Studies on densities (ρ) and viscosities (η) of electrolyte solutions are of great importance in characterizing the properties and structural aspects of solutions. The addition of an electrolyte to an aqueous organic solution alters the pattern of ion solvation and causes phenomenal changes in the behaviour of the dissolved electrolyte. Hence studies on the limiting apparent molar volume and viscosity B-coefficients of electrolyte provide us valuable information regarding ion-ion, ion-solvent and solvent-solvent interactions [1-3]. It has been found by a number of workers [4-6] that the addition of an electrolyte could either make or break the structure of a liquid. As the viscosity of a liquid depends on the intermolecular forces, the structural aspects of the liquid can be inferred from the viscosity of solutions at various electrolyte concentrations and temperature.

In this paper we have attempted to report the limiting apparent molar volume ( ), experimental slopes ( ) and viscosity B-coefficients for potassium chloride, potassium bromide and potassium iodide in aqueous mixture of L-Proline at 298.15, 308.15 and 318.15 K. Since potassium ion being a common cation for all of the electrolytes under investigation, the present work enables us to have a qualitative comparison of the role of anion in aqueous L-Proline in terms of various derived parameters obtained from viscosity (η) and density (ρ) measurements.

Materials

L-Proline (SD. Fine Chemicals) was purified by standard methods [7]. The purity of the solvent was checked by measuring the viscosity (η) and density (ρ) at 298.15 K which was in good agreement with the literature values. Doubly distilled, degassed and deionised water with a specific conductance of 1 × 10^-6/Ω/cm was used. Potassium chloride, potassium bromide and potassium iodide (Sigma-Aldrich, Germany) were purified by re-crystallizing twice from conductivity water and then dried in a vacuum desiccator over P2O5 for 24 h before use. The experimental values of viscosity (η) and density (ρ) of aqueous mixtures of 0.01 M, 0.03 M and 0.05 M L-Proline at different temperatures are listed in Table 1.

Table 1: Density (ρ, kg/m) and viscosity (η, mPa.s) of aqueous mixtures of 0.01 M L-proline, 0.03 M L-proline, 0.05 M L-proline at different temperatures.

Apparatus and procedure

Densities (ρ) were measured with an Anton Paar density-meter (DMA 4500M) with a precision of 0.0005 g/cm3. The calibration was done by double-distilled water and dry air and uncertainty in density was ± 0.00005 g/cm.

The measurements were done in a thermostat bath controlled to ± 0.01 K. Viscosity (η) was measured by means of Brookfield DV-III Ultra Programmable Rheometer with spindle size-42 having an accuracy of 1.0% and fitted to a Brookfield Digital Bath TC-500 at 298 K using density and viscosity values from the literature [8-10]. The uncertainty in viscosity measurements is within ± 0.003 mPa s. The mixtures were prepared by mixing known volume of pure liquids in airtight-stopper bottles and each solution thus prepared was distributed into three recipients to perform all the measurements in triplicate, with the aim of determining possible dispersion of the results obtained. Adequate precautions were taken to minimize evaporation loses during the actual measurements.

The electrolyte solutions studied here were prepared by mass and the conversion of molality into molarity was accomplished [3] using experimental density values. The experimental values of concentrations (c), densities (ρ), viscosities (η), and derived parameters at various temperatures are reported in Table 2.

c(mol/dm3)	ρ×10-3( kg/m)	η (mPa.s)	×106 ( m3/mol )	(kg1/2 mol-1/2)
Potassium chloride in aqueous mixture 0.01 M L-proline
298.15K
0.0151	0.99804	0.918	40.7724	0.081
0.0302	0.99862	0.923	38.4219	0.089
0.0452	0.99924	0.927	36.6533	0.093
0.059	0.99985	0.931	35.1369	0.1
0.0752	1.00059	0.935	33.7388	0.104
0.0903	1.00132	0.938	32.4331	0.106
Potassium chloride in aqueous mixture 0.01 M L-proline
308.15K
0.0151	0.99651	0.779	44.1206	0.084
0.0302	0.99709	0.784	40.094	0.097
0.0452	0.99773	0.789	37.3202	0.11
0.059	0.99835	0.793	35.4719	0.117
0.0752	0.99914	0.798	33.3263	0.128
0.0903	0.99988	0.802	31.9725	0.134
Potassium chloride in aqueous mixture 0.01 M L-proline
318.15K
0.0151	0.99509	0.647	46.1479	0.102
0.0302	0.99565	0.653	41.7753	0.126
0.0452	0.99628	0.659	38.6617	0.147
0.059	0.99693	0.664	35.9803	0.161
0.0752	0.99774	0.67	33.4493	0.177
0.0903	0.99858	0.675	30.951	0.187
Potassium bromide in aqueous mixture 0.01 M L-proline
298.15K
0.0153	0.99853	0.922	53.5574	0.116
0.0303	0.99959	0.931	50.8657	0.139
0.0454	1.00071	0.94	48.7479	0.16
0.0607	1.00189	0.948	46.9048	0.174
0.0759	1.00311	0.957	45.1597	0.192
0.0912	1.00438	0.966	43.5138	0.208
Potassium bromide in aqueous mixture 0.01 M L-proline
308.15K
0.0153	0.99692	0.783	62.103	0.126
0.0303	0.99796	0.792	55.8276	0.156
0.0454	0.99911	0.801	51.3776	0.183
0.0607	1.00036	0.81	47.6963	0.205
0.0759	1.00168	0.819	44.4546	0.226
0.0912	1.00305	0.828	41.8137	0.245
Potassium bromide in aqueous mixture 0.01 M L-proline
318.15K
0.0153	0.99542	0.651	69.3779	0.152
0.0303	0.9964	0.661	61.4873	0.198
0.0454	0.99754	0.671	55.3595	0.235
0.0607	0.9988	0.681	50.4901	0.267
0.0759	1.00015	0.69	46.2727	0.29
0.0912	1.00162	0.701	42.2061	0.321
Potassium iodide in aqueous mixture 0.01 M L-proline
298.15K
0.0151	0.99887	0.926	77.1285	0.152
0.0302	1.00029	0.941	74.3642	0.203
0.0451	1.00175	0.955	72.0751	0.238
0.0604	1.00329	0.968	70.1716	0.264
0.0755	1.00486	0.982	68.3442	0.292
0.0906	1.00647	0.995	66.6517	0.314
Potassium iodide in aqueous mixture 0.01 M L-proline
308.15K
0.0151	0.99714	0.788	93.7994	0.179
0.0302	0.99851	0.803	84.3485	0.239
0.0451	1.00001	0.818	77.8489	0.287
0.0604	1.00168	0.833	72.2998	0.327
0.0755	1.00341	0.847	67.9012	0.359
0.0906	1.00525	0.861	63.7203	0.388
Potassium iodide in aqueous mixture 0.01 M L-proline
318.15K
0.0151	0.99559	0.656	104.5438	0.217
0.0302	0.99691	0.672	91.3819	0.297
0.0451	0.99839	0.688	82.9873	0.361
0.0604	1.0001	0.703	75.4455	0.408
0.0755	1.00189	0.718	69.5956	0.45
0.0906	1.00387	0.733	63.5555	0.489
Potassium chloride in aqueous mixture 0.03 M L-proline
298.15K
0.015	0.99921	0.923	61.945	0.062
0.0301	0.99949	0.927	58.9421	0.069
0.0451	0.99984	0.931	56.3604	0.077
0.0603	1.00027	0.935	53.7704	0.085
0.0754	1.00075	0.938	51.5348	0.087
0.0907	1.00137	0.942	48.5495	0.094
Potassium chloride in aqueous mixture 0.03 M L-proline
308.15K
0.0151	0.99772	0.785	68.7139	0.073
0.0301	0.99795	0.79	64.0361	0.089
0.0451	0.99829	0.795	59.9847	0.103
0.0603	0.99871	0.799	56.6853	0.11
0.0754	0.99924	0.804	53.2104	0.122
0.0907	0.99979	0.808	50.6903	0.128
Potassium chloride in aqueous mixture 0.03 M L-proline
318.15K
0.0151	0.99619	0.652	64.8067	0.088
0.0302	0.99659	0.658	56.4412	0.116
0.0451	0.99712	0.664	50.6549	0.139
0.0604	0.99784	0.669	44.7281	0.151
0.0756	0.99866	0.675	39.7752	0.169
0.0907	0.99964	0.679	34.7144	0.175
Potassium bromide in aqueous mixture 0.03 M L-proline
298.15K
0.015	0.99923	0.928	105.103	0.107
0.0301	0.99983	0.938	92.0902	0.138
0.0452	1.00074	0.947	80.857	0.159
0.0603	1.00193	0.956	70.5692	0.178
0.0754	1.00332	0.965	61.7272	0.195
0.0905	1.00486	0.975	54.1642	0.214
Potassium bromide in aqueous mixture 0.03 M L-proline
308.15K
0.0151	0.99774	0.79	111.9319	0.126
0.0302	0.99842	0.802	92.8868	0.178
0.0453	0.99948	0.812	78.0739	0.205
0.0604	1.00089	0.822	64.8203	0.23
0.0755	1.00258	0.832	53.1259	0.253
0.0906	1.00448	0.842	42.9908	0.273
Potassium bromide in aqueous mixture 0.03 M L-proline
318.15K
0.0151	0.99612	0.657	114.1186	0.152
0.0302	0.99691	0.669	90.3578	0.214
0.0453	0.99818	0.68	71.7285	0.255
0.0604	0.99985	0.691	55.7207	0.29
0.0755	1.00185	0.702	41.6985	0.322
0.0906	1.00412	0.713	29.3384	0.35
Potassium iodide in aqueous mixture 0.03 M L-proline
298.15K
0.0149	0.99928	0.934	148.643	0.161
0.0302	1.00032	0.95	122.8749	0.214
0.0456	1.0018	0.965	104.7895	0.251
0.0604	1.00356	0.98	90.4247	0.284
0.0755	1.00571	0.996	76.8781	0.318
0.0906	1.00802	1.012	66.0675	0.348
Potassium iodide in aqueous mixture 0.03 M L-proline
308.15K
0.0149	0.99781	0.796	154.2313	0.19
0.0302	0.99878	0.813	127.989	0.259
0.0456	1.00027	0.829	107.9414	0.307
0.0605	1.00225	0.845	89.2142	0.35
0.0756	1.00455	0.861	73.9113	0.388
0.0907	1.00701	0.879	61.9273	0.431
Potassium iodide in aqueous mixture 0.03 M L-proline
318.15K
0.0149	0.99612	0.663	161.2379	0.229
0.0302	0.99723	0.68	126.7934	0.312
0.0456	0.99899	0.697	101.1548	0.378
0.0605	1.00115	0.714	81.1575	0.435
0.0756	1.00374	0.731	63.5879	0.485
0.0907	1.00674	0.749	47.3012	0.535
Potassium chloride in aqueous mixture 0.05 M L-proline
298.15 K
0.0147	1.00058	0.931	66.192	0.071
0.0298	1.00085	0.937	61.6009	0.088
0.0449	1.00122	0.942	57.8579	0.097
0.0602	1.00167	0.947	54.712	0.106
0.0754	1.00215	0.952	52.421	0.114
0.0904	1.00271	0.957	49.9364	0.123
Potassium chloride in aqueous mixture 0.05 M L-proline
308.15 K
0.0147	0.99955	0.794	65.5701	0.084
0.0298	0.99998	0.8	55.9323	0.103
0.0449	1.00064	0.806	47.6291	0.12
0.0602	1.00141	0.812	41.7613	0.135
0.0754	1.00234	0.817	36.1032	0.144
0.0903	1.00348	0.823	29.8654	0.157
Potassium chloride in aqueous mixture 0.05 M L-proline
318.15 K
0.0147	0.99787	0.66	64.981	0.101
0.0298	0.99833	0.667	54.6447	0.133
0.0449	0.99899	0.674	46.7675	0.159
0.0603	0.99985	0.681	39.6604	0.181
0.0754	1.00089	0.688	32.9022	0.201
0.0904	1.00199	0.695	27.6887	0.219
Potassium bromide in aqueous mixture 0.05 M L-proline
298.15 K
0.0152	1.00061	0.935	108.4123	0.105
0.0304	1.00135	0.945	89.2875	0.137
0.0454	1.00251	0.954	73.4616	0.158
0.0605	1.00389	0.964	61.9252	0.181
0.0756	1.00556	0.974	51.1534	0.201
0.0907	1.00753	0.983	40.6693	0.216
Potassium bromide in aqueous mixture 0.05 M L-proline
308.15 K
0.0152	0.99958	0.798	107.8545	0.124
0.0304	1.00046	0.809	84.4013	0.168
0.0454	1.00177	0.82	66.8686	0.203
0.0605	1.00345	0.83	52.0043	0.228
0.0756	1.00542	0.841	39.243	0.254
0.0907	1.00756	0.851	28.872	0.275
Potassium bromide in aqueous mixture 0.05 M L-proline
318.15 K
0.0152	0.99789	0.664	108.0181	0.149
0.0304	0.99906	0.676	74.8907	0.211
0.0454	1.00072	0.687	52.7317	0.252
0.0605	1.00279	0.698	34.9094	0.287
0.0756	1.00533	0.709	18.0126	0.318
0.0907	1.00822	0.721	2.945	0.351
Potassium iodide in aqueous mixture 0.05 M L-proline
298.15 K
0.0146	1.00065	0.941	152.2114	0.161
0.0303	1.00155	0.959	129.5148	0.224
0.0452	1.00301	0.976	109.0648	0.27
0.0607	1.00488	0.993	92.6019	0.308
0.0759	1.00713	1.009	77.4794	0.338
0.0908	1.00958	1.025	64.8759	0.367
Potassium iodide in aqueous mixture 0.05 M L-proline
308.15 K
0.0146	0.99962	0.804	151.6684	0.19
0.0303	1.00072	0.823	122.6546	0.27
0.0452	1.00236	0.841	100.5065	0.329
0.0607	1.00433	0.859	84.5345	0.377
0.0759	1.00688	0.876	67.0643	0.416
0.0908	1.00963	0.893	52.8721	0.452
Potassium iodide in aqueous mixture 0.05 M L-proline
318.15 K
0.0146	0.99794	0.67	151.2114	0.228
0.0303	0.99944	0.69	109.1685	0.335
0.0453	1.00165	0.709	78.8492	0.411
0.0607	1.00446	0.728	54.4682	0.473
0.0759	1.00769	0.746	34.0835	0.523
0.0908	1.01154	0.764	13.2989	0.57

Table 2: The concentration (c), density (ρ), viscosity (η), apparent molar volume and of potassium chloride, potassium bromide and potassium iodide in different aqueous mixtures 0.01M L-proline , 0.03M L-proline, 0.05M L-proline at different temperatures.

Density calculation

The apparent molar volumes ( ) were determined from the solution densities using the following Equation [3]:

Where M is the molar mass of the solute, c is the molarity of the solution; ρ0 and ρ are the densities of the solvent and the solution respectively. The limiting apparent molar volumes was calculated using a least-squares treatment to the plots of versus √c using the following Masson equation [11]:

Where ( ) is the apparent molar volume at infinite dil ution and ( ) is the experimental slope. The plots of ( ) against square root of molar concentration (√c) were found to be linear as depicted graphically in Figures 1-9 with negative slopes. Values of and are reported in Table 3.

Figure 1: The apparent molar volume (φ_V ) and the square root of concentrations (√c) for potassium chloride in different aqueous mixtures 0.01 M L-Proline at 298.15 K (—♦—), 308.15 K (—■—) and 318.15 K (—▲—).

Figure 2: The apparent molar volume (φ_V ) and the square root of concentrations (√c) for potassium chloride in different aqueous mixtures 0.01 M L-Proline at 298.15 K (—♦—), 308.15 K (—■—) and 318.15 K (—▲—).

Figure 3: The apparent molar volume (φ_V ) and the square root of concentrations (√c) for potassium chloride in different aqueous mixtures 0.01 M L-Proline at 298.15 K (—♦—), 308.15 K (—■—) and 318.15 K (—▲—).

Figure 4: The apparent molar volume (φ_V ) and the square root of concentrations (√c) for potassium bromide in different aqueous mixtures 0.01 M L-Proline at 298.15 K (—♦—), 308.15 K (—■—) and 318.15 K (—▲—).

Figure 5: The apparent molar volume (φ_V ) and the square root of concentrations (√c) for potassium bromide in different aqueous mixtures 0.01 M L-Proline at 298.15 K (—♦—), 308.15 K (—■—) and 318.15 K (—▲—).

Figure 6: The apparent molar volume (φ_V ) and the square root of concentrations (√c) for potassium bromide in different aqueous mixtures 0.01 M L-Proline at 298.15 K (—♦—), 308.15 K (—■—) and 318.15 K (—▲—).

Figure 7: The apparent molar volume (φ_V ) and the square root of concentrations (√c) for potassium iodide in different aqueous mixtures 0.01 M L-Proline at 298.15 K (—♦—), 308.15 K (—■—) and 318.15 K (—▲—).

Figure 8: The apparent molar volume (φ_V ) and the square root of concentrations (√c) for potassium iodide in different aqueous mixtures 0.01 M L-Proline at 298.15 K (—♦—), 308.15 K (—■—) and 318.15 K (—▲—).

Figure 9: The apparent molar volume (φ_V ) and the square root of concentrations (√c) for potassium iodide in different aqueous mixtures 0.01 M L-Proline at 298.15 K (—♦—), 308.15 K (—■—) and 318.15 K (—▲—).

Molarity of L-Proline	(m3/mol)			(m2/mol3/2 L1/2)
	298.15 K	308.15 K	318.15 K	298.15 K	308.15 K	318.15 K
0.01	46.56	52.19	56.59	-46.89	-68.46	-84.87
0.03	71.52	81.51	85.63	-73.76	-102.01	-167.33
0.05	77.16	89.67	90.59	-90.67	-196.92	-208.7
Potassium bromide
0.01	60.6	75.84	87.98	-56.09	-113.71	-151.83
0.03	141.42	160.01	173.02	-288.91	-388.12	-477.11
0.05	155.11	162.3	178.49	-379.72	-446.42	-584.88
Potassium iodide
0.01	84.48	113.9	131.59	-58.75	-168.01	-227.4
0.03	203.81	218.21	237.49	-460.91	-521.5	-634.07
0.05	212.82	217.51	241.73	-489.42	-545.32	-758.61

Table 3: Limiting apparent molar volumes ( ) and experimental slopes ( ) of potassium chloride, potassium bromide and potassium iodide in different aqueous mixtures 0.01M L-proline , 0.03M L-proline, 0.05M L-proline at different temperatures.

In these systems the ion-solvent and ion-ion interactions can be interpreted in terms of structural changes between various components of the solvent and solution systems. can be used to interpret ionsolvent interactions. A perusal of Table 3 reveals that the values are positive and increase with a rise in both the temperature and amount of L-Proline in the mixtures. This indicates the presence of strong ionsolvent interactions and these interactions are further strengthened at higher temperatures and higher molar mass of L-Proline in the mixtures, suggesting larger electrostriction at higher temperatures and in enhanced amount of L-Proline.

A perusal of Table 3 also reveals that the values are negative for all the solutions at all the experimental temperatures and values decrease as the experimental temperature and amount of L-Proline in the mixtures increases. Since is a measure of ion-ion interactions, the results indicate the presence of weak ion-ion interactions in the solutions at all the experimental temperatures and these interactions further decrease with a rise in temperature and increase in molar mass L-Proline in the mixtures. In other words, it may be said that the solvation of electrolyte/ions increases with the increase of L-Proline content in water. This is probably due to more violent thermal agitation at higher temperatures, resulting in diminishing the force of ion-ion interactions (ionic-dissociation) [12]. This suggests that ion-solvent interactions dominate over ion-ion interactions in all the solutions and at all experimental temperatures.

The variation of with temperature of potassium chloride, potassium bromide and potassium iodide in aqueous mixture of LProline follows the polynomial,

Over the temperature range under study where T is the temperature in K. Values of coefficients of the above equation for potassium chloride, potassium bromide and potassium iodide in aqueous mixture of L- Proline are reported in Table 4.

Table 4: Values of the coefficients of Eq. (4) for potassium chloride, potassium bromide and potassium iodide in different aqueous mixtures 0.01 M L-proline, 0.03 M L-proline, 0.05M L-proline at different temperatures.

The apparent molar expansibilities ( ) can be obtained by the following equation:

The values of for different solutions of the studied electrolytes at 298.15, 308.15 and 318.15 K are reported in Table 5. From the table it is evident that the values of for potassium chloride increases with the increase in the amount of L- Proline in the mixture. However, for potassium bromide and potassium iodide the values were found to be rather complicated to explain. During the past few years it has been emphasized by a number of workers that is not the sole criterion for determining the structure making or breaking tendency of any solute. Hepler [13] developed a technique of examining the sign of for the solute in terms of long-range structure-making and breaking capacity of the electrolytes in the mixed solvent systems. The general thermodynamic expression used is as follows

Molarity of L-Proline		(m3/mol)		(m3/mol/k)
	298.15 K	308.15 K	308.15 K
Potassium chloride
0.01	0.0595	0.47064	0.34664	-0.0124
0.03	1.3223	1.7363	0.34664	-0.0586
0.05	1.8603	0.7023	0.4556	-0.1158
Potassium bromide
0.01	1.6789	1.3690	1.0589	-0.0310
0.03	2.1379	1.5799	1.0220	-0.0558
0.05	0.3286	1.2306	2.1326	0.0902
Potassium iodide
0.01	3.5582	2.3862	1.2142	-0.1172
0.03	1.1960	1.6840	2.1720	0.0488
0.05	-0.5373	1.4146	3.3666	0.1952

Table 5: Limiting partial molar expansibilities for potassium chloride, potassium bromide and potassium iodide in different aqueous mixtures 0.01 M L-proline , 0.03 M L-proline, 0.05 M L-proline at different temperatures.

If the sign of is positive or small negative [14,15] the electrolyte is a structure maker and when the sign of is negative, it is a structure breaker. As is evident from Table 5, the electrolyte under investigation generally acts as a structure breaker.

The viscosity data of solutions for the electrolytes in 0.01 M, 0.03M, 0.05M L-Proline have been analyzed using Jones-Dole [16] equation:

Where η₀ and η are the viscosities of the solvent and solution respectively. A and B are the constants estimated by least square method and are reported in Table 6. From the table it is evident that the values of the A-coefficient are very small for all the solutions under investigation at all experimental temperatures. These results indicate the presence of weak ion-ion interactions, and these interactions further decrease with both rise of experimental temperatures and amount of L-Proline suggesting an increase in ion-solvation. Interestingly, values are found to be smallest for Potassium iodide and hence it may be concluded that solubility in aqueous L-Proline solutions is maximum for Potassium iodide and minimum for Potassium chloride.

Table 6: Values of A and B coefficients for potassium chloride, potassium bromide and potassium iodide in different aqueous mixtures 0.01M L-proline , 0.03M L-proline, 0.05M L-proline at different temperatures.

The effects of ion-solvent interactions on the solution viscosity can be inferred from the B-coefficient [17,18]. The viscosity B-coefficient is a valuable tool to provide information concerning the solvation of the solutes and their effects on the structure of the solvent. From Table 6 it is evident that the values of the B-coefficient of potassium chloride, potassium bromide and potassium iodide in the studied solvent systems are more positive than A-coefficients, thereby suggesting the presence of strong ion-solvent interactions, and these types of interactions are strengthened with a rise in both temperature and amount of L-Proline in solutions. These conclusions are in excellent agreement with those drawn from values discussed earlier. It has been reported in a number of studies [19,20] that dB/dT is a better criterion for determining the structure-making/breaking nature of any solute rather than simply the value of the B-coefficient. It is found from Table 6 that the values of the B-coefficient increase with a rise in temperature (positive dB/dT) suggesting the structure-breaking tendency of potassium chloride, potassium bromide and potassium iodide in the solvent systems.

Extensive study of potassium chloride, potassium bromide and potassium iodide in aqueous mixture of L- Proline reveals that potassium iodide is more associated in L-Proline than the other two halides. The ion-association is found minimum in the case of potassium chloride in L- Proline. The said interaction of potassium bromide arises in the intermediacy of potassium iodide and potassium chloride. The present study reveals the predominance of ion-solvent interaction over the ion-ion interaction in all the solution under investigation.

The authors are grateful to the UGC supported Major research project, Ref. No. RP/5032/FCS/2011 New Delhi for financial support in order to continue this research work.

One of the authors, Prof. M. N. Roy is thankful to University Grant Commission, New Delhi, Government of India for being awarded one time grant under Basic Scientific Research via the grant-in-Aid No. F.4-10/2010 (BSR) regarding his active service for augmenting of research facilities to facilitate further research work.