protic ionic liquids

Molecular dynamics simulation of room-temperature protic ionic liquid mixture of bis (2-hydroxyethyl) ammonium acetate and water by a refined force field Dheiver Santos1, Charlles Abreu2, Frederico Tavares2 and Silvana Mattedi1 1Graduate program in Chemical Engineering, Federal university of Bahia, Brazil 2School of Chemistry, Federal university of Rio de Janeiro, Brazil

Protic ionic liquids - PILs Proton transfer from the acid to the base

Presence of proton-donor and aceptor sites

Used to build up a hydrogen network

PILs Characteristics Formed by a stoichiometric acid-base Brønsted reaction (Presence of protondonor and –aceptor sites)

Initial little attention from academia besides first ILs were protic and BASIL process and caprolactam process use PILs

Called BAD IONIC LIQUIDS

PILs Characteristics Amphiphilic species in the solution may induce selfaggregation phenomena Micelles Greaves and Drummond. Chem. Rev. 2008, 108, 206-237

Disadvantages Advantages Simple synthesis and purification Low cost

Walden Plot Greaves and Drummond. Chem. Rev. 2008, 108, 206-237

Lower stability, more degradable

toxicity and biodegradability ??? Peric et al. EnvironmentalToxicology and Chemistry 2011, 2802-2809

Renewable interest in PILs

Gabriel and Weiner, Ber. 21 (1888) 2669 Walden, Bull. Acad. Sci. St. Petersburg (1914) 405

Literature review .... Year 1888: First known Ionic liquid (Gabriel and Weiner (1888)) [EtOHNH3+][NO3-]

Year 1992: Ionic liquids with anions PF6- ,BF4Year 2002: BASIL process

Years 50: Caprolactam process an ionic liquid intermediate is used (Fábos et al (2008))

1888

1910

1950 1970

Year 1914: First published synthesis of an ionic liquid (Walden, 1914) [EtNH3+][NO3-]

1980

1990

Years 1975-1982: Synthesis of 1,3-dialkylimidazoles, 1alkylpiridines and halogenated aluminates of imidazol

2000

2010

Year 2005: Synthesis of 2hydroxyethyl ammonium formiate (Bicak, 2005) Bicak (2005) J. Mol. Liq. 116, 15–18

Fabos et al. (2008) Chem Sus. Chem. 1:189

Works on ILs (Web of Science) Published works (protic ionic liquids)

Cited works

Results found: 670 Sum of the Times Cited : 11430 Sum of Times Cited without self-citations :8607 Citing Articles : 6851 Citing Articles without self-citations : 6356 Average Citations per Item : 17.06 h-index :51

Objectives Understand association and self aggregation phenomena charactheristics of PILs and water Develop and test a force field to PIL (chosen IL: BHEAA) Study pure and aqueous solution properties through MD

IL studied Bis(2hydroxyethyl)amine

Acetic acid

Bis(2hydroxyethyl)ammonium acetate (BHEAA)

Available data of pure properties Kurnia et al (2009a) Density for IL + water (Taib et al. (2010)) Studied as possible media for CO2 absorption (Kurnia et al (2009b)) Kurnia et al (2009a) J. Chem. Thermodyn. 41; 517-521 Taib et al(2010) J.Chem.Eng.Data 55, 5910-5913 Kurnia et al. (2009b) J. Chem. Thermodyn. 41; 1069-1073

Force Fields BHEAA: CHARMM (Zoete et al, 2011) Water force field TIP3P Vtotal = Vbonded + Vnonbonded Vbonded =

∑

bonds

Vnonbonded

K b (b − b0 ) 2 +

∑

Kθ (θ − θ 0 ) 2 +

angles

∑

K χ [1 + cos(n χ − σ )]

dihedrals

  R = ∑  ε ij  min,ij  rij nonbonded   pairs ij 

12

  Rmin,ij  − 2*    rij

  

6

 qq + i j  rij 

   

Zoete et al (2011) J. Comput. Chem., 32, 2360-2368

Parametrization (1) Choose the atom type based on available force fields.The initial set of parameters (bonded and VDW) are deduced using CHARMM. (2) Obtain optimized molecular geometry by performing QM calculations (using ChemBio3D). The equilibrium bond lengths and angles are determined (3) Adjust bond and angle force constants to fit the vibrational frequency data (using FTIR data ) (4) Validate the force field using QM training sets, such as the minimum interaction energies and geometries of ion pairs. Return to step 3, since the vibrational frequencies are dependent on the parameters adjusted in step 4. Repeat until the deviations between the two sets of parameters are small enough. (5) Validate the force field through liquid-phase simulation. Refine the parameters if necessary

Parameters from FTIR spectra 1

c = λυ

λ

=

υ c

λ = wavelength υ = frequency c = speed of light in a vacuum

1

10000 υ = wavenumber = = λ λ ( µm)

Characteristic Vibrational Frequencies of Bonds Bonds are not rigid but behave like a spring with a mass at either end. Obey Hooke’s Law: F = -kx This gives rise to a characteristic frequency for the vibration: k 1 reduced _ mass υ= 2π The frequency is affected by: (1) the mass of bonded atoms m1m2 (2) the bond strength reduced _ mass = m1 + m2 k is the force constant, mi is the mass of bonded atoms

FTIR

FTIR of BHEAA done in a FTIR Shimadzu Prestige 21 with ATR

Simulation Protocol MD calculations using LAMMPS (Plimpton, 1995) NPT (Nosé-Hoover thermostat) Cutoff ratio + skin: 11+2 Å time step:0.5 fs run time: 1ns Integration algorithm: verlet 100 molecules IL: 2900 atoms pure BHEAA, BHEAA+ water Plimpton(1995) J Comp. Phys. 117, 1-19 http://lammps.sandia.gov

Speed of sound elastic _ property The speed of sound can be u= = inertial _ property estimated from Hooper et al (2009) :

E

ρ

Normally, the speed of sound is determined using the isentropic bulk modulus. However, for most materials the difference between the isothermal and isentropic modulus is small and we have therefore utilized. The isothermal bulk modulus is defined as:  ∂P  E = −V0   ∂ V  T From NPT simulations at any pressure it could be determined from volume fluctuations: kb ⋅ T ⋅ V E=

kb is Boltzmann’s constant, V is the instantaneous volume.

1 n Vi − V ( ∑ n − 1 i =1

)

2

Hooper et al. (2009) J. Chem. Phys. 10, 144904

Results Density data

BHEAA Water

Figure 1. Density for BHEAA (1) + water (2) at 298 K by molecular dynamics

Exp ρ/g.cm-3 (298 K)

MD ρ/g.cm-3 (298 K)

1.167483

1.065

0.9970

1.020

Results Sound velocity data

BHEAA Water

Figure 2. Speed of sound for BHEAA (1) + water (2) at 298 K by molecular dynamics

Exp u/ms-1 (298 K)

MD u/ms-1 (298 K)

1863.35

1759.2

1496.7

1523.2

Results Intermolecular energy

Figure 3. Internal energy for BHEAA (1) + water (2) at 298 K by molecular dynamics

Results Enthalpy of vaporization

Figure 4. Enthalpy of vaporization for BHEAA (1) + water (2) at 298 K by molecular dynamics

Results Cohesive energy

Figure 5. Cohesive energy density for BHEAA (1) + water (2) at 298 K by molecular dynamics

Results Hildebrand solubility parameter Values comparable with correlation results of viscosity data from similar ILs using Eyring theory coupled with regular solution theory. (Santos et al 2014)

Figure 6. Hildebrand solubility parameter for BHEAA (1) + water (2) at 298 K by molecular dynamics (Santos et al. 2014. To be presented in ILSEPT 2014, in Toronto P094 Monday June 30th 17-18 Metro Ballroom Centre )

Results Radial distribution function ion-ion 2.5

3.5

2

ion-ion 0.9 (303 K)

2.5

1.5

ion-ion 0.9 (313 K)

2

ion-ion 0.9 (323 K)

1

ion-ion 0.9 (333 K) 0.5

ion-ion 0.8 (293 K)

3

g(r)

g(r)

ion-ion 0.9 (293 K)

ion-ion 0.8 (303 K) ion-ion 0.8 (313 K)

1.5

ion-ion 0.8 (323 K)

1

ion-ion 0.8 (333 K)

0.5

0

0 0

5

10 r(A)

15

0

5

10

15

r(A)

Indicates that a liquid (ionic liquid) system is present. g(r) exhibits peaks, indicating that at certain radial values, it is more likely to find particles than at others. This is a result of the attractive nature of the interaction at such distances.

Results 4 3.5 3 2.5 2 1.5 1 0.5 0

2.5

ion-water 0.9 (293 K)



2



1.5


ion-water 0.9 (323 K) ion-water 0.9 (333 K)

g(r)

g(r)

Radial distribution function ion-water


1


0.5 0 0

5

10 r(A)

15

0

5

10

15

r(A)

Indicates strong interaction between these sites. Moreover, the well defined positions and sharpness of these distributions indicate that these correlations can be associated to hydrogen bond like interactions between the pairs

Conclusions A force field was developed to BHEAA Force field parameters optimization methodology using FTIR results were derived and tested MD simulation were performed Methodology to calculate sound velocity was tested. Obtained pure density and sound velocity for IL and water are comparable to experimental data

Conclusions The obtained Hildebrand parameter are of the same magnitude of similar ILs derived from experimental viscosity data and Eyring theory Radial distribution functions are coeherent and indicates that hydrogen bonds are formed between IL and water Further simulations will be performed to confirm and expand the study

Acknowlegdments:

Thank You!!!

Group of Applied Thermodynamics UFBA

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