J. Phys. Chem. 1995, 99, 269-273 269

Conductometric, Viscometric, and Spectroscopic Investigations on the Solvation Phenomena of Alkali-Metal Ions and Ion Pairs in 2-Methoxyethanol

Bijan Das and Dilip K. Hazra* Department of Chemistry, North Bengal University, Darjeeling 734 430, India

Received: March 31, 1994; In Final Form: September 16, 1994@

Precise measurements on electrical conductances, relative viscosities, and laser-Raman spectra of solutions of some alkali-metal salts in 2-methoxyethanol (ME) have been reported. The conductance data were analyzed by the 1978 Fuoss conductance equation and the viscosity data by the Jones-Dole equation for associated electrolytes as well as by the transition-state treatment. The ionic contribution to the limiting equivalent conductance, viscosity B coefficient, and other thermodynamic parameters have been determined using the "reference electrolyte" Bu4NBPb. The results indicate strong association of all these salts in ME. Among the alkali-metal ions, Na+ ion is found to be most solvated in this medium. Raman spectral data indicate that Li+ and Na+ ions get attached to the ME molecules through the ethereal oxygen atom rather than through the alcoholic oxygen atom of the solvent.

Introduction

2-Methoxyethanol (ME) has drawn much attention in recent years as a solvent medium for electrochemical investigations as well as for various industrial processes.'J It has unique solvating properties associated with its "quasi-aprotic" charac- ter.3 As it is a monomethyl ether of ethylene glycol, it is very likely to show physicochemical characteristics midway between protic and aprotic solvents. Hence, it is of much interest to study the behavior of electrolytes in such a solvent medium. Conductometry and viscometry are the two important classical methods which give us valuable informations regarding ion- ion and ion-solvent interactions in solution?-6 In recent years, various spectroscopic techniques have also been increasingly used to explore the types of interactions arising in electrolytic solution^.^-'^ Here we have applied the viscometric, conduc- tometric, and Raman spectroscopic techniques to study the solvation behavior of some alkali-metal ions in 2-methoxyetha- nol. The spectroscopic data in conjunction with the conduc- tometric and viscometric data have elucidated the nature of interactions of alkali-metal ions with this solvent medium.

Experimental Section

2-Methoxyethanol (G.R.E. Merck) was distilled twice im- mediately before use and the middle fraction collected. The purified solvent had a density of 0.960 02 g ~ m - ~ , a coefficient of viscosity of 1.5414 cP, and a specific conductance of ca. 1.01 x low6 S cm-' at 25 "C. These values are in good agreement with the literature values.15 The solvent properties are recorded in Table 1.

The salts used were of Fluka purum or puriss grade. Lithium perchlorate was recrystallized three times from conductivity water and then dried under vacuum for several days. Sodium perchlorate was recrystallized several times from water f methanol mixtures and dried in vacuum. Potassium, rubidium, and cesium perchlorates were prepared by precipitation by adding sodium perchlorate to a solution of the corresponding chloride salts in anhydrous methanol and were recrystallized 8- 10 times from a mixture of water + methanol and dried under vacuum at 200 "C for several days. Lithium tetrafluoroborate

@Abstract published in Advance ACS Abstracts, November 15, 1994.

0022-365419512099-0269$09.00/0 0

TABLE 1: Physical Properties of 2-Methoxyethanol temp, "C eo, g c n i 3 E 90, cp

25 0.960 02 16.93 1.5414 16.15 1.2579 35 0.953 56

45 0.947 15 15.39 1 .woo

and lithium chloride were dried under vacuum at high tempera- tures for 48 h and were used without further purification.

The kinematic viscosities were measured at the desired temperature (accuracy f O . O 1 "C) using a suspended Ubbelohde- type viscometer.16 The densities were measured with an Ostwald-Sprengel type pycnometer of about 25 cm3 capacity. The precisions of the viscosity and density measurements were *3 x g cm-3 and 0.05%, respectively. The kinematic viscosities were converted into the absolute viscisities by multiplying the former with density.

Conductance measurements were carried out on a Pye- Unicam PW 9509 conductivity meter at a frequency of 2000 Hz using a dip-type cell of cell constant 0.75 1 cm-' and having an accuracy of &0.1%. Measurements were made in an oil bath maintained at 25 f 0.005 "C, as described Solutions were prepared by weight for the conductance runs, the molalities being converted to molarities by the use of densities. Several independent solutions were prepared and runs were performed to ensure the reproducibility of the results. Due correction was made for the specific conductance of the solvent.

Laser Raman spectroscopic measurements were made at 25 "C with a DILOR 224 spectrometer using 4880 8, excitation. The spectral slit width was kept at 4 cm-'. The laser power used was 300 mW. The spectra were recorded by the Regional Sophisticated Instrumentation Centre, Indian Institute of Tech- nology, Madras. All spectra were scanned at least twice to ensure repeatability.

Conductance measurements have been carried out with all the alkali-matal perchlorates and LiBF4. However, in the viscosity and Raman spectral measurements, KC104, RbC104, and CsC104 were omitted because of their low solubility in 2-methoxyethanol. The Raman spectrum of LiCl in this solvent has also been recorded in order to study the effect of anion.

The dielectric constants of 2-methoxyethanol at 25, 35, and 45 "C were taken from the 1iterat~re.l~

1995 American Chemical Society

210 J. Phys. Chem., Vol. 99, No. 1, 1995 Das and Hazra

TABLE 4: at 25 "C

A", KA, Aoqo, electrolyte S cm2 mol-' dmw3 mol-' S cm2 mol-' P R, 8, u

Conductance Parameters in 2-Methoxyethanol

LiC104 42.30 f 0.15 217 f 11 0.652 8.41 0.17 NaC104 37.76 f 0.14 300 f 13 0.582 8.65 0.13 KC104 42.59 f 0.24 342 f 18 0.656 8.97 0.17 RbC104 45.42 f 0.24 389 -f 18 0.700 9.12 0.16 CSC104 50.71 f 0.36 432 f 28 0.782 9.31 0.35 LiBF4 41.48 f 0.10 141 f 7 0.639 8.03 0.14

TABLE 5: Theoretical A Coefficients and Viscosity B Coefficients of Electrolytes in 2-Methoxyethanol

temp, A, E , &20Sr electrolvte O C dm3n mol-'" dm3 mol-' W mol-' LiC104

NaC104

LiBF4

Results an1

25 35 45 25 35 45 25 35 45

0.0227 - - 0.0264 - - 0.0231 - -

0.260 0.480 0.693 0.394 0.322 0.256 0.273 0.493 0.713

19.82 24.62 29.42 24.14 22.15 20.21 20.06 25.44 31.79

Discusun

The equivalent conductances (A) of electrolytes measured at the corresponding molar concentrations (c) are given in Table 2s and the relative viscosities (173 and densities (e) are collected in Table 3s of the supplementary material (see paragraph at the end of the paper).

The conductance data have been analyzed by the Fuoss conductance equation20,21 which can be expressed as

A = p[Ao( 1 + R,) + EL] (1)

p = 1 - a(l - y ) (2 )

y = 1 - K A c y y (3)

In f = ,&/2( 1 + KR) (4)

where Rx and EL are relaxation and hydrodynamic terms, respectively, and the other terms have their usual significance. The parameters A", KA, and R were obtained by solving the above equations. Initial A" values for the iteration procedure were obtained from Shedlovskyz2 extrapolation of the data.

In practice, calculations were made by finding the values of Ao and a which minimize

C? = XIAj(calc) - Aj(obsd)12/(n - 2 ) I

(7)

for a sequence of R values and then plotting (7 (%) = 100u/A" against R; the best-fit R corresponds to the minimum in the u (%) vs R curve. However, since a rough scan using unit increment of R values from 4 to 20 gave no significant minima in the u (%)-R curves, the R value was assumed to be R = u

TABLE 6: Ionic Values in 2-Methoxyethanol A+", I * O q o ,

r,, temp, S cm2 S cm2 rs, E , dm3 &zo*, R*, ions A oc mol-' mol-' P A mol-' ~ m o l - 1 A ns

Lit 0.93 25 15.79 0.243 3.37 0.181 9.41 3.06 0.89 35 - - - 0.346 12.37 - - 45 - - - 0.508 15.42 - -

35 - - - 0.188 9.90 - - 45 - - - 0.071 6.21 - -

Na' 1.17 25 11.25 0.173 4.73 0.315 13.73 3.68 1.50

- - - K' 1.49 25 16.08 0.248 3.31 - Rb' 1.64 25 18.91 0.291 2.81 - CS' 1.83 25 24.20 0.373 2.20 - - - - clod- 2.40 25 26.51 0.409 2.00 0.079 10.41 2.32 -0.01

35 - - - 0.134 12.25 - - 45 - - - 0.185 14.00 - -

35 - - - 0.147 13.07 - - 45 - - - 0.204 16.37 - -

- - -

BF4- 2.02 25 25.69 0.396 2.07 0.092 10.65 2.44 -0.32

+ d, where a is the sum of the crystallographic radii and d is given by21

d = 1.183(M/~~)"~ 8, (8)

where M is the molecular weight of the solvent and eo its density.

The values of A", KA, and R obtained by this procedure are reported in Table 4.

The relative viscosities for the unassociated electrolytes are generally analyzed by the Jones-Dole equation29 in the form

q, = q/q0 = 1 + Ac'" 4- Bc (9)

where 7;1 and 170 are the viscosities of the solution and the solvent, respectively, and c is the molar concentration.

As these electrolytes have been found to be strongly associ- ated in 2-methoxyethanol from conductivity measurements, the viscosity data were analyzed24 by eq 10 instead of eq 9:

[rr - 1 - A(ac)'"]/ac = Bi + B - 1 - a P a

Here A, Bi, and BP are the characteristic constants and a is the degree of dissociation of the ion pair. The values of a were calculated from the conductance data (for LiC1, these were obtained from our earlier work2s) using the equations as described in the literature.24 The A values were calculated theoretically from the physical parameters of the solvent and the limiting ionic equivalent conductances using Falkenhagen and Vernon equation26 and are given in Table 5. These A values have been used for the analysis of the data. In view of the weak temperature dependence of the A coefficients, the A values at 25 OC were utilized at the other temperatures. The plots of [vr - 1 - A(a~) l /~ ] / ac against (1 - a)/a were linear in all cases. The intercept at (1 - a)/a = 0 was taken as the required value of Bi. The B coefficients, i.e., Bi values, reported in Table 5 for the electrolytes were obtained from these plots using the least-squares method.

The viscosity data have also been analyzed on the basis of the transition-state treatment of the relative viscosity of elec- trolytic solutions by applying the equation27

where TI" and V2" are the partial molal volumes of solvent and solute, respectively. A p 2 " * , the contribution per mole of

Solvation Phenomena of Alkali-Metal Ions J. Phys. Chem., Vol. 99, No. 1, I995 271

I I I I I 1 I

3500 3000 2500 2000 1500 1000 500 100

Figure 1. Raman spectrum of 2-methoxyethanol.

1 I I I I I I I 3500 3000 2500 2000 1500 1000 500 100

Figure 2. Raman spectrum of LiC104 in 2-methoxyethanol.

solute to the free energy of activation for viscous flow of the solution, was determined from the above relationship.

The activation parameters for viscous flow for the electrolytes are given in Table 5.

The ionic values were calculated from the reference electro- lyte BuqNBPhl6 and are reported in Table 6.

In Figures 1-4 are reproduced the Raman spectra for the pure solvent and of LiC104, NaC104, and LiCl solutions. Some important bands are listed in Table 7.

From Table 4, we see that with the exception of the sodium salt, the limiting equivalent conductances (A") of the alkali- metal perchlorates increase as the size of the cation increases. The result shows that the conductance of Na+ ion is much lower on the basis of its dimension. It seems that, though the Na+ ion has a lower surface charge density than that of the Li+ ion, it is experiencing greater interaction between the charge on the ion and the dipoles of the adjacent solvent molecules, which leads to a reduction in its mobility. In other cases, the structure- forming effect increases with decreasing dimension and con- sequently the mobility is in the reverse order.

All these salts are found to be moderately associated (Table 4) in this medium. This is quite expected owing to the low dielectric constant of the solvent. Further, for the perchlorate salts, KA increases as the size of the cation increases, though the association constant decreases with decreasing size of the anion [K~(LiC104) > Ka(LiBF4)I. These observations can be accounted for by the assumption that the stabilizations of the cations are in the order Li+ Na+ < K+ Rb+ Cs+ and the stabilization of the anions decreases in the order BF4- >

The Walden products for alkali-metal perchlorates (Table 4) are substantially lower than those in aqueous solution^^^^^^ and show considerably less variation with the crystallographic size compared with aqueous solutions. The apparent excess of mobility in aqueous solutions has been attributed to far greater solvation in the nonaqueous solvent. It is generally accepted that larger alkali-metal ions possess an excess mobility in aqueous solution owing to their ability to break hydrogen bonds in their immediate vicinity and thereby reduce the local v i s ~ o s i t y . ~ ~ , ~ ~ Thus, the changes from Li+ to Cs+ may be

C104-.

212 J. Phys. Chem., Vol. 99, No. 1, 1995

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I

Das and Hazra

I

4000 3500 3000 2500 2000 1500 1000 SO0 100

Figure 4. Raman spectrum of LiCl in 2-methoxyethanol.

TABLE 7: Raman Frequencies in cm-' vas(C-O)cther,

species c , mol dm-3 vaS(0-H) ~aS(C--o)alcohal vA0-W vAC-0) ME 3380 (br, w) 1128 (m) 1075 (m), 1023 (m) 897 (s), 839 (s) LiC104 1.001 3428 (m) 1123 (m) 1068 (m), 1019 (w) 894 (m), 838 (s) NaC104 1.003 3461 (m) 1118 (m) 1060 (m), 1009 (m) 882 (m), 826 (s) LiCl 1.001 3377 (br, w) 1109 (m) 1055 (sh, w), 1002 (m) 876 (m), 822 (s)

br = broad, w = weak, m = medium, s = strong, sh = shoulder.

considered to have been associated with the change of ions from strongly solvated to structure-breaking.

Table 6 shows that the Stokes radii decrease with increasing size of the cations (with the exception of Na+ ion) and this is most likely due to greater ionic mobilities of these ions. The order of anionic conductances in 2-methoxyethanol is C104- > Br-, which is the same as that found in l - p r ~ p a n o l , ~ ~ 2-propan01,~' and l - b u t a n ~ l . ~ ~ The Stokes radius (rJ for BF4- is approximately equal to its crystallographic radius and, for c104-, r, is less than its crystallographic radius showing that these ions are poorly solvated in this medium. However, with the exception of Na' ion, the Stokes radii are much less than

the sum of the radii of the alkali-metal ions and the solvent molecule (TME = 3.14 indicating that these ions are only slightly solvated in this solvent.

The viscosity B coefficients shown in Table 5 are large and positive. With the rise of temperature, the B values of LiC104 and LiBF4 increase with the exception of NaC104. The changes in Apz0* with temperature follow the same pattern as the B values. Positive enthalpies of activation of NaC104 (the values are negative for the other salts) imply that attainment of the transition-state for viscous flow is accompanied by the bond- breaking and distortion of intermolecular bonds and the reverse is true for the other salts studied.

Solvation Phenomena of Alkali-Metal Ions J. Phys. Chem., Vol. 99, No. 1, 1995 213

much pronounced effect) which indicates that the bands may be due to the vibration of ion pairs (solvent separated or solvent shared) or at least that the anion has entered the inner solvation shell of the alkali cations. This view is also consistent with the moderate association constants of these salts (Table 4) and the solvation numbers of the ions (Table 6 ) in this medium.

The ionic B values (Table 6 ) increase with the rise of temperature with the exception of Na+ ion, Le., dB+ldT < 0 for Na+ ion (structure maker) and dB+/dT > 0 (structure breaker) for the other ions. The R+ values agree well with the corrected Stokes' radii obtained from conductance studies. The R+ values of Li+ and Na+ ions are found to be about 3 times higher than their corresponding crystallographic radii, indicating that they form ion pairs with solvated cations in 2-methoxy- ethanol. The C104- and BF4- ions are, however, found to be scarcely solvated in this solvent medium.

2-Methoxyethanol may be considered as a combination of an ether and an alcohal. It shows v,(O-H) in the range 3000- 4000 cm-', v,(O-H) in the range 1000-1100 cm-', vas(C- O),, and va(c-o)&ohd in the range of 1100-1150 cm-', and v,(C-O) in the region 800-950 cm-' (Figure 1).

Figures 2-4 represent the Raman spectra of the electrolytic solutions which show notable changes from that of pure 2-methoxyethanol.

In 2-methoxyethanol, hydrogen bonding appears at 3380 cm-'. After dissolution of salts, complexation may arise and changes in the spectra may appear. The probability of com- plexation of alkali-metal ions with 2-methoxyethanol molecules through the -OCH3 group makes the -OH moiety rather free and in this case, v,(O-H) and v,(O-H) bands should be shifted respectively to the higher and lower regions from 3380 cm-'. In case of linkage through the -OH group, the reverse trend should be observed.

The asymmetric C-0-C stretching and symmetric C-0-C mode of 2-methoxyethanol appear at 1128 and 897 cm-', re~pect ively.~~ The complexation of 2-methoxyethanol with the alkali-metal salts has been shown to shift the frequencies of both the symmetric and asymmetric C-0-C stretching modes of 2-methoxyethanol to lower value^.^^^^ As a result of this complexation, the -OH group of 2-methoxyethanol becomes quite free and hence the frequency of the band of normal -OH of 2-methoxyethan01(3380 cm-') is being shifted to the higher value, as observed.

Further, the deformation vibration of -CH3 in 2-methoxy- ethanol occurs at 1075 and 1023 cm-', and when -OCH3 group of 2-methoxyethanol is strongly complexed with the alkali-metal ions, these bands are probably shifted to the lower regions.

Thus, it appears that the alkali metal ions have linkages preferably with the -OCH3 group of the solvent molecule and not with the -OH group.

Again, the shifts in all cases are always found to be higher with NaCl04 than with LiC104. This indicates that Na+ ion interacts more strongly with the solvent molecules compared to the Li+ ion. This is in agreement with the conductometric data of these salts in this solvent.

In case of LiCl solution, v,(O-H) band appears at 3377 cm-'. This shift of 3380 cm-I of 2-methoxyethanol to lower frequency region may be attributed to the probability of forming hydrogen bonds of C1- ion with the solvent molecule. This type of behavior has also been observed from the Raman spectrum of LiCl in methanol.12 Thus, we see that the salts LiC104 and NaC104 shift the 3380 cm-I band toward the higher frequency region, whereas for LiCl the reverse trend is observed. This observation obviously suggests that c104- ion is more effective in breaking down the hydrogen bonds than C1- ion.

From Table 7, we see that the frequencies of all the bands under consideration are dependent both on the nature of cations and anions (the anions seem to have a greater effect but not

Acknowledgment. The authors are thankful to CSIR, New Delhi, for financial assistance.

Supplementary Material Available: Equivalent conduc- tances and relative viscosities of electrolytes in 2-methoxyetha- no1 in Tables 2 s and 3S, respectively (3 pages). Ordering information is given on any current masthead page.

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