO 1.1964[37]

EC 1.9a [23]

EMC 0.6478[25]

EMIOTf 45b [32]

EP 0.5323b [24]

MPN 1.1[22]

NM 0.6260[38]

TOMATFA

a AtT 313 K.b AtT 293 K.

FIGURE 5. Plot of the standard deviation rTofhc (j) andhm(N) against viscosity gfor several substances fromtable 4.

FIGURE 6. Plot of the amount of supercooling dT against viscosity g for severalsubstances fromtable 4without (j) and with added carbon fibres (N).

1546 P. Wachter et al. / J. Chem. Thermodynamics 40 (2008) 15421547


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factors can reinforce or diminish each other, which leads to the

behaviour shown infigure 6.

In general, supercooling is strongly reduced by addition of car-

bon fibres as crystallisation aids. A second observation is the in-

crease in supercooling with increasing viscosity for symmetric

molecules (AN, DMC, DO, and DMSO) as well as for asymmetric

molecules (EMC, NM, and GBL), for which the magnitude of

supercooling is several times larger than for symmetric molecules.

5. Conclusion

A new automatic computer-controlled device for fast and

simultaneous determination of phase transition points for up to

30 samples is presented. Its performance is illustrated and the reli-

ability of the determined phase transition points evaluated by

comparison with literature. The new equipment, which is able to

record temperaturetime functions of up to 30 samples at very

low heating and cooling rates down to 1.5 K h1, enabled a highly

accurate reproduction of well-known phase transition points, a

verification of only rarely given phase transition points and a

first-time determination of so far unknown phase transition points.

Investigations of the influence of cooling-rates, viscosity, and

application of crystallisation aids on the magnitude of the superco-

oling resulted in a decrease of supercooling with decreasing cool-

ing-rate, decreasing viscosity, and application of carbon fibres as

crystallisation aids, respectively.

Acknowledgments

The authors thank the Bundesministerium fr Bildung und

Forschung (BMBF) under contract No. 01SF0304, the Deutsche

Forschungsgemeinschaft (DFG Priority Project Ionic Liquids SPP

1191) for funding and D. Moosbauer for editing our paper.

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