Indian Journal of Chemistry VoI.38A, August 1 999, pp. 778-782

Physical properties of some molten hydrated calcium salts

S K Jain*, Sachin Prashar & Shailesh K Jain Department of Chemistry, Hindu College, University of Delhi, Delhi 1 \ 0 007; India

Received 16 November 1998; revised 5 March 1999

Density, conductivity, viscosity, refractive index and surface tension of molten hexahydrates of calcium salts of the type caX2 (X=CI, Br, I, NO), CNS) have been measured as a function of temperature. Both, the conductivity and fluidity exhibit non-Arrhenius temperature-dependence for all the systems, and have been described in terms of the equation,

based on the GOoperative rearrangement theory. The dependence of the parameters Ay, By and To.y on the proton capturing power of the anions has been discussed. The correlation observed between To (or Tg) with the strengths of the acids of the anions indicates the tendency of the anions to bind to the proton of the water molecules already associated with the cation sheath.

I n the recent past considerable i nterest has been shown in the studies of hydrated melts and solutions of electrolytes at h igher concentrations, particularly where water is j ust sufficient to fi l l the first coordinat ion sphere of the ionsl - 1 5 • In most of these stud ies, the major focus was on the effect of cations on the solution behaviour. The effect of anions on the behaviour of such systems appears to have not beeQ studied in detai l . Here, we present the results of one such study concerning molten hydrates of some calcium salts.

Materials and Methods Analytical reagent grade salt-hydrates avai lable

commercial ly were used as such in the present study. The water content of each salt-hydrate was estimated by gravimetric methods.

Densities were measured by measuring the volume of a known amount of the melt using a precalibrated densitometer. The densitometer design, its cal ibration, and the measuring technique were simi lar to those reported earlier9 . The reported results are e'stimated to he accurate to ± 0. 1 %.

V iscosities were measured by using a suspended type viscometer calibrated w ith aqueous solution of glycero l . The viscometer constants were 0.23 52 and 0.08 1 96 CPS- I ( I cP= 1 0-J Nm-2 s). I nherent accuracy of these values is estimated to be better than ± 0 .5%.

Conductivities were measured by employing the four-e lectrode cell method . The detai ls about the circu it design, and the operational usefu lness of this method have been described earl ierl6 . The cell

resistance was observed to be independent of frequency over the range 1 0- 1 600 Hz. The reported results correspond to the measurements made at 1 kHz. The expected accuracy of the results is ± 0.3%.

The refractive index measurements were made on the Abbe' type refractometer whose prisms were maintained at ca. 70°C, by c irculating water from a constant temperature bath through the prism compartments. A drop of the melt was ,directly placed onto the lower prism and the prisms were immediately c lamped into the position and the measurement was made. The measurements were continued on the descending temperature scale at intervals of :::; 1 0 deg, unti l crystal l ization occurred. The values were reproducible to ±0.0002 .

The surface . tensions were measured by the d ifferential capi l lary rise method, as described earlier. The reported values are estimated to be accurate to ±O.5%.

The density, viscosity, and the conductivity measurements were carried out s imultaneously in the same constant temperature bath (capacity 20 dm·1) and the temperatures were control led and measured to ± O. I °C.

Al l measurements reported here were begun at h igher temperatures and continued downwards. At each temperature, the constancy of the cell resistance for :::; 1 0 min was taken as the proof for the thermal equ i l ibr ium between the melt and the bath l iquid. �eproducibi l ity of results in the heating and cool ing cycles was taken as evidence that there was no apprec iable loss of water during the experiments.

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JAIN et al. : PHYSICAL PROPERTIES OF MOLTEN HYDRATED CALCIUM SALTS 779

Table I - Density/equivalent volume - temperature equations for some molten hydrated salts of calcium

System Data Temp. e {g cm-3)=a-bT{oq 1 04b \ 03SE

Ve {cmJ eguiv�lrA +BT{OC) 1 04a (40°C) Vm (40°C) points range a

(0C)

CaCI2·6.09H2O 1 0 1 3-60 1 .5390 6.55 CaBr2·6.05H20 7 26-60 1 .9625 8.49 CaI2 ·6.03H2O 5 42-6 1 2.2822 9.43 Ca(CNSh·6.03H2O I I 7-58 1 .4049 7.97 Ca(N03h ·6.04H2O 1 3 5-59 1 .6356 8.44

0.3 0.2 0.7 0.4 0.2

A 1 0 B SE

7 1 .69 3 . 1 5 0 .01 80.96 3.64 0.07 93.66 4.04 0.03 96.79 5 .70 0.03 83 .40 4.45 0.05

deg-I cmJ mol-I

4.3 1 45.90 4.4 1 64.84 4.2 1 90.55 5.7 1 98. 14 5 .2 1 70.36

Table 2 - Parameters of Eq. 3 for fluidity - temperature and conductivity - temperature data for some molten hydrated salts of calcium

System Temp. B� Temp. B/\ T � pK : In � = In A, - range In i\ = In A/\ -range (T - To,/\ ) K (0C) (T - To,, ) (0C)

In A ; B ; To� SE In A A B A TO.A SE

CaCI2·6.09H2O 1 9-60 1 .369 625. 1 1 73 0.002 1 9-54 5 .399 545.5 1 7 1 0.005 1 1 75 -6. 1 CaBr2·6.05H20 1 9-60 1 .447 646.2 . 1 58 0.0039 1 3-60 5.343 478.8 1 7 1 0.0056 1 60 -8.0 Cah·6.03H2O 42-6 1 2. 1 92 1 073 . 1 1 00 0.0057 42-6 1 5.949 82 1 .4 1 09 0.01 1 1 04 -9.0 Ca(CNSh·6.03H2O 7-58 1 .623 579.0 1 70 0.0023 7-58 4.377 330.4 1 94 0.0086 1 70 - 1 .85 Ca(N03)2·6.04H20 4-59 1 .373 5 1 4.0 1 89 0.0020 5-59 4.842 422.5 1 93 0.0065 1 90 - 1 .4

'Estimated from ref. 20; * Adrien Albert & E. P. Serjeant, The determination of ionisation constants, 2nd Edition (Chapman & Hall Ltd, London), page 97.

Results and Discussion The density, conductivity and viscosity data for the

molten hydrates of five calcium salts are presented in the form of least-squares fitted equations. The densities (p) and the equivalent volumes ( Ve) for al l the salts varied l inearly with temperature and can be satisfactori ly described by Eqs. I and 2 respectively. p (g cm-3) = a - bT (°C) . . . ( 1 )

Ve (cm) equ iv- ' ) = A + B r(°C) . . . (2)

The values of the coefficient a, b, and A, B are presented in Table 1 . The expansivities (a) expressed as

1 ( d Ve ) a = Ve dT I' for n itrate, and thiocyanate salts are appreciably higher than those for calcium hal ides. Such an observation appears to be logical in v iew of somewhat loose packing in the salts with d istorted anions l i ke CNS- and NO; . Alternatively, it may also be a reflection of the existence of the water-separated ion­pairs of the type Ca(H20)CNS+ and Ca(H20) NO� in the melts of these salts due to greater polarizabi l ity of these anions as compared to the hal ide ions, resulting in weaker eLectrostriction of the water molecules in the coordination of the cation .

The conductivities and fluid ities of the systems investigated in the present study exhibited a non­Arrhenius temperature dependence. The data were fitted i nto the equation of the type,

B In Y (A, ¢) = In A y - y . . . (3)

(T - To,y ) based on the cooperative rearrangement theory of Adam and Gibbs ' 7 . The terms Ay, By and To,y are constants characteristic of the system. The parameter To,v has been interpreted as theoretical glass transition temperature and its thermodynamic nature has been d iscussed by Adam and Gibbs 1 7, Cohen and Turnbu l l ' 8 ana Ange l l and Ra�r9. The exponential coefficient By according to Adam and Gibbs ' 7 is expressed as , B y = N A Ll � S; / R Ll Cp where NA is Avogadro's number, S :. is the m inimum configurational entropy per particle required for a mass transporting rearrangement, Ll� is a free-energy barrier opposing the rearrangements, and LlC1, is the change in heat capacity of the l iquid at the glass transition. The present data were fitted into Eq. 3 with al l three parameters as adjustable parameters. The best fit parameters are presented in Table 2. For each process, the To,y values are in reasonably good agreement between themselves. The experimental

780 INDIAN J CHEM SEC A, AUGUST 1 999

190

170

130

110

90 o 2 3

I .. 5 6 7 8 9

Fig. I - Plot of To. v (Y=A+> against pK. of the conjugate acid of the anion in the corresponding salt

Table 3 -Refractive index-temperature equatioos, molar polarization and the calculated expansivity values for molten hydrates of some calcium salts

System

CaClr6.09HzO CaBrl·6.05H2O Cal2·6.03H1O Ca(CNS)z·6.03H:zO Ca(N01)z-6.04HzO

"at 4O"C_

Data Temp. points range

eC)

5 30-70 5 30-70 3 50-70 5 30-70 5 30-70

n = p -qT (0C)

1 .4750 2.0 1 .5145 2.2 1 .591 7 2.5 1 .52 10 2.8 1 .4490 2.2

glass transition temperature (Tr:'s) for these systems were estimated from the work of Angell and Sare20

and are included in Table 2 for a facile comparison with To.y's obtained in the present study_ The relatively much lower values ofTo_y for Cah-6-0JH10 are noteworthy_ Similar behaviour is seen in the � values estimated from the work of Angell and Sare . The only explanation we can give at this stage is that this behaviour of iodide salt may be due to lower

0.5 40.48 40.43 3.7 4.3 1 .00 0.6 48.94 48.87 3.7 4.3 1 .02 0.4 63.55 63.47 3.5 4.3 0.98 1 .0 59.24 59. 1 0 4.6 5.5 1 .04 0.3" 44.93 44.83 4.3 ·5.0 1 .04

electrostatic charge concentration in this system probably due to some covalent character in Ca2f,_r interactions. Angell and Sare20 had observed that the salts of spherical or quasi-spherical singly charged anions have low Tr:'s. while those of asymmetric anions have higher T,'s roughly in the inverse proportion to the number of axes about which they can rotate without requiring adjustments in their nearest neighbour environment. Such a dependence of

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JAIN et al. : PHYSICAL PROPBRTIBS OF MOL TBN HYDRATED CALCIUM SALTS 781

Table 4 - Surface tension-temperature equations for molten hydratel of lome calcium, salts

System Data Temp. points ranse

(OC)

CaCl1·6.09H1O 3 20-60 CaBrl·6.0'H20 2 30-'0 CaI2·6.03H20 2 '0-60 Ca(CNS)2·6.03H20 3 30·70 Ca<:NO]lr6.04H;O 3 20-60

Til on configurational restrictions is in general accordance with the entropy theory of the glass transition developed Gibbs and Dimarzi02J • The T O.Y values obtained in the present study also indicate similar anion-dependence of the parameter.

Another factor which might play a role in the anion-dependence of T� . and hence T O.Y is the effectiveness of the anion with which it could hold a proton from a neighbouring H20 molecule, thereby ordering their environment. As a consequence of this ordering tendency the total configurational entropy is lowered, thereby resulting in an increase in the values of the glass-transition temperatures. Since this tendency is given by the pKa value of the respective acids, T O.Y values obtained in this study are plotted against pKa of the conjugate acid of the anion in Fig. 1 . The resulting linear plot does confirm that the parameter T O,Y is governed by the proton capturing tendency of the anion present in the salt. The Calr6.03H20 system, however, is an exception,

The refractive index was found to vary linearly with temperature. The n-T"C equations are presented in Table 3. The molar refractions at two typical temperatures, 20°C and 40°C were calculated using the Lorenz-Lorentz equation, Ru. (em3 morl) = [(� - I )I(� + 2)] Vm . . . (4) in which Vm is the molar volume. The calculated values are included in Table 3 . The molar refractions estimated in this study follow the order, ,- > CNS- > Br- > NO» cr. The electronic polarizabiJities for some of these ions in the molten state as estimated by Iwadate et a/.12 exhibit the same order, thereby suggesting that the behaviour of these anionJ in hydrated melts is similar to that observed in the molten state. The Ru. values show a slight (*' 0.2%) variation over the range 2�C. As a first approximation, if this variation of Ru. with temperature is ignored, the expaIliivities of dJeje systems may be calculated from the temperature

CI 'mN m-Il-e.' -g,'T '0C} C1 (40°C)

p' 1 02q, SE mN m�1

1 12.2 '.6 0. 1 4 1 1 0.0 1 0'.2 1 0. 1 1 0 1 .2 96.6 1 0.4 92.' 87.9 1 0.6 0.0' 83.7 96.0 1 3 .7 0. 1 3 90.'

coefficient of the refractive index and the differential forms of the Gladstone-Dale and the Lorenz-Lorentz equations, through the expressions, aOD = [I /(n - 1)]dnldT . . . (5) [ 6n ] dn au" =

(n2 _ I) (n 2 + 2) dT . . . (6)

The at..L values are invariably lower. The ratio a / £roD in all cases is nearly 1 ; a value typical of the ionic systems. This, thus, suggests another method for estimating the expansivities for the systems containing limited amount of water, through refractive index measurements.

The surface tension (a')-temperature (1) data were fitted into the �uation C1 (mN m-') -p'-q'T(OC)

The coefficients p' and q' are given in Table 4. The surface teJlJion valuel at a typical temperature, viz. , 40°C follows the order: cr > Br� > J= > NO ; > CNS-. The order appears to be consistent with the adsorption characteristics of the anion on the Jurface of the hydrated cationt I.e, t the tendency to form water separated ion-pairs of the type Caz* (Hz<»X=. Relerenea

• An"U C: A, J eleclnJChem Soc, HZ (t%�, t2Z,t 2 An",. C A. J pltyl Chem, ., (I %�, 2.n. 3 An"U C A, J pltyl Chem, 70 (1%6) 398S,. .. Brau�n J, Orr L It. Mac�d W, J chgm En8"8 DolO, Jl

( '%1) 4'5, 5 An"U C A It. Gruen f) M, JAm chgm Soc, " ( 1966) �192 .. (, BrauMkin J, Alvarez..f"une§ A It It. BflWn�n H, J pliy§

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.0 Jain S K,. J chem £118n8 Daw, U (1911) 383" • • Jain S K. J chgm £118ft8 Dow. lJ (1918) 170" 12 .lain S K,. J chem £118"8 D(JItJ, lJ (1918') 216 .. • 3 .lain S K,. J chgm £118ftI /)(JItJ, lJ ( ' 918) U6 1 4 .lain S K.. J ph}'»' Chem, 8% ( ' 918) 1272,

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