J . Chem. Soc., Faraday Trans. I , 1982, 78, 1507-1514 Ion-Solvent Interactions in Water-rich Binary Mixtures. Viscometric Behaviour of Sodium Salt Solutions in Water + Sulpholane Mixtures at 30, 40 and 50 O C BY ANTONIO SACCO,* GIUSEPPE PETRELLA,* ANGELO DELL'ATTI* AND ANGELO DE GIGLIO" Institute of Physical Chemistry, University of Bari, Via Amendola 173, 70126 Bari, Italy Received 911, June, 198 1 The relative viscosities of NaC1, NaBr, NaI and NaClO, have been measured at 30, 40 and 50 OC in water + sulpholane mixtures, in the water-rich region. The resulting Jones-Dole B coefficients and their dependence on temperature provide useful information as regards changes in water structure when small amount of organic cosolvent is added. The results are also discussed in terms of the transition-state treatment.Numerous studies have shown that viscosity measurements are very useful in providing information regarding ion-solvent interactions and particularly as regards the modifications induced by ions on the solvent structure. The viscometric technique has generally been used in studying the behaviour of electrolytes in pure solvents,l-10 while little research has been carried out on electrolytic solutions in mixed s o l ~ e n t s . ~ l - ~ ~ We decided to study systematically the viscometric properties of a number of electrolytes dissolved in binary water + organic-solvent mixtures in the water-rich region, since we believe such investigations can provide useful information on the effects induced by the cosolvent on the water structure. In the present study viscometric measurements are given for NaCl, NaBr, NaI and NaCIO, in water + sulpholane mixtures in mole fractions of sulpholane [X(sulpholane)] equal to 0.0208, 0.0744 and 0.1586 at 30, 40 and 50 OC.EXPERIMENTAL The apparatus and the experimental technique used for viscometric and densimetric measurements have been described el~ewhere.~~ * The estimated precision of the measured viscosity data was always better than 0.05%. NaC1, NaBr, NaI were ultrapure grade (Merck). These salts, after drying, were used without further purification. NaClO, (Fisher reagent grade) was recrystallized several times from water + methanol mixtures and dried at 150 O C in a vacuum oven for 4 days. The purification of water and sulpholane has been described previou~ly.~~ RESULTS For the calculation of the B coefficients by means of the Jones-Dole equation15 [eqn qr = 1 +AC:+BC (1) (1>1 we followed the method based on the principle of orthogonal polynomials suggested by Vincent et aZ.,16 who have studied in detail the effects on the B coefficient of 15071508 I ON-S 0 LV E N T I N T E R A C T I ON S I N BINARY MIXTURES higher-order concentration terms in ' extended ' Jones-Dole equations.We note at this point that in order to achieve the highest possible degree of uniformity among the viscometric data, it is advisable to use only one method for calculating B coefficients, and we feel that the method suggested by Vincent et al. is the most suitable for this purpose. In order to determine the B coefficients we always used the theoretical value of the A coefficients obtained by means of eqn (2)17 The conductometric data and the values of the physical properties of the solvent mixtures were taken from a previous study of ours.14 Note that the viscometric calculations were carried out at the same percentages of solvent mixtures used in measuring conductances in order to be able to use the values of limiting equivalent conductances.Moreover, since it was not possible to obtain limiting conductance values of ions at temperatures other than 30 OC, we assumed that the A coefficients did not vary significantly with temperature; therefore the same A coefficients used at 30 OC were also used at 40 and 50 OC. TABLE 1 .-THEORETICAL A COEFFICIENTS IN WATER + SULPHOLANE MIXTURES AT 30 "C A/dmt mol-4 X( sulp holane) NaCl NaBr NaI NaC10, 0.0208 0.0062 0.0062 0.0063 0.0068 0.0744 0.0062 0.0062 0.0066 0.0073 0.1586 0.0064 0.0062 0.0066 0.0073 In table 1 the A values used in the calculation are reported.The experimental results for NaCl, NaBr, NaI an NaClO, in the water+sulpholane mixtures at the three temperatures are reported in the Appendix in table 6. The calculated B coefficients at 30, 40 and 50 O C are reported in table 2. On applying transition state treatment to the relative viscosity of electrolytic an equation can be obtained from which it is possible to calculate the molar free energy of activation of the solution for viscous flow: where ApY#, the molar free energy of activation of the pure solvent, can be obtained by means of eqn (4): and and are the partial molar volumes of the solvent and solute, respectively.In order to calculate the molar volume of the solvent we treated each solvent mixture as a pure material, and thus obtained by means of eqn ( 5 ) :A. SACCO, G. PETRELLA, A. DELL'ATTI AND A. DE GIGLIO 1509 The values of the partial molar volumes of the electrolyte, K O , in solvent mixtures at different temperatures were calculated by means of precise density measurements.* Moreover, by measuring the B coefficients at different temperatures the enthalpies and entropies of activation can be obtained by means of the equations The values of the activation parameters of the solveni tables 3 and 4, respectively. L (6) and solutes are reported in TABLE 2.-vISCOSITY B/dm3 m0l-l COEFFICIENTS IN WATER -t SULPHOLANE MIXTURES AT 30, 40 AND 50 OC 30 "C 40 OC 50 "C NaCl NaBr NaI NaC10, NaCl NaBr NaI NaClO, NaCl NaBr NaI NaCIO, NaCl NaBr NaI NaCIO, 0.088 & 0.001 0.058 fr 0.001 0.014 f 0.001 0.022+0.001 0.1 05 f 0.00 1 0.076 f 0.001 0.029 f 0.001 0.041 f 0.002 0.1 77 f 0.00 1 0.152 f 0.002 0.113f0.001 0.107 f 0.002 0.285 f 0.001 0.253 & 0.001 0.205 f 0.002 0.186 +_ 0.003 X(sulpho1ane) = 0" 0.099 fr 0.002 0.069 f 0.001 0.033 f 0.001 0.044 0.001 0.1 11 fO.OO1 0.094 fr 0.001 0.048 f 0.00 1 0.055 fr 0.00 1 0.167 f 0.002 0.162 & 0.002 0.I28 f 0.002 0.1 11 f 0.002 0.276 f 0.001 0.253 f 0.003 0.209 f 0.002 0.186 fr 0.002 X(sulpho1ane) = 0.0208 X(sulpho1ane) = 0.0744 X(sulpho1ane) = 0.1586 0.109 f 0.002 0.084 f 0.002 0.048 & 0.001 0.060 f 0.001 0.121 fO.001 0.107f0.002 0.060 f 0.001 0.063 f 0.001 0.162 f 0.002 0.168 fO.OO1 0.139 & 0.002 0.126 + 0.002 0.267 f 0.00 1 0.250 f 0.003 0.2 15 k 0.003 0.1 80 k 0.00 1 a From ref.(8). TABLE 3.-sOLVENT ACTIVATION PARAMETERS AH,9$13 313 ASPf &4,$03 'p;,$13 A&,%3 X(sulpho1ane) /kJ mol-l /kJ mol-l /kJ mol-1 /kJ mol-l /kJ mol-l 0" 14.78 5.95 9.04 8.83 8.66 0.0208 15.21 5.79 9.62 9.42 9.25 0.0744 15.74 5.01 10.9 1 10.73 10.59 0.1586 16.38 4.07 12.46 12.31 12.20 a From ref. (8). * The values of the salts will be reported and discussed in a later paper.1510 ION-SOLVENT INTERACTIONS IN BINARY MIXTURES TABLE 4.-sOLUTE ACTIVATION PARAMETERS AT 40 * c salt X(su1p holane) 313 AS,"# /kJ mol-l NaCl 0.0208 0.0744 0.1586 NaBr 0.0208 0.0744 0.1586 NaI 0.0208 0.0744 0.1586 NaCIO, 0.0208 0.0744 0.1586 23.8 28.4 35.3 22.6 28.0 33.3 18.3 26.2 30.8 20.3 25.6 29.6 -40.7 12.5 17.2 - 70.4 - 26.6 - 7.8 - 64.2 - 50.1 - 23.5 - 56.4 -42.3 0.3 - 16.9 40.9 52.5 1.4 25.5 - 45.9 - 23.9 7.3 - 36.1 - 16.7 -47.8 29.9 DISCUSSION The behaviour of electrolytes in water + sulpholane mixtures in water-rich regions can first be analysed by observing their B and dB/dT coefficients.In pure water it is well-known that the Na+ ion has a low B value;' moreover, we have recently showns that its differential dB/dT, while not being virtually zero as had been thought,l is only slightly influenced by a variation in temperature between 30 and 50 OC. Therefore we can state that the noteworthy positive values shown by dB/dT for NaCl, NaBr, NaI and NaClO, salts in pure water are due exclusively to the anion. For this reason all the anions that we have taken into consideration are defined as 'sts-ucture breakers' in pure water.On the other hand, the behaviour of the Na+ ion and of C1-, Br-, I- and C10, ions in pure sulpholane is completely In fact, the high value of the B coefficient found for Na+ ion, with dB/dT< 0, shows that it is strongly solvated in sulpholane and behaves as a 'structure maker'. In contrast in the case of anions, the low B coefficient values, with dB/dT z 0, show that they interact weakly with the solvent molecules. On the basis of the behaviour of the ions in water and in sulpholane, the noticeable increase in B observed for all salts as the percentage of sulpholane is increased in the mixtures (table 2) can be explained by supposing that interactions between the Na+-sulpholane molecules become more and more important.Let us now consider the dependence of B on temperature (table 5). Up to mole fraction 0.0208 all salts show a positive dB/dT value, and the same applies in pure water, although the values are smaller. When the mole fraction rises to 0.0744, while NaCl shows a small negative dB/dT value, the dB/dT values for the other salts are still positive, although smaller when compared with the values at X(sulpho1ane) = 0.0208. Lastly, at X(sulpho1ane) = 0.1586, while dB/dTfor NaCl is appreciably less than zero, NaBr and NaClO, have dB/dT z 0, and only NaI still shows a low positive value of dB/dT. This behaviour can be explained if one bears in mind that the structure- breaking ability of the anions generally increases with increasing size of the ion, and if one supposes that sulpholane acts as a breaker of water structure.Thus as the percentage of sulpholane in the solvent increases, the anions become less effective structure breakers. It can thus be understood how the dB/dT coefficients, which areA. SACCO, G. PETRELLA, A. DELL'ATTI AND A. DE GIGLIO 151 1 positive for all the salts in pure water and X(sulpho1ane) = 0.0208, change their sign at higher mole fractions of sulpholane and how this happens for the more able structure-breaking anions only in mixtures richer in sulpholane. The hypothesis that sulpholane has hydrophilic properties is also proved by the results of studies on water + sulpholane mixtures carried out using various experimental methods.18-21 The analysis of the solute activation parameters reported in table 4 shows that in the region 0 < X(sulpho1ane) < 0.0208 both AH:# and AS:# are negative for all TABLE 5.--dB/dT COEFFICIENTS IN WATER + SULPHOLANE MIXTURES IN THE RANGE 30-50 "C (dB/dT)/dm3 mol-l K-' X(sulpho1ane) NaCl NaBr NaI NaClO, 0" 0.001 05 0.001 30 0.001 70 0.001 90 0.0208 0.000 80 0.001 55 0.001 55 0.001 10 0.000 75 0.000 80 0.001 30 0.000 95 0.0744 0.1586 -0.000 90 -0.000 15 0.000 50 -0.000 30 a From ref.(8). salts; this fact indicates that, in this range, the average transition state is associated with bond making and an increase in order. On the other hand, with an increase in the percentage of sulpholane in the mixtures, the values of AH:# and AS:# are increasingly less negative, and in some cases they are both positive at X(sulpho1ane) = 0.1586.In the latter cases and with the highest concentrations of sulpholane we can suppose that the transition state for viscous flow is accompanied by the breaking and distortion of intermolecular bonds. Moreover, the smooth progression of the values of electrolyte activation parameters found in the water + sulpholane mixtures (table 4) confirms the absence of structural enhancement of the water by the sulpholane; in fact when the water structure is increased by the addition of a cosolvent, maxima or minima in the values of activation parameters of viscous flow are obtained, as can be seen for electrolytes in water + methano112 and water + acetonez2 mixtures.In order to be able to discuss more fully the behaviour of electrolytes it would undoubtedly be useful to know the ionic values of the B coefficients in the solvent mixtures; nevertheless, we feel that it is possible to conclude that the viscometric behaviour of suitably chosen electrolytes can provide information as regards the effect caused by small additions of organic solvents on the structure of water. Lastly, note that in the above discussion the solvent mixtures are treated as average pure solvents. Obviously this does not allow us to point out the possible occurrence of preferential solvation of one of the components of the mixture toward the ions.1512 ION-SOLVENT INTERACTIONS IN BINARY MIXTURES APPENDIX TABLE 6.-cONCENTRATION, C/mOl dmP3, RELATIVE DENSITY, d,., AND RELATIVE VISCOSITY, Vr, FOR NaC1, INCLUDED IN THE CALCULATION OF THE B VALUES ARE MARKED WITH AN ASTERISK.NaBr, NaI AND NaC10, IN WATER+SULPHOLANE MIXTURES AT 3, 40 AND 50 "C. MEASUREMENTS NOT C dr tlr C dr tlr c 4- tlr 30 OC 40 OC 50 OC 0.015 948 0.030 041 0.036 61 1 0.051 526 0.068 716 0.083 613 0.018 574 0.027 863 0.033 745 0.046 294 0.064 374 0.077 613 0.012 71 1 0.020 166 0.024 727 0.034 071 0.047 181 0.056 141 0.014 730 0.021 158 0.026 178 0.038 193 0.052 644 0.064 471 0.017 728 0.029 836 0.034 899 0.047 854 0.071 724 0.080 971 0.021 357 0.030 279 0.036 170 0.050 344 0.068 827 0.083 501 0.012 991 0.020 251 0.025 591 0.033 956 0.047 484 1.000 65 1.001 18 1.001 44 1.002 02 1.002 71 1.003 25 1.001 43 1.002 11 1.002 56 1.003 50 1.004 87 1.005 87 1.001 38 1.002 21 1.002 70 1.003 73 1.005 16 1.006 11 1.001 07 1.001 54 1.001 92 1.002 80 1.003 84 1.004 75 1.000 69 1.001 11 1.001 29 1.001 77 1.002 62 1.003 05 1.001 45 1.002 11 1.002 53 1.003 57 1.004 87 1.006 13 1.001 38 1.002 05 1.002 66 1.003 51 1.004 88 0.057 304 1.005 86 1.002 38 1.004 15 1.004 95 1.006 68 1.008 82 1.010 70 1.002 25 1.003 00 1.003 61 1.004 75 1.006 61 1.007 56 1.001 07 1.001 47 1.001 78 1.002 19 1.002 73 1.003 10 1.001 35 1.001 52 1.002 20 1.002 88 1.003 78 1.004 52 1.003 98 1.006 06 1.007 21 1.009 89 1.014 52 1.016 11 1.004 02 1.005 57 1.006 35 1.009 34 1.012 11 1.014 45 1.002 23 1.003 22 1.004 00 1.005 22 1.006 75 1.008 02 X(sulpho1ane) = 0.0208 0.015 879 0.029 91 1 0.036 451 0.051 302 0.068 416 0.083 253 0.0 18 494 0.027 742 0.033 598 0.046 093 0.064 093 0.077 273 0.012 656 0.020 079 0.024 619 0.033 924 0.046 975 0.055 894 0.014 667 0.021 066 0.026 065 0.038 026 0.052 414 0.064 187 NaCl 1.OOO 66 1.001 21 1.001 42 1.002 05 1.002 70 1.003 30 NaBr 1.001 40 1.002 10 1.002 53 1.003 49 1.004 83 1.005 82 NaI 1.001 40 1.002 22 1.002 70 1.003 77 1.005 15 1.006 05 NaClO, 1.001 08 1.001 55 1.001 92 1.002 77 1.003 80 1.004 68 1.002 55 1.004 32 1.005 02 1.007 05 1.009 32 1.011 14 1.002 52 1.003 67 1.004 28 1.005 55 1.007 64 1.008 95 1.001 29 1.001 83 1.002 14 1.002 89 1.003 61 1.004 15 1.001 64 1.002 18 1.002 55 1.003 54 1.004 29 1.005 35 X(sulpho1ane) = 0.0744 0.015 801 1.000 70 0.029 763 1.001 24 0.036 273 1.001 51 0.051 049 1.002 06 0.068 079 1.002 73 0.082 838 1.003 26 0.018 402 1.001 48 0.027 605 1.002 16 0.033 433 1.002 64 0.045 866 1.003 58 0.063 777 1.004 90 0.076 889 1.005 87 0.012 594 1.001 42 0.019 980 1.002 24 0.024 498 1.002 73 0.033 756 1.003 77 0.046 742 1.005 16 0.055 625 1.006 19 0.014 595 1.001 08 0.020 963 1.00V56 0.025 937 1.001 92 0.037 837 1.002 74 0.052 154 1.003 77 0.063 867 1.004 61 0.017 628 0.029 666 0.034 700 0.047 579 0.071 314 0.080 504 0.021 236 0.030 106 0.035 963 0.050 055 0.068 439 0.083 028 0.012 917 0.020 135 0.025 444 0.033 761 0.047 212 0.056 977 NaCl 1.000 75 1.001 13 1.001 30 1.001 73 1.002 62 1.002 99 NaBr 1.001 36 1.002 02 1.002 42 1.003 45 1.004 85 1.006 07 NaI 1.001 42 1.002 05 1.002 66 1.003 50 1.004 87 1.005 90 1.003 66 0.017 523 1.005 86 0.029 488 1.006 78 0.034 491 1.009 00 0.047 293 1.013 63 0.070 888 1.015 46 0.080 016 1.004 36 0.021 107 1.005 83 0.029 924 1.006 70 0.035 744 1.009 70 0.049 754 1.012 74 0.068 026 1.015 36 0.082 551 1.002 03 0.01 2 838 1.003 26 0.020 014 1.004 42 0.025 290 1.005 75 0.033 557 1.007 64 0.046 937 1.008 42 0.056 628 1.000 88 1.001 23 1.001 39 1.001 82 1.002 75 1.003 03 1.001 36 1.002 04 1.002 39 1.003 51 1.004 88 1.006 42 1.001 41 1.002 09 1.002 70 1.003 52 1.005 13 1.005 83 1.002 70 1.004 66 1.005 60 1.007 83 1.010 01 1.011 60 1.002 98 1.004 05 1.004 94 1.006 35 1.008 30 1.009 82 1.001 47 1.002 03 1.002 40 1.003 23 1.004 15 1.004 85 1.001 71 1.002 23 1.002 63 1.003 82 1.004 81 1.005 79 1.003 31 1.005 71 1.006 64 1.008 87 1.013 08 1.014 99 1.004 35 1.005 94 1.006 95 1.009 76 1.013 03 1.015 88 1.003 15 1.003 66 1.004 63 1.005 75 1.008 28 1.008 92A.SACCO, G. PETRELLA, A. DELL’ATTI A N D A. D E GIGLIO 1513 TABLE 6.-(continued) C 4 rtr c 4 rtr C 4 4% 30 OC 40 OC 50 OC 0.014 056 0.022 348 0.026 048 0.035 979 0.049 742 0.059 997 0.011 436 0.018 985 0.032 986 0.045 083 0.049 400 0.072 926 0.019 201 0.026 431 0.034 284 0.045 723 0.063 164 *0.075 746 0.019 075 0.031 242 0.035 646 0.048 005 0.065 419 0.080 557 0.016 113 0.023 018 0.030 201 0.040 035 0.055 314 0.067 617 1.000 88 1.001 38 1.001 65 1.002 22 1.003 16 1.003 90 1.000 72 1.000 97 1.001 41 1.001 68 1.001 78 1.002 55 1.000 90 1.001 25 1.001 77 1.002 54 1.003 83 1.004 57 1.001 85 1.002 84 1.003 54 1.004 62 1.006 26 1.007 67 1.000 81 1.001 23 1.001 66 1.002 25 1.003 13 1.003 91 X(sulpho1ane) = 0.0744 NaClO, 1.002 54 0.013 976 1.000 86 1.002 52 0.013 893 1.000 99 1.003 41 0.022 221 1.001 38 1.003 44 0.022 088 1.001 49 1.004 12 0.025 900 1.001 62 1.004 28 0.025 745 1.001 74 1.005 51 0.035 775 1.002 25 1.005 36 0.035 564 1.002 41 1.006 94 0.049 457 1.003 14 1.006 85 0.049 163 1.003 26 1.007 93 0.059 653 1.003 86 1.008 48 0.059 296 1.003 94 X(sulpho1ane) = 0.1586 1.004 02 1.006 37 1.010 61 1.014 42 1.015 43 1.022 32 1.005 80 1.007 81 1.009 81 1.012 75 1.017 63 1.020 10 1.005 13 1.007 79 1.008 75 1.011 51 1.014 90 1.018 29 1.004 05 1.005 78 1.007 16 1.009 09 1.01 1 87 1.014 25 0.011 361 0.018 862 0.032 767 0.044 790 0.049 078 0.072 442 - 0.026 256 0.034 057 0.045 423 0.062 748 0.075 247 0.018 951 0.031 038 0.035 413 0.047 690 0.064 982 0.080 033 0.016 006 0.022 870 0.030 001 0.039 770 0.054 956 0.067 182 NaCl 1.000 81 1.001 11 1.001 45 1.001 84 1.001 92 1.002 58 NaBr 1.001 53 1.002 05 1.002 86 1.004 14 1.004 87 NaI 1.001 96 1.002 91 1.003 62 1.004 67 1.006 34 1.007 79 NaClO, 1.000 72 1.001 30 1.001 55 1.002 16 1.003 16 1.004 00 - 1.003 74 0.01 1 283 1.000 86 1.006 06 0.018 730 1.001 08 1.010 28 0.032 544 1.001 57 1.013 74 0.044 481 1.001 88 1.015 21 0.048 739 1.001 93 1.021 50 *0.071 943 1.002 61 - - - 1.007 67 0.026 079 1.001 48 1.009 88 0.033 832 1.002 11 1.012 90 0.045 132 1.003 14 1.017 96 0.062 328 1.004 11 1.020 17 *0.074 741 1.004 82 1.004 97 0.018 823 1.002 05 1.007 91 0.030 830 1.003 07 1.008 81 0.035 174 1.003 70 1.01 1 79 0.047 370 1.004 83 1.015 05 0.064 551 1.006 55 1.018 37 0.079 492 1.007 88 1.004 21 0.015 896 1.000 55 1.005 27 0.022 710 1.OOO 98 1.006 91 0.029 797 1.001 44 1.008 81 0.039 496 1.001 95 1.01 1 88 0.054 578 1.002 98 1.014 34 0.066 711 1.003 67 1.002 80 1.003 90 1.004 45 1.006 03 1.007 61 1.009 27 1.003 60 1.005 83 1.009 81 1.013 15 1.014 54 1.020 15 - 1.007 57 1.009 41 1.012 86 1.017 00 1.019 68 1.004 99 1.007 97 1.009 0 1 1.012 24 1.015 36 1.018 62 1.003 98 1.005 16 1.006 77 1.008 32 1.011 44 1.013 97 1 2 3 4 5 6 7 8 B 10 11 M.Kaminsky, Discuss. 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