3880
NOTES
the experimental error and is attributed to either: (a) a difference in the matrix (one containing hydrogen peroxide, the other little or none), or (b) more probably to traces of ozone or nitrogen oxides in the ozonehydrogen atom product. There were traces of ozone visible in the product. The ozone can react with hydrogen peroxide or decompose to liberate 57 kcal. or 34 kcal./mole of ozone, respectively. The heat of decomposition of Hz04a t -196' is -17.8 f: 2.2 kcal./mole. This differs considerably from the value of -44 kcal./mole reported by Ghorinley4 and - 39 kcal./mole reported by Skorokhodov, et a1.6
Ghormley assumed a superoxide assay without product analysis. Skorokhodov's calculation of the heat of decomposition is not clear. The heat of formation of Hz04 at -196' was estimated from data obtained in this research and those available in the l i t e r a t ~ r e . ~The heat capacity of HzOz from -196 to 25' was obtained from GiguBre's data. The average C, of H202 is 5.50 cal. mole-' deg.-l from -196 to -173O, 6.85 cal. mole-' deg.-' from -173 to -143') 9.40 cal. mole-' deg.-l from -143 to -53', 2.31 0.03967T (OK.) cal./mole from -53 to -2', 21.36 cal. mole-' deg.-' from 0 to 25'. The heat of fusion of HzOza t -0.43 is 2.9 kcal./mole. The average C, of H, is 6.25 cal. mole-' deg.-' from - 196 to 25'. The average C, of 0 2 is 6.98 cal. mole-' deg.-l from -1183 to 25' and 13.0 cal. mole-' deg.-' from -183 to -196'. The heat of vaporization of 0 2 a t - 183' is 1.6 kcal./mole. It is assumed that there is no interaction of the matrix with Hz04 or H202 upon decomposition at -196'. The heat of formation of HzOz at -196' is -45.7 kcal./mole. Using the value of -17.8 kcal./mole for the reaction
+
Hz04. (~Hzoz * y HzO) (s)
-
HzOz (XHZOZ * yHz0) (9)
+ Oz(1)
a t -196', the heat of formation of H204 is -27.9 kcal./mole. The combination of HOz radicals is thought to lead to the formation of Hz04.a Using the data of Graylo for the heat of formation of gaseous HOz radicals ( + 5 kcal./mole) and assuming this radical has the
same thermal properties as hydrogen peroxide, the heat of reaction at - 196' of
2HOz(s) is -4 kcal./inole H2O4.
Viscoelastic Properties of Plasticized Gelatin Films
by J. B. Yannas' and A. V. Tobolsky Friek Chemical Laboratory, Princeton University, Princeton, New Jersey (Received June BO, 1964)
Features of the molecular structure of gelatin have been studied by a great variety of physicochemical approaches. In particular, the related problems of chain conformation, chain aggregation, and crystallinity of gelatin have received attention. Investigations of such as yet unresolved questions have been performed on gelatin samples which have covered the entire concentration range-from dilute solutions through gels to glassy films.2 Our own effort has centered on the measurement of viscoelastic properties of plasticized gelatin sheets and sheets of pure, unplasticized protein. So far, these measurements have given us useful insight into the characteristics of this interesting biopolymer.
Experimental Limed ossein gelatin in powder form and sheets was obtained through the courtesy of Dr. D. Tourtellotte of Kind and Knox Gelatin Co., Camden, N. J. (Lot 4181-1). Gelatin prepared in identical fashion has been characterized by Scatchard, et al. (Knox Special Gelatin Type A).a It is a relatively monodisperse grade, the value of M,/M, being 1.61. Gelatin of this type has M n = 45,700 and an isoionic point of 4.92. The moisture content is of the order of 10 wt. 7 0 and the ash content is of the order of 1wt. %. Dimethyl sulfoxide (J. T. Baker Chemical Go., purity 99.9%) was the main plasticizer used. Glycerol ~~
(9) (a) P. A. Gigubre, I. D. Liu, J. S. Dugdale, and J. A. Morrison, Can. J . Chem., 32, 117 (1954); (b) P. A. Giguhre, B. G. Morissette, A. W. Olmos, and 0. Knop, ibid., 33,804 (1955); (c) "Selected Values of Chemical Thermodynamic Properties," National Bureau of Standards Circular 500, U. S. Govt. Printing Office, Washington, D. C., 1952; (d) H. W. Woolley, J . Res. Natl. Bur. Std., 40, 163 (1948); (e) H. W. Woolley, R. B. Scott, and F. G. Brickwedde, ibid., 41, 379 (1948). (10) P. Gray. T r a n s . Faraday Soe., 5 5 , 408 (1959).
T h e Journal of Physical Chemistry
Hz04(~),
~~
~
(1) National Institutes of Health predoctoral fellow, Department of
Chemistry, Princeton University, Princeton, N. J. (2) J. D. Ferry, A d v a n . Protein Chem., 4, 21 (1948); A. G. Ward and P . R. Saunders, "Rheology," F. R. Eirich, Ed., Val. 2, Academic Press, Inc., New York, N. Y., 1958; W. R. Harrington and P. H. van Hippel, A d v a n . Protein Chem., 16, 1 (1961). (3) G. Scatchard, J. L. Oncley, J. W. Williams, and A. Brown, J . Am. Chem. Soc., 66, 1980 (1944).
KOTES
3881
(J. T. Baker Chemical Co., purity 96.0%) was also uscd as an alternate plasticizer in several instances. Plasticized gelatin filnis were obtained by weighing amounts of gelatin powder, previously pulverized finely and dehydrated in a vacuum oven at 65' for several days, into aluminum dishes containing :m excess of plasticizer. The initial protein concentration was of lhe order of 5 wt. yo. The dishes were placed in a drying oven at 65 f 1' and the plasticizer was allowed to evaporate slowly. Dishes were removed from the oven ~7heiithe protein concentration in the films increased to the desired level (40-80 mt. Oi;,). Several filnis were also prepared using undried gelatin powder. In the undried form gelatin dissolved much more readily into the plasticizer giving films of superior clarity; these possessed identical viscoelastic properties with films that were prepared using dried gelatin powder. Immediately after removal from the oven , the films weie cut up into rectangular strips; these were inimerwd in silicone oil (polydirnethylsiloxane, Dow Corning Corp.) wherein they were tested. Strips of unplasticized protein were prepared by cutting up sheet gelatin after brief heating on a hot metal surface to impart flexibility; strips so obtained were dehydrated initially in a drying oven a t 105' overnight and subsequently in a vacuum oven a t 105' over a period of 4 days. Prolonged exposure of sheet gelatin to 105' (2 wli.) did not affect the magnitude of the properties measured. The torsion inodulus G of the strips measured after 10 sec. was obtained by a Clash-Berg torsional creep apparatus4 in the region where the modulus exceeds lo9 dynes/cin2. Below that level of the modulus, nzeasurements were made in a modified Gehinan app a r a t u ~ . ~Curves of log 3G vs. temperature were thus
-
GELATIN
-
T
-- DIMETHYLSULFOXIDE SYSTEM
obtained for the various specimens within the explerimentally accessible temperature range from - 80 to 220'. The lower temperature limit was set by limitations of the apparatus used; the upper temperature range was imposed by the decoinposition characteristics of the protein and by the volatility of the plasticizer. The buoyant weight of specimens immersed in silicone oil was also measured over a considerable temperature range. A discontinuity in the slope of the buoyant weight os. temperature curve indicated the presence of a glassy transition in the specimen. Such measurements of the glass transition temperature are entirely equivalent to those where a discontinuity in the slope of a specific volume vs. temperature curve is used as the criterion for the transition.
Results and Discussion The log modulus us. temperature curves for five specimens of gelatin plasticized with varying amounts of dimethyl sulfoxide (DMSO) are displayed in Fig. 1. The curve for the pure, unplasticized protein sheet is also shown; this curve is incomplete due to decomposition of the protein at ca. 220'. Arrows indicate the position of the glass transition temperature, T,, as measured by the procedure illustrated in Fig. 2. The T , of the pure protein is in the vicinity of 190'; its precise determination has eluded us so far owing to the experimental conditions which a l l o ~sufficient time for decomposition to occur and invalidate the data in that temperature region. Values of T , for the system gelatin-DR4SO are listed in Table I. A tabulation of values of T c (the
I I 1
TEMPERATURE, ( " C )
Figure 2 . Buoyant weight us. temperature for gelatin-dimethyl sulfoxide 80-20. -100
0
IO0
TEMPERATURE, ("C)
200
F. Clash and R. M. Berg, Ind. Eng. Chem., 34, 1218 (1942:. (5) S.D. Gehman, D. ]E. Woodford, and C . S.Wilkinson, Jr., itk?., 39, 1108 (1947). (4) R.
Figure 1. Ten-second modulus us. temperature for gelatin--dimethyl sulfoxide sheets.
Volume 68, S u m b e r 12
December, 19194
NOTES
3882
Infrared Spectral Observation of Surface States
Table I : Viscoelastic Parameters of Gelatin Films Gelatin concn., wt. %
Gelatin -D M SO--TBI"C. Ti,OC.
7--
40 ... -33 45 ... -31 ... -20 50 55 ... - 13 60 -3 17 65 23 ... 69 35 50 75 60 ... 80 80 112 100 ( 180-200)a (220)a a Data uncertain owing to decomposition.
Gelatinglycerol Ti,'C.
...
Department of Chemistry, University of Arkansas, Fayetteville, Arkansas (Received J u l y 16,1964)
... ,.. -11 30 .
.
I
53
... 110 (220)Q
temperature at which 3G equals lo9 dynes/cm.2) for the systems gelatin-DMSO and gelatin-glycerol is also given there. The results of Fig. 2 display a discontinuity in the slope of specific volume us. temperature, rather than a discontinuity of the specific volume itself. This is an indication of a glass transition rather than a first-order transition (melting). The shape of the modulustemperature curve in the neighborhood of the transition shows a change from glassy to leathery behavior, which is also indicative of a glass transition (see Fig. 1). It is also observed (Fig. 1) that extended rubbery plateaus, where the modulus is relatively invariant with temperature, can be obtained even when the plasticizer concentration is as high as 55% wt. In no case was any melting or flow observed; instead, the specimens remained dimensionally stable and truly rubbery until loss of plasticizer started affecting the measurements. The persistence of rubbery behavior in linear polymers over an extended temperature range is frequently evidence of the presence of a pseudo three-dimensional network structure. We believe that, in the case of plasticized gelatin, small protein crystallites are present even a t very high plasticizer concentrations; these microcrystallites act as cross links which do not melt even at teniperatures as high as 200' (Fig. 1). This interpretation is consistent with our belief that T , for pure gelatin is in the vicinity of 190'. In summary, we believe that gelatin plasticized with dimethyl sulfoxide and glycerol exhibits viscoelastic behavior typical of a plasticized microcrystalline polymer such as polyvinyl (6) A. V. Tobolsky, D. Carlson, and N. Indictor, J . A p p l . Polymer
Sei.,7 , 393 (1963). (7) A. V. Tobolsky and R. R . Taylor, ibid., in press. ( 8 ) M.Shen and A. V. Tobolsky, I n d . E n g . Chem., in press
T h e Journal of PhyRical Chemistry
by George Blyholder and Edwin A. Richardson1
I n the course of studies2 related to the surface interactions of a-Fez03and polar adsorbates such as water and ammonia, a number of absorption bands in the infrared region (2 to 15 p ) were noted. These bands appear to be directly related to the solid adsorbent and only indirectly to the particular adsorbates employed. The purpose of this note is to describe the occurrence of these bands and their changes upon cheiiiisorption and to propose several possible origins.
Experimental A Perkin-Elmer Model 21 infrared recording spectrometer is employed. To expand the low transmission range of the instrument, the reference beam is partially blocked off with screen wires. The sample disk is mounted in a modified gas cell as described by Blyholder and Neffe3 The effective range of the system is 2 to 13 p. A conventional vacuum apparatus capable of maintaining 0.001 p pressure is employed in degassing the disk. Certified reagent grade iron(II1) oxide obtained from Fisher Scientific Co. was employed without further treatment. Particle sizes are in the 5 p range and the material has a surface area of about 15 mS2/g. The farinfrared spectrum of this material indicates that it is essentially a-Fe203.4 About 0.3 g. of a-FezOs is transferred to a 2.5-cm. diameter die and pressed a t 8000 p.s.i. Before pressing, the die is heated in an oven a t 140" to prevent sticking of the disk to the die face. By careful handling, the disk can be removed from the die without breaking and transferred to the cell described above. The disks as prepared above contain considerable water and thus are not active in chemisorption. The activation process consists of degassing the disk a t room temperature for several hours or until a pressure in the order of 0.05 p can be maintained. The disk is then heated t o 375" in an atmosphere of O2 and maintained for several hours. This oxidizes any impurities which might cause subsequent reduction. The cell is then (1) Abstracted in part from a thesis submitted in partial fulfillment of the Doctor of Philosophy degree, University of Arkansas. (2) G . Blyholder and E. A. Richardson, J . P h y s . Chem., 66, 2597 (1962). (3) G . Blyholder and L. D. Neff, ibid., 66, 1464 (1962). (4) Shell Oil Co., Houston, Texas, private communication.
