MELTING TEMPERATURE OF IONIC LIQUIDS

CLAUDIA AGUIRRE CESPEDESMSc. in Chemistry – P. Universidad Católica de Chile

Doctoral Student in Mineral Processing Eng. – Chemical Engineering Department, Universidad de Antofagasta

October, 2008

International Workshop

Overview

Why Research the Melting Point of Ionic Liquids?Melting TemperatureMelting Temperature of Ionic LiquidsDifficulties in the Measurement of Ionic LiquidsExperimental Melting Point Data of Ionic LiquidsGroup Contribution Methods to Predict Melting Point of Ionic liquidsConclusions

Why Research the Melting Point of Ionic Liquids?

Melting points (Tm) are significant properties

Tm of ILs represents the lower limit of the liquid gap and defines the interval of temperature to use them as solvents.

Few useful guidelines exist for understanding the relationship between the melting point of a compound and its chemical structure

Difficult to predict if a compound will be a solid or liquid

Melting Temperature

Tm for a pure substance is the temperature at which the solid and liquid phases are in equilibrium at one atmosphere

Tm is determined by strength of a crystal lattice: intermolecular forces, molecular symmetry, conformational degrees of freedom of a molecule

Melting Point of Salts

Related with a lattice energy

)1(345.012.287

00⎟⎟⎠

⎞⎜⎜⎝

⎛−=

−+

rrZvZU

v

0r

−+ ZZ ,

U lattice energy

number of ions per molecule

sum of ionic radii

Charge of the ionic species

Relationship between the size of similar anions containing halides and melting point of various sodium salts

Melting Temperature of Ionic Liquids

Ionic liquids consist of a salt (large ions and a symmetric cation) which reduce the lattice energy and lower the melting point

Solid-liquid transition temperatures can be below ambient and as low as -100°C

Efficient method for measuring the melting temperature is differential scanning calorimetry(DSC)

DSC Melting and Crystallization Experiment of EMIM-Chloride, dT/dt=10K/min

Melting-point and Crystal Structure

Since the Physical property can be adjusted through variations on the cation and/or anion to correlate the structure of the ILs with their Tm

With the properties of a series of imidazoliumcation- based ILs, the Tm decreases when the size and asymmetry of the cation increase

The Anion Effect

The presence of Hydrogen bonding between counter ions may explain the physico-chemical properties of imidazolium ILs

Hydrogen bonding between counter ions has been established between the protons of the imidazolium ring and basic counter anions

Optimized molecular structures of ionic liquids: Emim BF4, BmimBF4, EmimPF6 y BmimPF6

TmEmimBF4=286.8K

TmEmimPF6=333.3K

Source: N.Sun et al./Fluid Phase Equilibria 246 (2006)

Deviations between experimental melting point data published by different authors for some ionic liquids. The value with* was used as reference to determine the deviations for the other values.

268.95

425.15

27.29

9.23

357.15

300.15

343.15

339.15

202.15

338.15

37.5195.65*

[dmim][BF4]

21.1351.15*

[bmpyrr][BF4]

7.14280.15*

[emim][AlCl4]

7.96314.15*

[bmim][Cl]

8.82185.77*

[bmim][BF4]

28.50263.15*

[bmim][AlCl4]

I%ΔTmITmIonic liquid

Relationship between the size of similar anions containing halides and the melting point of their [C2-mim] salts

Source: D.W. Rooney, K.R. Seddon, Handbook of Solvents (2001)

[Emim] Salts

Br PF6

AlCl4

BF4

Cl

250

270

290

310

330

350

370

170 190 210 230 250 270 290 310 330 350

Anion radius (pm)

Mel

ting

Poin

t (K

)

Br PF6 AlCl4 BF4 Cl

Tm (K) of [Emim] Salts from IL-Thermo Database – Showing Inconsistencies

Tm (K) of [Bmim] Salts

[BMIM]

Cl

AlCl4

AlCl4

BF4

050

100150200250300350400

170 190 210 230 250 270 290 310

Anion radius (pm)

Mel

ting

Poin

t (K)

Good trend and Accuracy

Trend Inconsistency & Inaccuracy

Estimation of Melting Point of Organic Compound

The development of structure-Tm relationships are not as straightforward as for other properties

Method of Joback and Reid: Based on a database of 388 organic compounds

and yielded an AAE of 22.6K

Tm(K) = 122.5 + ∑ni(ΔTm)i

n =388, s = 24.7K

Joback and Reid, Group Contributions

Application of J.R. model to cyclopropylmethyl ether

ARE(%) = 17.6

Ciclopropil metil eter Tm = -119° C C0 (ec.3) 122.5 122.5 -CH3 1(-5.10) -5.10 -CH2- (ring) 2(7.75) 15.50

CH (ring) 1(19.88) 19.88 O 1(22.23) 22.23

------- Tm = 175.01 K

Tm = -98°C

O CH3

Selected Methods to Predict the Melting Point of Organic Compounds Using GCM

Simamora and Yalkowsky(1994)

Proposed a method to estimate the Tm of aromatic compounds from chemical structure.Tm equation is developed from a data set containing 1690 aromatic compounds which gives a standard error of 37.45 K

The model equation is:

Where:ni is the number of occurrences of group i in the molecule,

mi is the contribution of group i,

σ is the rotational symmetry, defined as the number of ways that a molecule can rotate to give indistinguishable images.

,1181log6.45.13

1)( =−

=° ∑ nmnCT iim σ

Group Contribution Approach for Tm

•The GC-method predicts both the melting point and the enthalpy of fusion of the saturated triglycerides• The GC-method takes into account the position of the acyl groups and the class to which the triglyceride belongs.

Zeberg-Mikkelsen and Stenby (1999)

• Proposed a method that utilizes both group contribution and simple molecular geometric parameters.• Approximately 1040 melting point data were compiled and analyzed by multiple regression with average absolute error of 20%.

Zhao and Yalkowsky (1999)

• Developed a group contribution methods which incorporate non-additive, non-constitutive properties • The root mean square errors for the estimation of melting points is 34.3 K.

Krzyzaniak et al. (1995)

Comments on the MethodReference

Group Contribution Approach for Tm

• Presented a modified group vector space (GVS) method for estimating the normal boiling points and melting points of hydrocarbons• Estimated the melting points of 1115 hydrocarbons with average percent deviations of 0.86%.

Li et al. (2006)

• Used a combination of additive group contributions and non-additive molecular parameters.• Estimate the normal melting points of 1215 organic compounds with average absolute error of 33.2 K.

Jain et al. (2004)

• Proposed a method in which the specific position of a group in the molecule has been considered.• Estimated the normal melting points of 529 compounds with average percent deviations of 6.8%.

Wen and Qiang (2002)

Comment on the MethodsReference

ConclusionsThermal behavior of many ILs is complex.

Thermodynamic properties of ILs are strongly dependent on the mutual fit of the cation and anion, in terms of size, geometry and charge distribution.

Additionally, measurements of Tm are affected by the purity of a compound and experimental error.

Based on the structure analysis, the main factors which influence Tm in ILs are the presence or absence of strong hydrogen bonds in the lattice and charge distribution.

ChallengesUnderstand the real variables that control the melting process for Ionic Liquids

Developing a method to predict the melting point of ionic liquids using a modified group contribution method

Thank you for your attention!

Phase Diagram for the [Cn-mim]Cl Ionic Liquids