Indian Journal of Chemistry Vol. 30A. April Jl!Y I. pp. 313-321 Effect of temperature on hydrogen isotope effects in dehydration of ion-exchange resins: Alkali metal ionic (Li +, K + and Cs +) forms of Dowex-50W resins Neeta C Inarndar, S K Sarpal* & A R Gupta Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085 Received 3 October 1990: accepted 12 December 1990 Hydrogen isotope effects in dehydration of alkali metal ionic (Li +, K + and Cs +.) forms of Dowex- 50W resins (with 2, 4, 8 and 12% divinylbenzene content), having n, (number of moles of water per equivalent of resin) values of - 2 to - 27, have' been determined at 303, 318 and 333K using a Rayleigh distillation type technique. For very low n, values, the observed single stage separation factor, a oo " for all the systems is independent of temperature and has a constant value of - 1.025 ± 0.004, which is much lower than a", the single stage separation factor for pure water distillation at that temperature. In the case of Li + resins at all temperatures and K + and Cs + resins at higher temperatures only, the a ohs varies with nwas follows: as n.,-value increases, a phs increases gradually, reaches a ~aximum value, a m • x (at nmaJ which is greater than a". At still higher n, -values the a ob , decreases gradually and finally ap- proaches the a w . In these systems where a maximum is observed in the EohJ = a obs - 1]versus n, plots, the data have been analysed in terms of the three-water sub-system-model for cation hydration and the E I, E2 and E" representing the separation factors for the water molecules in the primary shell, the secondary shell and the bulk water. respectively have been obtained. Analysis of the results for different tempera- tures shows that E I and E 2, originating from electrostatic interaction, are temperature independent. The fact that E, = E" (within the experimental error), shows that the effect of cationic charge does not extend beyond the second layer of water molecules. At lower temperature (303K), E ol " versus n; plots for K + and Cs " resins do not exhibit a maximum and E ohs approaches e; at high n , values. Further, E2 (353 or 303K) < E" (303K) for these systems. These data show that water structure is preferentially formed at high n., values at the expense of hydration shells, i.e., the hydration shells are destroyed by the more stable water structure at these temperatures. For any cationic form of the resin, the sharpness in the maxi- mum of E o1 " versus n, curve 'increases with increase in temperature, implying that with increasing temper- ature the bulk water structure is destroyed more rapidly than the hydration shell water structure. These observations are in consonance with the views reported in literature (see ref. 13) on electrolyte solutions. Recent studies 1.2 on the hydrogen isotope effects in the dehydration of alkali metal ionic (Li +, Na +, K + and Cs +) forms of Dowex-50W resins at 353K have emphasized that in the absence of strong interac- tions between counterions and the ionogenic groups, resin matrix provides a medium to study specifically the cation-water interactions without being influenced by the anions unlike in an electro- lyte solution. The magnitudes of D/H isotope ef- fects in these studies have revealed that water mole- cules in the secondary hydration shell are more strongly H-bonded to the water molecules in the primary hydration shell than the water molecules in the bulk water and that the effect of the cationic charge does not extend beyond the second hydra- tion shell. The well defined characteristics of these hydration shells decrease with increase in size of the alkali metal ion, following the sequence Li + > Na + > K + > Cs +, with Li -e- exhibiting the fea- tures associated with well defined first and second hydration shells and Cs + of very distorted first and second hydration shells. Similar studies on the elec- trolyte solutions have generally been made at am- bient temperature. In order to correlate, directly, the information obtained from these studies with that from the studies on electrolyte solutions, we have' extended the earlier investigations to lower temperatures, viz. 303, 318 and 333K. Materials and Methods All chemicals used were of AR or GR (E. Merck) grade. Hydrogen form of Dowex-50W resins (of 2, 4, 8 and 12% DVB contents and of 50-100 mesh), supplied by J.T. Baker & Co, USA, were treated in the usual manner' .. H and finally converted into Li ". K + or Cs + forms (see refs 1 & 2). Characterisation of the air-dried resin (ADR) samples was done by determining their moisture contents and ion-ex- 313
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Indian Journal of ChemistryVol. 30A. April Jl!Y I. pp. 313-321
Effect of temperature on hydrogen isotope effects in dehydration ofion-exchange resins: Alkali metal ionic (Li +, K + and Cs +) forms of
Dowex-50W resins
Neeta C Inarndar, S K Sarpal* & A R GuptaChemistry Division, Bhabha Atomic Research Centre, Bombay 400 085
Received 3 October 1990: accepted 12 December 1990
Hydrogen isotope effects in dehydration of alkali metal ionic (Li + , K + and Cs +.) forms of Dowex-50W resins (with 2, 4, 8 and 12% divinylbenzene content), having n, (number of moles of water perequivalent of resin) values of - 2 to - 27, have' been determined at 303, 318 and 333K using a Rayleighdistillation type technique. For very low n, values, the observed single stage separation factor, aoo" for allthe systems is independent of temperature and has a constant value of - 1.025 ± 0.004, which is muchlower than a", the single stage separation factor for pure water distillation at that temperature. In thecase of Li + resins at all temperatures and K + and Cs + resins at higher temperatures only, the aohs varieswith nwas follows: as n., -value increases, aphs increases gradually, reaches a ~aximum value, am•x (atnmaJ which is greater than a". At still higher n, -values the aob, decreases gradually and finally ap-proaches the aw. In these systems where a maximum is observed in the EohJ = aobs - 1]versus n , plots, thedata have been analysed in terms of the three-water sub-system-model for cation hydration and the E I, E 2
and E" representing the separation factors for the water molecules in the primary shell, the secondaryshell and the bulk water. respectively have been obtained. Analysis of the results for different tempera-tures shows that E I and E 2, originating from electrostatic interaction, are temperature independent. Thefact that E, = E" (within the experimental error), shows that the effect of cationic charge does not extendbeyond the second layer of water molecules. At lower temperature (303K), Eol" versus n; plots for K +
and Cs " resins do not exhibit a maximum and Eohs approaches e; at high n , values. Further, E2 (353 or303K) < E" (303K) for these systems. These data show that water structure is preferentially formed athigh n., values at the expense of hydration shells, i.e., the hydration shells are destroyed by the morestable water structure at these temperatures. For any cationic form of the resin, the sharpness in the maxi-mum of Eo1" versus n, curve 'increases with increase in temperature, implying that with increasing temper-ature the bulk water structure is destroyed more rapidly than the hydration shell water structure. Theseobservations are in consonance with the views reported in literature (see ref. 13) on electrolyte solutions.
Recent studies 1.2 on the hydrogen isotope effects inthe dehydration of alkali metal ionic (Li+, Na +, K +
and Cs +) forms of Dowex-50W resins at 353K haveemphasized that in the absence of strong interac-tions between counterions and the ionogenicgroups, resin matrix provides a medium to studyspecifically the cation-water interactions withoutbeing influenced by the anions unlike in an electro-lyte solution. The magnitudes of D/H isotope ef-fects in these studies have revealed that water mole-cules in the secondary hydration shell are morestrongly H-bonded to the water molecules in theprimary hydration shell than the water molecules inthe bulk water and that the effect of the cationiccharge does not extend beyond the second hydra-tion shell. The well defined characteristics of thesehydration shells decrease with increase in size of thealkali metal ion, following the sequenceLi + > Na + > K + > Cs +, with Li -e- exhibiting the fea-
tures associated with well defined first and secondhydration shells and Cs + of very distorted first andsecond hydration shells. Similar studies on the elec-trolyte solutions have generally been made at am-bient temperature. In order to correlate, directly,the information obtained from these studies withthat from the studies on electrolyte solutions, wehave' extended the earlier investigations to lowertemperatures, viz. 303, 318 and 333K.
Materials and MethodsAll chemicals used were of AR or GR (E. Merck)
grade. Hydrogen form of Dowex-50W resins (of 2,4, 8 and 12% DVB contents and of 50-100 mesh),supplied by J.T. Baker & Co, USA, were treated inthe usual manner' ..H and finally converted into Li ".K + or Cs + forms (see refs 1 & 2). Characterisationof the air-dried resin (ADR) samples was done bydetermining their moisture contents and ion-ex-
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INDIAN J CHEM. SEe. A. APRIL 1991
change capacities as described earlier!". The ion-exchange capacities were computed on the basis ofoven dried resin (ODR), the moisture content ofADR being known. The maximum water uptake[obtained from moisture content of surface driedresin (SDR)], expressed in terms of nw, the numberof moles of water per equivalent of ODR and theion-exchange capacities of the various resin samplesare given in Table 1.
Resin samples of different n; values (lower thanthe maximum attainable) were prepared as de-scribed earlier+" by equilibrating isopiestically theODR with LiCI or H2S04 solution of appropriatewater activity. For larger n., values the SDR sampleswere used as such.
The equipment used for Rayleigh type distilla-tions, has been described elsewhere 1.5. Before doingthe actual distillation experiment with a resin sam-ple, its water content and the D/H composition ofthis water were determined by distillating, for - 1-2hr, the entire water from a known weight of the resinin single step under running vacuum at 383K. Theactual experiment involved multistep distillation ofwater from resin samples at the desired temperature(303,318 or 333Kbuch that - 0.05 to - 0.25 g wa-ter was collected in each of the 3-4 fractions, the lastfraction containing - 15-20% of the total water inthe resin. A reasonable rate of distillation at a parti-cular temperature was obtained by performing theexperiment at an appropriate constant lower pres-sure. The last fraction was distilled at a raised tem-perature of 383K and under running vacuum for- 1-2 hr to ensure removal of last traces of water.Total weight of water in all the fractions (water col-lected) matched with the weight loss of the flask after
Table I-Characteristics of the various samples of Dowex-50W(50-100) resins
Cross linking Exchange capacity Max water uptake(%DYB) (meq/gODRa) (n~)
Li t-forrn
2 4.90 35.0
4 4.91 22.0
8 4.87 11.5
12 5.03 9.5
K +-form
2 4.34 27.54 4.25 18.6
Cs + -forrn
2 3.03 27.1
4 3.21 17.8
(a) Oven-dried resin.(b) Number of moles of water per equivalent of ODR.
314
distillation (water expected). Pure water was alsodistilled in an identical fashion at 303, 318 and333K and the a" (single stage separation factor forwater distillation) values obtained (1.064 at 303K,1.057 at 318K and 1.044 at 333K) were in goodagreement with literature values" (1.070 at 303K.1.057 at 318K and 1.046 at 333K).
The water fractions were analysed for theirD-contents on a D/H ratio mass spectrometer mod-el-6607•
ResultsThe single stage separation factor, a, defined by
Eq. (1)
[D IH]Oiquid water/water in resin .1
a=[D/Hlwater vapour>
... (1)
is calculated using the Rayleigh formula' Eq. (2)(N/No)U1lu-I)=(WolW) ... (2)
In Eq. (2) Wand N are the weight and the D/H iso-topic composition of the last fraction and Woand Noare the corresponding values for the total water in-itially present in the resin. The Wo values, obtainedfrom 'water collected' and 'water expected' were inexcellent agreement with each other for all the ex-periments. No, the initial D/H composition, valuesobtained in the two ways, i.e. from the D/H isotopicanalysis of water distilled in single step distillationand from the multistep distitlation data by mass bal-ance (the weights and the isotopic compositions ofthe individual fractions being known), were, like-wise, in good agreement. For the computation of a,the Wo and No obtained from multistep distillationdata were used, i.e. the value of Wo used corre-sponded to 'water collected' and that of No was ob-tained from the mass balance equation. The a-values obtained from these experimental data are la-belled as aobs'
The aobs-values along with other relevant data forLi +, K + and Cs + forms of Dowex-50W resins, arelisted in Tables 2, 3 and 4, respectively. A goodmatch between the values of the 'water collected'and 'water expected' indicates that the water lossesduring the experiments were negligible.
Variation of aobs as a function of n, for Li + , K +
and Cs + ionic forms of resins, at different tempera-tures, is shown in Figs 1 to 3, as plots of Eobs[= aobs-1] versus nw' In Figs 1-3 the data for 353K(from refs 1 and 2) are also included for comparison.The general pattern of the curves is similar to thatobtained earlierl-', for Li + resins at all temperaturesand for K + resins at 333K. For K + -form at 303Kand for Cs + -forrn at 303 and 333K, there are dis-
INAMDAR et al.: EFFECT OF TEMPERATURE ON HYDROGEN ISOTOPE EFFECT
tinct differences, namely, Cobs increases steadily forthe lowest n; -values and approaches cw( = a.; - 1)without exhibiting any maximum values.
Analysis of dataThe hydration model for the alkali metal cations
in the resin phase, used earlier':", predicts that forsystems having stable primary and secondary hydra-tion shells, distinguishable from the bulk water, theCobs at very low n, values is constant and very muchlower than Cw' Further, a well defined maximum isobserved in the Cot;s versus n , plots. The Cobs at anyn;value is given in terms of individual contributionsof cI' c2 and c3 from the three-water sub-systems(primary hydration shell, secondary hydration shelland the bulk water, respectively), by Eq. (3)
... (3)
where nJ = n 1 +n2 + n3] is the total number ofmoles of water per equivalent resin; n, and n2 arethe number of moles of water in primary and secon-dary hydration shells, respectively; and n3( = n , - n, - n2) represents the bulk water. In ref. 1,fixed values of n1( =4) and n2( = 2n1, i.e. 8) wereused for Li + ions with the following boundary con-ditions:nw~44<nw ~12n,'> 12
: n, =nw,n2=0, n3=0: n. =4, n2 =n., -n1, n3 =0: n, =4,n2=8, n3=nw'"-12
As the number of water molecules in the first andsecond hydration shells vary from cation to cationand are expected to be temperature dependent, thepresent data have been analysed without assigningany value for n , and n2• For this purpose a computer
Table 2 - Hydrogen isotope effects in dehydration of Li + ionic forms of Dowex-50W resins at different temperatures
Resin nw Water (g) Initial Water in D-content aab,sample D-content last fr. last fr. (±.004)
program based on pattern search method was deve- The Eobis values for each experimental point, i, (i = Iloped in which the values of n, and n2 were not as- to k), were calculated giving a range of values for EI,sumed. If nmaxis the number of water molecules at E2• E3, n, and nma•. The error sum square defined bythe maximum in the Eobs versus n., curve, tne boun- Eq. (4)dary conditions then become: k
nw<nl : n, =nw,n2=O, n3=OS = I [Ei(obo) - E;(caI)f ... (4)
i-I
n. <n; ~nmax: n, =nl,n2 =nw-n., n3=O was minimised. The program was executed on ND
n., > nmax : n, =n.,n2=nmax-n.,n3=nw-nmaxcomputer. The optimised values of the various para-meters for different temperatures, where this model
Table 3 - Hydrogen isotope effects in dehydration of K + ionic form of Dowex-50W resins at different temperatures
Resin Water (g) Initial Water in D-content aobssample n •.. D-content last fr. last fr. (±.004)
is applicable, were obtained. The data for the var-ious alkali metal ionic resins at 353Kl,2 were alsoanalysed using the same program.
Discussion(i) Lithium ion
The optimised values of the various parameters(E'S and n's) for Li + resins at different temperaturesare given in Table 5. The values for the parametersat 353K are in good agreement with those reportedearlier 1• The primary hydration number for Li + ionin the resin (n, in Table 5) is 4.9±0.4 at 303, 318and 333K. This is in agreement with the hydrationnumber for Li + ion in aqueous electrolyte solutionwhich vary from 4 ± 1 (determined by X-ray diffrac-tion8-11) to 5.7 (from molecular dynamics (MD) si-mulation studies+) at temperature close to 303K.The hydration numbers reported in the presentstudy are for lithium ion in the resin phase wheredisturbing effect of the anions, which are invariablypresent in aqueous electrolyte solutions, are absent.This implies that the Li + -water interactions are sostrong that the effect of the anions even in moder-ately concentrated electrolyte solutions (as in X-raydiffraction or MD simulation) do not affect them.The hydration number of lithium ions in the resinobtained by this technique is 4.0 ± 0.4 at 353K asreported in Table 5 and is lower than that at 303K.Busleava and Samoilov" have also observed thathydration number of Li + decreases as temperatureincreases and have reported a value of the tempera-
nw -Fig. 2 - Variation of tobs with n., for K + form of Dowex-50Wresins. [A: 303K (~); B: 333K (0, exptl.; -, calcd. from opti-mised parameters inoTable 5) and C: 353K (ref. 2) (-, calcd .
nw --Fig. 3 - Variation of tobs with n; for Cs + form of Dowex-50Wresins. [A: 303K (~); B: 333K), 0: explt, -: calcd. from opti-mised parameters in Table 5) and C: 353K (ref. 2) (-: calcd.
from optirnised parameters in Table 5)J.
ture coefficient for hydration number of Li + as- 0.005 deg - 1. This would lead to a change in hy-dration number by about 0.3 for Li ". This is withinthe limits of accuracy of the present data ( ± 0.4). Assuch no quantitative conclusions regarding the ef-fect of temperature on hydration numbers can bedrawn. However, qualitatively one can say that hy-dration number does decrease when the tempera-
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Table 5 - Optimised parameters (s's and n's) at different temperatures for alkali metal ionic forms of Dowex-50W resins
ture increases from 303 to 353K. These featurescan also be seen in the variation of nmax with temper-ature, i.e. nmax qualitatively increases when the tem-perature decreases, e.g. from. 353 to 303K. How-ever, the well-defined maximum at 353K, indicativeof a very stable second hydration shell, gradually be-comes less sharp as temperature decreases. Besides,the number of water molecules in the second hydra-tion shell (equal to 2nl at 353K) becomes less than2n 1 as temperature decreases. Both these observ-ations suggest that the stability of second hydrationshell decreases as temperature decreases.
The e 1 represents the magnitude of H-bondingamong the water molecules within primary hydra-tion shell. Table 5 shows that it is independent oftemperature and very much lower than e; at anytemperature. This implies that water molecules inthis shell are strongly oriented under the effect ofelectric field of the cation with very little possibilityof H -bonding among themselves as in the crystal hy-drates 14. The f2 represents the strength of H-bondsformed by water molecules in the second hydrationshell with the water molecules in the primary hydra-tion shell. Data in Table 5 show that E2 is independ-ent of temperature, within the limits of experimentalerror, and has an average value of 0.096 ± 0.005.The E, is equal to fw' within experimental error, atall temperatures, i.e. it increases in the same way asdoes e;...with decreasing temperature. As fl and f2
are independent of temperature, fmax should be thesame at all temperatures. The distinctly larger valueof fmax at 303K arises because nma, is larger and thecontribution of f2n2 term to the f"b, becomes grea-ter. This supports the above conclusion that thenumber of water molecules in the first and second
31~
hydration shells of Li + ions are greater at 303K thanat 353K.
The sharpness of the maximum in the Eobs versusn; curves is governed by the difference in the Emax
and fw' As fmax is in turn determined by f2' thesharpness of the maximum is controlled by f2 - fw
at any temperature. As f2 is constant at all tempera-tures and e; increases with decrease in temperature,it follows that the maximum in the Eobs versus n.,curve becomes progressively less sharp with dec-reasing temperature, as is actually observed. For thesame reason the approach of Eobs to e; at high n;va-lues becomes progressively slower as the tempera-ture increases, e.g. at n., = 24, the differencefobs - e; at various temperatures is: 303K (0.0018),318K (0.0020), 333K (0.0065) and 353K (0.0190).It implies that the H-bonded structure of water be-come stronger with decreasing temperature but thehydration shell remains unaffected, or in otherwords, hydration shell gets more structured relativeto the bulk water as temperature increases. Thesame conclusion was reached by Samoilov and co-workers in their study on the effect of temperatureon ionic hydration in aqueous solutions.
As f2 depends upon the nature of cations, a situa-tion can arise when, for a given cation at some tem-perature, E2 may become smaller than Ew' The con-sequence of this on the nature of the Eons versus n.,curves and (cation) hydration will be discussed asand when such situations arise.
(ii) Sodium ionA sharp maximum was obtained in the Eobs versus
n, plots and the fons' for very low rr; values, wasconstant and very much lower than e; at 353K (see
INAMDAR et al.: EFFECf OF TEMPERATURE ON HYDROGEN ISOTOPE EFFECT
ref. 2). Thus, the three-water sub-system-model forionic hydration, applicable for Li + ions, can be ap-plied to this system also. Using the computer pro-gram described in an earlier section, optimised va-lues of f), n., f2' nmax, n2 and f3 were obtained andare listed in Table 5. As discussed for Li + ions, f)and f2 are independent of temperature and n) andn2 values increase slightly at 303K as compared tothose at353K. As e.> e; at 303K, one would expectthat the same model would apply to Na + resins at303K. Thus the behaviourofNa + in resin phase wouldbe very similar to that of Li + resins. No detailed studyof Na + resins at different temperatures was carriedout. All the same, fobsfor the Na + resin at 303K wasdetermined at one n., value ( = 20.3) and was foundto be 0.069. The fobsat 303K for this n., value wascomputed using Eq. (3) with f) = 3.0, f2 = 0.074,n2 = 7.0 and f3 = 0.070. Computed value of fobs(= 0.065) agrees well with the observed value withinthe limits of experimental error, confirming thatNa + resins behave like Li" resins at 303K. In parti-cular this establishes that Na + ions are able to retaintheir primary and secondary hydration shells in theresin even at 303K.
(iii) Potassium ionThe fobsversus nw plots for K + ions at 353K and
333K show a maximum and a flat portion at verylow n., values where fobs< e; and has a constant va-lue. These are the features required for the applica-tion of the three-water sub-system-model (Eq. 3).The data for K + resins at these temperatures wereanalysed using the computer program describedabove and the values of the optimised parametersare listed in Table 5.
It was pointed out while discussing K'" resin dataat 353K (ref. 2), that due to weaker electrostatic in-teraction between K + and water dipoles, even be-fore the primary hydration shell gets completed, wa-ter molecules start forming the H-bonds amongstthemselves. Thus the distinction between the firstand second hydration shells is lost. The maximum inthe fobsversus n., plots marks the completion of thiscomposite hydration shell. Therefore, n, is not theprimary hydration number but refers to the nearestneighbour water molecules which do not formH-bonds amongst themselves and n2 refers to thenumber of water molecules in the composite hydra-tion shell which forms l+bonds. So far as the f2'which is a measure of the H-bond strength formedby these n2 water molecules, is greater than fw,stable hydration shell is formed (a situation realisedat 353K and 333K). At 303K, the fobsversus n , plotfor K + resin does not show a maximum and there isno indication of a flat portion at very low n., values
indicating that the three-water sub-system-model isnot applicable. Also, at 303K, f2 < e; (303K). If thethree-water sub-system-model is applied to the dataat 303K, it is found that f2 (303K» E2 (353 or333K), as well as E2 becomes equal to e; at 303K.This clearly shows that the model breaks down atthis temperature. The fact that fw> E2 at 303K sug-gests that water structure is more stable than the hy-dration shell structure. All the same, at low n, va-lues ( < nmax at 353 or 333K) where normal waterstructure cannot be formed, a composite hydrationshell around K + in the resin will exist at 303K as inthe cases at 353 or 333K.
To ascertain whether the hydration shell structureat high n., values is retained and only the extra watermolecules [= n, - nm•x (353K)] form the normal wa-ter structure, Eobsat 303K was computed using E),
n» f2' and n2 (353K) and f3[ = e; (303K)]. The com-puted values are much lower than the observed va-lues (Table 6). It means that at n, > nmax (353K), thenumber of water molecules contributing to E"bswithindividual contribution of e; is greater thann., - nm•x (353K), i.e. the composite hydration shellhas less than nmax (353K) water molecules. This im-plies that the hydration shells which are stahle at353K and 333K, are being destroyed at 303K andnormal water structure is progressively formed attheir expense. This means that nmax value observedat 353K (the overall hydration number, nh) progres-sively decreases. To test this hypothesis, nh valuesfor n.,'> nmax (353K) were computed using Eq. (5)
fobs-n , = E I' n , + f2' n2 + E.l·(nw - n , - n I)
= Em•x 'nh + E.,·(n" - nh) ... (5)
In Eq. (5) E), n., E2 and n2 are taken from Table 5and EJ = e; (303K). These computed values for n, atnw> nm•x (353K) are given in Table 6 which clearlydemonstrates that hydration shell of K + ion is des-troyed as bulk water structure is progressivelyformed, i.e. the bulk water structure is formed at theexpense of hydration shells structure.
(iv) Caesium ionThe EObsversus n., plots for Cs ' resins at 353K
show a maximum and conform to the system wherethe three-water sub-system-model can be applied.The calculated values for n I, E I> n max- E 2 and E, arcgiven in Table 5. Though the foosversus n., plots forCs + resins at 333K do not show a clear maximum, itis observed that f2 = e; (333K), indicating that themodel could be applicable. The values of E'S and n'sat 333K for Cs + resins (Table 5) show that n J, e J, n~and f2 are the same as ohserved for 353K and E, isequal to e; (333K), confirming that the model is in-
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INDIAN 1 CHEM, SEe. A, APRIL 1991
deed applicable. However, at 303K, the £2obtainedfrom data at higher temperatures is much less thane; (303K). Also there is no maximum in £obsversusn., plot at 303K. Thus this system behaves like K +
resins at 303K and the same arguments apply. As n.,value increases beyond nm•x (obtained from data at353 and 333K), hydration shell structure is progres-sively destroyted and bulk water structure is increa-singly formed at their expense. The n, vaiues whichsatisfy £obsat higher n; (Eq. 5) are listed in Table 6,confirming the above conclusions.
(v) Comparison of results for various ionic resins atdifferent temperatures
The overall s-value at any temperature is thesmallest for Cs + system and the largest for Li + sys-tem, that for K+ falling in between the two. The £m.x,which is largely composed of £2-value is, likewise,the highest for Li + system and the lowest for Cs +
system. The magnitude of s-value reflects thestrength and/or number of H-bonds between thewater molecules. In other words the e-value is ameasure of the extent of H-bonded structureformed amongst the water molecules, more thestructuring the greater is the e-value and vice-versa.The ~£( = £max- £w),which is a measure of the en-hanced structuring of water in the hydration shellsas compared to the bulk water, follows the sequ-ence: ~£(Li')> ~£(K')> ~£(c") for all temperatures.
The influence of temperature on the degree ofstructuring of water molecules in the hydrationshells around any alkali metal ion and the bulk watercan, similarly, be expressed in terms of the ~£, and itis observed that the relative structuring of water-inthe hydration shells. (as compared to the bulk water)is larger at higher temperature than at the lower, i.e.
Table 6 - Variation in overall hydration numbers (nh) of K + andCs ' ions in Dowex-50W resins a\3.o.3K at high n; values
for any metal ionic system:Af(353K'> Af(333K» Af(318K}> Af(303K)' This obser-vationis in consonance with the conclusions of Bus-leava and Sarnoilov'? that with the increase in tem-perature the bulk water structure is destroyed morerapidly than the hydration shelled water structure.
ConclusionHydrogen isotope effects in liquid-vapour equi-
librium in any water system, which area measure oftheir H-bonded structure, have been effectivelyused to get an insight of the delicate balance be-tween alkali metal cation-water interactions (ion hy-dration) and water-water interactions (H-bondedwater structure). Ion-water interactions being elec-trostatic in character are insensitive to temperaturechanges (separation factors £1 and £2 for primaryand secondary hydration shell water molecules areindependent of temperature, within the experimen-tal errors) and depend primarily on the nature of thecation. However, the bulk water structure is verysensitive to temperature (e; decreases with increas-ing temperature). At high ionic concentrations (lown., values) and at all temperatures where normal wa-ter structure is not present, all cations form their hy-dration shells in the resin phase where the anioniceffects, which destroy the hydration shells at highconcentrations in aqueous electrolyte solutions, areabsent. Small cations, like Li + or Na ", interact verystrongly with water molecules and their hydrationshells have stronger H-bonded structure than liquidwater in the temperature range 303K to 353K(£2> e; in this temperature range). Thus they retaintheir hydration shells at all temperatures even at lowionieeoncentrations, i.e. at high n , values (maximaare observed in the £obsvs n., curves). K+ and Cs +
ions are able to form only distorted hydration shellswhere the distinction between the primary and thesecondary shells is lost due to weak electrostatic in-teractions between these cations and water mole-cules. For these cations £2> e; at 353 and 333K but< e; at 303K. Thus these cations retain.hydrationshells at low ionic concentrations or at high n; va-lues (where bulk water structure can he formed) on-ly at 353 and 333K. temperatures. At 303K, for K +
and Cs + at low ionic coneentration (high rr; values),water structure is preferentially formed at the ex-pense of hydration shells, i.e. their hydration shellsare progressively destroyed. In general, hydrationshells get more structured relative to the bulk waterstructure as temperature increases because waterstructure is more rapidly destroyed with increasingtemperature whereas hydration shells are relativelyinsensitive to temperature. Qualitatively, these stud-ies support the observation that primary or overall
INAMDAR et at.: EFFECT OF TEMPERATURE ON HYDROGEN ISOTOPE EFFECT
hydration number of cations decreases with increas-ing temperature.
AcknowledgementThe authors wish to express their sincere thanks
to Dr J P Mittal, for his interest in these investig-ations.
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