Viscosity of NaNO3 and NaSCN in formamide solutions in the temperature range 35–80°C

00~94686i79/om1bo2m

Elecwocbmlco Arra, Vol. 24. pp. 209-212. Ltd. 1979. Printed in Great Brltsin 0 Peqamon Press

VISCOSITY SOLUTIONS

OF NaNO, AND NaSCN IN THE TEMPERATURE

amoop

IN FORMAMIDE RANGE 3540°C

MARIO DELLA MONICA,G. PETRELLA, A.SACCO and M. CASTAGNOLU Istituto di Chimica-fisica dell’llniversita’, Via Amendola 173, Bari, Italy (Receiued 11 May 1978)

Abstract -The temperature dependence of viscosity of concentrated salt solutions of NaNO, and NaSCN in formamide has been investigated in this work. The viscosity data were analyzed with a three parameters equation proposed by Angel1and describing the transport processes in glass-forming liquids. The glasstransition temperature T, of NaNO, and NaSCN depend linearly on the salt concentration. Alternatively. experimental viscosity data were treated following a model derived for the viscosity of suspensions of rigid spheres, and the salvation of the cations at differenttemperatures was evaluated.

INTRODUCTION

In recent years a systematic investigation on the physico-chemical properties of concentrate electrolyte non-aqueous solutions has been carried out in our laboratory. The viscosity of several salts in formamide has been interpreted following a model proposed by Angell[l] for aqueous systems. This model assumes that the energy of a mass transfer process is temperature dependent ; change occurring till the glasstransition temperature is reached. The results of measurements on solutions of uni-univalent electrolytes in formamide[2] showed that also nonaqueousconcentrated salt solutionscan be analysed in terms of concepts for relaxation processes in glassforming liquids. Viscosity data have also been treated by a model proposed by Vand for solutions of electrolytes whose ions have a large size compared to the dimension of the solvent molecules, thus obtaining a good agreement between the proposed equation and the experimental results. In the present work a previous investigation on the viscosity of NaNO, and NaSCN in formamide at 25°C has been extended up to 80°C temperature in order to study the influence of the temperature on the factors which control mass transfer processes.

0.002; 45 + 0.005; 60 f employed. The solutions were solvent to a given quantity prepared. All operations were filled with dry nitrogen. RESULTSAND

0.05 ; 80 * 0.1; was prepared by addition of of solution previously performed in a dry-box

DISCUSSION

Experimental viscosity data of NaNO, and NaSCN in formamide solutions at the four temperatures experienced are reported in Table 1. To interpret the experimental viscosity data two promising models can be applied. In one of these the viscosity increase found for solutions of large ions was explained with an Einstein’s proposal[4]. In the corresponding equation, firstly derived for the relative viscosity of dilute suspensions of rigid spheres, the proposed equation for concentrated water solutions was[5] log q,., = 2.5@/2.3(1

- Q@)

2.5cj2.3 log of,., = l/i’EXPERIMENTALSECTION

The formamide used (Baker Analysed Reagent Grade) was dried with molecular sieves and subsequently deionized with a mixed bed of ion-exchange resins before use. The above treatment gives a formamide with a specific conductance of (2-7) x lo-’ ohm-‘cm-‘. Sodium nitrate (Reagent Grade) was recrystallized twice from water; sodium thiocyanate (Reagent Grade) was recrystallized twice from methanol and twice from absolute ethanol. Both salts were finally dried in zu~cuoover phosphoric oxide at 110°C. The techniques used for the determination of the viscosity of the solutions have been described in a previous work[3]. A thermostat controlled at 35 f

(I)

where Q, is the volume fraction of the solute and Q is a parameter which accounts for the mutual interference b$ween the ions. Replacing in (1) th_eterm @ with the c Vterm (c = molar concentration ; V = effective rigid molar volume of the salt)and rearranging one obtains : Qc

(2)

which is the equation of a straight line in the report of 2.5c/2.3 log vrcl against the concentration c. Equation (2) has been successfully applied to some formamide solutions in the temperature range 25-8O”C[6] and the results allowed to calculated the salvodynamic dimension of different ions in this solvent. Viscosity data of NaNO, and NaSCN in formamide solutions have been treated with (2) and the results plotted in Fig. 1. On these figures some considerations can be made : (1) equation (2) appears inadequate to describe the viscosity of solutions in the low concentration region ; (2) the extrapolation to zero concentration of the function gives temperature dependent valuesof the l/ Pterm; 209

MARIO DELLA

210

MONICA, G. PETRELLA, A. SACCOAND M.

Table 1. Viscosities of NaN&

(35”) C(IIlOl/l)

solutions at various temperatures

WJ”)

CP

0.1655 O-4287 1.0574 I.5282 2.0171 2.9768 3.6818

2.7911 3.1082 4.0122 4.8324 5.8963 8.8383 11.970

0.43 19 1.0656 1.5401 2.0312 2.9981 3.7075

in formamide

cb-m)

CP

0.1668

and NaSCN

C~AGNOLCJ

2.2551 2.4951 3.1894 3.7350 4.5582 6.6085 8.7410

0.1636 0.4239 1.0457 1.5107 1.9969 2.9443 3.6428

1.7198 1.8905 2.3370 2.7640 3.2330 4.4974 5.7880

cbol/l) 0.1611 0.4174 1.0308 1.4896 1.9690 2.9065 3.5983

(f-30”)

CP 1.2700 1.3850 1.6810 1.9354 2.2407 2.9823 3.7247

NaSCN (35”)

c(mol/l)

c(mol/J)

CP

0.1713 0.4434 0.8926 1.8259 2.5494 3.7056 4.7166 5.7676

0.1699 0.4398

2.76% 3.0869 3.6430 5.4050 7.5451 13.830 26.030 54.770

(45”)

CP 2.2435 2.4800 2.8961 4.1734 5.6788 9.7970 17.460 34.060

0.8857 1.8121 2.5302 3.6758 4.6813 5.7258

(3) the slope of the straight lines are temperature

independent, but differs appreciably for the two investigated systems. As point (1) is concerned large deviations from linearity have also been observed for other s&s in the dilute region of formamide solutions[3]. On the other hand experimental viscosity data of some tetraalkylammonium salts are well interpolate by (2) in solutions IO

*

SOY

a

h

ci

.

u

u

q

A

ey

-

.-.4??

I_.-._

6-

L

I I

1 2

c

3yc I 3

m0t.i’

J-

I

‘0

0

c(mWU 0.1681 0.4353 0.8765 1.7922 2.5059 3.6402 4.6371 5.6712

WY)

CP 1.7052 1.8732 2.1644 2.9744 3.9129 6.3838 10.310 18.302

c(mol/l) 0.1654 0.4283 0.8628 1.7679 2.4733 3.5959 4.5838 5.6103

@O”)

CP 1.2620 1.37 10 1.5659 2.0697 2.6267 3.995 1 5.9730 9.7352

as dilute as 0.03 mole/l[6]. This different behaviour is understandable if one considers that while large tetraalkylammonium cations are unsolvated also at the lowest concentrations, alkaline and alkaline-earth metal cations, in dependence of their high charge density, change in size, because of different amount of solvent available for solvation processes in the various concentration regions. To explain the temperature dependence of the extrapolated 1/ Pterm, it must consider that Prepresents the volume of the ions with the associate solvent molecules ; therefore a gradual desolvation of the ions with increasing temperature appears likely. From the extrapolated l/i’ values, in the hypothesis that the volume occupied by the cations is the whole volume of the salt, it is possible to calculate the molar volumes of the salts at different temperatures. The results are reported in Table 2 as radii of a single cation. A survey of this table shows that, likewise to Nal in formamide system, one solvent molecule on average surrounds each sodium cation at room temperature. The last point (3) requires some valuations about the meaning of the Q term in (2). According to Vand[S] the viscosity of solution of uncharged spheres rises from a series of fields of velocity of deformation which develop an additional streaming of surrounding spherical particles. Considering the interactions of all the spheres present, a theoretical value of the Q term equal to 0.609 can be calculated. The results of

c Fig. I. Plots of (2) for NaNO,

m0i.i’ and NaSCN

in

systems at diRerent temperatures.

formamide

measurements reported in this work and in a previous workr71 show that: (1) in the investigated systems up to 80°C no appreciable variation of the Q term with the temperatureis observed ; (2) while NaNO, is characterized by a very small value of the Q term (0.04); NaSCN, NaClO,, NaI, NaBr, show values (0.42; 0.42; 0.56; 0.28 respectively) which in some cases moderately agree with the theoretical one. The five electrolytes

Viscosity of NaNO,

Table 2. Effective rigid molar volume of the

salt at

and NaSCN various

in formamide solutions

Table

constants T,, A and K of (3) relative to NaNO, and NaSCN in formamide systems

3. Values of

temperatures calculated by (2)

NaNO,

NaNO,

;c, (L) ;A,

25 171 4.1

35

45

60

80

156

146

135

124

3.9

3.8

3.8

3.6

NaSCN 35

211

45

60

c

-A

(mole%)

(as hi9

0.6689 1.7312 4.2604 6.1435 8.0950 11.9043 14.6996

150 152 154 156 156 159 162

8.3 x ,oo-4 1.1 Y 10-j 2.4x lo-’ 5.1 2.0 1.9 1.6

x x x x

iA)

156 3.9

140 3.8

132 3.7

122 3.6

1.08 1.06 1.0s

10-a 10-J 10-S 1O-3

1.10 1.09

247 248 262 266 284 308 319

NaSCN

80 c

(L,,

1.10 1.08

(mole%1

110 3.5

considered are characterized by a common cation and by anions which develop with the solvent molecules ion-dipole and hydrogen-bonding interactions. In this occurrence differences of the Q term from the theoretical values could be due to the presence in solution of charged particles which differently interact with the solvent, thus more affecting the structure of the solution than thenoncharged particles. The approach to the interpretation of viscosity data of concentrated solutions can be also made following an entirely different model. According to Angell[8] any supersaturated electrolyte solution passing through a glass transition, if sufficiently undercooled; consequently the temperature dependence of the mass transfer processes ofconcentrated electrolyte solutions can be described by a modified Arrenhius equation:

0.6879 I.7840 3.6027 7.4134 10.407 1 15.2298 19.5297 24.0529

151 150 155 162 162 171 175 185

2.6 5.2 7.6 1.2 1.4 1.5 3.7 4.8

x x x x x Y x x

LO-’ 10-4 lo-’ 10-s 10-S 10-s 10-S 10-S

-A @log) 1.10 1.11 1.06 1.05 1.08 1.05 1.13 1.09

244 254 250 262 288 303 342 351

(3)

where X represents the mass transfer process involved, A and K and the T, temperature are constants. The glass-transition temperature T, assumes different meanings according to the different .interpretative models employed to deduce (3) In the Gibbs-Adam entropic model[9], T,, represents the temperature where the configurational entropy of the system vanishes; so mass transfer processes require an infinite energy under this temperature. The analysis of the experimental viscosity of NaNO, and NaSCN in formamide systems has been firstly made with a report of the logarithm of the viscosity against the reciprocal of the absolute temperature; since the graphs showed a curvilinear dependence, a report against a corrected term l/IT-T,) has been subsequently made, To values being checked with an IBM 360/65 computer and a least square program. Values of the glass-transition temperature calculated in this way and corresponding standard deviations are shown in Table 3 with the A terms (expressed as logarithm) and the K terms of (3). These figures (T, and K) are also reported in graph (Fig. 2) against the salt concentration expressed in bl 24-2--G

mol

%

NaSCN

Fig. 2. Glass-transition temperatures and constants K of (3) reported against the salt concentration.

mole %. A survey of Fig. 2 shows a linear dependence, on the concentration, of the glass-transition temperature; the extrapolation to zero concentration gives a glass-transition temperature value of pure solvent of 148 K and 150 K, respectively, values that well agree

212

MARIODELU MONICA,G.

FETRELLA,

with the corresponding quantity found for Nal in formamide system[10]. This last result strengthens the hypothesis that mass transport processes in concentrated non-aqueous systems can he successfully treated in terms of equations and theories proposed for pure salts and concentrated aqueous solutions. Nevertheless more work is needed to fully understand the intimate mechanism of transport in these systems.

A. S~cco AND M.

CMTAGNOLQ

glags-transition temperature of pure solvent obtained in this work, confirms the analogous value previously found in the NaI in formamide system. REFEQENCES

1. C. A. AngeU. J. phys. Chem. lo.3988 (1966); C. A. Angel1 and E. J. Sare, ibid. 52, 1058 (1970). 2. P. Bruno and M. Della Monica, J. phys. Chem. 76,3034 (1972);

P. Bruno and M. Della

Monica.

Electrochim.

M. Della Monica,~ibid. 21, 641 (1976); M. Della Monica and S. Bnfo, ibid. 22, 1213 Acta .20, 179 (1975);

(1977). 3. P. Bruno and M. Della Monica, Electruchim. Acta lot. cit. CONCLUSIONS

The results of viscosity data, analysed with the Einstein-Vand model, allow to calculate a radius of solvated cation decreasing with temperature. In an alternative manner the viscosity data have been analysed with a model proposed for the treatment of the temperature dependence of the transport properties in fused salts and in concentrated aqueous solutions. The

in (2). 19, 289 (1969). 4. A. J&stein, Ann. Phys. (Leipzig) 5. V. Vand, J. phys. Chem. 52, 277 (1948). 6. M. Della Monica and S. Bufo, lot. cit. in (2). 7. M. Della Monica, lot. cir. in (2). II. C. A. Angel& .I. phys. Chem. 68, 218 (1964); 68,

1917

( 1964). 9. G. Adam and J. H. Gibbs,. them. Phys. 43, 139 (1965). 10. P. Bruno, G. Gatti and M. Dell.. Monica, Elecrmchim. Acto 20, 533 (1975).