Viscometric Properties of Aqueous Sodium and Potassium Salt of L-Leucine

Introduction

Global warming is the most alarming environmental problem. Excessive emissions of carbon dioxide (CO₂) from several sources, including the burning of fuels, both human and other industrial activities, are the primary cause of global warming^1,2. The reduction of CO₂ from the flue gas is a vital step in many industrial activities required for economic and environmental aspects. There are many post- combustion capture technologies to separate CO₂ from flue gas, before it enters the atmosphere. The goal is to collect pure CO₂ channels that could be compressed and transported to a storage area. In general, this process is known as CO₂ capture and storage(CCS).

For CO₂ capture chemical absorption using alkanolamine absorbents have been extensively employed³. Some of the demerits associated with alkanolamines in CO₂ capture process are their degradation in oxygen-rich atmosphere and corrosion in the process equipment⁴. These drawbacks of alkanolamine solvents limits their application for carbon dioxide capture procedures.

Recently, salts of amino acid have been investigated as good substitutes for amines and many researchers have examined their interaction with CO₂ ^5,6. Amino acids resembles amines in their functional group and acts in similar manner in the CO₂ capture process. Amino acids have certain unique advantages over alkanolamines like fast reaction kinetics, very low volatility, more stability towards oxidative degradation⁷, higher surface tension, and having a significant absorption capability⁸.

Physicochemical properties are essential in the simulation and design of gas-liquid contactors, heat exchangers, working of the equipment, process modelling, and for the evaluation of proper absorbents for CO₂ capture^9-12. These properties are also essential for the deduction of chemical reaction kinetics from CO₂ absorption rate experiments^13,14.  

For aqueous sodium and potassium leucinate solutions, such properties still have not been described in the existing literature at a lower concentration range. Thus, we provided fresh experimental data on viscosity, thereafter Jones-Dole coefficient (B), temperature derivative of B i.e. (dB/dT), free energy of activation(Δ𝐺̅^0#), the enthalpy and entropy of activation (Δ𝐻̅^0# and Δ𝑆̅^0#) for sodium and potassium salt solutions of above mentioned amino acid.

Experimental

Materials and preparation of salt amino acid

L-leucine (CAS No. 61-90-5, 99.0% purity) was supplied by LOBA CHEMIE PVT. LTD., NaOH (CAS No. 1310-73-2, GR, 98 % purity) and KOH (CAS No. 1310-58-3, GR, 98% purity) were bought from Merck. All chemicals were used as received.

The aqueous salt solutions of leucine were made by neutralizing leucine with an equimolar amount of NaOH and KOH in triply distilled water. Using the weight/weight method the solutions were made in dry, airtight glass bottles with stoppers. Electronic balance accurate to 0.1 mg was used to record the masses.

Structure of sodium and potassium salt of L-leucine

Viscosity measurements

Dynamic viscosity values of aqueous solutions of sodium and potassium salt of L-leucine were measured using Ubbelohde suspended-level viscometer^{15, 16} for concentrations range 0.01 to 0.15 m and at temperatures (298.15, 303.15, 308.15, and 313.15) K at atmospheric pressure. Triply distilled water and pure AR grade solvents were used to calibrated Viscometer. The viscometer was placed straight in a transparent glass water bath with a openings above the water level, and a thermostat was used to maintain the constant temperature at  0.01 K. a digital stopwatch that is electronic. The flow-time(t) measurements were performed using an electronic digital sport stopwatch   with a precision of 0.01 s. Minimum three readings were recorded of each viscosity data point with a 0.05 s reproducibility and their averaged values were considered.

Results and Discussion

Dynamic viscosities (η) were determined via

where t, ρ, η, and t_o, ρ_o,η_o are flow time, density, and viscosity of solution and solvent, respectively.

The measured values of viscosity for aqueous solution of sodium and potassium salt of L-leucine at concentration range (0.01 to 0.15) m and temperatures at (298.15, 303.15, 308.15, and 313.15) K are placed in Table 1. It is observed that viscosity increases with concentration. Table 1, Figure 1, Figure 2 indicates that density increases with molality of the solutions and decreases with the increase in the temperature.

The viscosity data were analysed using the Jones-Dole equation¹⁷

𝐴 is a constant term that provides data on the strength of ion-ion interactions in a solution. Above equation can be modified as

Equation 3 resembles the straight line equation (y = mx + c) with 𝐵 as a slope and A as an intercept. The values of [(η /η ₀)-1] /m^0.5 presented in Table 2. Least square method was employed to obtain Jones-Dole coefficient (B). B values for sodium and potassium salt of leucine at (298.15, 303.15,308.15 and 315.15) K are presented in Table 3.  B value of sodium and potassium salt of amino acid decreases with an increasing temperature.

Activation Free energy of solvent and activation free energy of solute  can be calculated by the following equations,

where η₀, 𝑁_𝐴, and ℎ, are the viscosity of the solvent, Avogadro number and Planck’s constant, respectively.

The enthalpy and entropy of activation Δ𝐻̅^0# and Δ𝑆̅^0#, respectively are calculated by the following equation

Equation 4 is similar to the straight line equation (y = – mx + C) where intercept is equal to Δ𝐻̅^0# and the slope is − Δ𝑆̅^0#. Δ𝑆̅^0#and Δ𝐻̅^0# values were calculated using Least square method. Figure 5 and Figure 6 show the variation of Δ𝐺̅^0# vs temperature.

 Jones-Dole coefficient (B) and (Δ𝐺̅^0#) values of sodium leucinate are found to be greater than potassium leucinate.

Table 1: Viscosities ( η ) of sodium and potassium leucinate at different temperatures.

m (kg.mol-1)	η (m-1.kg.s-1)				m (kg.mol-1)	η (m-1.kg.s-1)
	298.15 K	308.15 K	308.15 K	313.15 K		298.15 K	303.15 K	308.15 K	313.15 K
sodium leucinate					potassium leucinate
0.00000	0.8903	0.7975	0.7195	0.6535	0.00000	0.8903	0.7975	0.7195	0.6535
0.01002	0.8943	0.8018	0.7240	0.6582	0.01007	0.8937	0.8012	0.7234	0.6576
0.03050	0.9043	0.8110	0.7327	0.6665	0.03010	0.9023	0.8092	0.731	0.6648
0.05098	0.9147	0.8205	0.7414	0.6746	0.05014	0.9112	0.8172	0.7385	0.6717
0.06973	0.9242	0.8291	0.7493	0.6819	0.07199	0.9210	0.8262	0.7467	0.6792
0.08976	0.9345	0.8385	0.7578	0.6896	0.09103	0.9297	0.8339	0.7538	0.6857
0.10979	0.9448	0.8478	0.7662	0.6974	0.11007	0.9382	0.8418	0.7608	0.6921
0.13074	0.9558	0.8575	0.7750	0.7055	0.12977	0.9473	0.8498	0.7681	0.6988
0.15169	0.9667	0.8674	0.7839	0.7133	0.14947	0.9561	0.8579	0.7751	0.7054

Table 2: [(h/hₒ)-1]/m ^0.5 values for sodium and potassium leucinate in water at different temperatures.

m (kg.mol-1)	[(η /ηₒ)-1]/m 0.5				m (kg.mol-1)	[(η /ηₒ)-1]/m 0.5
	298.15 K	308.15 K	308.15 K	313.15 K		298.15 K	303.15 K	308.15 K	313.15 K
sodium   leucinate					potassium    leucinate
0.01002	0.04488	0.05386	0.06248	0.07185	0.01007	0.03806	0.04623	0.05402	0.06252
0.03050	0.09004	0.09693	0.10505	0.11391	0.0301	0.07769	0.08456	0.09213	0.09967
0.05098	0.12138	0.12773	0.13481	0.14300	0.05014	0.10484	0.11032	0.11793	0.12438
0.06973	0.14420	0.15005	0.15685	0.16457	0.07199	0.12852	0.13413	0.14090	0.14657
0.08976	0.16571	0.17160	0.17768	0.18438	0.09103	0.14668	0.15128	0.15801	0.16331
0.10979	0.18475	0.19035	0.19589	0.20274	0.11007	0.16217	0.16743	0.17302	0.17804
0.13074	0.20347	0.20807	0.21333	0.22007	0.12977	0.17773	0.18205	0.18751	0.19243
0.15169	0.22033	0.22504	0.22981	0.23495	0.14947	0.19117	0.19590	0.19988	0.20542

Table 3: (B), (A), dB/dT, (ΔG), (ΔH), and (ΔS) values for sodium and potassium leucinate in water at different temperatures.

Parameter	298.15 K	308.15 K	308.15 K	313.15 K	298.15 K	303.15 K	308.15 K	313.15 K
sodium   leucinate					potassium   leucinate
B	0.60587	0.59228	0.5784	0.56508	0.5339	0.52163	0.50942	0.49762
A	-0.01572	-0.00597	0.00429	0.01531	-0.0148	-0.00591	0.00392	0.01308
dB/dT	-0.0027244				-0.0024522
ΔG	104.6072	104.1042	103.4897	102.9127	96.2864	95.8459	95.3761	94.8414
ΔH	138.6106				124.9556
ΔS	-0.11396				-0.09608

Figure 1: Plot of η/(m-1,kg.s-1) versus m/(kg.mol-1) of sodium leucinate in water at different temperatures (T). Click here to View figure

Figure 2: Plot of η/(m-1,kg.s-1) versus (m/kg.mol-1)0.5 of potassium leucinate in water at different temperatures (T) Click here to View figure

Figure 3: Plot of [(η/ηo)-1]/m0.5 versus (m/kg.mol-1)0.5 of potassium leucinate in water at different temperatures (T) Click here to View figure

Figure 4: Plot of [(η/ηo)-1]/m0.5 versus (m/kg.mol-1)0.5 of potassium leucinate in water at different temperatures (T) Click here to View figure

Figure 5: Plot of ∆G/KJ.mol-1) of aqueous Sodium Leucinate vs T/K Click here to View figure

Figure 6: Plot of ∆G/KJ.mol-1) of aqueous Potassium Leucinate vs T/K Click here to View figure

The observations of above Tables and Figures shows that

Viscosity of aqueous solution of sodium and potassium salt of leucine increases with increase in the concentration of sodium and potassium of salt amino acid.

Viscosity decreases with an increase in the temperature. Temperature increases cause thermal energy to rise, which causes the breakdown of amino acid salt/water aggregates.

Conclusion

B value for sodium and potassium salt of leucine decreases as the temperature rises. The positive B and ΔG values indicate strong water and amino acid salt interactions. The negative value of (dB/dT) for investigated amino acid salt -water systems confirms structure making behavior of studied amino acid salts. As a result, it can be said that sodium and potassium salt of leucine is a water structure maker.

Reported data and the related parameters presented in this work may be helpful in the design, improvement, and evaluation of techniques and processes that use the amino acid salt systems for CO₂ capture.

Acknowledgment

Author wants to thank the Director of Government Vidarbha Institute of Science, and Humanities, Amravati for permission for the research facility and the Principal, HPT Arts and RYK Science College of Nashik for his kind support.

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