NOTES
4040
background information, eq. 2 may be employed to calculate relative k, values in different salt sohtions. It is interesting that for octyl glucoside, for which k, = k, in NaC1, the ratio of k, values (calculated from ref. 7 ) in NaCl and NazSOc is 1:2.7, in fair agreement with the ratio 1:3.2 predicted by eq. 2 and the ratio of 1:2.7 observed for benzene.15
gelatin. We have studied the mechanical loss and shear properties of gelatin and vitreous gelatin-water systems at low temperatures and wish to present the results to provide more information on transitions that occur in this biopolymer.
Salting Out of Ionic Association Colloids
Gelatin obtained from Atlantic Gelatin (a division of General Foods Corp.) was used without any further treatment. The properties of the gelatins studied are listed in Table I. The gelat,in-water systems were prepared from Gelatin-I and distilled water by combining the two compounds and warming them in lightly stoppered test tubes in a water bath at about 90". The desired concentrations were obtained by checking the solution weight and adding solvent to make up any weight loss occurring during the solution period. Concentrations are calculated on an oven-dry gelatin basis. Vitrification of the gelatin-water systems was accomplished by rapidly cooling in liquid nitrogen and mounting the specimens while they were in a glassy state. The unplasticized gelatin was molded into plaques a t 110' and 3000 p.s.i. These plaques were desiccated until the experiment was performed. The moisture content of the Gelatin-I plaque was 9.6y0 and that of the Gelatin-I1 plaque was 8.3%.
Although the major effects of added electrolytes on the c.m.c. of ionic association colloids are due to interionic interactions, the above analysis suggests that salting-out effects, generally neglected, can be quite substantial. The calculated k, value for a dodecyl group is about 0.5. Therefore, the activity coefficient of an amphiphatic ion containing a dodecyl group should be increased by factors of 1.12, 1.78, and 3.2 in 0.1, 0.5, and 1 M NaCl because of the salting out of the chain.
Concluding Remarks The above analysis suggests that the problem of qalt effects on nonionic association colloids can be profitably attacked in terms of some fairly wellestablished ideas developed for simple nonelectrolytes. Previous explanations, based on changes in water activity,j the presence of charged impurities,6 or the unavailability of solute molecule^,^ need not be involved. Schick, Atlas, and Eirich8 have suggested salting out of the ethylene oxide chains in some ethylene oxide condensates in terms of a dehydration mechanism. As indicated above, the salting out of the hydrophobic chains seems to be the most important factor to be considered for polyethylene oxide systems also. Acknowledgments. The revision of this paper was done at the University of Southern California and was supported in part by P.H.S. Research Grant GM 10961-01 from the Division of General Medical Services, Public Health Service.
Experimental Section
Table I: Properties of Gelatin Used in Study
Gelatin-I Gelatin-I1
Moisture content,= %
PH
Bloomb
10.8 9.0
6.5 4.35
200 200
TypeC
B A
'
a As-received basis. Bloom is a standard measure of the gel strength of gelatin. It is defined as the number of grams required to force a 0.5-in. plummet of a Bloom gelometer 4 mm. into an aqueous 6.67% solids gelatin gel that has been chilled 17 hr. a t 10' (J. F. Suter, U. S. Patent 3,164,560(Jan. 5, 1965)). Type B gelatin is obtained from lime-conditioned calfskin, beef hides, or ossein. Type A gelatin is obtained from acidconditioned pigskin.
Transitions in Gelatin and Vitrified Gelatin-Water Systems
by Joseph V. Koleske and Joseph A. Faucher Research and Deaelopment Department, Unwn Carbide Corporation, Chemicals Division,South Charleston, West Virginia 95'58058 (Receited April 15, 1955)
Yannas and Tobolskyl recently reported on the viscoelastic properties of plasticized and UnplaStiCiZed The Journal o j Physical Chemistry
Mechanical loss measurements were made a t about 1-5 C.P.S. with a recording torsion pendulum similar to that described by Nielsen. * These measurements were used to calculate the real, G', and the loss, G", components of the complex shear modulus. (1) J. B. Yannas and A. V. Tobolsky,
J. Phys. Chem., 68, 3880
(19w). (2)
L. E. Nielsen, R ~ Osci. . Instr., 2 2 , 690 (1951).
NOTES
404 1
Discussion 0.60
Figure 1 is a plot of the mechanical loss spectrum of Gelatin-I and an initially vitreous 54: 46 gelatin-water gel. Examining the low-temperature data, we note that a peak occurs at about -85' for both specimens. It seems this peak is associated only with the gelatin and it is affected by the water in an unusual manner. The position of maximum loss is unaltered, but the temperature range over which the transition occurs is markedly narrowed. Keeping in mind that a 65.5: 34.5 gelatin-water solution cannot be frozena-that is, only vitrification will take place regardless of the manner of cooling-most of the water in the 54:46 system must be bound to the gelatin in a very strong fashion. It is possible to argue that this bound water inhibits the onset of the energy-absorbing mechanism that produces the -85' peak. Yet, if this were the case, one would expect the peak to be obscured, to occur at a higher temperature, or at least to be decreased in magnitude. If the water acted as a plasticizer for this mechanism, typical behavior would predict the transition to occur at a lower temperature and the breadth of the transition to be increased. Thus far we have found no way out of this dilemma and can only point out what seems to be a paradox-the peak is sharpened, increased in magnitude, and not shifted by the diluent. One wonders if the term plasticizer would be apt in this case. The -10' peak in the gelatin-water system can be ascribed to the devitrification which has been found by others to occur a t this t e m p e r a t ~ r e . ~ We made no attempt to examine this sample above room temperature since in our apparatus the specimen would lose moisture at these temperatures. Table I1 summarizes the lowtemperature transition data for the systems studied.
Table I1 : Low-Temperature Transitions in Gelatin and Ge1at.h-Water Syetems
System
Gelatin-I Gelatin-I1 Gelatin-I-water (54:46) Gelatin-I-water (49:51)
Transition temp., "C.
Frequency at transition, C.P.S.
Devitrification temp., OC.
- 85
2.7 3.4 4.3 4.2
... ...
- 85 -88 -85
- 10
- 10
Turning to the higher temperatures, we find from the data in Figure 1 that a large loss peak occurs a t 130' and another a t 180' for Gelah-I. Gross melting seems to be occurring a t 220' or perhaps somewhat higher. Okamot0 and SaekiY5 in their investigation of gelatin, found
0.10
0.30
0.20
e
0 0 8
0 06
cj *
0.04
2
0.03
5
002
0 01
0 008
7i33EEz 0 003
0 0 0-180 2
-140
-100
-60
Temperature -20 20
C
60
100
143
180
220
Figure 1. Mechanical loss characteristics of Gelatin-I (0) and 54:46 Gelatin-I/water ( 0 )in an initially vitreous state. Measurements are independent of sample dimensions. Pee text.
that a second-order transition occurred in the amorphous regions at 120' and that small crystallites of widely distributed stability melt over a temperature range of 80-100' and at 180'. The stable crystallites melted a t about 220'. The previously cited work' ascribed the transition in the vicinity of 190' to the glass transition temperature. Above 220' these authors found that decomposition occurred and the possible melting of gelatin was thus obscured. In the case of our gelatin specimens, we found that G' began to increase at moderately high temperatures as shown in Figure 2. Such behavior indicates that a change of sample dimensions is occurring or that the compound studied is crystallizing, cross linking, or possibly decomposing. Dimensional measurements made on the specimen after completion of the run (it is not practical for us to make these measurements during the run) indicate that the first reason is the most plausible, since the sample thickness increased by a factor of about 3. When the final data point is corrected by this factor, the solid point shown in Figure 2 at 210' is obtained and the data may actually be more accurately described by the dashed curve shown in this figure. However, the cross-linking and/or decomposition factors cannot (3) T. Moran, Proc. Roy. SOC.(London), A112, 30 (1926); cf. ref. 4. (4) B. J. Luyet and P. M. Gehenio, "Life and Death at Low Tem-
peratures," Biodynamica, Normandy, Mo., 1940, p. 214. ( 5 ) T.Okanioto and K. Saeki, Kolloid-Z., 194, 124 (1964).
Volume 69, .Vumber 11
Soaember 1966
4042
NOTES
ties (perhaps this is not too surprising when the complex structure of this biopolymer is considered) and that the previous history of the sample can result in different mechanical properties at higher temperatures. Although differences in the high-temperature properties are found by different investigators, there seems to be a persistent loss peak at 180’ and most sources agree that the major melting occurs near 220’.
A Rotating-Disk Thermocell. I. Theory -.
by Benson R. Sundheim and Werner Sauerwein
=
9 0
u
8.8
Department of Chemistry, ;Vew York University, A-ew Y o T ~ , N e w York 10003 (Received April 29, 1466)
8 6
”r:’
8.4
8 0
~
,
‘ r - -
~, 1
~
7 8
-180
-140
-100
-60
-20
20
Iempernture,
~
60 ‘C
100
140
1:
180
220
Figure 2 . Real and loss components of the complex shear modulus for Gelatin-I. Note (see text) that there is considerable uncertainty about the sample dimensions at elevated temperatures and the open circle G’ and G” data are apparent values. The solid point shows the true value of G‘ after correction for dimensional changes that have occurred.
be disregarded, for the specimen was very brittle after the run and seemed to have frothed during the latter stages of the experiment. It is well to note that although G’ and G” are dependent on sample dimensions, which obviously are in doubt at the higher temperatures, the mechanical loss, ,“/GI, is independent of dimensional changes. Results similar to these were also obtained with Gelatin-I1 except that we found peaks a t 90 and 115’ with G’ starting its increase at 120’. There were also indications of a melting or R large loss peak in the neighborhood of 180-190’. To summarize, this torsion pendulum work indicates that gelatin has a mechanical loss peak, whose temperature position is unaffected by water, occurring at -85’. For the gelatin-water systems studied, devitrification occurred at the expected temperature of - 10’. When the high-temperature data we obtained are compared with the current literature, there are indications that gelatin from different sources can have different properThe Journal of Physical Chemistry
Studies of thermoelectric properties entail the determination of electrode potentials across accurately known temperature and concentration distributions. A major experimental problem is the maintenance of these distributions free of convective perturbations.’ A related problem, that of determining the concentration profile produced under certain circumstances by the passage of electricity through an electrochemical cell, has been satisfactorily treated by establishing a known forced convection pattern.? The rotating-disk electrode was used for this purpose since complete solutions to the hydrodynamic and diffusion problems are known in this case.2 Here we explore the properties of the rotating-disk electrode system when the electrode and solution are held at different temperatures. One-Component Systenz. For a one-component electrolyte, e.g., fused AgKO3, no compositjon gradients can occur. The thermopotential is readily expressed in terms of the temperature gradient3 so that a knowledge of the temperature distribution is sufficient. A multicomponent system in which the convectioii is sufficient to prevent the establishment of the Sor&t effect will behave in the same way. The meaning of the various symbols employed below is given in the Glossary. (1) H. J. V. Tyrell, “Diffusion and Heat Flow in Liquids,” Butterfield Scientific Publications, London, 1961. This work gives a critical review of experiment and theory as well as extensive references. (2) (a) V. G. Levich, Acta Physicochim. URSS, 17, 257 (1942); (b) V. G. Levich, ibid., 19, 117, 133 (1944); (c) Y. G. Levich, “Physicochemical Hydrodynamics” (in English), Prentice-Hall, Inc., New York, N. Y., 1962. (3) (a) H. Holtan, Thesis, University of Utrecht, 1953; (b) J N. Agar in “The Structure of Electrolytic Solutions,” W.Hamer, Ed., John ‘Niley and Sons, Inc., New York, N. Y., 1959, p. 200; (c) B. Sundheim, “Fused Salts,” JfcGraw-Hill Book Co., Inc., New York. N. Y., 1964, p. 201.
