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Partial molar heat capacities and volumes of transfer of some saccharides from water to aqueous urea solutions atT = 298.15 K

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transfer of some saccharides from water to aqueous urea solutions at T = 298.15 K

P. K. Banipal," T. S. Banipal,6 J. C. Ahluwalia

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi, 110016, India

and B. S. Lark

Department of Chemistry, GuruNanakDev University, Amritsar, (Ph.), 143005, India

Apparent molar heat capacities <t>Cp and volumes <j>V of seven monosaccharides {D(—)- ribose, D(—)-arabinose, D(+)-xylose, D(+)-glucose, D(+)-mannose, D(+)-galactose, D(—)-fructose}, seven disaccharides {sucrose, D(+)-cellobiose, lactulose, D(+)-melibiose hemihydrate, D(+)-maltose monohydrate, D(+)-lactose monohydrate, D(+)-trehalose dihydrate} and one trisaccharide {D(+)-rafEnose pentahydrate} have been determined in (0.5, 1.0, 1.5, and 3.0) mol • kg"1 aqueous urea solutions at T = 298.15 K from specific heat and density measurements employing a Picker flow microcalorimeter and a vibrating-tube densimeter, respectively. By combining these data with the earlier reported partial molar heat capacities C° 2 and volumes V£ in water, the corresponding partial molar properties of transfer (C° 2 tr and V^^) from water to aqueous urea solutions at infinite dilution have been estimated. Both the C° 2 tr and F2°tr values have been found to be positive for all the sugars and to increase with increase in concentration of the cosolute (urea), suggesting that the overall structural order is enhanced in aqueous urea solutions.

This increase in structural order has been attributed to complex formation between sugars and urea molecules through hydrogen bonding and to a decreased effect of urea on water structure. The transfer parameters have been rationalized in terms of solute-cosolute interactions using a cosphere overlap hydration model. Pair, triplet and higher-order interaction coefficients have also been calculated from transfer functions and their sign and magnitude have been discussed.

KEYWORDS: saccharides; heat capacity; volume; interaction coefficients; stereochemical effects

(2)

The hydration characteristics of saccharides(1~6) and their interactions with electrolytes and nonelectrolytes(7~15) in aqueous media are of significant biological and thermodynamic importance. It has been widely reported(1622) that sugars and polyols act as efficient stabilizing agents for proteins/enzymes because of their ability to enhance the structure of water. Lee and Timasheff^17) attributed the stabilizing effect of sucrose to the positive Gibbs free energy required to form a cavity in the solvent due to an increase in the solvent cohesive force when sucrose was added to water. Bull and Breese(23) suggested that sugars enhanced the structure of water in the immediate neighbourhood of the protein.

On the other hand, urea forms nearly ideal mixtures because of possible compensating interactions with water and can exhibit a chaotropic action on ordered systems such as micelles, folded globular proteins and water soluble synthetic polymers/24'25) It has also been reported that urea enhances the solubility of the nonpolar compounds due to the weakening of hydrophobic interactions, which contribute to the stability of micelles and folded globular proteins and thus devoid water of its unique property of promoting hydrophobic interactions/26) Furthermore, it is well known(27~30) that the extent of denaturation of certain proteins by urea-type denaturants is reduced in the presence of sugars or polyhydroxy alcohols. However, the mechanism by which this renaturating effect on protein structure is induced, either by the direct binding of urea with the polyhydroxy compound/protein molecule or through the alteration of water structure, is not clearly understood. Thus the investigation of interactions between urea and saccharides having different stereochemistry will help to clarify the nature and specificity of these interactions.

In continuation of our investigations of the solution behaviour of saccharides, we report here, apparent molar heat capacities <fiCp and volumes <fiV of various mono-, di- and tri- saccharides in (0.5, 1.0, 1.5 and 3.0) mol • kg"1 aqueous urea solutions at T = 298.15 K.

By combining these data with the earlier reported1^ partial molar heat capacities C° 2 and volumes F2° in water, the corresponding partial molar properties of transfer (C° 2 tr and V^ respectively) at infinite dilution have been determined. These transfer parameters are interpreted in terms of (solute + cosolute) interactions. The effect of cosolute concentration has also been discussed. Coefficients of pair, triplet and higher-order interactions in terms of (solute + cosolute) have also been calculated from transfer functions.

2. Experimental

A picker differential flow microcalorimeter (Sodev Inc., Sherbrooke, Canada) was employed for measuring the heat capacities per unit volume of the solutions. Its oper- ating principle and procedure have been described elsewhere/31) The precision of the microcalorimeter used was ±0.5 per cent with a limit of detectability of 7 • 10~5 J K "1 g"1. A programmable circulating thermostat supplied with the in- strument was used to maintain the temperature within ±1 • 10~3 K. The current and voltage were measured with accuracies of ±1 • 10~4 A and ±1 • 10~3 V, respectively using a Systronics digital multimeter. The signals were recorded by using a Bryans-2800 potentiometric strip chart recorder. The specific heat capacity for the reference water at

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T = 298.15 K was taken(31) as 4.1796 J • K"1 • g"1. The performance of the calorimeter was checked by measuring the apparent molar heat capacities of aqueous sodium chloride solutions at T = 298.15 K. A good agreement with the literature was found.(31)

A digital vibrating-tube densimeter (model DMA 60/602, Anton Paar, Austria) was used to measure the densities of the solutions. The details of its principle and operation have been described elsewhere.(32) A Tronac PTC-40 proportional temperature controller and an MK-70 ultracryostat were used to control the temperature of the bath used. The temperature was checked by using an NBS thermometer with an accuracy of ± 0.01 K and the thermal stability of the bath was found to be better than ±1•10~3 K. The instrument was calibrated with dry air and water to yield density values accurate to within ±3 • 10~6 g • cm"3. All the measurements of the densities of the various solutions were made with reference to pure water having a density of 0.997047 g • cm"3 at T = 298.15 K.(33) The densities of aqueous sodium chloride solutions were in excellent agreement with the literature values/32' 34)

The various saccharides whose mass fraction purities are given in parentheses were purchased from Sigma Chemical Company: D(-)-ribose (0.98), D(-)-arabinose (0.98), D(+)-glucose (0.997), D(-)-fructose (0.98), sucrose (0.995), lactulose (0.98), D ( + ) - melibiose hemihydrate (0.98) and D(+)-raffinose pentahydrate (0.99). D(+)-xylose, D(+)-mannose, D(+)-galactose, D(+)-cellobiose, D(+)-maltose monohydrate, D ( + ) - lactose monohydrate and D(+)-trehalose dihydrate were obtained from the American National Institute of Standards and Technology (NIST). Their characterization has been reported elsewhere.(35~38) These sugar samples were used without further purification, however, before use they were dried over P2O5 in a desiccator. Analar grade urea obtained from BDH was used after drying for 48 h at T = 333 K. All the solutions were prepared using distilled and deionized water obtained by passing double distilled water through a Cole-Parmer ion-exchange resin mixed bed column. The water was then degassed before use. The solutions were made by weight using a Mettler balance with a resolution of 0.01 mg.

3. Results

The apparent molar heat capacity <$>CP and volume <$> V of the sugars studied were obtained from the experimentally measured specific heat capacity and density data in (0.5, 1.0, 1.5 and 3.0) mol • dm"3 aqueous urea solutions using the following relations:

<f>Cp = Mcp - {1000(C; - cp)/m] (1)

<PV = (M/p) - {1000(p - pl)/mppl} (2) where M is the molar mass of the solute, m is the molality of the solution, c° cp and po, and p are the corresponding specific heat capacities and densities of the solvent and the solution, respectively.

The values of p, cp, <fiCp and <j> V for the various sugars in the aqueous urea solutions at T = 298.15 Kas a function of molality are summarized in table 1. At infinite dilution, the partial molar heat capacities (<t>C° = C° 2) and partial molar volumes {<t>°V = F2°) were calculated from the corresponding 4>CP and <p V data by taking the average of all the data

(4)

tions of urea at T = 298.15 K m

(mol -kg^1)

0.05802 0.09617 0.11059 0.14072 0.18505 0.24352 0.25941 0.27101 0.06995 0.10319 0.13966 0.18317 0.22142 0.06233 0.11660 0.15133 0.21193 0.07369 0.10320 0.16397 0.21262 0.23912

0.06157 0.10612 0.18934 0.19993 0.26444 0.07546 0.12830 0.13247 0.15121 0.25999 0.05300 0.12187 0.15469 0.25944

P (g-cm~3)

1.007983 1.010019 1.010784 1.012380 1.014711 1.017744 1.018562 1.019150 1.015932 1.017693 1.019602 1.021875 1.023844 1.022555 1.025402 1.027214 1.030320 1.042211 1.043733 1.046843 1.049267 1.050592

1.008301 1.010769 1.015323 1.015910 1.019384 1.016386 1.019288 1.019507 1.020525 1.026410 1.022170 1.025932 1.027711 1.033270

Cp

( J - K - i - g -1) ( J - K - D(-)-Ribose 0.5 mol • kg urea solution

4.0834 4.0717 4.0675 4.0582 4.0278 4.0229 4.0199

1.0 mol • kg urea solution 4.0109

4.0019 3.9927 3.9801 3.9714

1.5 mol • kg urea solution 3.9504

3.9376 3.9296 3.9145

3.0 mol • kg urea solution 3.7843

3.7786 3.7673 3.7586 3.7543 D(—)-Arabinose 0.5 mol • kg urea solution

4.0825 4.0688 4.0441 4.0410 4.0226

1.0 mol • kg urea solution 4.0095

3.9947 3.9940 3.9893 3.9610

1.5 mol • kg urea solution 3.9530

3.9365 3.9281 3.9046

pcP

1 -mol-1)

298 299 301 300 301 300 302 320 323 328 322 329 344 348 350 345 377 375 376 377 379

300 301 303 303 305 322 320 323 326 327 351 350 345 350

<pV (cm3 -mol^1)

95.67 95.70 95.70 95.69 95.68 95.72 95.74 95.77 95.68 95.69 95.77 95.75 95.80 95.78 95.83 95.80 95.90 95.82 95.84 95.83 96.00 96.01

93.61 93.62 93.64 93.58 93.60 93.59 93.61 93.68 93.70 93.58 93.80 93.80 93.78 93.92

(5)

TABLE 1—continued m

(mol-kg"1) 0.05752 0.10458 0.15043 0.19998 0.24205

0.06089 0.10993 0.13413 0.16519 0.20696 0.21926 0.35378 0.05219 0.10050 0.14996 0.21206 0.25318 0.10297 0.11079 0.11152 0.15814 0.22970 0.06415 0.10814 0.14657 0.18790 0.25411

0.07032 0.10271 0.13799 0.15206 0.19072 0.25917 0.06701 0.15602 0.17828 0.20566

P (g-cm~3)

1.041361 1.043797 1.046154 1.048683 1.050794

1.008123 1.010727 1.012002 1.013636 1.015805 1.016461 1.023309 1.014970 1.017511 1.020111 1.023339 1.025398 1.024635 1.025054 1.025080 1.027473 1.031110 1.041666 1.043910 1.045840 1.047900 1.051135

1.009574 1.011719 1.014040 1.014964 1.017480 1.021870 1.016657 1.022497 1.023918 1.025670

Cp

( J - K - L g - 3.0 mol • kg"1

3.7876 3.7790 3.7701 3.7613 3.7540

c l) ( J - K - urea solution

D(+)-Xylose 0.5 mol • kg"1

4.0824 4.0674 4.0603 4.0504 4.0385 4.0343 3.9946 1.0 mol-kg"1

4.0159 4.0024 3.9892 3.9731 3.9623 1.5 mol-kg"1

3.9399 3.9381 3.9377 3.9263 3.9097 3.0 mol • kg"1

3.7857 3.7775 3.7706 3.7631 3.7511

urea solution

urea solution

urea solution

urea solution

D(+)-Glucose 0.5 mol • kg"1

4.0757 4.0642 4.0515 4.0459 4.0326 4.0092 1.0 mol-kg"1

4.0074 3.9778 3.9708 3.9618

urea solution

urea solution

pcP

1 -mol"1) 380 382 378 379 380

296 298 301 297 301 298 297 322 320 323 325 325 339 340 338 339 342 370 374 376 377 377

365 367 366 362 364 366 376 378 380 379

<pv

(cm3 -mol"1) 96.00 95.90 95.82 95.76 95.79

95.89 95.90 95.91 95.89 95.92 95.84 95.95 95.98 96.08 95.97 95.91 96.11 96.33 96.21 96.31 96.38 96.40 96.55 96.50 96.57 96.61 96.76

112.33 112.36 112.37 112.35 112.36 112.40 112.42 112.39 112.50 112.54

(6)

m (mol-kg"1)

0.07244 0.10044 0.16091 0.18884 0.22862 0.23128 0.07307 0.11273 0.16319 0.18896 0.23751

0.05480 0.09644 0.12070 0.16129 0.22839 0.05933 0.10237 0.15323 0.20742 0.07823 0.10855 0.17976 0.18305 0.05352 0.11757 0.12513 0.15785 0.24343

0.05958 0.07434 0.11964 0.13044 0.16151 0.17502 0.24060

1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1

P

; • cm"3) .024144 .025864 .029778 .031556 .034090 .034256 .042986 .045440 .048549 .050133 .053078

.008562 .011348 .012961 .015635 .020006 .016173 .019033 .022387 .025909 .024440 .026439 .031113 .031306 .041827 .045907 .046414 .048481 .053733

.008945 .009963 .013036 .013755 .015832 .016726 .021083

1.

(.

,5

Cp

I ' K - ' - g "

mol • kg"1

3.9437 3.9354 3.9175 3.9093 3.8972 3.8971 3.0 mol • kg"1

0.

1.

1.

,5 3.7797 3.7703 3.7584 3.7515 3.7402

c l) ( J - K "

urea solution

urea solution

D(+)-Mannose mol • k g "

4.0814 4.0664 4.0578 4.0431 4.0192 .0 mol • kg"1

,5 4.0101 3.9956 3.9790 3.9616 mol • k g "

3.9408 3.9313 3.9096 3.9084 3.0 mol • kg"1

0.,5 3.7849 3.7685 3.7668 3.7598 3.7384

urea solution

urea solution

urea solution

urea solution

D(+)-Galactose mol • kg"1

4.0793 4.0739 4.0573 4.0534 4.0427 4.0375 4.0147

urea solution

pcP

1 -mol"1) 404 406 405 405 402 405 425 430 432 428 429

365 367 367 365 363 377 378 380 381 389 390 391 390 430 425 426 433 427

359 360 360 360 363 361 362

<pv

(cm3 -mol"1)

112.59 112.60 112.66 112.64 112.65 114.41 114.57 114.55 114.49 114.47

111.87 111.88 111.87 111.89 111.90 111.97 111.96 111.89 111.90 112.21 112.14 111.88 111.99 113.17 113.20 112.98 112.98 113.28

110.70 110.70 110.77 110.80 110.84 110.74

(7)

TABLE 1—continued m

(mol-kg"1) 0.04202 0.10676 0.15371 0.22018 0.23792 0.05558 0.12378 0.13717 0.15418 0.19877 0.23100 0.05789 0.10942 0.15656 0.20675 0.23974

0.06127 0.10401 0.16252 0.20306 0.23864 0.06247 0.11493 0.16148 0.20572 0.26706 0.07920 0.13181 0.15410 0.18734 0.21417 0.24153 0.05335 0.10470 0.12297 0.15092 0.20482 0.25038

P (g-cm~3)

1.015069 1.019461 1.022584 1.026970 1.028124 1.023063 1.027601 1.028490 1.029615 1.032530 1.034619 1.042145 1.045473 1.048471 1.051623 1.053690

1.009032 1.011909 1.015793 1.018466 1.020802 1.016414 1.019918 1.022984 1.025895 1.029866 1.024606 1.028050 1.029486 1.031651 1.033427 1.035187 1.041880 1.045219 1.046389 1.048197 1.051590 1.054435

Cp

( J - K - L g - 1.0 mol-kg"1

4.0162 3.9943 3.9791 3.9575 3.9523 1.5 mol-kg"1

3.9483 3.9280 3.9239 3.9191 3.9059 3.8972 3.0 mol • kg"1

3.7836 3.7709 3.7598 3.7477 3.7401

c l) ( J - K - urea solution

urea solution

urea solution

D(—)-Fructose 0.5 mol • kg"1

4.0796 4.0646 4.0447 4.0310 4.0193 1.0 mol-kg"1

4.0095 3.9927 3.9780 3.9649 3.9454 1.5 mol-kg"1

3.9415 3.9255 3.9186 3.9091 3.9019 3.8943 3.0 mol • kg"1

3.7849 3.7722 3.7678 3.7604 3.7488 3.7377

urea solution

urea solution

urea solution

urea solution

pcP

1 -mol-1

381 380 382 381 383 395 402 401 403 402 405 426 428 431 430 431

374 376 378 378 379 385 390 391 395 392 402 401 399 401 404 405 428 429 430 426 433 431

) (cm3 -mol^1) 110.78 110.73 110.87 110.88 110.90

110.87 110.89 110.90 110.99 111.04 112.60 112.54 112.59 112.64 112.60

111.30 111.33 111.42 111.39 111.34 111.47 111.48 111.54 111.44 111.43

111.46 111.64 111.63 111.45 111.46 112.05 112.07 112.13 112.03 112.20 112.25

(8)

m (mol-kg"1)

0.06984 0.09623 0.10764 0.14762 0.24110 0.05598 0.09529 0.18814 0.25733 0.05635 0.09410 0.18372 0.22611 0.05016 0.08710 0.10746 0.14887 0.17975 0.26012

0.04744 0.09385 0.10446 0.11262 0.07521 0.07828 0.11609 0.13121 0.15111 0.21738 0.04669 0.08905 0.14331 0.24637 0.05784 0.09254 0.14807 0.19697 0.21028

P (g-cm~3)

1.013785 1.017091 1.018505 1.023425 1.034606 1.019319 1.024220 1.035497 1.043619 1.026344 1.030996 1.041766 1.046722 1.044535 1.049125 1.051418 1.056284 1.059848 1.068933

1.010955 1.016814 1.018125 1.019142 1.021720 1.022100 1.026914 1.028631 1.031034 1.038942 1.025140 1.030400 1.036998 1.049110 1.045307 1.049671 1.056227 1.061871 1.063417

Cp

( J - K - L g -

c l) ( J - K - Sucrose

0.5 mol • kg"1

4.0516 4.0331 4.0253 3.9981 3.9375 1.0 mol-kg"1

3.9923 3.9658 3.9066 3.8644 1.5 mol-kg"1

3.9290 3.9051 3.8509 3.8266 3.0 mol • kg"1

3.7700 3.7500 3.7391 3.7178 3.7012 3.6617

urea solution

urea solution

urea solution

urea solution

D(+)-Cellobiose 0.5 mol • k g -1

4.0678 4.0359 4.0287 4.0234 1.0 m o l - k g -1

3.9797 3.9529 3.9431 3.9301 3.8898 1.5 m o l - k g -1

3.9353 3.9085 3.8753 3.8154 3.0 mol • k g -1

3.7661 3.7469 3.7179 3.6928 3.6862

urea solution

urea solution

urea solution

urea solution

pcP

1 -mol-1

669 668 668 667 667 682 678 678 677 690 691 692 694 724 728 728 731 726 728

678 680 680 682 686 684 683 680 684 692 693 694 695 730 726 729 728 728

<pv

) (cm3 -mol-1)

212.11 212.12 212.16 212.13 212.15 212.33 212.39 212.36 212.36 213.04 213.13 213.11 213.10 214.32 214.32 214.33 214.38 214.36

212.00 211.91 212.03 211.99 212.46 212.49 212.40 212.49 212.43 212.99 212.88 212.86 212.90

214.03 214.10 214.13 214.00

(9)

TABLE 1—continued m

(mol-kg"1)

0.04955 0.08902 0.12674 0.14464 0.20935 0.03633 0.05876 0.15240 0.05153 0.09942 0.10364 0.15472

P (g-cm~3)

1.011347 1.016426 1.021204 1.023455 1.031396 1.016938 1.019836 1.031631 1.025891 1.031938 1.032469 1.038784

Cp

( J - K - L g -

<pcP -1) ( J - K -1 -mol-1

Lactulose 0.5 mol • kg~'

4.0664 4.0392 4.0136 4.0019 3.9610 1.0 mol • kg~'

4.0060 3.9913 3.9321 1.5 mol-kg"1

3.9336 3.9044 3.9018 3.8719

urea solution 680 680 679 680 684 urea solution

695 697 700 urea solution

720 718 717 718

<pv

) (cm3 -mol^1)

209.50 209.50 209.48 209.41 209.55 209.56 209.54 209.62 210.25 210.28 210.25 210.22 D(+)-Melibiose hemihydrate

0.05882 0.08512 0.13249 0.13722 0.06385 0.10122 0.12770 0.12914 0.05209 0.08699 0.09505 0.16012

0.05626 0.10848 0.11039 0.17513 0.20939 0.26548 0.06400 0.11573 0.13612 0.22902

1.012411 1.015724 1.021580 1.022147 1.020331 1.024971 1.028233 1.028412 1.025811 1.030129 1.031110 1.038966

1.012130 1.018675 1.018951 1.026878 1.031040 1.037546 1.020364 1.026807 1.029296 1.040362

4.0605 4.0425 4.0113 4.0082 1.0 mol-kg"1

3.9884 3.9646 3.9482 3.9471 1.5 mol-kg"1

3.9335 3.9125 3.9078 3.8705

urea solution

urea solution

D(+)-Maltose monohydrate 0.5 mol • kg"1

4.0610 4.0248 4.0236 3.9811 3.9582 3.9239 1.0 mol-kg"1

3.9873 3.9541 3.9406 3.8833

urea solution

urea solution 726 724 727 727 740 741 742 740 760 760 762 764

740 741 742 746 741 744 760 764 759 756

220.70 220.74 220.90 220.99 220.80 221.00 220.92 220.90 221.82 221.81 221.88 221.79

228.82 229.23 228.88 228.99 228.74 229.15 229.40 229.35 229.40 229.49

(10)

m (mol-kg"1)

0.06156 0.09654 0.15077 0.22690 0.07616 0.10828 0.15560 0.20718 0.25793

P (g-cm~3)

1.027020 1.031332 1.037905 1.046870 1.047736 1.051595 1.057162 1.063023 1.068757

Cp

( J - K - ' . g -1) ( J - K - 1.5 mol • kg^1 urea solution

3.9262 3.9043 3.8717 3.8279

3.0 mol • kg urea solution 3.7568

3.7398 6.7158 3.6905 3.6666

pcP

1 -mol-1) 769 769 770 771 808 806 808 809 810

<pv

(cm3 -mol^1) 230.10 230.20 230.13 230.05 230.81 230.75 230.78 231.12 230.95 D(+)-Lactose monohydrate

0.5 mol • kg"1 urea solution 0.07458

0.07755 0.10314 0.12864 0.17251 0.20363 0.23002 0.07734 0.12541 0.14742 0.15330 0.17729 0.20970 0.06064 0.09953 0.16688 0.22619 0.06867 0.11311 0.15672 0.21809 0.24352

0.05751 0.10843 0.15750 0.21362 0.24565

1.014521 1.014896 1.018123 1.021321 1.026733 1.030499 1.033644 1.022127 1.028126 1.030832 1.031542 1.034440 1.038308 1.026993 1.031844 1.040051 1.047067 1.046953 1.052361 1.057560 1.064690 1.067600

1.012362 1.018841 1.024938 1.031744 1.035530

4.0470 4.0293 4.0125 3.9835 3.9637 3.9475 1.0 mol-kg"1

3.9785 3.9481 3.9343 3.9306 3.9157 3.8970 1.5 mol-kg"1

3.9269 3.9035 3.8628 3.8298 3.0 mol • kg"1

3.7620 3.7391 3.7170 3.6881 3.6763

urea solution

urea solution

urea solution

D(+)-Trehalose dihydrate 0.5 mol • kg"1

4.0601 4.0249 3.9929 3.9567 3.9359

urea solution 753 750 752 750 751 752 760 765 764 764 763 767 772 779 774 778 825 823 820 823 823

812 814 820 818 814

227.99 228.04 228.14 228.04 227.99 228.02 228.07 228.28 228.31 228.31 228.36 228.42 228.48 228.70 228.78 228.79 228.83 229.12 229.15 229.17 229.23 229.20

245.35 245.31 245.29 245.24 245.31

(11)

TABLE 1—continued m

(mol • kg-1) 0.05208 0.10490 0.16399 0.22728 0.05844 0.10182 0.15469 0.24359 0.05538 0.10800 0.13425 0.22519

0.05185 0.10515 0.15050 0.23703 0.06700 0.10378 0.15566 0.23359 0.05513 0.09204 0.10111 0.16722 0.05514 0.09525 0.14754

1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

P

; • cm-3) .018931 .025598 .032849 .040383 .026737 .032174 .038648 .049114 .045305 .051717 .054847 .065388

.014759 .024533 .032536 .047061 .024783 .031417 .040458 .053364 .029532 .036180 .037782 .049120 .048298 .055244 .063976

Cp

( J - K - L g - 1.0 mol-kg-1

3.9950 3.9602 3.9232 3.8852 1.5 mol-kg"1

3.9281 3.9012 3.8697 3.8206 3.0 mol • k g -1

3.7687 3.7419 3.7288 3.6864

<pcP l) ( J - K -1 -mol-1) urea solution

827 827 829 830 urea solution

840 840 842 849 urea solution

890 892 892 897 D(+)-Raffinose pentahydrate 0.5 mol • k g -1

4.0470 3.9942 3.9516 3.8766 1.0 m o l - k g -1

3.9638 3.9295 3.8832 3.8189 1.5 m o l - k g -1

3.9133 3.8800 3.8720 3.8166 3.0 mol • k g -1

3.7535 3.7227 3.6847

urea solution 1350 1352 1352 1355 urea solution

1360 1362 1362 1364 urea solution

1373 1374 1373 1376 urea solution

1417 1418 1420

<pv

(cm3 -mol-1) 245.72 245.71 245.75 245.81 245.87 245.78 245.72 245.90 246.49 246.50 246.55 246.62

398.50 398.47 398.49 398.54 398.89 398.90 398.93 399.05 399.56 399.49 399.50 399.57 401.65 401.65 401.67

points where a negligible concentration dependence within the experimental uncertainty was observed. However, in cases where a concentration dependence was observed, these properties were determined by least-squares fits of the corresponding data to the following equations:

<$>c

p

= 4>ci -

<pv = <p°v-

• sytn.

(3) (4)

(12)

sugars in different concentrations of urea are presented in tables 2 and 3 along with their standard deviations. The uncertainties in (pCp and <pV vary in the range ±(1 to 3) J • K"1 • mol"1 and ±(0.02 to 0.10) cm3 • mol"1, respectively.

The partial molar heat capacities of transfer C° 2 tr and partial molar volumes of transfer

^"tr from w a t e r to aqueous urea solutions have been determined as follows:

2 tr(water -> aqueous urea) = C° 2(aqueous urea) - C° 2(water), (5) F2°tr(water -> aqueous urea) = V2°(aqueous urea) - F2°(water). (6) The transfer parameters are given in tables 2 and 3.

4. Discussion PARTIAL MOLAR HEAT CAPACITIES OF TRANSFER C° 2 tr

The C° 2 values for the sugars in aqueous urea solutions are higher than the corresponding values in water resulting in positive heat capacities of transfer (table 2). These values increase systematically with the concentration of urea in all the cases. Very few studies on partial molar heat capacities of sugars in urea solutions are available in the literature.

Jasra and Ahluwalia(11) have reported C° 2 t r values at T = 303.15 K for some saccharides such as D(+)-glucose, sucrose, D(+)-cellobiose and D(+)-maltose monohydrate from enthalpy of solution data at T = 298.15 K and T = 308.15 K and these are: (17, 10, and 42) J • K"1 • mol"1 for D(+)-glucose; (54, 88, and 100) J • K"1 • mol"1 for sucrose; (9, 94, and 85) J • K"1 • mol"1 for D(+)-maltose monohydrate and (13, 30, and 61) J • K"1 • mol"1 for D(+)-cellobiose in (2, 4, and 6) mol • kg"1 urea solutions, respectively. The present C° 2 tr value at T = 298.15 K in 3 m urea, 80 J • K"1 • mol"1, for sucrose is quite comparable while the values of (87, 112, and 63) J • K"1 • mol"1 for D ( + ) - glucose, D(+)-maltose monohydrate and D(+)-cellobiose, respectively are comparatively higher than those reported in the literature/1 X) The present values emanating from direct measurements are believed to be more accurate.

A simple examination of the data in table 2 reveals that the C° 2 values of the various sugars increase from mono- to di- to trisaccharides, however this trend is not found in the corresponding transfer values. Furthermore, among the monosaccharides it can be seen that the C° 2 tr values decrease somewhat from the aldopentoses to aldohexoses and that for a single ketohexose the decrease is quite appreciable. The dependence of the C° 2 tr

values for various sugars on the concentration of urea is depicted in figure 1. It can be seen that the C° 2 tr values for the studied monosaccharides tend to level off except for D ( + ) - mannose and D(-)-fructose where the increase is almost linear. However, in the case of di- and tri-saccharides the C° 2 tr values increase almost linearly except for D(+)-maltose monohydrate and D(+)-melibiose hemihydrate, for which the values tend to level off.

These trends indicate that the level of saturation of the interactions of these solutes with urea vary in the different cases.

Significant positive transfer values of heat capacities suggest that the overall structural order is enhanced in aqueous urea solutions. The cosphere overlap model developed

(13)

TABLE 2. Partial molar heat capacities C° 2 of some saeeharides in water and aqueous urea solutions at infinite dilution along with their corresponding transfer values Cp;2,tr a* T = 298.15 K,

m represents the molality of urea solutions

Compound Monosaccharides D(-)-Ribose D(—)-Arabinose D(+)-Xylose D(+)-Glucose D(+)-Mannose D(+)-Galactose D(—)-Fructose

Disaccharides Sucrose

D(+)-Cellobiose Lactulose D(+)-Melibiose- hemihydrate D(+)-Maltose monohydrate D(+)-Lactose monohydrate D(+)-Trehalose dihydrate

Trisaccharide D(+)-Raffinose pentahydrate

Water"

279(2) 284(3) 279(2) 342(2) 346(3) 345(3) 365(2)

648(3)

665(3) 663(3) 660(3)

694(3)

729(3) 799(3)

1337(4)

C

h

0.5 300(2) 302(2) 298(2) 365(2) 365(2) 361(2) 373(3) 26.76*

±8.41 668(3)

675(3) 51.19*

±31.63 681(4) 726(2)

740(2) 16.72*

±23.60 751(3) 813(3) 18.53*

±43.96 1349(3)

25.54*

±8.62

/ ( J - K -1-

1.0 324(3) 324(2) 323(2) 378(2) 379(3) 381(3) 385(2) 36.89*

±31.22 679(2)

683(2) 697(3) 741(4)

760(3)

764(4) 826(4) 18.95*

±8.20 1359(3)

21.30*

±10.54

mor1)

m/(mol 1.5 347(3) 349(3) 340(3) 405(3) 390(3) 401(4) 402(2)

689(3) 20.92*

±8.45 692(4) 14.63*

±4.36 718(3) 758(3) 39.24*

±22.54 770(4)

776(3) 839(3) 17.95*

±7.86 1371(4)

27.26*

±18.32

•kg"1) 3.0 377(2) 380(2) 369(2) 35.25*

±22.19 429(3) 428(3) 425(3) 26.31*

±15.50 427(2) 19.59*

±27.50 728(4)

728(3)

806(3) 16.04*

±13.92 823(2) 887(4) 41.18*

±11.76 1415(2)

32.76*

±7.36

Cp,2,tr/

0.5 21 18 19 23 19 16 8

20

10 18 66

46

22 14

12

(J-K-1

1.0 45 40 44 36 33 36 20

31

18 34 81

66

35 27

22

1 -mor1

1.5 68 65 61 63 44 56 37

41

27 55 98

76

47 40

34 3.0 98 96 90 87 82 80 62

80

63

112

94 88

78

Parentheses contain standard deviations.

Uncertainties in C° 2 tr values range from 3 J • K^1 • mol^1 to 5 J • K^1 • mol^1, estimated by taking the square root of the sum of the squares of the standard deviations in aqueous and mixed aqueous solutions.

* Values of Sc /unit.

a Reference 6.

(14)

298.15 K, m represent the molality of the urea solutions

Compound Monosaccharides D(-)-Ribose D(—)-Arabinose D(+)-Xylose D(+)-Glucose D(+)-Mannose D(+)-Galactose D(—)-Fructose

Disaccharides Sucrose

D(+)-Cellobiose Lactulose D(+)-Melibiose- hemihydrate

D(+)-Maltose monohydrate D(+)-Lactose monohydrate

Water"

95.26 (0.02) 93.43 (0.05) 95.68 (0.09) 111.91 (0.08) 111.70 (0.07) 110.29 (0.04) 111.06 (0.06) 211.92 (0.08)

211.31 (0.04) 208.65 (0.07) 214.17 (0.01)

228.14 (0.07) 227.03 (0.04)

V2

0.5 95.72 (0.03) 93.61 (0.02) 95.90 (0.03) 112.36 (0.02) 111.88 (0.01) 110.76 (0.05) 111.36 (0.05) 212.11 (0.02) 0.17*

±0.29 211.98

(0.04) 209.49 (0.04) 220.46 (0.05) 3.45*

±1.34 228.97

(0.17) 228.04 (0.05)

/(cm • mol

1.0 95.74 (0.05) 93.63 (0.05) 96.05 (0.13) 112.46 (0.06) 111.93 (0.04) 110.83 (0.06) 111.47 (0.04) 212.36 (0.02)

212.45 (0.03) 209.57 (0.03) 220.90 (0.07)

229.41 (0.05) 228.13 (0.03) 1.57*

±0.64 -1)

m/(mol 1.5 95.83 (0.04) 93.82 (0.05) 96.33 (0.07) 112.63 (0.03) 112.05 (0.13) 110.94 (0.06) 111.53 (0.09) 213.10 (0.02)

212.91 (0.05) 210.25 (0.02) 221.80 (0.03)

230.12 (0.06) 228.68 (0.03) 0.68*

±0.42

•kg"1) 3.0 95.90 (0.13) 95.85 (0.09) 96.60 (0.09) 114.50 (0.06) 113.12 (0.12) 112.59 (0.05) 112.12 (0.08) 214.34 (0.03)

214.06 (0.05)

230.88 (0.14) 229.09 (0.02) 0.55*

±0.25

2,tr

0.5

0.46 0.18 0.22 0.45 0.18 0.47 0.30

0.19

0.67 0.84 6.31

0.83

1.01 /(cm3

1.0

0.48 0.20 0.37 0.55 0.23 0.54 0.41

0.44

1.14 0.92 6.73

1.27

1.10

•mol

1.5

0.57 0.39 0.65 0.72 0.35 0.65 0.47

1.18

1.60 1.60 7.63

1.98

1.65 -1)

3.0

0.64 2.42 0.92 2.59 1.42 2.30 1.06

2.42

2.75

2.74

2.06

(15)

TABLE 3—continued

K°/(cm3 • mor1) K°tr/(cm3 • mol"1) m/(mol - k g " ' )

Compound Water" 0.5 1.0 1.5 3.0 0.5 1.0 1.5 3.0 D(+)-Trehalose 243.75 245.30 245.67 245.82 246.43 1.55 1.92 2.07 2.68 dihydrate

(0.06) (0.04) (0.02) (0.08) (0.02) 0.54* 0.81*

±0.49 ±0.29 Trisaccharide

D(+)-Raffinose 397.10 398.50 398.80 399.53 401.66 1.40 1.70 2.43 4.56 pentahydrate

(0.08) (0.02) (0.03) (0.03) (0.01) 0.97*

±0.42 Parentheses contain standard deviations.

Uncertainties in V£tr values range from (0.04 to 0.10) cm • mol , estimated by taking the square root of the sum of the squares of the standard deviations in aqueous and mixed aqueous solutions.

* Values of Sy /unit.

a Reference 6.

by Gurney(39) is invoked to explain the transfer heat capacity and volume data. The properties of the water molecules in the hydration cosphere depend on the nature of the solute species.(40'41) The region occupied by the solvent that is markedly affected by the presence of the solute molecules is termed the cosphere. According to this model, when two molecules approach each other, their hydration cospheres overlap and some of the cosphere material is displaced resulting in changes in the thermodynamic properties/42 44)

This overlap comes into play because of the following interactions between the saccharide (solute) and urea (cosolute) molecules: the interaction between the hydrophilic urea molecules and the polar/hydrophilic, hydroxy sites of the sugars; and the interaction between the hydrophilic urea molecules and the hydrophobic parts/groups of the sugars.

The first type of interaction contributes positively, whereas the second type contributes negatively to C° 2 ^ values. The significant positive C° 2 tr values observed for all the sugars suggest that the hydrophilic-hydrophilic interactions dominate over the hydrophobic-hydrophilic interactions. The increase in C° 2 tr values with increase in concentration of urea, indicates a further strengthening of the hydrophilic-hydrophilic interactions. Since urea can form hydrogen bonds easily both with proton donors and acceptors, and as saccharides contain embedded in them large numbers of potential hydrogen bonding sites, the mutual overlap of the hydration cospheres of urea and sugar molecules will lead to an increase in the magnitude of hydrogen-bonding interactions.

For hydrophobic solutes, negative heat capacities of transfer(45"52) and, positive en- thalpies and entropies of transfer(47"50'53"55) from water to aqueous urea solutions have been reported, indicating a net bond breakage and thus an increase in disorder of the solvent

(16)

1

'o

li

°

i

1

it -^o

on oU 70 60 50 40 30 20 10 0

140 120 100

on

60 40 20 0

1 1.5 2 m/(mol • kg"1)

1 1.5 2 2.5 m/(mol • kg"1)

FIGURE 1. Partial molar heat capacity transfer values C° 2 tr against the molality m of urea solutions for some saccharides at T = 298.15 K. a, 0, D(—)-ribose; •, D(+)-arabinose; A, D(+)-xylose;

O, D(+)-glucose; X, D(+)-galactose. b, •, D(+)-mannose; •, D(—)-fructose. c, •, D(+)-maltose monohydrate; •, D(+)-melibiose hemihydrate. d, •, sucrose; •, D(+)-cellobiose; A, lactulose;

x, D(+)-lactose monohydrate; *, D(+)-trehalose dihydrate; •, D(+)-raffinose pentahydrate.

system. On the other hand, the ionic solutes (e.g. alkali metal halides)(56) are accompanied by positive heat capacities of transfer and, negative enthalpies^5'51'57) and entropies of transfer from water to aqueous urea solutions which result in a decrease in the disorder of the solvent system and an increase in hydrogen bonding. Similarly, positive C° 2 tr values have also been reported(50) for hydrophilic groups such as the peptide group (-CONH-), and the peptide backbone unit (-CH2CONH-) with potential hydrogen-bonding sites, from water to aqueous urea solutions. Therefore, the significant positive C° 2 tr values observed in this work for sugars from water to aqueous urea solutions indicate a behaviour similar to ionic or hydrophilic solutes. This strengthens the view that positive values of C

!,trin-

References

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