NOTES
818
viously been employed from the above point of view, and, in fact, rarely a t alL4 I n Table I we have listed observed values of the viscosity of water from Dorsey’s selection.6 The uncertainties of values between 40 and 100” are thought6 to be 0.5 t o 1%. Below 40” the uncertainty is a few tenths of a per cent. We have shown in Table I the possibility of fitting the observations with equation 2, using n = 5 (found by trial and error), A = 5.109 X 10-17 and B = 3337.7. Calculated viscosities are correct from less than 40” to over 175”. Alternatively one could use n = 10 to improve the calculations from 0 t o 100°, destroying, however, the fit above 100”. We prefer the equation with n = 5 because it is suitable for a wider temperature range. This rather arbitrary criterion is justified below for the case of water. Data6and calculations for mercury are presented in a similar way in Table I, where we have used n = l/4, A = 0.9827 and B = 393.1. It is seen that the three constants are quite adequate over the available temperature range. Unfortunately, measurements have not been extended to the freezing point of mercury.
Vol. 60
are too low, much as in the case of water (Table I). In interpreting the above results it is convenient to use the formulation of E ~ r i n g . We ~ have B = A H * / R and AT” =
-* WaX
e-AS*/R
where AH* is the heat of act,ivation for flow, A S * is the entropy of activation, A is the distance between equilibrium positions in the direction of motion, and the other thvee A’s are intermolecular distances in mutually perpendicular directions. It will be expected that the factor containing the A’s will change with temperature in approximately the same way as the reciprocal of molecular volume. that is, the factor should decrease by 9% for wat8cr and 6.5% for mercury for the temperature ranges of Table I. Using molecular volumes and the values of AT” already quoted, entropies of activation for water and mercury are found to be positive and negative, respectively. I n an entirely similar way, we conclude that A#* is small and positive for methanol and small and negative for heptane. Only in the case of water does the entropy change strikingly in the temperature range considered. . These Tesults are not entirely unexpected. For TABLE I water and methanol, the formation of the activated state presumably involves, primarily, the breaking COMPARISON OF OBSERVEDVISCOSITIES (MILLIPOISES) WITH of a number of hydrogen bonds. I n contrast, for THOSE CALCULATED USINGEQUATION 2 the other two liquids, the mechanisms of flow are Water Mercury 1, c. Oobsd Ooslcd f, c. nobsd Ocalcd evidently cooperative ones. For example, one 10 13.10 12.20 -20 18.55 18.51 may note the possibility of viscous flow by the 20 10.05 9.71 0 16.85 16.84 rotation of double molecules. lo 30 8.00 7.89 20 15.54 15.54 The inadequacy of calculated values of the vis40 6.54 6.53 40 14.50 14.50 cosities near the freezing points may be attributed 50 5.49 5.49 60 13.67 13.66 to significant changes in heats of activation (and in 60 4.70 4.69 80 12.98 12.96 the temperature dependencies of entropies) which, 80 3.57 3.56 100 12.40 12.38 in turn, arise from major changes in the liquid 100 2.84 2.83 150 11.30 11.28 structures. A rather definite illustration can be 120 2.32 2.33 200 10.52 10.52 given according to the successful liquid water model 140 1.96 1.98 250 9.95 9.96 of Bernal and Fowler,” who concluded that, in the 160 1.74 1.73 300 9.50 9.55 approximate range 5 to 200°, a quartz-like struc175 1.58 1.58 340 9.21 9.28 ture predominates which, however, passes smoothly 200 1.36 1.40 into a characteristic low temperature form, as well a high temperature form. As we have pointed as Of other substances, for which accuracies and/or temperature ranges are much lower, we may men- out, equation 2 with n = 5 gives an adequate reption, with less certainty: heptane,’ for which we resentation of the viscosity of water over the range find, in the range -62 to +goo, n = 1, A = 1.850 35” to more than 175”, which is strikingly central X lo-* and B = 1266.7; and methanol* for which, to the Bernal-Fowler region. Evidently A H * and AS* are very sensitive t o the appearance of in the range -73 to +60°, n = 0, A = 8.00 X and B = 1262.0. Observed viscosities may be both the low and the high temperature forms. I am indebted to my wife, Ruth Craig Innes, for calculated (millipoises) to within about 1% over the indicated temperature ranges. Within 25 to assistance with the calculations. (9) 8. Glasstone, K. J. Laidler and H. Eyring, “The Theory of Rate 30” of the melting points, the calculated viscosities O
(4) The only references that have been found are: J. 8. Dunn, Trans. Faraday SOC..2 2 , 401 (1926); F. Hovorka, H. P. Lankelma and 6 . C. Stanford, J . A m . Chem. SOC.,60, 820 (1938); H. G. de Carvalho, Anais. assoc. quim. BrasiE, 2, 21 (1943). (5) E. N. Dorsey, “Properties of Ordinary Water Substance,” Reinhold Publ. Corp., New York, N. Y . , 1940, pp. 183-185. (6) S. Erk, Z. Physik. 47, 886 (1928). (7) J. F.Johnson and R. L. LeTourneau, J . A m . Chem. S o c . , 7S, 1743 (1953), kinematic viscosities below O n ; G. Egloff, “Physical Constants of Hydrocarbons,” Vol. I, Reinhold Publ. Corp., New York, N. Y. 1939, p. 41, densities below 0 ” ; “International Critical Tables,” Vol. 7 , McGraw-HillBook C o . . Inc., New Y o r k . N . Y., 1929. p. 219, viscosities above 0’. (8) “Handbook of Chemistry and Physics,” 34th Ed., Chemical Rubber Publishing Co., Cloveland, Ohio, 1952, p. 188G; J. R . Partington, “An Advanced Treatise on Physical Chemistry.” Val. 11, Longmans. Green and Co., N e n Yorli. N. Y.. 1951. p. 109.
Processes,” McGraw-Hill Book Co., Inc., New York, N. Y.,1941, P. 484. (10) J. 0.Hirschfelder, C. F. Curtiss and R. B. Bird, “Molecular Theory of Gases and Liquids.” John Wiley and Sons, Inc., New York, N. Y., 1954, p. 629. (11) J. D.Bernal and R. H. Fowler, J . Chem. Phys., 1, 515 (1933).
THE EFFECT OF OPTICAL ISOMERS ON T H E MELTING POINT OF GELATIN GELS B Y J. BELLo A N D J.
R.
VINOGRAD
Gales and Crellin Laboralories of Chemislru. California Instilute of Terh-
nolog,u, Pasadeua. California Receiued January 6 . 1966
It, has long been known that, cert8nincompounds
.
June, 1956
.
NOTES
8 19
added to gelatin may raise or lower the melting We have determined the melting point of gelatin point of gelatin gels. It has been suggested that gels in the presence of several sets of optical isothe effect of these substances, or at least those ions mers, both melting point raisers and reducers. Inthat lower the melting point, is a result of binding cluded is sodium acetyltryptophan, one of the most of the additive by the gelatin’ or due to other spe- effective melting point reducers reported.13 The cific interactions.2.3 The gelation of gelatin is ac- results are shown in Table I. The largest differcompanied by a large increase in the specific rota- ence observed between isomers was 0.2”, a differtion; this has been reviewed by Ferry.a The ence not larger than the experimental error. change of specific rotation on gelation must cer’ TABLEI tainly be the result of a change in the configuration of the gelatin molecule,4 probably an increased or- MELTINGPOINTSOF 5% GELATINGELS CONTAINING dering. OPTICAL ISOMERS Concn.. hl.p., It has been reported that additives that lower moles/l. OC. Added substanne the melting point of gelatin gels also lower the spe30.2 None cific rotation of gelatin a t temperatures at which 32.1 0.5 Disodium &malate gelation occurs in the absence of At 32.1 .5 Disodium nbmalate sufficiently high additive concentrations, additives .5 32.9 Disodium L-glutamate lower the specific rotation of gelatin to that of pure 33.0 .5 Disodium nL-glutamate gelatin solutions at 35” or higher, where a limiting 32.9 .5 Disodium L-aspartate value of rotation is reached and no gelation o ~ c u r s . ~ 32.9 .5 Disodium D-aspartate The decrease of rotation caused by additives that .8 24.6 Sodium acetyl-bphenylalanine reduce the melting point represents a reversal of 24.8 .8 Sodium acetyh-phenylalanine the configurational change attending gelation. At .5 19.1 Sodium acetyl-Irtryptophan 35” or in the presence of 2 M potassium thiocya.5 19.1 Sodium acetyl-i)~-tryptophan nate (one of the more effective melting point reduc.91 30.5 L-Arabinose ers), aqueous gelatin exists as discrete molecules1o .91 30.4 D- Arabinose having the form of a random coil.11,12 Additives that raise the melting point also raise the specific Had there been large differences between the efr~tation.~ fects of optical isomers, one could have attributed If the additives that raise.or lower the melting the melting point changes to binding of the addipoint of gelatin gels do so by promoting or inter- tives at or close to asymmetric centers or to being fering, respectively, with the attainment of an or- bound at more distant sites which approach these dered configuration necessary for gelation as a re- centers during the configurational change attending sult of being bound by the gelatin or as a result of gelation or to binding at non-asymmetric sites some other specific interaction at or near the asym- which react intermolecularly with asymmetric metric center it might be anticipated that optical centers. None of these explanations for the effects isomers might exert different effects. A similar of additives can be excluded if gelatin has a high effect might be anticipated if additives act by being degree of “configurational ada~tability,”’~ being bound near asymmetric centers that are cross-link- able to accommodate enantiomorphs equally well. ing sites in the gelation process. Alternatively, if There are other explanations for the effects of the effect of additives is solely the result of chang- enantiomorphs which are not excluded by these data. ing the nature of the solvent, then D and L isomers Additives may act solely on the solvent, or they may will have identical effects on the melting point of occupy non-asymmetric sites which are involved in gelatin gels since the effects of enantiomorphs on cross-linking or in the configurational changes. an optically inactive solvent must be identical. A Acknowledgment.-This work was performed DL mixture, however, might have a different effect under contract No, DA 49-007-MD-298 with the from either pure enantiomorph as a result of forma- Office of the Surgeon General, Department of the tion of a racemic compound. Army. We wish to thank Miss Helene Riese for (1) J. Bello and J: R. Vinograd, Paper No. 10 presented before the technical assistance. Division of Colloid Chemistry at the 127th Meeting of the American Chemical Society, Cincinnati, Ohio, March 29-April 7. 1955. (2) G. A. Feipen and I. L. Trapani, Arch. Biochem. Biophys., 68, 184 (1954). (3) J. D. Ferry, “Advances in Protein Chemistry,” Vol. IV, M. L. Anderson and J . T. Edsall, editors, Academic Press, Inc., New York, N. Y., 1948, p. 1. (4) C. Robinson and M. V. Bott, Nature, 168,325 (1951). (5) E. Stiasny. Rolloid Z.,86, 353 (1924). (6) A. B. Manning, Bio-hem. J . , 18, 1085 (1924). (7) J. R. Katz and J. F. Wienhoven, Rec. trau. chim., 62, 36 (1933). (8) D. C. Carpenter and F. E. Lovelace, J. Am. Chcrn. Soc., 67,2337 (1935). (9) This is restricted to the commonly investigated concentration range, up to 10% gelatin. At higher concentrations the melting point increases sharply: J. Russel, “The Theory of the Photographic Process,“ C. E. K. Mees, editor, The Macmillan Co., New York, N. Y., 1954, p. 03. (IO) E. 0. Kraemer, THISJOURNAL,45, A60 (1941); 46, 177 (1942). ( 1 1 ) J. W. Williams, W. M . Saunders and J. 8. Cicirelli, ibid., 68, 774 (1954). (12) H. Boedtker and 3’. Doty. ibid., 68, 968 (1954).
Experimental Sodium salts were prepared from the corresponding acids by reaction with sodium hydroxide, the final pH being between 8 and 9 to ensure complete conversion to the salt. The pH of pairs of isomers were adjusted to the same value to within 0.1 pH unit. Acetyltryptophan and acetylphenylalanine were prepared according to du Vigneaud and Sealock’s and du Vigneaud and Meyer,16 respectively. The remaining amino acids were the best quality sold by the California Foundation for Biochemical Research. The gelatin used was Wilson Laboratories’ U-COP-co, Special Non-Pyrogenic Gelatin, an acid extracted pigskin gelatin of isoelectric point 9 2.
(13) R. S. Gordon, Jr., and J. D. Ferry, Federation PTOC.,6 , 186 (1946). (14) F. ICarush, J. Am. Chem. SO:.. 72, 2705 (1980). (15) V. du Vigneaud and R . R. Sealock, J . B i d . Clrem., 96, 511 (1932). (10) V. du Vigneaud and C. E. Rleyer, ibrd., 98,295 (1932).
NOTES
820
Melting points were meawred as followe. About 10 ml. a 150 x 20 mm. loosely stoppered test-tube was warmed at 50" for ten minutes to erase the sample': thermal history, stoppered tightly and stored a t 0 f 0.01 for 20 hours. A Neoprene ball, dig 1.17, and diameter 0.5 cm. was inserted under the surface of the gel, the stopper replaced and the tube heated a t the rate of 5" per hour. The melting point was taken as the temperature at which the ball reached the bottom of the tube. The reproducibility of the method is about *0.3". af solution in
THERMAL DIFFUSION NEAR THE CRITICAL SOLUTION TEMPERATURE * BY L. J. TICHACEK A N D H. G. DRICEAMER Department of Chemisfru and Chemical Engineering, University of IlEinoie, Urbana, Illinois Received Janlurrlr Id, 1966
The thermal diffusion ratio CY for a binary system is defined by the flux equation -.F
JI
- pD
--c
[grad XI
- axl (1 - XI] 1 grad 2'1
(1)
Vol. 60
t ~ r e .Since ~ there is no necessity for the numerator to go to zero a t this point, CY should increase greatly in magnitude as we approach the critical solution temperature. To test this hypothesis thermal diffusion measurements were made on the system isooctane (2,2,4trimethylpentane) perfluoroheptane. The isooctane was Phillips pure grade. The perfluoroheptane was obtained from Minnesota Mining and Manufacturing Co. It was carefully redistilled and a cut boiling from 82.2 to 83" was retained. I n order to operate with a AT of 1 - 2 O , a special cell was designed as a modification of our standard cell.4 The cell is shown in Fig. 1, and is largely self-explanatory. The fritted glass was seated in a groove etched in the glass body of the cell and sealed in with sauereisen. The AT was measured using iron-constantan thermocouples. The cell is in many ways more useful than our standard cell. The results are shown in Fig. 2. Because of the small AT'S used, considerable scatter was inevi-
From the thermodynamics of irreversible processes it is possible to derive an expression for a2 where
0
- 7.0the Vi's are partial molar volumes, P is the average volume and Oi* is the heat of transport of component i. For our present purposes, the factor is of most interest. in the denominator XI bpccl/bXl In an ideal solution it equals RT. It approaches zero as one ripproaches the critical solution tempera-
- 6.0d.
- 5.0-
-
- 4.0- 3.0a
-7
-2.0-
I
6
I
I
- 1.0-
b
0
I
I
I
0
20
I
40 60 VOLUME FRACTION
I
80
I(
nC,Fl,.
Fig. 2.-Thermal diffusion ratio cy us. composition system n-perfluoroheptane (2,2,4-trimethylpentane).
table. X ( b p / b X ) can be written using ScatchardHildebrand theory.
bx;
= RT [I
x1
Fig. 1.-Glass walled thermal diffusion cell: 1, I , thermocouples; 2,2, sample taps; 3, sample tap scaling screw; 4,4, agitators; 5, fritted glass diaphragm, cemented in; 6, wall of cell chamber-glass tubing; 7,7, Teflon gaskets; 8,8, holes for connecting screws; 9, coolant takes. (1) This work was supported in part by the AEC. (2) L. J. Tiohacek, W. 5. ICmak and H. G. Drickamer, THIEJOURN A L , 60, 060 (1956).
where pi
"'"I
- VRTR
(3)
= volume fraction i
7 = average molar volume.
The subscript R refers to the value relative to the value at the C.S.T.
(3) J. H. Hildebrend and R. L. Scott, "Solubility of Non-Electrolytes," 3rd Ed., Reinhold Publ. Corp., New York N. Y.,1950, p. 253. (4) R. L. Saxton, E. L. Dougherty, Jr., and €1. G. Driokamer. J . Chem. Phya.. 11,1166 (1954).
