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Open Access

Peer-reviewed

Research Article

Thermodynamic and Ultrasonic Properties of Ascorbic Acid in Aqueous Protic Ionic Liquid Solutions

Abstract

In this work, we report the thermodynamic and ultrasonic properties of ascorbic acid (vitamin C) in water and in presence of newly synthesized ammonium based protic ionic liquid (diethylethanolammonium propionate) as a function of concentration and temperature. Apparent molar volume and apparent molar isentropic compression, which characterize the solvation state of ascorbic acid (AA) in presence of protic ionic liquid (PIL) has been determined from precise density and speed of sound measurements at temperatures (293.15 to 328.15) K with 5 K interval. The strength of molecular interactions prevailing in ternary solutions has been discussed on the basis of infinite dilution partial molar volume and partial molar isentropic compression, corresponding volume of transfer and interaction coefficients. Result has been discussed in terms of solute-solute and solute-solvent interactions occurring between ascorbic acid and PIL in ternary solutions (AA + water + PIL).

Citation: Singh V, Sharma G, Gardas RL (2015) Thermodynamic and Ultrasonic Properties of Ascorbic Acid in Aqueous Protic Ionic Liquid Solutions. PLoS ONE 10(5): e0126091. https://doi.org/10.1371/journal.pone.0126091

Academic Editor: Chris Lorenz, King's College London, UNITED KINGDOM

Received: November 22, 2014; Accepted: March 19, 2015; Published: May 26, 2015

Copyright: © 2015 Singh et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Data Availability: All relevant data are within the paper.

Funding: The authors are thankful to Council of Scientific and Industrial Research (CSIR, http://www.csirhrdg.res.in/), India for the financial support through grant numbers 01(2525)/11/EMR-II. Dr. Vickramjeet Singh acknowledges the financial support from CSIR through his post-doctoral fellowship. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Ionic liquids (ILs) are molten salts with melting point below 100°C. ILs have unique physicochemical properties such as broad liquid temperature range, negligible vapor pressure, wide electrochemical window,high thermal stability, and high specific solvent abilities [1–5]. The thermo-physical properties of ILs can be tuned by appropriate selection of cation and anion, as a result ILs can be made biocompatible and these are found to be very attractive in various analytical applications, particularly, in the fabrication of various modified electrodeswhichcan be used to extract chemicals or compounds (synthetic colors) from food samples. Reports are available [6–12]were ILs have been used for the extraction of Sudan and Para Red dyes from chilli powder employinghigh performance liquid chromatography (HPLC), thus showing potential application of ILs in food industries [10–11,13–15].

Chailapakul et al. [16] have used IL based carbon electrode for the analysis of sudan I, sudan II, sudan III and sudan IV. Recently, a novel carbon composite electrode of an IL (n-octylpyridinum hexafluorophosphate) and single-walled carbon nanotube (SWCNT) was designed to determine the levels of ascorbic acid from food samples [11]. Ionic liquids have also been proposed as an effective compound to be used in the formation of aqueous biphasic system (ABS) [17–18]. ABS are considered as an alternative to liquid-liquid extraction techniques, as ABS are being used as powerful technique in bioseparation processes, purification, extraction and enrichment [18–19]. ABS with ILs in combination with salts, amino acids, polyhydroxy compounds (saccharides), and polymers has been reported [18–24]. More recently, it has been reported [25] that the ABS employing ILs with various solutes (saccharides, amino acids, vitamins, etc) requires reliable and systematic thermodynamic data. The acquaintance of thermodynamics of ILs in aqueous solutions with various biomolecules (saccharides, amino acid, etc) is of crucial importance to improve the process design and to understand the molecular interactions between ILs and biomolecules, thus serving with the design of ABS systems.

Furthermore, the increased utilization of ILs for various applications (chemical or separations processes) requires accurate determination of thermodynamic data [25]. The nature and strength of interactions between AA and various co-solutes (electrolytes, non electrolytes, surfactants etc.) have been studied [26–31] by evaluating the thermodynamic properties of ternary systems (AA + water + co-solutes). These thermodynamic properties are useful in characterizing the solvation behaviour of vitamins and to further understand solute-solute and solute-solvent interactions [32–33]. Ascorbic acid (vitamin C) one of the most important vitamin for human health and nutrition, is found in fruits and vegetables [34–39]. It is a sugar acid, having antioxidant properties and can prevent or treat common cold and scurvy. It also acts as a cofactor and thus maintain activity of various enzymes [26–27,32,40–41]. The degradation of AA is very important and is considered to be the major cause of color and quality change during storage or processing of food materials [40, 42].

However, to the best of our knowledge there is no report on thermodynamic and ultrasonic properties of AA in aqueous solutions of ammonium based protic ionic liquid (PIL) as a function of concentration and temperature. So, in order to understand the molecular interactions occurring between AA and PIL, we report herein the volumetric propertiesof AA in aqueous solutions of newly synthesized protic ionic liquid i.e. diethylethanolammonium propionate([DEEA][Pro]) at different temperatures (293.15 to 328.15) K. Various parameters such as partial molar volumes and isentropic compression, transfer volumes, interaction coefficients, and thermal expansion coefficients have been evaluated and discussed in terms of solute-solute and solute-cosolute interactions.

Materials and Methods

2.1 Materials

Ascorbic acid (mass fraction purity; 0.99) was purchased from Sisco Research Laboratory Pvt. Ltd. India, N,N-diethylethanol amine (0.99), propanoic acid (0.99)and methanol (0.99) were purchased from Sigma Aldrich. AA was used after drying in a vacuum desiccator (over anhydrous CaCl2) for 48 h at room temperature and all the other chemicals were used without further purification.

2.2. Synthesis and characterization of ([DEEA][Pro])

PIL, ([DEEA][Pro]) was synthesized by exothermic neutralization of bronsted acid (propanoic acid) by base (N,N- diethylethanol amine). To10 ml of methanol, N,N- diethylethanol amine (0.1mol) was added in round bottom flask and this mixture was kept in ice bath for few minutes. Further, propanoic acid (0.11mol) was added slowly and drop wise (by using dropping funnel) to the above reaction mixture. Addition of acid was completed in 2 hrs at a temperature below 5°C and then reaction mixture was stirred continuously for 24 hrs at room temperature. The excess amount of starting material and solvent were removed by putting the reaction mixture into rotavapor for 4 hrs. The resultant product (PIL) was dried at room temperature under high vacuum for 36 hrs, in order to remove moisture and excess of amineand further IL was kept in N2 atmosphere.

Synthesized PIL was characterized by 1H NMR, 13C NMR (Bruker Avance 400 MHz) and FTIR (JASKO FT/IR-4100) spectroscopictechniques. NMR was recorded in CDCl3. 1H NMR of [DEEA][Pro], δ = 4.725ppm (broad, 2H, OH and NH+), δ = 3.863ppm (t, 2H), δ = 3.049ppm (q, 3H), δ = 2.972ppm (t, 2H), δ = 2.287ppm (q, 2H), δ = 1.250ppm (t, 6H) and δ = 1.119ppm (t, 3H). IR was recorded using KBr disk, the JASKO FT/IR-4100 spectrometer has a maximum resolution of 0.9 cm-1 and signal to noise ratio of 22000:1. The IR broad band appeared in range of 3400–2800 cm-1 correspondsto the characteristic ammonium peak, ν(N-H) and ν(O-H) stretching vibration. The broad band centered around 1600 cm−1corresponds to the characteristic carbonyl, ν(C = O) stretching and δ(N-H) plane bending, vibrations. The Karl Fischer titrator from Analab (Micro Aqua Cal 100) was used to measure the water content. This instrument operates on conductometric titration principle using dual platinum electrodes that permits detection of water content from less than 10 ppm to 100%. The water content in the synthesized [DEEA][Pro] was ≈ 7000 ppm. The amount of water present in PIL has been taken into account for the molality correction of stock solutions (water + PIL).

2.3 Density and speed of sound

The vibrating-tube digital density meter and sound velocity analyzer (Anton Paar, DSA 5000M) was used to measure simultaneously the densities, ρ and speeds of sound, u of AA in water and in mB (molality of PIL) = (0.10, 0.15, 0.20, and 0.25) mol·kg-1 aqueous solutions of [DEEA][Pro] at temperatures, T = (293.15, 298.15, 303.15, 308.15, 313.15, 318.15, 323.15 and 328.15) K and at atmospheric pressure. The instrument is equipped with a density cell and a sound velocity cell, which are temperature controlled by a built-in Peltier thermostat (PT-100) with an accuracy of ±0.01 K. It can measure the density in the range of (0 to 3000) kg·m-3 and speed of sound from (1000 to 2000) m·s-1. At regular intervals, instrument was calibrated at atmospheric pressure with dry air and deionized, double distilled, and freshly degassed water according to the procedure mentioned in the instrument manual. The uncertainties in the measurement of density and speed of sound were ± 7×10ˉ3 kg∙mˉ3 and ±0.5 m∙sˉ1, respectively. The solutions were made fresh in Millipore quality freshly degassed water on mass basis in air tight glass vials by using Sartorius balance (Model CPA225D) having a precision of ±0.01 mg. The uncertainty in molality was ± 1.03·10-5 mol·kg-1.

Results and Discussion

3.1 Apparent molar volume and apparent molar isentropic compression

The understanding of molecular interaction between a solute and solvent (water) andthe packing efficiency of solute within the structure of water has been studied in aqueous [43–44] and mixed aqueous solutions [3,45–47]. The packing efficiency of a solute which is governed by solute-solvent interactions can be measured by employing apparent molar volume. Apparent molar volume is smaller for heavily hydrated molecules as compared to those which are weakly hydrated, and this may be due to greater interaction of solute molecules with water [48]. The solvation behaviour of a solute has been studied by two most important parameters i.e.apparent molar volume, V2,ϕ and apparent molar isentropic compression, Ks,2, ϕ. In this study, the apparent molar volume, V2, ϕ and apparent molar isentropic compression, Ks,2, ϕ of AAin water and in mB = (0.10, 0.15, 0.20, and 0.25) mol·kg-1 aqueous solutions of [DEEA][Pro] at different temperatures (Table 1) were determined form density and speed of sound data (Table 1) by using the following Eqs (1) and (2): (1) (2) where M and m are respectively, the molar mass and molality of AA; ρ and ρo are the densities of solution and solvent (water or water + [DEEA][Pro]), κsandκs° are the isentropic compressibilities of solution and solvent, respectively.

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Table 1. The densities, ρ, apparent molar volumes, V2, ϕ, speeds of sound, u and apparent molar isentropic compression, Ks,2, ϕ° of ascorbic acid in water and in aqueous [DEEA][Pro] solutions at temperatures, T = (293.15 to 328.15) K and at ambient pressure.

https://doi.org/10.1371/journal.pone.0126091.t001

The experimentally measured densities and speeds of sound of the solutions were used to evaluate the isentropic compressibility, κs as:κs = u-2.ρ-1. The combined uncertainties in V2, ϕvalues resulting from experimentally measured quantities[u(m) = 1.03·10-5 mol·kg-1, u(ρ) = 7.0·10−3kg·m−3,u (T) = 0.01 K]ranges from (0.20 to 0.04). 106 m3·mol-1at (≤ 0.05 mol·kg-1) and high concentration range of AA, respectively and combined uncertainties in Ks,2, ϕ values ranges from(0.60 to 0.12)·10−15m3·mol-1·Pa-1at low and high concentration range of AA, respectively (level of confidence, k = 0.95).

The V2,ϕand Ks,2,ϕvalues (Table 1) of AAin water increase with increase in concentration of AA and temperature, whereas in aqueous [DEEA][Pro] solutions V2,ϕ values decrease with increase in concentration of AA. The variation of V2,ϕversus molality of AA in water at different temperature is shown in Fig 1. In this plot, V2,ϕ value increase as color of the figure change from dark blue region to dark orange region. TheKs,2,ϕvalue of AA in water and in aqueous solutions of [DEEA][Pro] are negative as well positive (at high temperature and cosolute concentration). For ionic compounds in water, highly negative Ks,2,ϕvalues have been observed, whereas for hydrophobic soluteKs,2,ϕvalues are positive and for uncharged hydrophilic solutes,Ks,2,ϕvalues are intermediate (small and negative). Negative Ks,2,ϕvalues suggest that, AA gets hydrated due to electrostricted water molecules in the vicinity of hydrophilic groups of AA. At high temperature and cosolute ([DEEA][Pro]) concentration, Ks,2,ϕvalues increase and become positive, which indicate the removal of electrostricted water around the solute (AA) molecules. Due to the presence of both hydrophilic and hydrophobic groups in AA, the Ks,2,ϕvalues obtained reflects the competition between various types of interactions between AA and solvent [32].

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Fig 1. Plot of apparent molar volumes, Vϕversus molalities, m of ascorbic acid in water at temperatures, T = (293.15, 298.15, 303.15, 308.15, 313.15, 318.15, 323.15 and 328.15) K.

https://doi.org/10.1371/journal.pone.0126091.g001

3.2 Infinite dilution partial molar volume and partial molar isentropic compression

Infinite dilution partial molar volume(V2° = V2°,ϕ)and partial molar isentropic compression(K °s,2 = K°s,2,ϕ) has been evaluated by least-square fitting to the corresponding data as Eqs (3) and (4): (3) (4) where Sv and SKare respective experimental slopes. Apparent molar volume, V2,ϕ of AA in water has been reported by various workers [33, 49–52] and in few reports,V2,ϕ values were determined without considering the ionization of AA. Ayranci et al. [33] studied the volumetric properties of AA in 0.01 M HCl solutions, by suppressing the ionization of AA. A modified Debye-Huckel equation was proposed by Hakin, Mudrack, &Beswick [49] to consider the partial dissociation of AA in water, since at high concentration of weak acid, degree of hydrolysis is small, later some workers reported the volumetric properties of AA in water [50–52] without considering the dissociation of AA. Dhondgeet al. [40] have reported that at low concentration the effect of hydrolysis of AA on volumetric properties are assumed to be negligible (as it lies in the range of uncertainties), whereas at high concentration it is smaller due to decrease in the degree of hydrolysis. In this work, V2,ϕ values of AA were determined by assuming that the effect of hydrolysis of AA on apparent molar volume is very small and negligible [40]. The V2° and K °s,2values of AA in water agree well the literature values [32,29,33,49–50,52] and are given in Tables 2 and 3, respectively. The V2°values of AA increase with concentration of [DEEA][Pro] and temperature, which indicate an increase in interactions between ions of [DEEA][Pro] (−NH3+, C2H5COO−) and AA, which may be due to the dominance of hydrophilic-ionic interactions. Both negative and positive K°s,2values were observed for AA in water and also in presence of [DEEA][Pro],which increase with concentration of PIL and temperature, thus indicating reduction in the electrostriction.

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Table 2. Infinite dilution partial molar volumes, V2ºof ascorbic acid in water and in aqueous solutions of [DEEA][Pro]at T = (293.15 to 328.15) K.

https://doi.org/10.1371/journal.pone.0126091.t002

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Table 3. Infinite dilutionpartial molar isentropic compression, K °s,2 of ascorbic acid in water and in aqueous solutions of [DEEA][Pro]at T = (293.15 to 328.15) K.

https://doi.org/10.1371/journal.pone.0126091.t003

The influence of PIL on solvation behaviour of AA can be studied on the basis of partial molar volumes of transfer, (ΔtV2°and ΔtK°s,2), which is considered to be free from solute-solute interactions, and werecalculated using Eq (5): (5) where ΔtX2° = (ΔtV2° or ΔtK°s,2), X2° = (V2° or K°s,2), the plot of ΔtV2°versus mB has been illustrated in Fig 2a and ΔtK °s,2 versus mB in Fig 2b. The transfer parameters (ΔtV2°and ΔtK°s,2) are positive and increase with concentration of newly synthesized PIL (synthesis is shown in Fig 3) and temperature. The ΔtV2° values increase with cosolute (PIL) concentration, however a slight decrease in ΔtV2° values from mB ≈ (0.10 to 0.15) mol·kg−1 have been observed, whereas ΔtK°s,2 values increase continuously with PIL concentration at all temperature. This behaviour in ΔtV2° values for AA is different (only between [0.10 to 0.15] mol·kg−1) as compared to ΔtK°s,2 values. The observed difference may be due to the fact that apparent molar isentropic compression is more sensitive parameter as compared to apparent molar volume [44] in measuring the structural changes occurring in solutions.

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Fig 2. Plots of volumes of transfer(ΔtV2°and ΔtK°s,2) versus molalities, mB of [DEEA][Pro] of ascorbic acid.

a) ΔtV2°versus mB; b) ΔtK°s,2 versus mB at temperatures, T = ♦, 293.15 K; ■, 298.15 K; ▲,303.15 K; ×, 308.15 K, *, 313.15 K, ☼, 318.15 K, +, 323.15 K,-, 328.15 K.

https://doi.org/10.1371/journal.pone.0126091.g002

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Fig 3. Synthesis of diethylethanolammonium propanoate [DEEA][Pro].

https://doi.org/10.1371/journal.pone.0126091.g003

Ascorbic acid has a five membered ring containing two carbonyl groups and exists as [53] conjugate ene-diol (having two unstable keto form and one stable ene-diol form) as shown in Fig 4. The enol form is stable and is delocalized, as shown in Fig 4.

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Fig 4. 1,2 Diketone (X) and 1,3-diketone (Y) form of ascorbic acid and their interconversion.

https://doi.org/10.1371/journal.pone.0126091.g004

The two possible forms of AAin solutions are: 1,2 diketone (X) and 1,3-diketone (Y) as shown in Fig 4, and these forms rapidly interconvert (Fig 4). AA contains two acidic protons, namely, Hα and Hβ(I of Fig 4), on dissociation of any of these protons, ascorbate ion is formed. The stability of respective ascorbate ion formed determines the acidity of proton (Hα or Hβ). As shown in Fig 4, structure A has one more equally contributing resonating structure B [53], therefore the stability of conjugate base on removal of Hβ is more (shown in I of Fig 4) as compared to conjugate base generated on removal of Hα (shown in II of Fig 4). The carboxylate anion of [DEEA][Pro] will probably interact more strongly with Hβ (most acidic hydrogen), which may be responsible for positive transfer volumes (ΔtV2° or ΔtK°s,2), further, it has been observed that these interactions dominate over the whole concentration range of PIL. Aryanci et al. [33] studied the solvation behaviour of AA in aqueous sodium chloride solutions (in 0.01 M HCl) and they observed no regular trend in ΔtV2° values for AA, and suggested that the irregular trend in ΔtV2° values indicate the presence of complex interactions between solute and cosolute in aqueous NaCl solutions (containing background 0.01 M HCl). However, in the present study, regular trend observed in ΔtV2° and ΔtK°s,2 values suggest that with increase in PIL concentration and temperature, the hydrophilic-ionic types of interactions become more favorable.

The ΔtV2° and ΔtK°s,2values can be interpreted in terms of structural interaction model [54] and group additivity model [55]. According to these models [54–55], the type of interactions between AA and [DEEA][Pro] in ternary solutions can be classified as: a)Hydrophobic—cation interactions between hydrophobic parts of AA and—NH3+ of PIL; b) Hydrophilic—cation interactions between (–OH,—C = O, and—O–) groups of AA and −NH3+; c) Hydrophobic—anion interactions between hydrophobic parts of AA and C2H5COO−; d) Hydrophilic—hydrophobic interactions between the hydrophilic groups of AA and hydrophobic parts of [DEEA][Pro]. According to the structural interaction model [54], type(a, c and d)interactions are repulsive as the groups involved are incompatible or inability of the groups to orient water, contribute negative volume. Interactions of type b contribute positive to transfer volume due to the overlap of hydration co-sphere of ions of [DEEA][Pro] and hydrophilic groups (–OH,—C = O, and—O–) of AA, which leads to a decrease in structure-breaking tendency of ion and thus reduction of electrostriction. Transfer volumes (ΔtV2° and ΔtK°s,2) increase with cosolute concentration and temperature, which indicate dominance of hydrophilic-ionic interactions (type b) over hydrophobic type of interactions. The positive ΔtV2° values of AA in presence of NaCl [33] and in PEG3350[29] have also been reported,which also suggests the dominance of hydrophilic type of interactions.

Thethermal expansion coefficients (∂V2°/∂T)P and its second derivatives (∂2V2°/∂T2)Phave been calculated by using the following Eq (6): (6) where νo, ν1 and ν2 are constants. The derivative of V2° with respect to temperature at constant pressure i.e. (∂V2°/∂T)Pfor AA in water and in presence of [DEEA][Pro] is given in Table 4. Hepler [56] used a mathematical equation, to deduce information regarding structure-making or-breaking ability of an ion in solution. According to Hepler’s [56] criteria: ( , negative (∂2V2°/∂T2)P values obtained for AA in water and in aqueous PIL solutions suggest that AA behave as structure breaker (chaotropes). Banipal et al.[32] and Dhonge et. al. [40] have also reported that AA behaves as structure breaker in water.

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Table 4. Partial molar expansion coefficients, (∂V2∞/∂T)P and (∂2V2∞/∂T2)P of ascorbic acid) in water and in aqueous [DEEA][Pro] solutions at temperatures, T = (293.15 to 328.15) K.

https://doi.org/10.1371/journal.pone.0126091.t004

3.3 Interaction coefficients

Pair (YAB) and triplet (YABB) volumetric and compressioninteraction coefficients have been calculated from corresponding volume of transfer (ΔtV2° or ΔtK°s,2) based on McMillan-Mayer theory [57] of solutionsby using Eq (7): (7) where A denotes AA and B denotes PIL ([DEEA][Pro]). Constants YAB(VAB or KAB) and YABB(VABB orKABB) are pair and triplet volumetric or compressioninteraction coefficients, respectively. Pair interaction coefficients, YAB contribute positively and triplet coefficients, YABBcan contribute negatively (Tables 5 and 6). Pair volumetric interaction coefficients, YABare found to be positive and triplet interaction coefficients, YABB are negative, at all temperatures. Overall, triplet volumetric interaction coefficients, YABB are small, which indicate that the interactions between AA and [DEEA][Pro] are mainly pair wise. Positive values of both VAB and KAB parameters suggest that interactions occur due to the overlap of hydration co-spheres of AA and ions of PIL.

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Table 5. The pair,VAB and triplet,VABB interaction coefficients for ascorbic acid in aqueous[DEEA][Pro]solutions at T = (293.15 to 328.15)K.

https://doi.org/10.1371/journal.pone.0126091.t005

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Table 6. The pair, KAB and triplet, KABB compressibility interaction coefficients for ascorbic acid in aqueous[DEEA][Pro] solutions at temperatures, T = (293.15 to 328.15) K.

https://doi.org/10.1371/journal.pone.0126091.t006

Conclusions

The volumetric and compressiontransfer volumes (ΔtV2° and ΔtK°s,2) for AA in presence of diethylethanolammonium propionate ([DEEA][Pro]) were found to be positive, which increase with increase in concentration of [DEEA][Pro] and temperature. The positive ΔtV2° and ΔtK°s,2values suggest the dominance of hydrophilic-ionic type of interactions between the ions of PIL and hydrophilic sites of AA (−OH, −C = O, −O−), due to the overlap of hydration co-sphere of PIL and AA.

The negative values of second derivative i.e. (∂2V2°/∂T2)P obtained for AA in water and also in aqueous solutions of [DEEA][Pro] suggest that AA behave as structure breaker. The values of pair interaction coefficients (VAB and KAB) were found to be positive, which further support the view that interactions occur due to the overlap of hydration co-sphere of AA and ions of PIL. The transfer parameters and interaction coefficients suggest that AA interacts strongly with [DEEA][Pro] in aqueous solution.

Author Contributions

Conceived and designed the experiments: VS RG. Performed the experiments: VS GS. Analyzed the data: VS GS RG. Contributed reagents/materials/analysis tools: RG. Wrote the paper: VS GS RG. Performed calculations: VS RG. Created figures: VS GS.

References

  1. 1. Swatloski RP, Spear SK, Holbrey JD, Rogers RD (2002) Dissolution of Cellulose with Ionic Liquids. J Am Chem Soc 124: 4974–4975. pmid:11982358
  2. 2. Singh V, Chhotaray PK, Gardas RL (2014). Solvation Behaviour and Partial Molar Properties of Monosaccharides in Aqueous Protic Ionic Liquid Solutions. J Chem Thermodyn 71: 37–49.
  3. 3. Singh V, Chhotaray PK, Gardas RL (2015) Effect of Protic Ionic Liquid on the Volumetric Properties and Taste Behaviour of Sucrose. Food Chemistry, 169, 478–483. pmid:25236254
  4. 4. Keshapolla D, Singh V, Gardas RL (2014) Volumetric, Acoustic and Transport Properties of Binary Mixtures of Benzyldimethylammonium Based Ionic Liquids with N, N-Dimethylformamide at Temperature from 293.15 to 328.15 K. J Mol Liq 199: 330–338.
  5. 5. Tsuda T, Mochizuki E, Kishida S, Iwasaki K, Tsunashima K, Kuwabata S (2014) SEM Observation of Hydrous Superabsorbent Polymer Pretreated with Room-Temperature Ionic Liquids. PLoS ONE 9(3): e91193. pmid:24621609
  6. 6. Lan W, Liu CF, Yue FX, Sun RS, Kennedy JF (2011) Ultrasound-Assisted Dissolution of Cellulose in Ionic Liquid, Carbohydr Polym 86: 672–677.
  7. 7. Pinkert A, Marsh KN, Pang SS, Staiger MP (2009) Ionic Liquids and Their Interaction with Cellulose. Chem Rev 109: 6712–6728. pmid:19757807
  8. 8. Attri P, Baik KY, Venkatesu P, Kim IT, Choi EH (2014) Influence of Hydroxyl Group Position and Temperature on Thermophysical Properties of Tetraalkylammonium Hydroxide Ionic Liquids with Alcohols. PLoS ONE 9(1): e86530. pmid:24489741
  9. 9. Attri P, Lee SH, Hwang SW, Kim JIL, Lee SW, et al. (2013) Elucidating Interactions and Conductivity of Newly Synthesised Low Bandgap Polymer with Protic and Aprotic Ionic Liquids. PLoS ONE 8(7): e68970. pmid:23874829
  10. 10. Elyasi M, Khalilzadeh MA, Karimi-Maleh H (2013) High Sensitive Voltammetric Sensor Based on Pt/Cnts Nanocomposite Modified Ionic Liquid Carbon Paste Electrode for Determination of Sudan I in Food Samples. Food Chem 141: 4311–4317. pmid:23993620
  11. 11. Ping J, Wang Y, Wua J, Ying Y, Ji F (2012) Determination of Ascorbic Acid Levels in Food Samples by using An Ionic Liquid-Carbon Nanotube Composite Electrode. Food Chem 135: 362–367. pmid:22868100
  12. 12. Hayakawa EH, Mochizuki E, Tsuda T, Akiyoshi K, Matsuoka H, et al. (2013) The Effect of Hydrophilic Ionic Liquids 1-Ethyl-3-Methylimidazolium Lactate and Choline Lactate on Lipid Vesicle Fusion. PLoS ONE 8(12): e85467. pmid:24392011
  13. 13. Tian M, Cheng R, Ye J, Liu X, Jia Q (2014) Preparation and Evaluation of Ionic Liquid-Calixarene Solid-Phase Microextraction Fibres for the Determination of Triazines in Fruit and Vegetable Samples. Food Chem 145: 28–33. pmid:24128445
  14. 14. Zhang H, Qi S, Dong Y, Chen X, Xu Y, Maa Y, et al. (2014) A Sensitive Colorimetric Method for the Determination of Nitrite in Water Supplies, Meat And Dairy Products using Ionic Liquid-Modified Methyl Red as A Colour Reagent. Food Chem 151: 429–434. pmid:24423553
  15. 15. Feng R, Zhao D, Guo Y (2010) Revisiting Characteristics of Ionic Liquids: A Review for Further Application Development. J Environ Prot 1: 95–104.
  16. 16. Chailapakul O, Wonsawat W, Siangproh W, Grudpan K, Zhao Y, Zhu Z (2008) Analysis of Sudan I, Sudan II, Sudan III, and Sudan IV in Food by HPLC with Electrochemical Detection: Comparison of Glassy Carbon Electrode with Carbon Nanotube-Ionic Liquid Gel Modified Electrode. Food Chem 109: 876–882.
  17. 17. Wu B, Zhang YM, Wang HP (2008) Aqueous Biphasic Systems of Hydrophilic Ionic Liquids + Sucrose for Separation. J Chem Eng Data53: 983–985.
  18. 18. Mourao T, Tome LC, Florindo C, Rebelo LPN, Marrucho IM (2014) Understanding the Role of Cholinium Carboxylate Ionic Liquids in PEG-Based Aqueous Biphasic Systems. ACS Sust Chem Eng 2: 2426–2434.
  19. 19. Raja S, Murty VR, Thivaharan V, Rajasekar V, Ramesh V (2011) Aqueous two phase systems for the recovery of biomolecules-A review. Sci Technol 1: 7–16.
  20. 20. Ventura SPM, Sousa SG, Serafim LS, Lima AS, Freire MG, Coutinho JAP (2011) Ionic Liquid Based Aqueous Biphasic Systems with Controlled pH: The Ionic Liquid Cation Effect. J Chem Eng Data 56: 4253–4260.
  21. 21. Zafarani-Moattar MT, Hamzehzadeh S (2011) Partitioning of Amino Acids in the Aqueous Biphasic System Containing the Water-Miscible Ionic Liquid 1-Butyl-3-Methylimidazolium Bromide and the Water-Structuring Salt Potassium Citrate. Biotechnol Prog 27: 986–997. pmid:21509956
  22. 22. Freire MG, Louros CLS, Rebelo LPN, Coutinho JAP (2011) Aqueous Biphasic Systems Composed of a Water-Stable Ionic Liquid + Carbohydrates and their Applications. Green Chem 13: 1536–1545.
  23. 23. Pereira JFB, Lima AS, Freire MG, Coutinho JAP (2010) Ionic Liquids as Adjuvants for the Tailored Extraction of Biomolecules in Aqueous Biphasic Systems. Green Chem 12: 1661–1669.
  24. 24. Freire MG, Neves CMSS, Marrucho IM, Lopes JNC, Rebelo LPN, Coutinho JAP (2010) High-Performance Extraction of Alkaloids Using Aqueous Two-Phase Systems with Ionic Liquids. Green Chem 12: 715–1718.
  25. 25. Wu B, Zhang YM, Wang HP (2009) Volumetric Properties and Conductivities of 1-Butyl-3-Methylimidazolium Tetrafluoroborate + Sucrose + Water Mixtures. J Chem Eng Data 54: 1430–1434.
  26. 26. Kant S, Kumar A, Kumar S (2009) Molar volume, Viscosity and Conductance Studies of Some Alkali Metal Chlorides in Aqueous Ascorbic Acid. J Mol Liq150: 39–43.
  27. 27. Jiang X, Zhu C, Ma Y (2013). Densities and Viscosities of Erythritol, Xylitol, and Mannitol in L-Ascorbic Acid Aqueous Solutions at T = (293.15 to 323.15) K. J Chem Eng Data 58: 2970–2978.
  28. 28. Banik I, Roy MN (2012) Study of Solute-Solvent Interaction of Some Bio-Active Solutes Prevailing in Aqueous Ascorbic Acid Solution. J Mol Liq 169: 8–14.
  29. 29. Sahin M, Ayranci E (2011) Volumetric Properties of (Ascorbic Acid + Polyethylene Glycol 3350 + Water) Systems at T = (288.15, 298.15, and 308.15) K.J Chem Thermodyn 43: 177–185.
  30. 30. Kant S, Kumar S (2013) Effects of Alkaline Earth Metal Ions on Thermodynamic and Ultrasonic Properties of Ascorbic Acid. J Chem Eng Data 58: 1294–1300.
  31. 31. Jiang X, Zhu C, Ma Y (2014) Volumetric and Viscometric Studies of Amino Acids in L-Ascorbic Acid Aqueous Solutions at T = (293.15 to 323.15) K. J Chem Thermodyn 71: 50–63.
  32. 32. Banipal TS, Singh H, Banipal PK, Singh V (2013) Volumetric and Viscometric Studies on L-Ascorbic Acid, Nicotinic Acid, Thiamine Hydrochloride and Pyridoxine Hydrochloride in Water at Temperatures (288.15–318.15) K and at Atmospheric Pressure. Thermochim Acta 553: 31–39.
  33. 33. Ayranci G, Sahin M, Ayranci E (2007) Volumetric Properties of Ascorbic Acid (Vitamin C) and Thiamine Hydrochloride (Vitamin B1) in Dilute HCl and in Aqueous NaCl Solutions at (283.15, 293.15, 298.15, 303.15, 308.15, and 313.15) K. J Chem Thermodyn 39: 1620–1631.
  34. 34. Sdiri S, Bermejo A, Aleza P, Navarro P, Salvador A (2012) Effects of Ascorbic Acid on the Microstructure and Properties of SDS Micellar Aggregates for Potential Food Applications. Food Res Int 50: 143–148.
  35. 35. Valente A, Albuquerque TG, Sanches-Silva A, Costa HS (2011) Ascorbic Acid Content in Exotic Fruits: A Contribution to Produce Quality Data For Food Composition Databases. Food Res Int44: 2237–2242.
  36. 36. Comunian TA, Thomazini M, Alves AJG, de Matos JFE, de Carvalho Balieiro JC, Favaro-Trindade CS (2013) Microencapsulation of Ascorbic Acid by Complex Coacervation: Protection and Controlled Release. Food Res Int 52: 373–379.
  37. 37. Monti DA, Mitchell E, Bazzan AJ, Littman S, Zabrecky G, Yeo CJ, et al. (2012) Phase I Evaluation of Intravenous Ascorbic Acid in Combination with Gemcitabine and Erlotinib in Patients with Metastatic Pancreatic Cancer. PLoS ONE 7(1): e29794. pmid:22272248
  38. 38. Zhu LL, Cao J, Sun M, Yuen T, Zhou R, Li J, et al. (2012) Vitamin C Prevents Hypogonadal Bone Loss. PLoS ONE 7(10): e47058. pmid:23056580
  39. 39. Kawada H, Kaneko M, Sawanobori M, Uno T, Matsuzawa H (2013) High Concentrations of L-Ascorbic Acid Specifically Inhibit the Growth of Human Leukemic Cells via Downregulation of HIF-1α Transcription. PLoS ONE 8(4): e62717. pmid:23626851
  40. 40. Dhondge SS, Deshmukh DW, Paliwal LJ, Dahasahasra PN (2013) The Study of Molecular Interactions of Ascorbic Acid and Sodium Ascorbate with Water at Temperatures (278.15, 288.15 and 298.15) K. J Chem Thermodyn 67: 217–226.
  41. 41. Wong SZH, Ching B, Chng YR, Wong WP, Chew SF, Ip YK (2013) Ascorbic Acid Biosynthesis and Brackish Water Acclimation in the Euryhaline Freshwater White-Rimmed Stingray, Himantura signifier. PLoS ONE 8(6): e66691. pmid:23825042
  42. 42. Yuan JP, Chen F (1998) Degradation of Ascorbic Acid in Aqueous Solution. J Agr Food Chem 46:5078–5082.
  43. 43. Birch GG (2002) Role of Water in Sweet Taste Chemoreception. Pure Appl Chem 74: 1103–1108.
  44. 44. Parke SA, Birch GG, Dijk R (1999) Some Taste Molecules and Their Solution Properties. Chem Sens 24: 271–279.
  45. 45. Seuvre AM, Mathlouthi M (2010) Solutions Properties and Solute-Solvent Interactions in Ternary Sugar-Salt-Water Solutions. Food Chem122: 455–461.
  46. 46. Banipal PK, Singh V, Aggarwal N, Banipal TS (2015) Hydration Behaviour of Some Mono-, Di-, and Tri-Saccharides in Aqueous Sodium Gluconate Solutions at (288.15, 298.15, 308.15 and 318.15) K: Volumetric and Rheological Approach. Food Chem 168: 142–150. pmid:25172693
  47. 47. Banipal PK, Singh V, Banipal TS, Singh H (2013) Ultrasonic Studies of Some Mono-, Di-, and Tri-Saccharides in Aqueous Sodium Acetate Solutions at Different Temperatures. Z Phys Chem 227: 1707–1722.
  48. 48. Jamal MA, Khosa MK, Rashad M, Bukhari IH, Naz S (2014) Studies on Molecular Interactions of Some Sweeteners in Water by Volumetric and Ultrasonic Velocity Measurements at T = (20.0–45.0°C). Food Chem 146: 460–465. pmid:24176368
  49. 49. Hakin AW, Mudrack SAM, Beswick CL (1993) The Volumetric and Thermochemical Properties of L-Ascorbic Acid in Water at 288.15, 298.15, and 308.15 K. Can J Chem 71: 925–929.
  50. 50. Apelblat A, Manzurola E (2007) Vapour Pressure and Volumetric Studies in Aqueous Solutions with Ascorbate Ions. J Mol Liq 131–132: 7–16.
  51. 51. Shamim M, Khoo SB (1979) Some Physical Properties of Aqueous L-Ascorbic Acid Solutions, Austral J Chem 32: 2293–2295.
  52. 52. Kaulgud MV, Dole HG, Rao KSM (1978) Apparent Molal Volume, Apparent Molal Compressibility and Transport Properties of Dilute Aqueous Solutions of Ascorbic Acid, Indian J Chem 16A: 955–958.
  53. 53. Clayden J, Greeves N, Warren S, Wothers P (2001) Organic Chemistry (2nd ed.) Oxford University Press. (Chapter 21).
    • 54. Desnoyers JE, Joly M, Perron G, Jolicoeur C (1969) Apparent Molal Volumes of Alkali Halides in Water at 25°C. Influence of Structural Hydration Interactions on the Concentration Dependence. J Phys Chem 73: 3346–3351.
    • 55. Savage JJ, Wood RH (1976) Enthalpy of Dilution of Aqueous Mixtures of Amides, Sugars, Urea, Ethylene Glycol, and Pentaerythritol at 25°C: Enthalpy of Interaction of the Hydrocarbon, Amide, and Hydroxyl Functional Groups in Dilute Aqueous Solutions. J Sol Chem 5: 733–750.
    • 56. Hepler LG (1969) Thermal Expansion and Structure in Water and Aqueous Solutions. Can J Chem 47: 4613–4617.
    • 57. McMillan WG Jr, Mayer JE (1945) The Statistical Thermodynamics of Multicomponent Systems. J Chem Phys 13: 276–305.