An experimental study on the surface properties of Protic Morpholinium-based ionic liquids

Accepted Manuscript An experimental study on the surface properties of Protic Morpholinium-based ionic liquids

Mohammad Hadi Ghatee, Tahereh Ghaed-sharaf PII: DOI: Reference:

S0167-7322(17)31267-9 doi: 10.1016/j.molliq.2017.06.045 MOLLIQ 7491

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

1 April 2017 28 May 2017 10 June 2017

Please cite this article as: Mohammad Hadi Ghatee, Tahereh Ghaed-sharaf , An experimental study on the surface properties of Protic Morpholinium-based ionic liquids, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.06.045

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An Experimental Study on the Surface Properties of Protic

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Morpholinium-Based Ionic Liquids

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Mohammad Hadi Ghatee*, Tahereh Ghaed-sharaf

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(Department of Chemistry, Shiraz University, Shiraz, 71946 Iran)

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Tel: +98 711 613 7174, Email: [email protected] ; [email protected]

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The author for correspondence 1

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Abstract In this work, ten novel protic morpholinium-based ionic liquids (ILs) with alkyl-carboxylate anion were synthesized and their densities and surface tensions were measured in the range 298.15 to 348.15 K. Surface tension was measured by capillary rise method, capillary apparatus

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facilitate measurement at liquid/vapor interface with very low water content and in the absence

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of water vapor. The meniscus contact angles were also measured under the same condition. The surface tension of each IL demonstrates highly accurate linear correlation with temperature,

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allowing the surface thermodynamics function (surface entropy and surface energy) to be evaluated accurately. From surface tension values, the critical point was estimated by

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Guggenhiem and Eötvös equations. Surface tension, density and contact angle of all ILs decrease linearly by increasing temperature and their variation is sharp for the anion with short alkyl chain

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length and smooth for long ones.

Keywords: Capillary rise method; Critical temperature; Density and surface tension

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measurement; Protic mopholinium-based ionic liquids; Surface entropy and energy

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Introduction ILs are a new type of solvents composed of large organic cations and organic or inorganic anions unable of forming an ordered crystalline lattice and thus remaining liquid at or near room

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temperature. The fact that ILs can be hydrophobic, hydrophilic, protic and aprotic has attracted

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high interest for tremendous potential applications. Due to a number of desirable properties, the

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use of ILs in different industrial process has increased in the past two decades [1-7]. In the light of their remarkable physicochemical characteristics, ILs have been used for electrodeposition

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[8], lithium batteries [9], electric double layer capacitors [10-12], solar cells [13, 14], biphasic catalyst in organic synthesis and as synthetic solvents and lubricants for green chemistry [15-17].

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Most studied ILs are based on imidazolium, pyridiniun, pyrrolidinium and quaternary

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ammonium [18-25]. In the category of ILs, the low toxic morpholinium-based ILs have a low

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cost [26], and are extremely fragile (comply with the Vogel–Tamman–Fulcher equation for temperature dependent viscosity)[27]. Morpholinium-based ILs have received much attention

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because they have been used for designing IL crystals [28, 29], solvent for cellulose[30],

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catalysts for organic synthesis[31], electrochemical applications[32] and decomposed at the high temperature[33]. Most of aprotic morpholinium ILs have high melting points [33, 34], so they

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are not suitable as a solvent or electrolyte in electrochemical applications. But, protic morpholinium ILs with carboxylate anions have attracted much attention due to their lower melting point and viscosity, so they might be utilized appropriately in electrochemical applications and play an essential role as solvents in different fields. Protic morpholinium-based ILs, N-methylmorpholinium, and N-ethylmorpholinium cations (Scheme 1), have been synthesized through a simple neutralization reaction with formic acid, and their densities, refractive indices, thermal properties, electrochemical windows, temperature dependent 3

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viscosities and ionic conductivities have been also measured [27]. These ILs exhibit a large electrochemical window as compared to other protic ones (up 2.91 V) and possess relatively high ionic conductivities of 10-16.8 mS.cm-1 at 298.15 K and 21-29 mS.cm-1at 373 K [27]. Among various properties, the data of surface/interfacial tension and wetability provides

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understanding the application of ILs in heterogeneous systems and transport in capillary and

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fibrous or porous environments [35]. While Surface tension of pure ILs and their solutions have been studied in some cases [36], the measurement of contact angles of the novel ILs on solid

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surfaces, is limited. Recently Freire[35] studied the contact angles and wettability of a broad range of ILs on polar and non-polar surfaces. The contact angle data were subsequently the interaction

energies of

cation-anion

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correlated with

implemented in COSMO-RS

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software[35]. Gao and McCarthy[37] reported the contact angle behavior of four relatively

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high surface tension ILs on seven hydrophobic surfaces and compared with water contact angle behavior. Restolho et al. [38-40] measured the contact angle of different ILs on various solid

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substrates and determined their dispersive and non-dispersive interactions with the solid surfaces.

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RH+N O-

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R1

R=CH3, C2H5 R1=H, CH3, C2H5, C5H11, C9H19. SCHEME 1. The studied morpholinium-based ionic liquids.

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According to literature data, the temperature dependence of surface properties for protic morpholinium ILs has not been reported to date. In this paper, we report the synthesis and the temperature dependent measurements of density and surface tension of ten protic morpholiniumbased ILs, N-methylmorpholine and N-ethylmorpholine with alkyl-carboxylate anions. The

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influences of anion and cation alkyl chain length on the measured properties are investigated.

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The surface tension is measured in the temperature range 298.15 to 348.15 K by capillary rise method using a capillary apparatus, which facilitate measurement at very low water content

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under liquid/vapor equilibrium. The surface thermodynamic properties are computed and the critical temperature of these ILs is predicted. The trend of density, contact angle, surface

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properties, and critical temperature with anion alkyl chain length will be discussed.

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2. Synthesis and Experimental Procedure

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2.1 Materials

Ten protic ILs based on morpholinium cations with carboxylate anions namely N-methyl

propionate([Mmorph][Pro]),

N-methylmorpholinium

hexanoate

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methylmorpholinium

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morpholinium formate ([Mmorph][For]), N-methylmorpholinium acetate ([Mmorph][Ace]), N-

([Mmorph][Hex]), N-methylmorpholinium decanoate ([Mmorph][Dec]), N-ethylmorpholinium

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formate ([Emorph][For]), N-ethylmorpholinium acetate ([Emorph][Ace]), N-ethylmorpholinium propionate ([Emorph][Pro]), N-ethylmorpholinium hexanoate ([Emorph][Hex]) and N-ethyl morpholinium decanoate ([Emorph][Dec]) were synthesized and used for this study. The materials and samples description in synthesis of morpholinium-based ILs are shown in Table 1. Deionized water was used where applicable.

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TABLE 1. Specification of chemicals used for synthesis.

Source

Purity

Purification Method

Reactants N-methylmorpholine N-ethylmorpholine formic acid acetic acid propionic acid hexanoic acid decanoicacid

Merck Merck Merck Merck Merck Merck Panreac

99% 99% 98% 100% 98% 98% 98%

none none none none none none none

Products N-methylmorpholinium Formate N-methylmorpholinium Acetate N-methylmorpholinium Propionate N-methylmorpholinium Hexanoate N-methylmorpholinium Decanoate N-ethylmorpholinium Formate N-ethylmorpholinium Acetate N-ethylmorpholinium Propionate N-ethylmorpholinium Hexanoate N-ethylmorpholinium Decanoate

synthesis synthesis synthesis synthesis synthesis synthesis synthesis synthesis synthesis synthesis

NMR NMR NMR NMR NMR NMR NMR NMR NMR NMR

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Chemical

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Under Vacuum Under Vacuum Under Vacuum Under Vacuum Under Vacuum Under Vacuum Under Vacuum Under Vacuum Under Vacuum Under Vacuum

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2.2 Synthesis of N-methylmorpholinium and N-ethylmorpholinium with Carboxylate Anions and Different Alkyl Chain Length

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The morpholine compound was put in a two-necked glass flask equipped with a reflux condenser and a dropping funnel. The flask was mounted in an ice bath. Then, acid was added drop-wise to the flask under magnetic stirring. Stirring was continued for 24 h at ambient temperature. Since every synthesis was performed quantitatively by the exact mole to mole ratio, the product was expected free from starting material impurity if the reaction goes to completion. Otherwise, a barely two phases liquid system would have been produced in the first stage of synthesis and the sign of incomplete reaction noted as evidenced by NMR chemical shift of the 6

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acid proton (COOH). Clearly, extraction with diethyl ether removes the acid and base impurity efficiently [41,42], as was indicated by disappearance of the peak at 𝛿~ 10.7 − 11.7 in CDCl3 for –COOH. The product was dried under vacuum for 24 h to remove any excess water content [27, 31, 41], before analysis and experimental measurements. The level of impurity was found to

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be lower than NMR detection as checked for all reactants and products. The concern about the

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incomplete reaction was resolved by NMR spectra for all synthesis. The loss of chemical shift of H (in the carboxylic acids, COOH, with 𝛿~ 10.7 − 11.7 in CDCl3 ) strictly confirmed the purity

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of product in all IL synthesis. Any product with trace of impurity was rejected. The 1HNMR and 13

CNMR chemical shifts for all ILs synthesized are listed below and the corresponding NMR

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spectra for all reactants and products are shown in the Supporting Materials (S1-S34).

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2.3 NMR chemical shifts

2.1.4.1 N-methylmorpholiniumformate[Mmorph][For]. 1H NMR (250 MHz, CDCL3): δ =

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13.54(s, 1H), 8.29 (s, 1H), 3.85(t, J =2.5, 4H), 2.11 (s, 4H), 2.73(s, 3H). 13C NMR (62.9 MHz,

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CDCL3): δ =166.29, 63.72, 52.95, 43.27. 2.1.4.2 N-ethylmorpholiniumformate [Emorph][For].1H NMR (250 MHz, CDCL3): δ =13.36

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(s, 1H), 7.89(s, 1H), 3.4 (s, 4H), 2.74- 2.65 (m, 6H), 0.85(t, J= 7.25,3H). 13C NMR (62.9 MHz, CDCL3): δ = 166.35, 63.67, 51.94, 50.85, 8.55. 2.1.4.3 N-methylmorpholiniumacetate [Mmorph][Ace]. 1H NMR (250 MHz, CDCl3): δ = 13.47(s, 1H), 3.15 (s, 4H), 2.06 (s, 4H), 1.81 (d, J = 3, 3H), 1.37 (d, J = 3.25, 3H).

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C NMR

(62.9 MHz, CDCl3): δ = 173.8, 64.3, 53.06, 43.7, 20.7.

2.1.4.4 N-ethylmorpholinium acetate [Emorph][Ace]. 1H NMR (250 MHz, CDCl3): δ = 13.22 (s, 1H), 3.27 (t, J = 4.75, 4H), 2.21- 2.12 (m, 6H), 1.47 (s, 3H), 0.65 (t, J = 7.25, 3H). 13C NMR 7

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(62.9 MHz, CDCl3): δ = 174.7, 64.9, 51.6, 51.5, 21.5, 9.6. 2.1.4.5 N-methylmorpholinium propionate [Mmorph][Pro]. 1H NMR (250 MHz, CDCl3): δ = 13.10 (s, 1H), 3.48-3.43 (m, 4H), 2.35 (s, 4H), 2.09-1.95 (m, 5H), 0.84-0.77 (m, 3H). 13C NMR

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(62.9 MHz, CDCl3): δ = 178.0, 65.3, 53.9, 44.6, 27.9, 9.2.

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2.1.4.6 N-ethylmorpholinium propionate [Emorph][Pro]. 1H NMR (250 MHz, CDCl3): δ = 13

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13.23 (s, 1H), 3.65 (t, J = 4.75, 4H), 2.57-2.46 (m, 6H), 2.19-2.10 (m, 2H), 1.05-094 (m, 6H). C NMR (62.9 MHz, CDCl3): δ =178.4, 65.2, 51.8, 51.7, 28.2, 9.8, 9.3.

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2.1.4.7 N-methylmorpholiniumhexanoate [Mmorph][Hex]. 1H NMR (250 MHz, CDCl3): δ = 13.61 (s, 1H), 3.23 (t, J = 4.75, 4H), 2.15 (s, 4H), 1.88 (s, 3H), 1.72 (t, J = 7.75, 2H), 1.08 (t, J = C NMR (62.9 MHz, CDCl3): δ = 177.5,

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65.2, 53.8, 44.4, 34.8, 31.2, 24.7, 22.1, 13.6.

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6.75, 2H), 0.82-0.77 (m, 4H), 0.367 (t, J = 7, 3H).

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2.1.4.8 N-ethylmorpholiniumhexanoate [Emorph][Hex]. 1H NMR (250 MHz, CDCl3): δ = 13.68 (s, 1H), 3.34 (t, J = 4.75, 4 H), 2.28-2.16 (m, 6H), 1.84-1.77 (m, 2H), 1.17 (t, J = 7.25, 2H), 0.89-0.68 (m, 7H), 0.47-0.42 (m, 3H).

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C NMR (62.9 MHz, CDCl3): δ = 177.7, 65.3,

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51.9, 51.7, 35.1, 31.3, 24.8, 22.2 13.7, 9.8.

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2.1.4.9 N-methylmorpholiniumdecanoate[Mmorph][Dec]. 1H NMR (250 MHz, CDCl3): δ = 13.72 (s, 1H), 3.33 (t, J = 4, 4H), 2.21 (s, 4H), 1.95 (d, J = 1.75, 5H), 1.18 (t, J = 6.5, 2H), 0.84

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(s, 12H), 0.45 (s, 3H).

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C NMR (62.9 MHz, CDCl3): δ = 177.6, 65.5, 54.1, 44.8, 34.9, 31.7,

29.3, 29.2, 29.1, 29.1, 25.1, 22.5, 13.9. 2.1.4.10 N-ethylmorpholiniumdecanoate [Emorph][Dec].1H NMR (250 MHz, CDCl3): δ =12.78 (s, 1H), 3.77 (t, J = 4.75, 4H), 2.84 (s, 4H), 2.18 (t, J =7.5, 2H), 1.53 (s, 2H), 1.16 (d, J = 1.16, 17H), 0.80 (s, 3H). 13C NMR (62.9 MHz, CDCl3): δ = 178.1, 65.0, 51.9, 51.6, 35.2, 31.7, 29.3, 29.2, 29.2, 29.1, 25.2, 22.5, 13.9, 9.5.

2.4 Water Content 8

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Protic ILs are highly hygroscopic and absorb moisture easily. It is well-established that even low water contents influences the thermodynamic properties of ILs to some extent and their dynamic properties to a great deal. Hence, each liquid sample was repeatedly freeze-dried under vacuum for about two hours and finally was kept at room temperature under vacuum until

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effervescence (due to dissolved gases) is completely seized. This process is done with great care

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to assure water and dissolved gases are removed as much as possible. The acidic anion of these ILs reacts easily with methanol present in Karl Fischer solvent and produces water. Hence, the

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well-known and widely used Karl Fischer method for accurate water content determination

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remains idle for the ILs in this work.

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2.5 Density measurements

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Density of all ILs was measured by a standard 5 mL pycnometer (Marienfeld) which was

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calibrated by using deionized water (with density of 0.9970 g.cm-3 at 298.15 K [43]). The pycnometer was filled with freeze dried IL sample, mounted in thermostat, and its temperature

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was controlled by glass thermometer within 0.5 K. Pycnometer volume calibration was done by

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the previous procedure[23].

2.6 Contact angle measurements Contact angle of ILs in the glass capillary apparatus was measured (by a Dino Capture 2.0 Lite camera) concurrent with the capillary height measurement, from 298.15 to 348.15K. The capillary was made of Borosilicate Glass. All standard precautions are essential for highly correlated contact angle as a function of temperature for all ILs.

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2.7 Surface tension measurements The surface tension of ILs was determined at liquid/vapor equilibrium using the capillary apparatus described in earlier works [21-23]. In order to produce highly accurate results, the

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same procedure was followed with extremely cleaned glassware’s. The capillary apparatus was

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cleaned by a mixture of hot nitric acid and hydrochloric acid for several hours, and finally

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washed with deionized water. The inner radii of capillaries were calibrated by the previous procedure[21-23], and are in the range (0.29 to 0.35) using a digital micrometer (±0.01mm).

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After loading the capillary apparatus with vacuum dried IL, the system was freeze dried for

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several times at liquid-nitrogen boiling temperature under low pressure (=10-3 mmHg). The capillary was sealed under vacuum after the complete evacuation and removal of water content

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and dissolved atmospheric gases. The sealed capillary apparatus was then mounted in an oil bath

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thermostat and the temperature was controlled by a glass thermometer within ±0.5 K. The measurements were performed in the range 298.15 to 348.15 K at interval of 5 K. At each

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temperature, thermal equilibration was allowed to be reached within 45 min as can be distinguish

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by the liquid sample height in the capillary stops changing completely. A camera (Sony Cybershot-DSC-H2) interfaced with a personal computer was used to determine the height of liquid

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samples raised in capillary. The height was determined by counting pick cells with the estimated standard uncertainty of ±0.005 mm. Then the Eq. (1) was used to calculate the surface tension: 𝑟

2𝛾 cos 𝜃 = 𝑟𝜌𝑔(ℎ + ), 3

(1)

where, 𝛾 is the surface tension, 𝜌 is the density, g is the acceleration of gravity, r is the capillary radius, h is the height of liquid in capillary and 𝜃 is the contact angle. The second term in parenthesis is the first order height correction factor, and takes into account the weight of liquid 10

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above the meniscus surface, assuming to be semi-spherical, in the capillary. The surface tension of ILs is calculated by measuring the capillary height, density, and contact angle.

3. Results and discussions

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3.1 Temperature Dependent Density

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The values of experimental densities for the ILs are shown in Table 2, including uncertainties

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and percent average absolute deviation (%AAD). We calculated uncertainties in densities as

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detailed previously[23].

Plot of densities of ILs versus temperature, measured in the range of 298.15 to 348.15 K, is

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shown in Figure 1. Linear temperature dependence with high correlation is seen for all ILs. The

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linear correlation coefficient (R2) of the fit is between 0.9982 and 0.9999, which indicates high correlations. In general, the density decreases by increasing cation and the anion chain length.

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This trend of in density versus temperature is due to the increase in the volume occupied by the

packing efficiency [44].

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anions and cations, which can be attributed to the increase in interionic separation and lower

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As it can be observed in Figures 2 and 3, the density of N-methylmorpholinium and N-

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ethylmorpholinium decreases with increasing anions chain length, and the variations of density is sharp for shorter anions chain length (from formate to acetate anions). The results indicate that this sharp variation diminishes at long chain length. We compare our results with analogue imidazolium to compensate for the lack of morphlinium data in literature. According to the Xu et al. [45], studies on the effect of alkyl chain length of anions on thermodynamic and surface properties of imidazolium-based carboxylate ILs, the density of 1-Butyl-3-methylimidazolium carboxylates changes sharper for shorter anion chain length.

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TABLE 2. Experimental values of density () of the ILs measured in the temperature range 298.15-348.15 K.a

/g.cm-3 [Emorph] [Pro] 0.9784 0.9734 0.9696 0.9652 0.9604 0.9556 0.9508 0.9462 0.9416 0.9364 0.9319

[Hex] 0.9287 0.9247 0.9202 0.9163 0.9118 0.9071 0.9034 0.8987 0.8947 0.8902 0.8857

[Dec] 0.8934 0.8893 0.8852 0.8817 0.8763 0.8741 0.8702 0.8669 0.8628 0.8585 0.8549

%AAD=

0.0233

0.0161

0.0299

0.0199

0.0377

[For] 1.1081 1.1040 1.1005 1.0968 1.0933 1.0895 1.0858 1.0820 1.0787 1.0749 1.0711

[Ace] 0.9841 0.9799 0.9754 0.9711 0.9666 0.9623 0.9579 0.9539 0.9491 0.9442 0.9396

0.0097

0.0194

[Pro] 0.9666 0.9634 0.9590 0.9548 0.9503 0.9462 0.9409 0.9368 0.9323 0.9277 0.9230

[Hex] 0.9334 0.9298 0.9257 0.9225 0.9179 0.9135 0.9091 0.9052 0.901 0.8962 0.8918

[Dec] 0.9014 0.8971 0.8932 0.8895 0.8856 0.882 0.8781 0.8739 0.8698 0.8664 0.8617

0.0357

0.0401

0.0232

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[Ace] 0.9973 0.9930 0.9885 0.9844 0.9804 0.9760 0.9713 0.9671 0.9625 0.9579 0.9539

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[For] 1.1299 1.1255 1.1223 1.1177 1.1140 1.1106 1.1064 1.1024 1.0992 1.0950 1.0905

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T/K 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15

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[Mmorph]

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Experimental density reported in literature at 298.15K for [Mmorph][For](=1.1260/g.cm-3 ) and [Emorph][For](=1.0620 /g.cm-3) [27]. %AAD=(1/n∑𝑛𝑖=1|𝑑𝑒𝑣|): n=number of data points, dev=(exp−fit)×100/exp a Standard uncertainties u are: u(T) = 0.5 K; u() = 0.0005 g.cm-3. The combined expanded uncertainty Uc() = 0.001 g.cm-3 (with 95% level of confidence)

1.15

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1.10

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Density/g. cm-3

1.05 1.00 0.95 0.90 0.85 0.80

298

308

318

328

338

T/K [Mmorph][For]

[Emorph][For]

[Mmorph][Ace]

[Emorph][Ace]

[Mmorph][Pro]

[Emorph][Pro]

[Mmorph][Hex]

[Emorph][Hex]

[Mmorph][Dec]

[Emorph][Dec]

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298.15K 303.15K 308.15 K 313.15K 318.15 K 323.15 K 328.15 K 333.15 K 338.15 K 343.15 K 348.15 K

1.09

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0.99

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Density/ g.cm-3

1.04

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Fig.1. Density of the ILs measured in the temperature range 298.15-348.15 K. Lines are linear fits.

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0.94

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0.89

0.84 1

3

5

7

9

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number of C atom(s) in anion

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298.15 K 303.15 K 308.15 K 313.15 K 318.15 K 323.15 K 328.15 K 333.15 K 338.15 K 343.15 K 348.15 K

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1.1

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Fig. 2. Density of N-methylmorpholinium versus number of C atom(s) in anion. Lines are trend lines.

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Density/ g.cm-3

1.05

1

0.95

0.9

0.85 1

3

5

7

number of C atom(s) in anion

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Fig. 3. Density of N-ethylmorpholinium versus number of C atom(s) in anion. Lines are trend lines.

3.2 Contact angle

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We measured the contact angle (θ) for the calculation of surface tension from Eq. (1), as well as

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for the evaluation of the wettability properties of the ILs over glass surface. The contact angle of

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ILs varies slightly with temperature over the range measured. Like most liquids, the extent of spreading of ILs over glass surface increases with temperature, as indicated by θ, shown in Table

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3 and plotted in Figure 4. Contact angle of all ILs decreases with temperature quite linearly. Linear correlation coefficient (R2) of the corresponding fits is between 0.9695 and 0.9965, which

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indicates high correlations. On the other hand, θ increases with the alkyl chain length of the

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cation and the anion. Interestingly, the rate of change of θ’s with respect to T for all ILs is close

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to one another (within 0.0162-0.0229 deg/K). The hydrophobicity of ILs increases by increasing the alkyl chain length, which causes a decrease of spreading on hydrophilic (glass) surface as a

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result of decrease in the solid-liquid dispersive interactions [46, 47]. Contact angle of ILs

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increases sharply in the short chain length domain of anion and increases smoothly in the long chain length domain, as shown in Figures 5 and 6. These observations are attributed to the trend

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of pKa of the corresponding anion acids, as will be discussed in section 3.3. The contact angle for both N-methylmorpholinium and N-ethylmorpholinium ILs increases almost with the same trend. Therefore, the higher the van der Waals interaction the lower the wettability of the IL. The contact angles of N-methylmorpholinium and N-ethylmorpholinium ILs at different temperatures (298.15 and 348.15) and various alkyl chain length (decanate and formate anions) are shown in Figures S35 and S36 (Supporting Information).

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TABLE 3. Experimental values of contact angle (θ) of the ILs measured in the temperature range 298.15348.15 K.

Contact angle (deg) [Emorph]

[For]

[Ace]

[Pro]

[Hex]

[Dec]

[For]

[Ace]

[Pro]

298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15

14.589 14.534 14.490 14.381 14.243 14.130 14.036 13.885 13.749 13.643 13.548

16.557 16.446 16.294 16.253 16.165 16.060 15.945 15.867 15.751 15.674 15.573

18.239 18.138 18.038 17.969 17.865 17.784 17.622 17.526 17.393 17.319 17.199

24.206 24.146 23.962 23.895 23.703 23.629 23.595 23.552 23.443 23.369 23.279

28.540 28.523 28.474 28.301 28.179 27.937 27.897 27.719 27.690 27.681 27.451

16.390 16.341 16.255 16.179 16.154 16.017 15.945 15.855 15.732 15.693 15.611

18.853 18.832 18.741 18.658 18.678 18.484 18.435 18.310 18.208 18.136 18.004

20.171 20.072 19.983 19.873 19.799 19.674 19.573 19.475 19.323 19.276 19.220

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M

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Standard uncertainties u are: u(θ) = 0.001 deg.

26.565 26.370 26.232 26.143 25.974 25.916 25.750 25.689 25.622 25.462 25.388

31

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29

23

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25

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Contact angle /deg

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21 19

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a

[Hex]

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T/k

T

[Mmorph]

17 15 13 298

308

318

328

338

T/K [Mmorph][For]

[Mmorph][Ace]

[Mmorph][Pro]

[Mmorph][Hex]

[Mmorph][Dec]

[Emorph][For]

[Emorph][Ace]

[Emorph][Pro]

[Emorph][Hex]

[Emorph][Dec]

15

348

[Dec] 30.651 30.592 30.493 30.414 30.358 30.236 30.192 30.058 29.982 29.880 29.745

ACCEPTED MANUSCRIPT

Fig.4. Contact angle of mopholinium-based ILs measured in range 298.15-348.15 K. Lines are linear fits.

29 27

T

298.15K 303.15K 308.15K 313.15K 318.15K 323.15K 328.15K 333.15K 338.15K 343.15K 348.15K

IP

23 21

CR

Contact angle/deg

25

19

US

17

AN

15 13 1

3

5

7

9

M

number of C atom(s) in anion

ED

Fig. 5. Contact angle of N-methylmorpholinium ILs versus number of C atom(s) in anion. Lines are trend lines.

PT

31 29

AC

Contact angle/deg

CE

27

298.15K 303.15K 308.15K 313.15K 318.15K 323.15K 328.15K 333.15K 338.15K 343.15K 348.15K

25 23 21 19 17 15

1

3

5

7

number of C atom(s) in anion

16

9

ACCEPTED MANUSCRIPT

Fig. 6. Contact angles of N-ethylmopholinium versus number of C atom(s) in anion. Lines are trend lines.

3.3 Surface Tension and its Temperature Dependent

T

The capillary rise method successfully produces accurate results if good temperature control,

IP

stable thermal equilibrium, high purity ILs and ultraclean glassware could be achieved [48].

CR

Water content is the major source of error in the measurement of surface tension of the ILs due

US

to their hygroscopic nature. The main difficulty is extensive adsorption of atmospheric water– vapor during the surface tension measurement [21-23]. The capillary apparatus facilitate the

AN

measurement at liquid/vapor interface in presence of very low water content and in the absence

M

of water vapor during the measurement.

We used the Eq. (1) and calculated the surface tension for each IL in the range of 298.15 to

ED

348.15 K, the results are shown in Table 5. For both N-methylmorpholinium and N-

PT

ethylmorpholinium ILs, surface tension is decreased by increasing anion chain length, as shown in Figures 7 and 8. It can be seen, like density, surface tension decreases sharply for short anion

CE

chain length, and decreases smoothly for long anion chain length. This can be attributed to the

AC

abrupt increase in pKa of the corresponding acids on going from formate to acetate, and the smooth change between propionate, hexanoate, and decanoate (pKa= 3.75, 4.75, 4.87, 4.88, 4.90, respectively). Therefore, the high surface tension of ILs containing formate anion is due to the high Columbic interaction between cation head group and the anion. Such a Columbic interaction diminishes sharply for acetate anion and smoothly thereafter for propionate, haxanoate and decanoate, resembling the variation of corresponding surface tension. This view point is applicable when we figure out the variation of surface tension of analog imidazolium protic ILs, 1-butyl-3-methylimidazolium carboxylate, versus anion chain length [45]. 17

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Accordingly, surface tension of 1-butyl-3-methylimidazolium carboxylate decreases by increasing anion chain length, with a sharper trend for anion of shorter chain length. We did repeat the syntheses and measurements at least twice. The results for surface tension measurements are reproducible within 2%.

IP

T

TABLE 5. Experimental values of surface tension (𝜸) measured for ILs in the temperature range 298.15348.15 K.

[Mmorph]

CR

γ/mJm-2 [Emorph]

[For]

[Ace]

[Pro]

[Hex]

[Dec]

[For]

[Ace]

[Pro]

[Hex]

[Dec]

298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15

48.4 47.9 47.3 46.8 46.3 45.9 45.3 44.9 44.3 43.8 43.3 0.05

28.8 28.3 28.1 27.6 26.9 26.2 25.9 25.3 24.9 24.4 23.9 0.36

28.1 27.7 27.3 26.9 26.6 26.1 25.6 25.3 24.9 24.3 23.9 0.17

25.3 25.0 24.6 24.1 23.8 23.5 23.0 22.6 22.3 21.8 21.5 0.13

22.2 22.0 21.8 21.6 21.3 21.2 21.0 20.5 20.5 20.3 20.1 0.2

44.2 43.8 43.3 42.8 42.4 41.9 41.4 40.9 40.4 40.0 39.4 0.07

27.7 27.4 26.9 26.5 26.1 25.5 25.2 24.5 24.1 23.7 23.2 0.24

25.7 25.4 24.9 24.5 24.2 23.8 23.4 23.0 22.6 22.1 21.7 0.15

23.1 22.8 22.4 22.1 21.7 21.4 21.1 20.7 20.4 20.0 19.7 0.13

19.7 19.5 19.2 19.0 18.8 18.7 18.4 18.2 17.9 17.6 17.5 0.02

AN

M

ED

%AAD=

US

T/K

%AAD=(1/n∑𝑛𝑖=1|𝑑𝑒𝑣|): n=number of data points, dev=(exp−fit)×100/exp

PT

Standard uncertainties u are: u(T) = 0.5 K, u()=0.2 mJ.m-2 and the combined expanded uncertainty is Uc()= 0.5 mJ.m-2 (at 95% level of confidence)

AC

CE

a

18

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50.0 298.15K 303.15K 308.15K 313.15K 318.15K 323.15K 328.15K 333.15K 338.15K 343.15K 348.15K

Surface tension/mJ.m-2

45.0

40.0

IP

T

35.0

CR

30.0

US

25.0

20.0

3 5 7 number of C atom(s) in anion

9

AN

1

M

Fig. 7. Surface tension of N-methylmorpholinium ILs versus number of C atom(s) in anion. Lines are trend lines.

PT

42.0

37.0

32.0

CE

Surface tension/mJ.m-2

298.15K 303.15K 308.15K 313.15K 318.15K 323.15K 328.15K 333.15K 338.15K 343.15K 348.15K

ED

47.0

AC

27.0

22.0

17.0 1

3

5

7

9

number of C atom(s) in anion Fig.8. Surface tension of N-ethylmorpholinium ILs versus number of C atom(s) in anion. Lines are trend lines.

19

ACCEPTED MANUSCRIPT

The variation of surface tension of these ILs with temperature is shown in Figure 9. Surface tension of all ILs decreases with increasing temperature, while keeps a high correlation in the whole range. The uncertainties of surface tension and percent average absolute deviations (%AAD) are shown in Table 5. The uncertainties of surface tension were calculated as detailed

CR

IP

T

in previous work [23], see Table 5.

50.0

US AN

40.0 35.0

M

30.0 25.0

ED

Surface tension/mJ.m-2

45.0

PT

20.0 15.0

AC

CE

298

308

318

328

338

348

T/K [Mmorph][For]

[Emorph][For]

[Mmorph][Ace]

[Emorph][Ace]

[Mmorph][Pro]

[Emorph][Pro]

[Mmorph][Hex]

[Emorph][Hex]

[Mmorph][Dec]

[Emorph][Dec]

Fig.9. Surface tension of mopholinium-based ILs measured in range 298.15-348.15 K. Lines are linear fits.

Essentially, surface tension decreases as the temperature is increased and fits to the linear equation: 𝛾 = 𝐸 𝑠 − 𝑇𝑆 𝑠 ,

(2) 20

ACCEPTED MANUSCRIPT

Therefore, from the linear temperature dependence of surface tension, the surface thermodynamics functions, namely the surface entropy 𝑆 𝑠 and surface energy 𝐸 𝑠 , can be calculated with high accuracy. In Eq. (2), it is assumed that 𝐸 𝑠 ≈ 𝐻 𝑠 , where 𝐻 𝑠 is the surface

T

enthalpy.

IP

The value of 𝑆 𝑠 is the most important thermodynamic properties of the liquid surface. Since

CR

surface entropy depends on the rate of change of surface tension with respect to temperature, i.e. 𝜕𝛾

𝑆 𝑠 = − ( ) , accurate surface yportne is guaranteed only if highly accurate surface tensions can

US

𝜕𝑇 𝑝

be made available. Far from critical point, the value of surface entropy is known to be almost

AN

constant independent of temperature. The importance of surface entropy turns to be a strong tool

M

in that it is an indication of microscopic ordering of surface molecules. The values of 𝑆 𝑠 and 𝐸 𝑠 obtained by fitting measured surface tensions in Eq. (2) are shown in Table 6. From the Figure 9

ED

and the corresponding R2 values (Table 6), it can be confirmed that a highly correlated surface

PT

tension was obtained over the whole temperature range for all ILs.

CE

TABLE 6. Surface entropy, surface energy for morpholinium-based ILs. R2’s are included.

[Mmorph]

AC

[For]

Ss/mJ m-2K-1 Es/mJ m-2 R2

[Ace]

[Emorph] [Pro]

[Hex]

[Dec]

[For]

[Ace]

[Pro]

[Hex]

[Dec]

0.0961

0.0919

0.0791

0.0683

0.0452

0.1019

0.1007

0.0839 0.0766

0.0432

78.752

58.929

53.172 48.158

35.11

72.91

55.232

49.341

43.453

33.192

0.9996

0.9935

0.9984 0.9991

0.9911

0.9995

0.9973

0.9989

0.9991

0.9954

Therefore, the surface tension of ILs (in this work) follows a perfect linear behavior as normal fluids (i.e., non-metallic and non-quantum liquid) [49], liquid metals[50], and ionic salts [51]. From Table 6, it can be seen that values of surface entropy for ILs are smaller than that of normal 21

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liquids. These indicates a high degree of organization of surface molecules of the present ILs, which is in agreement with the results reported by Santos et al.[52]. The plot of surface entropy versus the number of carbon atoms of the anion alkyl chain are shown in Figure 10. Surface entropy indicates the structural order of surface molecules increases as the anion chain length of

IP

T

the two types of ILs is increased. No big differences exist between 𝑆 𝑠 values of N-

CR

methylmorpholinium and N-ethylmorpholinium ILs. This is quite understandable in term of loss of symmetry of the anion with increasing alkyl chain length, which in turn limits the molecular

US

motions at the interface. Surface entropy decreases rather sharply for low anion alkyl chain

AN

lengths and decreases smoothly thereafter.

It would be valuable to have a comparison of the surface properties of different class of ionic

M

liquid. Surface entropy of protic 1-butyl-3-methylimidazolium carboxylate ILs with different

ED

anion chain length was calculated by Wang et al. [45]. The found the highest surface entropy (Ss=0.067 mJm-2K-1) for [C4mim][HCOO] among others. It can be seen that the surface entropy

PT

values for morpholinium carboxylate ILs (this work) is higher than imidazolium carboxylae ILs

CE

analog [45]. Surface properties of protic ILs such as 1-alkyl-3-methylimidazolium-based ILs ([C4mim]+, [C6mim]+, and [C8mim]+) with I−, Cl− , PF6− , and BF4− anions have been already

AC

reported [21]. Their surface entropy was found to decrease almost smoothly with the cation chain length. The surface entropies of those ILs are in the range 0.0820–0.0480 mJm−2K−1, which are lower than that of normal fluids. This indicates that the surface entropy does not depend very much on the nature of the liquid. Therefore, like normal liquids, the surface entropy can be considered as a generic property of an IL [21]. Surface tension of quaternary ammonium-based ILs (with [N222n]+ cation and (trifluoromethylsulfonyl)imide (NTf2− anion) have been measured and the effect of cation chain length (n=5,6,8,10 and 12) on surface properties studied. Values of 22

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𝑆 𝑠 for these ILs, contrary to the imidazolium-base ILs, was found to be an increasing function of number of carbon atoms of the cation alkyl chain, between 0.0713 to 0.0884 mJm-2 K-1 [23]. 0.11 N-methylmorpholinium

T

N-ethylmorpholinium

IP

0.09

CR

0.08 0.07

US

0.06 0.05 0.04 1

3

AN

Surface entropy/mJ. m-2 K-1

0.1

5

7

9

M

number of C atom(s) in anion

PT

ED

Fig.10. Surface entropy of morpholinium-based ILs obtained by surface tension measurement. Lines are trend lines.

CE

In Figure 11, the surface energy of morpholinium-based ILs is plotted as a function of alkyl chain length of the carboxylate anion. (See Table 6 for the numerical values). As it can be seen in

AC

this figure, the surface energies of both types are also decreasing functions of the anion chain length, while they show some systematic differences. No big differences can be seen between 𝐸 𝑠 of two types of ILs. According to Table 6 and Figure 11, the surface energy of [Mmorph][For] and [Emorph][For] ILs are the highest, in agreement with the results for imidazolium analog, 1butyl-3-methylimidazolium carboxylate ILs [45], where the highest surface energy was obtained for formate anion (𝐸 𝑠 [C4mim][HCOO]= 65.7 mJm-2). It is noteworthy to mention that energies of morpholinium carboxylate ILs (this study) are higher than imdazolium carboxylate ILs analog 23

ACCEPTED MANUSCRIPT [46]. For 1-alkyl-3-methylimidazolium-based ILs ([C4mim], [C6mim], and [C8mim]) with I−, Cl− , PF6− , and BF4− anions, the surface energy decreases more sharply than the surface entropy as the alkyl chain length increases. This indicates that surface energy is a specific property of an IL [21]. In quaternary ammonium-based ILs, [N222(n)]+ cation with NTf2− anion (with n=5, 6, 8, 10

IP

T

and 12), the values of 𝐸 𝑠 increase somewhat as the alkyl chain length increases [23]. Therefore,

CR

the results of measurement in the present and previous works allow concluding that the designer solvent ILs known to date may possess diverse surface thermodynamic properties, which are

US

essential in their capability as a potential solvent for diverse solutes.

AN

90 80

M

N-ethylmorpholinium

ED

70 60 50

PT

Surface energy mJ m-2

N-methylmorpholinium

CE

40

AC

30

1

3

5

7

9

number of C atom(s) in anion

Fig.11. Surface energy of morpholinium-based ILs obtained by surface tension measurement. Lines are trend lines.

3.4 Critical temperature According to Guggenheim [52], a certain exponent describes the vanishing of surface tension 24

ACCEPTED MANUSCRIPT

close to the critical point, which universally applies to compounds of the same universality class, irrespective of their chemical nature. Parallel to the scaling law, the temperature-dependent surface tension can be described by Guggenheim equation: 𝛾 = 𝑎(𝑇𝑐 − 𝑇)11/9 ,

(3)

IP

T

where, a is a substance-dependent constant. However, a simple and accurate power law such as

CR

Guggenheim relation, valid over all temperatures range down to the triple point is not guaranteed. One of the earliest and best-known empirical relations that involve the temperature

= 𝑘(𝑇𝑐 − 𝑇),

(4)

AN

𝑀 2/3

𝛾 (𝜌)

US

dependence of surface tension is that of Eötvös:

M

where, k is a constant, 𝜌 is liquid density (in g.cm-3), and M is molecular mass (in g·mol-1).

ED

We used Eqs. (3) and (4) and calculated Tc by fitting the measured surface tensions as a function of temperature [54]. The predicted Tc values and parameters of Eqs. (3) and (4) for the

PT

ILs are shown in Table 7. Values of Tc determined by the two methods are somewhat similar.

CE

According to R2’s, our surface tension data is best fit in Eq. (3). The differences can be attributed to the differences in the accuracy of the methods and to the accuracy of the measured liquid

AC

density (required for the application of Eötvös method). Despite this observation, close agreement between these two methods confirms the validity of the approaches [54]. TABLE 7. Predicted Tc and Parameters of Guggenheim and Eötvös Equations for Morpholinium-Based ILs.

Guggenheim Ionic Liquid [Mmorph][For] [Emorph][For] [Mmorph][Ace] [Emorph][Ace]

Eötvös

a

T c/ K

R2

K

T c/ K

R2

0.02051 0.01948 0.02286 0.02060

873.0 855.6 643.4 662.5

0.9996 0.9994 0.9933 0.9970

2.092 2.159 2.558 2.669

892.2 864.9 633.4 646.7

0.9994 0.9989 0.9906 0.9954

25

ACCEPTED MANUSCRIPT

0.01833 0.01743 0.01680 0.01490 0.00855 0.00928

703.0 690.1 696.7 705.8 920.3 825.9

0.9982 0.9988 0.9990 0.9991 0.9911 0.9952

2.160 2.201 2.381 2.210 1.426 1.619

712.2 692.1 701.8 710.0 1005.0 866.3

0.9972 0.9982 0.9984 0.9986 0.9817 0.9915

IP

T

[Mmorph][Pro] [Emorph][Pro] [Mmorph][Hex] [Emorph][Hex] [Mmorph][Dec] [Emorph][Dec]

CR

The behaviors of Tc as a function of alkyl chain length are shown in Figures 12 and 13. The trend for both N-methylmorpholinium and N-ethylmorpholinium are almost the same and shows a

US

minimum for acetate. Then the critical temperature is increased steadily with the anion chain

AN

length. So it seems that there are sort of irregularities in the trend of Tc's for both type of ILs. According to the discussion for the trend of surface tension and density, strong Columbic

M

interaction between cation and anion with short chain length and van der Waals interaction at

ED

long chain length of the anion are responsible for such behaviors. Certainly, at short chain length, the Columbic interaction is effectively occurred between anion and the cation head group, and at

PT

long chain length, the van der Waals interaction would be the predominant interaction between

CE

the alkyl chains. Some ILs was known to not effectively dissociate and do not contribute to the electrostatic screening [55]. The carboxylate anion of short alkyl chain length may present

AC

screening effect to a higher extent than the long ones.

26

ACCEPTED MANUSCRIPT

950 N-methylmorpholinium

900

N-ethylmorpholinium

800

T

Tc/K

850

IP

750

CR

700 650

1

US

600 3

5

7

9

AN

number of C atom(s) in anion

M

Fig12. Critical temperatures of morpholinium-based ILs according to Guggenheim equation. Lines are trend lines. 1050

ED

1000 950

PT

N-ethylmorpholinium

850 800

CE

Tc/K

900

N-methylmorpholinium

AC

750 700 650 600

1

3

5

7

9

number of C atom(s) in anion

Fig.13. Critical temperatures of morpholinium-based ILs according to Eötvös equation. Lines are trend lines.

27

ACCEPTED MANUSCRIPT 𝐸𝑠

Earlier a relationship has been proposed between Tc and 𝑇𝑐′ = ( 𝑆𝑠 )[54], where 𝑇𝑐′ is the critical temperature obtained by assuming that the surface tension decreases linearly up to the 𝐸𝑠

critical temperature. Tc is related to 𝑇𝑐′ linearly: 𝑇𝑐 = 𝛼 + 𝛽( 𝑆𝑠 ) . Imidazolium and quaternary

T

ammonium-based ILs follow the same linear trend with β=1.222 [54] and β=1.220 [23],

IP

respectively. These values are close to the 11/9 exponent of the Guggenhiem relationship.

CR

Interestingly, the values of the slope β(=1.2185) for the N-methylmorpholinium-based and

US

β(=1.2253) for N-ethylmorpholinium-based ILs obtained in this work are also close to the 11/9 exponent. The plot of Tc, predicted by the Guggenheim approach, versus 𝑇𝑐′ for these ILs along

AN

with imidazolium and quaternary ammoniumim-based ILs of earlier works [23, 54] is shown in

M

Figure 14. It can be seen that the three different type of ILs are characterized, based on their critical temperatures and surface thermodynamic functions, by the same slope (1.2109) with high

ED

correlation (R2=0.9997). It is interesting to note that imidazolium’s have the highest and the

PT

morpholinium’s have the lowest critical temperatures. 1300

CE

1200

Tc/K

AC

1100 1000 900 800

Quaternary ammonium Imidazolium Morpholinium

700 600 500 490

590

690

790

(Es/Ss)/K

28

890

990

1090

ACCEPTED MANUSCRIPT

𝐸𝑠

Fig. 14. Plot of Tc values predicted by the Guggenheim approach versus 𝑇𝑐′ = ( 𝑠 ) for morpholinium, 𝑆 imidazolium and quaternary ammonium. Line is the fit to data points.

4. Conclusions

T

The capillary rise method has been used to measure the surface tension of a new series of N-

IP

methylmorpholinium and N-ethylmorpholinium-based protic ILs consisting carboxylate anions

CR

in the range of 298.15-348.15K. Also, the density of these ILs was measured and studied in the same temperature range. The surface tension values obtained for all ILs are between 48.4 and

US

17.5 mJ.m−2. Surface thermodynamic properties, Ss and Es, were calculated and Tc’s were

AN

estimated from the measured surface tensions. The results for both N-methylmorpholinium and N-ethymorpholinium indicate that, as the anion alkyl chain length increases the density, surface

M

tension, surface energy, and surface entropy decreases while contact angle increases.

ED

[Mmorph][For] has the highest surface tension, density, surface energy, surface entropy and the lowest contact angle. The properties of morpholinium-based ILs are very sensitive to the alkyl

PT

chain length of the anion particularly at low chain length. The variation of density, contact angle,

CE

and surface properties versus anion chain length is sharp for ILs with anion of short chain length (formate and acetate), and is smooth for those of long chain length (propionate, hexanoate and

AC

decanoate). The trends of variation of these properties are in accord with the trend of pKa of their corresponding anion’s acids. These ILs can be considered as high surface energy ILs. Morphlinium-based ILs, like imidazoluim and quaternary ammonium-based ILs, can be categorized as low surface entropy compared with the normal liquids. Therefore, they are expected to demonstrate high solubility of a wide range of organic and inorganic materials. According to surface entropies, molecules are more orderly positioned at the IL surface as the alkyl chain length of anion and cation increases. Highly correlated surface tensions and densities 29

ACCEPTED MANUSCRIPT

measured as a function of temperature support these conclusions. The provided data in this work would be suitable for the establishment of the structure-property relationship of morpholinium ILs. This plus the trend of other properties suggest that a concerted selection of anion and cation with particular alkyl chain length ratio may be used to expand the character of the designer

CR

IP

T

solvent ILs.

Acknowledgements

US

The authors are indebted to the research council of the Shiraz University for financial supports.

AN

One of the authors (TGH) is thankful to Maryam Bahrami for many helps on synthesis and experimentations. Camera for contact angle measurement (DinoCapture 2.0 Lite Camera) was

M

provided by Dr. A.R. Zolghadr. Also we are thankful to Mr. Moghadam for glassblowing art

ED

works for fabricating capillary apparatuses.

PT

References

AC

CE

[1] M.C. Buzzeo, R.G. Evans, R.G. Compton, Non‐haloaluminate room‐temperature ionic liquids in electrochemistry—A review, ChemPhysChem. 5 (2004) 1106-1120. [2] K.R. Seddon, Ionic liquids for clean technology, J. Chem. Technol. Biotechnol, 68 (1997) 351-356. [3] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99 (1999) 2071-2084. [4] D. Zhao, M. Wu, Y. Kou, E. Min, Ionic liquids: applications in catalysis, Catal. Today. 74 (2002) 157-189. [5] R. Sheldon, Catalytic reactions in ionic liquids, Chem. Commun, DOI (2001) 2399-2407. [6] Z. Yang, W. Pan, Ionic liquids: Green solvents for nonaqueous biocatalysis, Enzyme Microb. Technol. 37 (2005) 19-28. [7] N. Jain, A. Kumar, S. Chauhan, S. Chauhan, Chemical and biochemical transformations in ionic liquids, Tetrahedron. 61 (2005) 1015-1060. [8] S.-P. Gou, I.-W. Sun, Electrodeposition behavior of nickel and nickel–zinc alloys from the zinc chloride-1-ethyl-3-methylimidazolium chloride low temperature molten salt, Electrochim. Acta. 53 (2008) 2538-2544. 30

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[42] https://toxnet.nlm.nih.gov/cgi-bin/sis/search2/f?./temp/~Lp0Nek:10) [43] D.R. Lide, CRC handbook of chemistry and physics, CRC press2004. [44] M. Montalban, C. Bolivar, F.G. Díaz Baños, G. Villora, Effect of temperature, anion, and alkyl chain length on the density and refractive index of 1-alkyl-3-methylimidazolium-based ionic liquids, J. Chem. Eng. Data. 60 (2015) 1986-1996. [45] A. Xu, J. Wang, Y. Zhang, Q. Chen, Effect of alkyl chain length in anions on thermodynamic and surface properties of 1-butyl-3-methylimidazolium carboxylate ionic liquids, Ind. Eng. Chem. Res. 51 (2012) 3458-3465. [46] M. Poleski, J. Luczak, R. Aranowski, C. Jungnickel, Wetting of surfaces with ionic liquids, Physicochem. Probl. Miner. Process. 49 (2013) 277-286. [47] G.V. Carrera, C.A. Afonso, L.C. Branco, Interfacial Properties, Densities, and Contact Angles of Task Specific Ionic Liquids†, J. Chem. Eng. Data. 55 (2009) 609-615. [48] A.W. Adamson, A.P. Gast, physical chemistry of surface, sixth ed., New York, 1997. [49] J.J. Jasper, The surface tension of pure liquid compounds, J. Phys. Chem. Ref. Data. 1 (1972) 841-1010. [50] M.H. Ghatee, A. Boushehri, Corresponding-states correlations for the surface tension of molten alkali metals, High Temp. High Pressures. 26 (1994) 507-514. [51] M.H. Ghatee, M.H. Mousazadeh, A. Boushehri, Corresponding states correlation for the surface tension of molten alkali halides, High Temp. High Pressures. 29 (1997) 717-722. [52] R. Lynden-Bell, Gas—liquid interfaces of room temperature ionic liquids, Mol. Phys. 101 (2003) 2625-2633. [53] E.A. Guggenheim, The principle of corresponding states, J. Chem. Phys. 13 (1945) 253261. [54] M.H. Ghatee, F. Moosavi, A.R. Zolghadr, R. Jahromi, Critical-Point Temperature of Ionic Liquids from Surface Tension at Liquid− Vapor Equilibrium and the Correlation with the Interaction Energy, Ind. Eng. Chem. Res. 49 (2010) 12696-12701. [55] M.A. Gebbie, M. Valtiner, X. Banquy, E.T. Fox, W.A. Henderson, J.N. Israelachvili, Ionic liquids behave as dilute electrolyte solutions, Proc. Natl. Acad. Sci. U.S.A. 110 (2013) 96749679.

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Protic Morpholinium Ionic liquids

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Surface tension/mJ.m-2

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Research highlights

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► Synthesis of 10 high purity N-methylmorpholinium- and N-ethylmorpholinium-based ionic liquids with alkyl-carboxylate anion.

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► Measurement of surface tension, contact angle, and density of the ionic liquids over 298 348K.

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► Achieving surface tension measurement at low water content under vacuum.

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► Observation of some irregularities in surface property with respect to cation and anion alkyl chain length

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► Understanding certain irregularity in the trend of predicted density and predicted critical temperatures.

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