Translational and Rotational Diffusion of Two Differently Charged

Subscriber access provided by UNIV OF CAMBRIDGE

Article

Translational and Rotational Diffusion of Two Differently Charged Solutes in Ethylammonium Nitrate – Methanol Mixture: Does the Nanostructure of the Amphiphiles Influence the Motion of the Solute? Niloy Kundu, Arpita Roy, Rupam Dutta, and Nilmoni Sarkar J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b02251 • Publication Date (Web): 26 May 2016 Downloaded from http://pubs.acs.org on June 3, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Translational and Rotational Diffusion of Two Differently Charged Solutes in Ethylammonium Nitrate – Methanol Mixture: Does the Nanostructure of the Amphiphiles Influence the Motion of the Solute? Niloy Kundu, Arpita Roy, Rupam Dutta and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India E-mail: [email protected] Tel: + 91-3222-283332, Fax: +91-3222-255303 Abstract In this article, we have investigated the translational and rotational diffusion of two structurally similar but differently charged solutes (Rhodamine 6G Perchlorate and Fluorescein sodium salt) in Ethylammonium Nitrate (EAN)-Methanol (CH3OH) mixture to understand the effect of added ionic liquid on the motion of the solutes. EAN and CH3OH both are amphiphilic molecules and characterized by extended hydrogen bonding network. Recently, Russina et al found that a wide distribution of clusters exist in CH3OH rich region (0.10≤ ≤ 0.15) and EAN molecules preserve their bulk-sponge like morphology (Russina, O.; Sferrazza, A.; Caminiti, R.; Triolo, A. J. Phys. Chem. Lett. 2014, 5, 1738−1742). The effect of this microheterogeneous mixture on the solute’s motion shows some interesting results compared to other PIL (Protic Ionic liquid)cosolvent mixtures. Analysis of the time resolved anisotropy data with the aid of StokesEinstein-Debye (SED) hydrodynamic theory predicts that the reorientation time of both the solutes appear close to the stick hydrodynamic line at methanol rich region. The hydrogen bond accepting solutes experience specific interaction with CH3OH and with increasing concentration of EAN, the specific interaction between the solute and solvent molecules is decreased while the decrease is more prominent in the low mole fraction of EAN due to the large size of cluster formation. The temperature dependent anisotropy measurements show that the hydrogen bonding interaction between EAN and CH3OH is increased with increasing the temperature. Moreover, Fluorescence Correlation Spectroscopy (FCS) shows the dynamic heterogeneity of the mixture which is due to the segregation of the alkyl chain of the PIL. Formation of large cluster at low mole fraction of IL (0.10≤ ≤ 0.15) can be proved by the insensitivity of the translational diffusion and rotational activation energy of the solutes to the concentration of EAN. Thus, the result of the work suggests that the addition of EAN to the CH3OH affect the specific interaction between solute and solvent and as a consequence, the translational as well as the rotational motion of the solutes are modulated.

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.1. Introduction. Today’s Chemistry mainly concern with the study of chemical reactions where solvent molecules play an important role by regulating the interaction between the dissolved atoms and molecules. Now, it is very much important to understand the structure in order to understand the role of the solvent. Thus, elucidation of solvent structure has received much attention in the scientific community and in recent times, theoretical, computational as well as experimental techniques provide unprecedented physical structure of liquid.1-4 Recently, room-temperature ionic liquids (RTILs) have received increase number of attention because of their unique physiochemical properties such as low volatility, high thermal stability, high ionic conductivity and the drawbacks associated with the nonaqueous solvents or organic solvents related to environment, health or safety can be conquered by using the RTILs.5-10Furthermore, the desired properties of the solvent can be obtained by tuning the cationic and anionic constituents of the RTILs and hence, they are often termed as “designer solvents” which leads their wide use in organic synthesis, catalysis, electrochemical studies and other chemical and technological applications.1113

Although the interest in ionic liquid over the other conventional liquid have rapidly grown up in 21st century the discovery of ionic liquid, ethyl ammonium nitrate (EAN) was made almost 100 years ago by the German chemist Walden. EAN is most extensively studied Protic ionic liquid (PIL) and in many aspects EAN is strikingly similar to water as it is clear, odorless and colourless. Besides, EAN also can form three dimensional hydrogen bonded network similar to water.14PILs are formed through the proton transfer reaction between the Brǿnsted acid and Brǿnsted base and thus, simultaneously hydrogen bond donor and acceptor sites are created on the ions which enable to form a 3D hydrogen bonded network.15-16 The proton transfer in the PILs have involved in many applications such as thermal stability17, catalytic activity8 and protein stabilization18 etc. However, unlike to water, EAN shows nanoscale heterogeneity where the ethyl chain is sufficient for the PILs to form a self assembled structure and the small angle neutron scattering (SANS) analysis also shows a disordered sponge like morphology of EAN.1920

Atkin et al. have provided the atomic detail of the ion rearrangement of EAN by neutron

diffraction measurement and they showed that the nitrate ion of EAN exclusively interact with the ammonium groups via hydrogen bonding and electrostatic interaction and the cationic alkyl 2 ACS Paragon Plus Environment

Page 2 of 27

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


groups are aggregated together due to the solvophobic interactions. Thus, a bicontinuous sponge like morphology is formed in bulk.19 The sponge like lamellar structure of EAN is also predicted by Henderson et al. from the crystal structure of EAN.21 The high viscosities of the RTILs compared to common solvents prevent their use in many chemical applications and their applicability can be enhanced by addition of different cosolvents which lead to change in the polarity, viscosity and the ionic conductivity of the medium.2224

Thus, the study of ionic liquid-binary mixture is the active field of research for theoretical as

well as experimental chemists.25-32 As we have mentioned earlier that PIL contains both proton donor and acceptor sites which lead to the formation of strong three-dimensional hydrogen bonded network in their bulk and thus, the study of binary mixtures between PILs and different molecular cosolvents with variable ability to form hydrogen bonds such as water, short chain as well as long chain alcohol rapidly grow interest.33-35 Previous study shows that in presence of high concentration of water the aggregated network of EAN is retained and the water molecules are mainly assembled in the polar domain of the EAN. The interaction between the ions lead to the increase in the effective head group size of the cation which changes the EAN structure from locally flat sponge like in bulk EAN to branched network in the mixture.35 Interestingly, EAN and alcohols are both amphiphiles with strong hydrogen bond donor and acceptor ability and surprisingly, EAN-methanol mixture shows some anomalous behavior which is not observed in EAN-water mixture or EAN-DMSO mixtures.36 Greaves et al. have suggested that in the binary mixture of PILs-alcohols with high mole fraction of alcohol ( ∼ 0.8) the nanostructure is analogous to the alcohol-water systems where some of the PILs are located at the alcohol nanostructure and with further additions of PIL to the alcohol results swelling of the nanostructure which causes an increase in the size of the structure.34 Recently, Russina et al. have performed the small angle X-ray and neutron scattering in different mole fraction of EAN in EAN-CH3OH binary mixtures and they have observed maximum excess scattering at ~ 0.15 which corresponds to the highly heterogeneous system and large distribution of cluster size is also observed at this mole fraction and with increasing concentration of EAN into the mixture, the cluster size is decreased ( ~ 0.30) which correspond to the development of the bicontinuous sponge like morphology similar to EAN in bulk.36-38



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Although the structural detail of EAN-CH3OH is well documented in the literature, the dynamics of a solute molecule dissolved in the mixture has not been explored before. With addition of EAN to the CH3OH the bulk viscosity of the medium is increased. Moreover, due to the extended hydrogen bonding network the rotational and translational motion of a solute can be modulated. For this reason, we have chosen two structurally similar but oppositely charged solutes; Rhodamine 6G (R6G, cationic) and Fluorescein sodium salt (Fl-Na, anionic). Recently, Dutt et al have studied the rotational dynamics of several structurally similar fluorophores in neat protic ionic liquids as well as in different PIL-cosolvent mixtures and they analyzed the data using Stoke-Einstein-Debye (SED) hydrodynamic theory.39-41 However, in our case, we have preferred to perform the rotational dynamics in the methanol rich region because of the formation of wide distribution of cluster in this region as indicated by recent studies.36 Besides the rotational dynamics, we have also studied the translational diffusion of the solutes in the mixture by Fluorescence Correlation Spectroscopy (FCS). A few FCS studies have been carried out to understand the diffusion behavior of the solutes in neat IL.42-44 Recently, Bhattacharyya et al have observed broad distribution in the diffusion coefficient values of the solutes in different ionic liquid based systems which can be attributed to the microheterogenity of the ILs.43 Thus, in this work, we aim to understand about the effect of mixing of two amphiphilic molecules (EAN and CH3OH) on the rotational and translational diffusion of two differently charged solutes and we find that the motion of the solutes are significantly modulated in this hydrogen bonded microheterogenous mixture. Finally, our results are consistent with the structural detail available in the literature for EAN-CH3OH mixture36,37. 2. Experimental Detail. -

2.1. Materials. Rhodamine 6G Perchlorate (R6G-ClO4 ) was purchased from Exciton and Fluorescein sodium salt (Fl-Na) was obtained from Sigma-Aldrich. Ionic Liquid, EAN was synthesized by the reaction of equimolar amount of ethylamine and nitric acid following the procedure of Evans et al.45 and water was removed by the rotary evaporation followed by lyophilization and finally, purity of the compound is checked with 1H NMR and 13C NMR. The water content of the ionic liquid was estimated by Karl Fischer titration and found to be < 100 ppm. The organic cosolvents methanol used was of spectroscopic grade (Spectrochem, Mumbai, India). The molecular structures of all the chemicals are shown in Scheme 1. 4 ACS Paragon Plus Environment

Page 4 of 27

Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.2. Solution Preparation. EAN and CH3OH are fully miscible in any proportion. For the anisotropy measurements, dye concentration was maintained as ~10-5 M and requisite amount of EAN was added to the cuvette under the nitrogen atmosphere. Then, calculated amount of methanol was added to the solution to achieve the desired mole fraction. The cuvette was sealed by the parafilm and it was allowed to equilibrate for sufficient time before each measurement. For the FCS measurement, the dye concentration was maintained as 1 nM. 2.3. Instrumentation. Instrumentation section is discussed in Supporting Information. 3. Results and Discussions. 3.1. Viscosity Measurements. The viscosity of the synthesized EAN is 31.68 mPa at 298K and it agrees well with the previous literature reports.39With addition of different mole fractions of EAN to CH3OH, the viscosity of the medium is gradually increased. The variation of viscosities of the EAN-CH3OH mixture at five different temperatures are shown in figure 1(a) and with increasing the temperatures the viscosities of the mixtures are gradually decreased. Now, to find out whether the mixtures show non-ideal behavior, the excess viscosities of the mixtures are calculated following the equation. = − [ + 1 − ]

(1)

In the above equation, is the viscosity of EAN-CH3OH mixtures at a given . and

!

are the viscosities of neat EAN and CH3OH respectively. The excess viscosities are

plotted against the mole fraction of EAN in figure 1(b) and it can be observed that the EANCH3OH mixtures show negative at all temperatures with the minima centered at = 0.6. The excess viscosity of the mixtures are fitted with the aid of Redlich-Kister equation which is defined as, = #$ 1 − #$ ∑*'+$ &' 2#$ − 1')$

(2)

where, #$ is the mole fraction of EAN and &' is the fitting parameter of the equation where a fifth order polynomial equation is used in order to fit the plot. The large difference in the shape or the size of the component molecules or loss of Coulombic attraction between the components within the mixture compared to the pure components results 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in decreased viscosity of the mixture. On the other hand, specific interactions between the components in the mixture (such as formation of hydrogen bonds or complex formation) increase the viscosity of the mixture compared to the pure components.46 In this case, we have observed negative excess viscosity when EAN is mixed with CH3OH and such negative excess viscosity is also observed when other imidazolium or pyridinium based ionic liquids are mixed with less viscous organic cosolvents.47-49 Over the whole mole fraction of EAN, with increasing the temperature the excess viscosity is also increased. Besides these, we have observed that at = 0.3, the viscosity of the medium is higher compared to that at = 0.4. Chagnes et al. have also observed non-Newtonian type of behavior at low content of EAN (0.2≤ ≤ 0.4)50. With increase in the temperature, the shape of the excess viscosity plot is changed; a flattening of the excess viscosity is observed at low mole fraction of EAN (at 318 K). The change in the shape of plot also indicates the change in the hydrogen bonding interaction or other interactions between the components. Thus, it would be interesting to find out the effect of this hydrogen bonded microheterogenous system on the translational and rotational diffusion of two differently charged solutes. 3.2. Rotational Dynamics of R6G and Fl-Na in EAN-CH3OH binary mixture. The emission spectra of positively charged solute, R6G in EAN-CH3OH mixtures are shown in figure S1(a) (Supporting Information). With increasing concentration of EAN, the emission spectra of R6G are red shifted. The terminal amino group of the R6G acts as hydrogen bond acceptor while protic solvents act as hydrogen bond donor. Thus, with increasing the proton donor ability of the solvent the hydrogen bonding interaction between the amino group of R6G and the solvent is increased. As a consequence, the conjugation between the aromatic group and the amino group of the dye is significantly reduced (scheme S1 of Supporting Information) and the planarity of the dye is decreased in both ground and excited state.51 Thus, the absorption and emission spectra of R6G become red shifted in non-protic solvents. The red shift in the emission spectra with increasing amount of EAN indicates the loss of hydrogen bonding interaction between R6G and EAN-CH3OH mixture. On the other hand, different protolytic form of Fl-Na exists when EAN is mixed with CH3OH which can be understood form the emission profile of Fl-Na in EANCH3OH mixtures. (Figure S1(b), Supporting Information).52


Page 6 of 27

Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The time resolved anisotropy measurements provide important information regarding the rotational dynamics of the probe molecules. Anisotropy decays of R6G and Fl-Na in neat CH3OH, EAN and EAN-CH3OH mixtures are adequately described by the single exponential decay function. With increasing concentration of EAN, the rotational relaxation time of both the solute molecules are gradually increased (figure S2(a) of Supporting Information). The increase in the rotational relaxation time is due to the increase in the viscosity of the medium with gradual addition of EAN. The rotational times of R6G and Fl-Na as a function of are given in table 1 and table 2 respectively. From the table, it is clearly evident that the rotational time of R6G in the mixture is higher than that of Fl-Na and it is also pictorially depicted in figure S2(b) (Supporting Information). The calculated Van der Walls volume for R6G (414 Å3) is much higher than that of Fl-Na (295.8 Å3). Therefore, the hydrodynamic friction experienced by R6G is higher than that of Fl-Na. As a consequence, the -. value is higher for R6G. Now to find out how EAN and CH3OH mixture influence the rotational time of both the charged solutes, viscosity normalized rotational relaxation time ( ./) is plotted against different mole fraction of EAN in EAN-CH3OH mixtures (figure 2) and the value of ./ decreases with increasing concentration of EAN. For instances, viscosity normalized rotational time is decreased almost 3.54 times for R6G, from 0.354 ns in neat CH3OH ( = 0.00) to 0.10 ns in pure EAN ( = 1.00). However, ./ value is decreased almost 4.34 times for Fl-Na, from 0.33 ns in neat CH3OH = 0.00) to 0.076 ns in pure EAN = 1.00). The Van der Waals volume calculated for CH3OH (36 A0) from the Edward’s volume increment method is much smaller than the volume obtained for EAN (90 A0). Moreover, we have already mentioned that EAN exhibits a long range structure, similar to a bicontinuous microemulsion. Thus, with increasing concentration of EAN in the mixture, the smaller CH3OH molecules are replaced by larger EAN molecules. Recently, Ghosh et al. have reported that small size solvent molecules provide good packing inside the solvent cluster compared to the large size solvent molecules.53 Thus, addition of EAN creates many spaces inside the solvent clusters where the solute molecules can easily rotate and it could be the possible reason for showing faster rotational relaxation time (viscosity normalized) after addition of EAN to the mixture. Another interesting observation we noted that the decrease in the viscosity normalized rotational time ./) for both the solute molecules are not monotonic in nature with varying concentration of EAN. 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

To get better idea about the rotational dynamics of the solute in the mixture, the measured rotational time has been analyzed within the theoretical framework of Stokes-Einstein-Debye (SED) hydrodynamic theory.54,55 According to the theory, solvent is treated as a structureless continuum which is characterized by its bulk property and the reorientation time of a solute is proportional to the solvent viscosity at a particular temperature and the proportionality constant is the ratio of the hydrodynamic volume (0 of the solute molecule to the Boltzmann constant (12 ). Thus, the reorientation time of the solute can be defined by the following expression, 4

9

-.3 = 5 8 ; 6 :

(3)

7

Hydrodynamic volume, 0 can be defined as the effective volume of the solute sensed in the solvent and it is described as 0 = 0'? ≤ 1). To calculate the reorientation time of R6G and Fl-Na with the aid of the SED theory, they are treated as asymmetric ellipsoid. The values of V and f for both the solutes are obtained from the earlier literature.56,57 Using the values of V, f, viscosity coefficient () and the rotational relaxation time (-. ) we have calculated the =>'? values for both the solutes in EAN-CH3OH mixture. Figure 3 shows the log-log plot of rotational relaxation time of both the solutes as a function of shear viscosity within the limit of stick and slip boundary lines. From figure 3, it is clearly evident that the rotational relaxation time for both the solutes appears close to the stick boundary line especially at low concentration of EAN which clearly describes the high solutesolvent interaction at low mole fraction of EAN. The difference in the rotational relaxation of R6G and Fl-Na can be better understood by A

B comparing the observed boundary condition parameter (=@> ). Thus, =@> (=ACDEFG ) is independent B

of viscosity and temperature and =@> as a function of is tabulated in table 1 and table 2 for both the solutes. The rotational relaxation time of R6G and Fl-Na in pure CH3OH lie outside the stick limit calculated from the SED theory (=@> >1) which indicates that both the solute molecules follow the superstick trend. It is generally observed that a polar solute molecule rotate much slower in a polar solvent than predicted by SED theory. The longer reorientation time can 8 ACS Paragon Plus Environment

Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


be attributed to the electrical aspects of the solute-solvent coupling which is known as dielectric friction.58 Thus, including the dielectric continium model the total rotational relaxation time can be written as -. = -HI + -IJ =

94K 67 :

LN

P)$

+ 6M : QPR$N -I O 7

(4)

In the above equation, S! is the excited state dipole moment of the solute, T is the dielctric constant, -I is the Debye dielectric relaxation time of the solvent and UV is the cavity radius of the solute. The value of S! , T and -I are taken from the literature.51,59 Thus, the -IJ value for R6G in neat CH3OH is obtained as 1.37 ps at 298K which is much smaller than the value we obtained for -HI (122 ps). Therefore, the overall -. value (123.37 ps) obtained considering both the SED theory and the dielectric friction theory is much lower than the rotational relaxation value of R6G in CH3OH (195 ps). This clearly indicates the specific hydrogen bonding interaction between the solute and solvent hinders the rotational relaxation of solute. However, this type of behavior has been reported for various polar molecules in polar solvents.60,61 Now, the =@> value of Fl-Na (2.04) is much higher than that the value obtained for R6G (1.60) in neat CH3OH. A closer look at the molecular structure of the Fl-Na reveals that it has carboxylate (COO-), carbonyl (C=O) and ONa groups. These functional groups act as a hydrogen bond acceptor (HBA) and CH3OH act as hydrogen bond donor (HBD). On the other hand, the central oxygen atom in the xanthene skeleton, nitrogen atoms and CO2Et are the main functional groups in the R6G. However, the oxygen atom in xanthenes skeleton is very poor hydrogen bonding interacting site as it does not conjugate with the W electron system. Interaction of R6G with HBD solvents such as linear alcohol increases the positive charge on the central carbon atom and the amine groups. Therefore, the distribution of the positive charge through the conjugation is restricted (scheme S1). Now, the monoethyl amino group of R6G has less tendency to accept the H bonds and ethoxy carbonyl group can effectively interact with the HBD solvents which decrease the contribution of the CO2Et group towards the stabilization of positive charge on the central carbon atom. Thus, compared to Fl-Na, the interaction with methanol is less for R6G. Therefore, the hydrogen bonding interaction between Fl-Na and CH3OH is much stronger than the interaction between R6G and CH3OH which is reflected in higher =@> value for Fl-Na in CH3OH. However, surprisingly, the solute-solvent interaction in neat EAN is more or less same



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

for both the solutes (=@> ~ 0.46). Recently, Dutt et al have also observed identical reorientation time for Fluorescein and Rhodamine 110 (R110) in PAN (n-propylammonium nitrate) and the trend has been explained by Kamlet-Taft and Abraham Model.41 However, there is a slight difference in the molecular structure between R6G and R110. R110 has NH2, NH2+(conjugated), CO2H groups which are hydrogen bond donors, while the functional groups in R6G act as hydrogen bond acceptor in protic solvents. Ionic liquid, EAN act as hydrogen bond donor as well as hydrogen bond acceptor due to the presence of ethylammonium cation and nitrate anion and the acidity and basicity of this PIL is well characterized. For the charged solutes, the aciditybasicity parameters of IL can be explained with the aid of Kamlet-Taft scale.62 According to Kamlet-Taft scale the hydrogen bond acidity (X) and basicity (Y) value of EAN are 0.85 and 0.46 respectively which indicates that the hydrogen bond donor ability of EAN is stronger compared to its hydrogen bond acceptor ability. Thus, Fl-Na should have higher solute-solvent interaction in EAN than R6G. However, we have observed that in both cases =@> are same. Kamlet-Taft polarity scale is valid for ionic solutes which is applicable for R6G and Fl-Na also. But, both of the solutes also contain the hydrogen bonding functional groups which are neutral in nature. In figure 3, the stick or slip boundary lines are plotted by connecting the two -.3 values which is calculated at complete slip or stick boundary condition in neat EAN and neat CH3OH respectively. With increasing concentration of EAN, the =@> value is decreased and the decrease is not monotonic in nature. Dutt et al have investigated the rotational diffusion of Rhodamine 110 and 9-phenylanthracence in PAN and Propylene Glycol mixture40 and they did not observe any significant variation in the =@> value throughout the mole fraction of PAN in the mixture which indicates the absence of any non-hydrodynamic effect. Samanta et al also observed no symmetric variation in the =@> value in 1-butyl-3-methylimidazolium hexafluorophosphate and toluene (and dioxane) mixture.63 However, recently, Maroncelli et al have performed the solvation and rotational relaxation study of C-153 in 1-butyl-3-methyl imidazolium tetrafluoroborate and acetonitrile mixture and they have found that the =@> value is decreased by a factor of 1.6 with increase in the mole fraction of the ionic liquid from 0.2 to 1.0.64 The different trend in the =@> value in different ionic liquid-cosolvent mixtures can be explained by the average size of the solvent molecules. If the sizes of the solvent molecules are comparable,


Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


no such difference in the =@> value (as a function of solvent composition) should be observed. However, we have mentioned earlier that the Van der Walls volume of EAN is much higher than that of CH3OH. Thus, the variation in =@> value is expected to depend on the variation of the solvent size as a function of composition. If we closely inspect the =@> value in table 1 and 2 it can be observed that a significant decrease in the =@> value at = 0.15 UZ[ 0.30 for both the solutes. The decrease in the =@> value indicates the weaker solute-solvent interaction at these mole fractions. In EAN-CH3OH mixture, the OH group of CH3OH strongly interacts with the ethylammonium cation or nitrate anion. Recently, Russiana et al found excess scattering in SAXS and SANS data at = 0.15 which is due to the formation of fluctuating cluster of large size distribution.36 The lamellar like structure of EAN is fragmented into large clusters due to the intrusion of methanol molecules and due to the strong hydrogen bonding interaction the aploar moiety of CH3OH is fitted into the polar matrix of EAN. However, with increasing concentration of EAN no such larger cluster is observed and the solution is turned into a percolating ionic network.37 A strong hydrogen bonding interaction is observed form the SANS measurement which almost saturate the hydrogen bond donor capacity of MeOH at = 0.30. Due to the strong hydrogen bonding interaction between the solvent mixture, the specific solute-solvent interaction is weakened at = 0.15 and 0.30 in the EAN-MeOH binary mixture. As a consequence, we have observed significant decrease in the =@> value at the two different mole fractions of EAN in the mixture. 3.3. Temperature Dependent Rotational Dynamics of R6G and Fl-Na in EAN-CH3OH binary mixture. With increasing temperature the rotational relaxation time of both the solutes is decreased which can be correlated to the decrease in the viscosity of the medium. To describe the temperature dependency of the anisotropy, the activation energy for the rotational motion of R6G and Fl-Na in different mole fraction of EAN in the binary mixture is calculated from the slope of the Arrhenius plot of ln(1/-.3 ) versus 1/T (figure 4). With increasing concentration of EAN, the activation energy for both the solutes are increased which indicates that the rotational motion of both the fluorophores is mainly viscosity guided. After carefully inspection of the dependency on the rotational activation energy in the EAN-CH3OH binary mixture an interesting characteristic in the range of between 0.10 - 0.15 is observed. In this concentration range, the rotational activation energy for both the solutes is insensitive to the concentration of EAN 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

(table S1, Supporting Information). A large distribution of cluster is formed in this region and as the size of the cluster is almost invariant in this region, the insensitivity of the rotational activation energy can be explained by the clustering effect. Shirota et al have also observed similar results in the solvation dynamics of Coumarin-153 in 1-propanol - water binary mixture.65 Further, SANS measurement shows that the size of the cluster becomes smaller with increasing concentration of EAN36 and accordingly, the rotational activation energy of the solutes is increased with increasing mole fraction of EAN. Temperature dependent anisotropy measurements were also performed to investigate the aggregation property of EAN and CH3OH molecules in the mixture. Chagnes et al pointed out the abnormal temperature dependent viscosity of the EAN-CH3OH binary mixture at the low concentration of EAN (0.2≤ ≤ 0.4) and the results indicate the hydrogen bonding interaction between EAN and methanol is increased with increasing the temperature.50 They have further proposed that the possible cluster is changed from 1EAN:2CH3OH at 293 K to 1EAN: 3CH3OH at 326 K. The anisotropy decays are collected at = 0.30 in the mixture by varying the temperature from 278 K to 318 K and the fitted rotational relaxation time of both the solutes are tabulated in table 3. The calculation for reorientation time of the solutes using SED theory is given in the Supporting Information. Figure 5 shows the theoretical stick and slip line obtained for R6G and Fl-Na at =0.30 in EAN-CH3OH mixture. The data points are the experimentally measured reorientation time of the solutes at different /]. To better understand the solute-solvent interaction we have also calculated the =@> values and they are tabulated in table 3. The =@> values varied in between 0.66 and 0.85 and the values approach more closely to the stick boundary with increasing the temperature. Above 308 K, the solute-solvent interaction is decreased which is due to the strong hydrogen bonding interaction between the solvent themselves. The change in the hydrogen bonding interaction between EAN and CH3OH is also confirmed from the viscosity measurements. The decrease in the =@> value is more prominent for Fl-Na than R6G (for R6G, the change in the =@> value falls within the experimental error limit) as we have showed that the hydrogen bonding interaction between CH3OH and Fl-Na is much stronger than that between R6G and CH3OH. Thus, introducing EAN in the mixture the interaction between Fl-Na and CH3OH is more affected than that of R6G and CH3OH.


Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.4. Translational Diffusion of R6G and Fl-Na in the EAN-CH3OH mixture: In this section, we have discussed the translational diffusion of two charged solutes in EAN-CH3OH binary mixture. The autocorrelation curve of R6G in neat EAN is showed in figure 6(a). The curves are fitted to a two component diffusion model and the residual is shown in figure S3 of Supporting Information. The diffusion coefficient of the probe molecules in neat EAN and the mixture are obtained from an average of 12 data sets and they are enlisted in table 4.

The bimodal

distribution of the diffusion coefficient of R6G clearly describes the dynamic heterogeneity in the EAN structure and it can be attributed to two distinct environment of EAN. The structure of EAN can be described by the alternating polar and non-polar layers which are very much disordered in nature. Recently, Samanta et al. have also observed bimodal distribution in the diffusion coefficient of different Rhodamine dyes in different imidazolium based ionic liquids.66 Bhattacharyya et al have also reported a broad distribution of the diffusion coefficient of the fluorophores in ionic liquid.43 Thus, considering these facts molecular motion of the solutes in ionic liquid can be ascribed in two different regions: domain formed by the alkyl chain and the polar region which is formed by the ionic constituents. The size of the focal spot in our set up is 0.6 ^/_& ~200 nm (Numerical aperture =1.2) and it is much higher than the domain formed by the segregation of the alkyl tail of EAN (which are of the order of few nanometer).19 Thus, the solute molecules can pass through both the polar and non-polar environments while passing through the confocal volume. For this reason, one can expect the diffusion coefficient of solutes in IL appear as single distribution (equal to the average of the diffusion coefficient in two environments). Thus, the bimodal distribution in the diffusion coefficient is only possible when each domain is interconnected and the interconnection between the polar and non-polar domain of EAN is established from the molecular dynamics simulation and SAXS study.36,37 Thus, the continuity in the different domains allow molecules to diffuse within an environment (polar/nonpolar) during passage through the confocal volume. Similar to rotational motion, the translational diffusion of the solutes is increased with increasing concentration of EAN. However, based on the rotational relaxation study we have mainly focused on the translational diffusion of the solutes in two specific regions in the mixture: 0.10≤ ≤0.15 and 0.30≤ ≤0.40. Surprisingly, we have observed that the autocorrelation curves of both the solutes are overlapped at =0.10 and 0.15 (figure 6(b)) and the diffusion coefficients are also close to each other (table 4). As we have already mentioned a large size 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

distribution of cluster is formed at this region, the transnational diffusion of the solutes is almost insensitive to the concentration of EAN. SANS data indicated that this type of cluster is highly heterogeneous36 and for this reason, two component diffusion model has been used to fit the autocorrelation curves and the residuals of fitted curves are shown in the supporting information (figure S3). However, the translational diffusion of Fl-Na is higher than that of R6G due to the strong solute-solvent interaction between Fl-Na and the mixture compared to R6G which has been discussed earlier (table 4, figure S4 and S5). Besides these, we have also observed that the translational diffusion of the solutes at =0.30 is slightly higher than the translational diffusion at =0.40 (figure 7). At =0.30, the mixture is characterized by percolating ionic network where CH3OH inserts into the polar network of EAN by strong and highly directional hydrogen bond interaction with nitrate ion of EAN. Thus, we have also observed higher viscosity at this mole fraction of EAN compared to =0.40. For this reason, the solute molecules experience slower diffusion at this mole fraction compared to that at =0.40. 4. Conclusion: CH3OH and EAN have similar amphiphilic character. However, the mixture cannot be characterized by simple segregation of polar and apolar moieties which originates from the CH3OH and EAN. In order to find out whether this microheterogeneous organized structure influence the motion of the solute molecule we have conducted this present study. Fluorescence anisotropy measurement of two structurally similar but differently charged solutes, R6G and Fl-Na showed that the addition of EAN to CH3OH decrease the solute-solvent interaction and the decrease is more prominent for Fl-Na compared to R6G as the hydrogen bonding interaction between Fl-Na and CH3OH is more stronger than the interaction between R6G and CH3OH. Enhanced interaction of EAN with CH3OH leads to the loss of interaction between solute and solvent and it is more prominent in the CH3OH rich region due to the formation of wide distribution of cluster. Moreover, the temperature dependent fluorescence anisotropy of the solute shows that the hydrogen bonding interaction between the two amphiphiles is temperature dependent and with increasing the temperature H bonding interaction is also increased. The microheterogenity of the mixture is also showed by the FCS study. Bimodal distribution in the diffusion coefficient can be attributed to the microheterogenous nature of the PIL resulting from the segregation of the alkyl chain of the IL. The formation of large cluster at low mole fraction of IL (0.10 ≤ ≤ 0.15) can be proved by the insensitivity 14 ACS Paragon Plus Environment

Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the translational diffusion and rotational activation energy of the solutes to the concentration of EAN. Thus, the result of the works is consistent with the structural detail of the EAN-CH3OH mixture available in the literature. Acknowledgment: N.S. gratefully acknowledges SERB, Department of Science and Technology (DST) Council of Scientific and Industrial Research (CSIR), Government of India for providing generous research grant. A.R. is thankful to CSIR, N. K. and D. B. are thankful to IIT Kharagpur, for providing their research fellowships. Supporting Information: Instrumentation, Calculation for temperature dependent anisotropy measurement, Different resonating structure of R6G, Table of activation energy of the solutes, Emission spectra of Fl-Na and R6G, Anisotropy decays of R6G and Fl-Na in the mixture, Residuals of the fitted autocorrelation curve of R6G and autocorrelation curves of Fl-Na are shown in the Supporting Information. References. (1) Nilsson, A.; Pettersson, L.G.M. The Structural Origin of Anomalous Properties of Liquid Water. Nat. Commun. 2015, 6, 8998. (2) Pothoczki, S.; Temleitner, L.; Pusztai, L. Structure of Neat Liquids Consisting of (Perfect and Nearly) Tetrahedral Molecules. Chem. Rev. 2015, 115, 13308–13361. (3) Crossley, J. Dielectric relaxation and molecular structure in liquids. R. Inst. Chem., Rev., 1971,4, 69-96. (4) Bohmera, R.; Gainaru, C.; Richert, R. Structure and Dynamics of Monohydroxy Alcohols Milestones Towards their Microscopic Understanding, 100 Years after Debye. Phys. Rep. 2014,545, 125−195. (5) El-Abedin, S. Z.; Endres, F. Ionic Liquids: The Link to High-Temperature Molten Salts? Acc.Chem. Res. 2007,40, (6) Dupont, J. From Molten Salts to Ionic Liquids: A “Nano” Journey. Acc. Chem. Res. 2011, 44, 1223−1231. (7) Rogers, R. D.; Seddon, K. R. Ionic Liquids Solvents of the Future? Science 2003, 302, 792−793. (8) Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Properties and Applications. Chem. Rev. 2008, 108, 206−237. (9) Castner, E. W., Jr.; Wishart, J. F. Spotlight on Ionic Liquids. J. Chem. Phys. 2010, 132, 120901−120909.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

(10) Hayes, R.; Warr, G.G.; Atkin, R. Structure and Nanostructure in Ionic Liquids. Chem. Rev. 2015, 115, 6357−6426. (11) Hallett, J. P.; Welton, T. Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. 2. Chem. Rev. 2011, 111, 3508−3576 (12) Parvulescu, V. I.; Hardacre, C. Catalysis in Ionic Liquids. Chem. Rev. 2007, 107, 26152665. (13). Bhattacharyya, K. Room-Temperature Ionic Liquid: A Nanostructured Liquid for HighVacuum and High-Energy Applications. J. Phys. Chem. Lett. 2010, 1, 3254−3255. (14) Greaves, T.L.; Drummond, C.J. Protic Ionic Liquids: Evolving Structure−Property Relationships and Expanding Applications. Chem. Rev. 2015, 115, 11379−11448. (15) Noda, A.; Susan, M. A. B. H.; Kudo, K.; Mitsushima, S.; Hayamizu, K.; Watanabe, M. Brønsted Acid−Base Ionic Liquids as Proton-Conducting Nonaqueous Electrolytes. J. Phys. Chem. B 2003, 107, 4024−4033. (16) Miran, M. S.; Yasuda, T.; Susan, M. A. B. H.; Dokko, K.; Watanabe, M. Binary Protic Ionic Liquid Mixtures as a Proton Conductor: High Fuel Cell Reaction Activity and Facile Proton Transport. J. Phys. Chem. C 2014, 118, 27631−27639. (17) Yoshizawa, M.; Xu, W.; Angell, C. A. Ionic Liquids by Proton Transfer: Vapor Pressure, Conductivity, and the Relevance of pKa from Aqueous Solutions. J. Am. Chem. Soc. 2003, 125, 15411−15419. (18) Mann, J. P.; McCluskey, A.; Atkin, R. Activity and Thermal Stability of Lysozyme in Alkylammonium Formate Ionic Liquids—Influence of Cation Modification. Green Chem. 2009, 11,785−792. (19) Atkin, R.; Warr, G.G. The Smallest Amphiphiles: Nanostructure in Protic RoomTemperature Ionic Liquids with Short Alkyl Groups. J. Phys. Chem. B 2008, 112, 4164-4166. (20) Hayes, R.; Imberti, S.; Warr, G.G.; Atkin, R. Amphiphilicity Determines Nanostructure in Protic Ionic Liquids. Phys. Chem. Chem. Phys., 2011, 13, 3237–3247. (21) Henderson, W. A.; Fylstra, P.; De Long, H. C.; Trulove, P. C.; Parsons, S. Crystal Structure of the Ionic Liquid EtNH3NO3—Insights into the Thermal Phase Behavior of Protic Ionic Liquids. Phys. Chem. Chem. Phys. 2012, 14, 16041−16046. (22) Seddon, K. R.; Stark, A.; Torres, M. J. Influence of Chloride, Water, and Organic Solvents on the Physical Properties of Ionic Liquids. Pure Appl. Chem. 2000, 72, 2275−2287. (23) Li, W.; Zhang, Z.; Han, B.; Hu, S.; Xie, Y.; Yang, G. Effect of Water and Organic Solvents on the Ionic Dissociation of Ionic Liquids. J. Phys. Chem. B 2007, 111, 6452−6456.


Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Fletcher, K. A.; Pandey, S. Solvatochromic Probe Behavior within Ternary RoomTemperature Ionic Liquid 1-Butyl-3-methylimidazolium Hexafluorophosphate + Ethanol + Water Solutions. J. Phys. Chem. B 2003, 107, 13532−13539. (25) Pramanik, R.; Rao, V.G.; Sarkar, S.; Ghatak, C.; Setua, P.; Sarkar, N. To Probe the Interaction of Methanol and Acetonitrile with the Ionic Liquid N,N,N-Trimethyl-N-propyl Ammonium Bis(trifluoromethanesulfonyl) Imide at Different Temperatures by Solvation Dynamics Study. J. Phys. Chem. B 2009, 113, 8626–8634. (26) Sarkar, S.; Pramanik, R.; Ghatak, C.; Setua, P.; Sarkar, N. Probing the Interaction of 1 Ethyl-3-methylimidazolium Ethyl Sulfate ([Emim][EtSO4]) with Alcohols and Water by Solvent and Rotational Relaxation. J. Phys. Chem. B 2010, 114, 2779–2789. (27) Rao, V.G.; Ghatak, C.; Pramanik, R.; Sarkar, S.; Sarkar, N. Solvation and Rotational Dynamics of Coumarin-153 in Ethylammonium Nitrate Containing γ-Cyclodextrin. J. Phys. Chem. B 2011, 115, 10500–10508. (28) Mandal, S.; Ghosh, S.; Banerjee, C.; Kuchlyan, J.; Sarkar, N. Roles of Viscosity, Polarity, and Hydrogen-Bonding Ability of a Pyrrolidinium Ionic Liquid and Its Binary Mixtures in the Photophysics and Rotational Dynamics of the Potent Excited-State Intramolecular ProtonTransfer Probe 2,2′-Bipyridine-3,3-diol. J. Phys. Chem. B 2013, 117, 6789−6800. (29) Verma, S.D.; Corcelli, S.A.; Berg, M.A. Rate and Amplitude Heterogeneity in the Solvation Response of an Ionic Liquid. J. Phys. Chem. Lett. 2016, 7, 504–508. (30) Araque, J.C.; Yadav, S.K.; Shadeck, M.; Maroncelli, M.; Margulis, C.J. How Is Diffusion of Neutral and Charged Tracers Related to the Structure and Dynamics of a Room-Temperature Ionic Liquid? Large Deviations from Stokes–Einstein Behavior Explained. J. Phys. Chem. B, 2015, 119, 7015–7029 (31) Zhang, X.X.; Liang, M.; Ernsting, N.P.; Maroncelli, M. Conductivity and Solvation Dynamics in Ionic Liquids. J. Phys. Chem. Lett. 2013, 4, 1205−1210. (32) Zhang, X.X.; Liang, M.; Hunger, J.; Buchner, R.; Maroncelli, M. Dielectric Relaxation and Solvation Dynamics in a Prototypical Ionic Liquid + Dipolar Protic Liquid Mixture: 1-Butyl-3 Methylimidazolium Tetrafluoroborate + Water. J. Phys. Chem. B 2013, 117, 15356–15368. (33) Álvarez, B.D.; González, V.G.; Morales, T.M.; Carrete, J.; Rodríguez, J.R.; Cabeza, O.; Gallego, L.J.; Varela, L.M. Mixtures of Protic Ionic Liquids and Molecular Cosolvents: A Molecular Dynamics Simulation. J. Chem. Phys. 2014, 140, 214502. (34) Greaves, T.L. Kennedy, D.F.; Kirby, N.; Drummond, C.J. Nanostructure Changes in Protic Ionic Liquids (PILs) Through Adding Solutes and Mixing PILs. Phys. Chem. Chem. Phys. 2011, 13, 13501–13509.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

(35) Hayes, R.; Imberti, S.; Warr, G.G.; Atkin, R. How Water Dissolves in Protic Ionic Liquids. Angew. Chem. Int. Ed. 2012, 51, 7468 –7471. (36) Russina, O.; Sferrazza, A.; Caminiti, R.; Triolo, A. Amphiphile Meets Amphiphile: Beyond the Polar−Apolar Dualism in Ionic Liquid/Alcohol Mixtures. J. Phys. Chem. Lett. 2014, 5, 1738−1742. (37) Russina, O.; Mariani, A.; Caminiti, R.; Triolo, A. Structure of a Binary Mixture of Ethylammonium Nitrate and Methanol. J Solution Chem. 2015, 44,669–685. (38) Mariani, A.; Russina, O.; Caminiti, R.; Triolo, A. Structural Organization in a Methanol:Ethylammonium Nitrate (1:4) Mixture: A Joint X-ray/Neutron Diffraction and Computational Study. J. Mol. Liq.2015, 212, 947-956. (39) Prabhu, S.R.;Dutt, G.B. Does Addition of an Electrolyte Influence the Rotational Diffusion of Nondipolar Solutes in a Protic Ionic Liquid? J. Phys. Chem. B 2015, 119, 6311−6316. (40) Prabhu, S.R.;Dutt, G.B. Rotational Diffusion of Nonpolar and Charged Solutes in Propylammonium Nitrate–Propylene Glycol Mixtures: Does the Organized Structure of the Ionic Liquid Influence Solute Rotation? J. Phys. Chem. B 2014, 118, 2738–2745. (41) Karve, L.; Dutt, G.B. Role of Specific Interactions on the Rotational Diffusion of Organic Solutes in a Protic Ionic Liquid–Propylammonium Nitrate. J. Phys. Chem. B 2012, 116, 9107– 9113. (42) Werner, J. H.; Baker, S. N.; Baker, G. A. Fluorescence Correlation Spectroscopic Studies of Diffusion within the Ionic Liquid 1-Butyl-3-Methylimidazolium Hexafluorophosphate. Analyst 2003, 128, 786−789. (43) Sasmal, D. K.; Mandal, A. K.; Mondal, T.; Bhattacharyya, K. Diffusion of Organic Dyes in Ionic Liquid and Giant Micron Sized Ionic Liquid Mixed Micelle: Fluorescence Correlation Spectroscopy. J. Phys. Chem. B 2011, 115, 7781−7787. (44) Guo, J.; Baker, G. A.; Hillesheim, P. C.; Dai, S.; Shaw, R. W.; Mahurin, S. M. Fluorescence Correlation Spectroscopy Evidence for Structural Heterogeneity in Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13, 12395−12398. (45) Evans, D. F.; Kaler, E. W.; Benton, W. J. Liquid Crystals in a Fused Salt: .Beta.,Gamma.Distearoylphosphatidylcholine in N-Ethylammonium Nitrate. J. Phys. Chem. 1983, 87, 533−535. (46) Yang, C.; Xu, W.; Ma, P. Thermodynamic Properties of Binary Mixtures of p-Xylene with Cyclohexane, Heptane, Octane, and N-Methyl-2-pyrrolidone at Several Temperatures J. Chem. Eng. Data 2004, 49, 1794-1801.


Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(47) Trivedi, S.; Pandey, S. Interactions within a [Ionic Liquid + Poly(ethylene glycol)] Mixture Revealed by Temperature-Dependent Synergistic Dynamic Viscosity and Probe-Reported Microviscosity. J.Phys. Chem. B 2011, 115, 7405−7416. (48) Tian, Y.; Wang, X.; Wang, J. Densities and Viscosities of 1- Butyl-3-methylimidazolium Tetrafluoroborate + Molecular Solvent Binary Mixtures. J. Chem. Eng. Data 2008, 53, 2056−2059. (49) Mokhtarani, B.; Sharifi, A.; Mortaheb, H. R.; Mirzaei, M.; Mafi, M.; Sadeghian, F. Density and Viscosity of 1-Butyl-3-methylimidazo-lium Nitrate with Ethanol, 1-Propanol, or 1-Butanol at Several Temperatures .J. Chem. Thermodyn.2009, 41, 1432−1438. (50) Chagnes, A.; Tougui, A.; Carré, B.; Ranganathan, N.; Lemordant, D. Abnormal Temperature Dependence of the Viscosity of Ethylammonium Nitrate−Methanol Ionic Mixtures. J. Solution Chem. 2004, 33, 247−255. (51) Zakerhamidi, M.S.; Moghadam, M.; Ghanadzadeh, A.; Hosseini, S. Anisotropic and Isotropic Solvent Effects on the Dipole Moment and Photophysical Properties of Rhodamine Dyes. J. Lumin.2012, 132, 931-937. (52) Sjoback, R.; Nygren, J.; Kubista, M. Absorption and Fluorescence Properties of Fluorescein. Spectrochim. Acta Part A 1995, 51, L7-L21. (53) Koley, S.; Kaur, H.; Ghosh, S. Probe Dependent Anomalies in the Solvation Dynamics of Coumarin Dyes in Dimethyl Sulfoxide−Glycerol Binary Solvent: Confirming the Local Environments are Different for Coumarin Dyes. Phys. Chem. Chem. Phys. 2014, 16, 22352−22363. (54) Fleming, G. R. Chemical Applications of Ultrafast Spectroscopy; Oxford University Press: New York, 1986. (55) Waldeck, D. H. The Role of Solute-Solvent Friction in Large Amplitude Motions. In Conformational Analysis of Molecules in the Excited States; Waluk, J., Ed.; Wiley-VCH: New York, 2000; pp 113−176. (56) Guo, J.; Han, K.S.; Mahurin, S.M.; Baker, G.A.; Hillesheim, P. C.; Dai, S.; Hagaman, E.W.; Shaw, R.W. Rotational and Translational Dynamics of Rhodamine 6G in a Pyrrolidinium Ionic Liquid: A Combined Time-Resolved Fluorescence Anisotropy Decay and NMR Study. J. Phys. Chem. B 2012, 116, 7883−7890. (57) Gangamallaiah, V.; Dutt, G.B. Rotational Diffusion of Nonpolar and Ionic Solutes in 1‑Alkyl-3- Methylimidazolium Bis(trifluoromethylsulfonyl)imides: Is Solute Rotation Always Influenced by the Length of the Alkyl Chain on the Imidazolium Cation? J. Phys. Chem. B 2012, 116, 12819−12825.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

(58) Nee, T.W.; Zwanzig, R. Theory of Dielectric Relaxation in Polar Liquids. J. Chem. Phys. 1970, 52, 6353. (59) Mashimo, S.; Kuwabara, S.; Yagihara, S.; Higasi, K. The Dielectric Relaxation of Mixtures of Water and Primary Alcohol. J. Chem. Phys. 1989, 90, 3292. (60) Mannekutla, J.R.; Inamdar, S.R.; Mulimani, B.G.; Savadatti, M.I. Rotational Diffusion of Coumarins: A Dielectric Friction Study. J Fluoresc 2010, 20,797–808. (61) Gayathri, B.R.; Mannekutla, J.R.; Inamdar, S.R. Rotational Diffusion of Coumarins in Alcohols: A Dielectric Friction Study. J Fluoresc 2008, 18, 943–952. (62) Poole, C. F. Chromatographic and Spectroscopic Methods for the Determination of Solvent Properties of Room Temperature Ionic Liquids. J. Chromatogr. A 2004, 1037, 49−82. (63) Paul, A.; Samanta, A. Effect of Nonpolar Solvents on the Solute Rotation and Solvation Dynamics in an Imidazolium Ionic Liquid. J. Phys. Chem. B 2008, 112, 947−953. (64) Liang, M.; Zhang, X. X.; Kaintz, A.; Ernsting, N. P.; Maroncelli, M. Solvation Dynamics in a Prototypical Ionic Liquid + Dipolar Aprotic Liquid Mixture: 1-Butyl-3-methylimidazolium Tetrafluoroborate + Acetonitrile. J. Phys. Chem. B 2014, 118, 1340−1352. (65) Shirota, H.; Castner, E. W. Solvation in Highly Nonideal Solutions: A Study of Aqueous 1Propanol Using the Coumarin 153 Probe. J. Chem. Phys. 2000, 112, 2367−2376. (66) Patra, S.; Samanta, A. Microheterogeneity of Some Imidazolium Ionic Liquids As Revealed by Fluorescence Correlation Spectroscopy and Lifetime Studies. J. Phys. Chem. B 2012, 116, 12275−12283.


Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Chemical Structure of Protic Ionic liquid EAN, Cosolvent Methanol and two fluorophores Rhodamine 6G perchlorate and Fluorescein sodium salt.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

Table 1: Parameters of Anisotropy decays of R6G in EAN-CH3OH binary mixtures at different mole fraction of EAN (at 298 K)

System

R6G

a

àbc

defg

0.00 0.05 0.10 0.15 0.20 0.30 0.40 0.50 0.70 1.00

0.55 0.92 1.14 1.65 1.8 3.88 3.83 5.11 7.38 31.68

experimental error ±5%

b

hji

0.30 0.32 0.33 0.35 0.36 0.36 0.38 0.38 0.38 0.39

khlm 0.20±0.01 0.24±0.02 0.30±0.02 0.34±0.03 0.41±0.03 0.68±0.05 0.74±0.07 1.0±0.1 1.7±0.2 3.2±0.3

knmoep h 0.12 0.20 0.25 0.37 0.40 0.86 0.85 1.1 1.6 7.0

qljr 1.7±0.05 1.2±0.08 1.1±0.06 0.92±0.09 1.0±0.08 0.79±0.07 0.87±0.09 0.88±0.1 0.94±0.1 0.46±0.09

initial anisotropy

Table 2: Parameters of Anisotropy decays of Fl-Na in EAN-CH3OH binary mixtures at different mole fraction of EAN (at 298 K)

a

System

àbc

defg

Fl-Na

0.00 0.10 0.15 0.20 0.30 0.40 0.50 0.70 1.00

0.55 1.14 1.65 1.8 3.88 3.83 5.11 7.38 31.68


b

hji

0.28 0.30 0.31 0.33 0.35 0.35 0.37 0.37 0.38

khlm 0.18±0.02 0.27±0.02 0.30±0.03 0.38±0.04 0.51±0.05 0.69±0.05 0.80±0.07 1.4±0.1 2.4±0.2

initial anisotropy


knmoep h 0.09 0.18 0.26 0.29 0.63 0.62 0.83 1.2 5.1

qljr 2.0±0.1 1.5±0.07 1.15±0.1 1.31±0.1 0.81±0.1 1.1±0.07 0.96±0.09 1.1±0.07 0.47±0.08

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3: Parameters of Anisotropy decays of R6G and Fl-Na at àbc =0.30 in EANCH3OH binary mixture System

Temp (K)

R6G

Fl-Na

a

d g s /tu

khlm

knmoep h

qljr

0.025 0.017 0.013 0.011 0.009

1.1±0.1 0.90±0.08 0.68±0.05 0.60±0.05 0.50±0.04 0.79±0.08 0.63±0.05 0.51±0.05 0.40±0.03 0.33±0.02

1.60 1.10 0.86 0.71 0.65 1.20 0.81 0.63 0.51 0.48

0.69±0.09 0.82±0.09 0.80±0.07 0.86±0.08 0.77±0.08 0.66±0.10 0.78±0.08 0.81±0.09 0.78±.08 0.69±0.06

278 288 298 308 318 278 288 298 308 318

0.025 0.017 0.013 0.011


Table 4: Diffusion Parameters of the solutes in different mole fraction of EAN in EANCH3OH mixture System

R6G

Fl-Na

= 1.00 = 0.10 = 0.15 = 0.30 = 0.40 = 0..10 = 0.15 = 0.30 = 0.40

&$

&Q

v3$

v3Q

0.20 0.50 0.37 0.38 0.34 0.39 0.41 0.30 0.15

0.80 0.50 0.63 0.62 0.66 0.61 0.59 0.70 0.85

8.05±2.00 33.3±9.50 31.8±10.0 19.5±5.00 26.8±7.00 20.8±8.32 19.8±10.32 11.3±5.26 16.7±4.35

55.3±8.00 135.2±15.0 150±15.0 153±20.0 159±50.0 115±25.0 125±20.0 120±15.0 115±20.0



60

(a)

0

50

278 K 288 K 298 K 308 K 318 K

30

-5

ηE

40

-10 0

20 10

318 K

278 K 288 K 298 K 308 K 318 K

-2

ηE

-15

-4

-20

0 0.0

0.2

0.4

0.6

0.8

χEAN

0.0

0.0

1.0

0.3

0.6

χ EAN

0.9

0.2

0.4

0.6

χEAN

0.8

1.0

Figure 1. (a) Plots of viscosities of EAN-CH3OH mixtures versus mole fraction of EAN from 278 K to 318 K. The curves passing through the data points represent the variation of η with EAN and (b) Plots of excess viscosities of EAN-CH3OH mixtures versus mole fraction of EAN form 278 K to 318 K and the curves are obtained by fitting with RedlichKister equation (The excess viscosity plot at 318 K is shown in the inset of the figure.)

0.40

0.35

R6G

0.35

/η

η (cp)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

Fl-Na

0.30

0.30

0.25

0.25

0.20

0.20 0.15 0.15 0.10 0.10 0.0

0.3 χ

0.6

0.9

0.0

0.3

0.6

0.9

χ

EAN

EAN

χ

Figure 2. Plot of viscosity normalized rotational relaxation time ( fraction of EAN for R6G and Fl-Na.


khlm /d against the mole

ry

ry un da

y -1

ou

-2

nd ar

0 -1

bo

pb

-2

ip

S li

-3

da

ic k

0

bo un

1

St i ck

log

bo

1

slip stick expt

2

St

slip stick expt

da ry

2

Fl-Na

Sl

-3 -4

R6G

-4

-5 -1

0

1

2

3

4

-1

0

1

log η

2

3

4

log η

Figure 3. Plots of log versus log d for R6G and Fl-Na in different mole fraction of EAN. Theoretical reorientation times calculated using SED theory with Stick, Slip boundary Conditions.

(a)

2.0

Ea (KJ/mol)

1.0 0.5 0.0

R6G

20

χEAN = 0.00

1.5

ln (1/τrot)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


un

Page 25 of 27

-0.5

Fl-Na

(b)

20

15 15

10

10

-1.0 -1.5

χEAN = 1.00 0.0031

0.0032

0.0033

0.0034

0.0035

5

0.0036

5 0.0

0.3

1/T (K-1)

0.6 χEAN

0.9

0.0

0.3

0.6

χEAN

Figure 4. (a) Effects of Temperature on the rotational reorientation dynamics of R6G in EAN-CH3OH mixtures. (Arrhenius Plot) (b) Activation energy plot of R6G and Fl-Na in different mole fraction of EAN obtained from Arrhenius Plot. The curves are fits: wx

ag 8ylz; = {|. |} − {~. }if −. {àbc

(for

R6G)

{. {if −~. {}àbc (for Fl-Na)


and

wx

ag 8ylz; = ~. } −

0.9


slip boundary stick boundary

1.5

(a) R6G

1.2

(b) Fl-Na

slip boundary stick boundary

Exp data

expt data 0.8

χEAN = 0.30

τrot

τrot

1.0

0.5

χEAN = 0.30

0.4

0.0 0.008

0.012

0.016

0.020

0.0 0.008

0.024

η/T

0.012

0.016

0.020

0.024

η/T

d Figure 5. Plot of khlm vs /t in EAN-CH3OH binary mixture for (a) R6G and (b) Fl-Na at àbc =0.30 in EAN-CH3OH binary mixture. Theoretical reorientation times calculated using SED theory with Stick, Slip boundary Conditions.

1.2

1.2

χEAN = 1.00

0.8 0.4

1.0

χEAN = 0.10

0.8

1.0

(a)

G(τ )

G(τ)

1.2

0.6 0.4 0.2 0.0 100

0.0 100

1000

10000

100000

Time (µs) 8

1000000

G(τ)

0.8

Events

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

1000

10000

Time (µ s)

100000

χEAN = 0.15

0.6 0.4

6

0.2

4 2

(b)

0.0 2

10

0 1

10

2

Dt (µm /s)

3

10

100

4

10

5

10

Time (µs)

2

10

3

10

4

10

Figure 6. (a) Normalized FCS traces of R6G in neat EAN and the lower panel shows the distribution of m values (b) Normalized FCS traces of R6G in EAN-CH3OH mixtures (àbc = i. {, àbc = i. {i) (Inset shows no variation is observed in the autocorrelation plot in two different mole fraction of EAN) 26 ACS Paragon Plus Environment

5

10

Page 27 of 27

1.2

(a)

χEAN = 0.3

6

χEAN = 0.4

1.0

(b)

χEAN = 0.3

3

Events

0.8

G(τ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.4

0 6

χEAN = 0.4 3

0.2 0 0.0

1 2

10

3

10

4

10

5

10

2

10

Time (µs)

3

10

4

10

10

100

Dt (µm2 s-1)

5

10

Figure 7. (a) Normalized FCS traces and (b) distribution of m values of R6G in EANCH3OH Mixture (àbc = i. i, àbc = i. i ).

Table of Content (TOC) only:


Translational and Rotational Diffusion of Two Differently Charged

Recommend Documents