57
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1930
A Study of Some Properties of Gelatin I-Hydration of Gelatin and Its Relation to Swelling1 Harvey A. Neville, Edwin R . Theis, and R. B. K’Burg WM.
H. CHAXDLER CHEMICAL LABORATORY, LEHIGHU S I V E R S I T Y , BETHLEHEM, PA.
The swelling of gelatin at the isoelectric point cannot material under such condiA R I O US substances be due to osmotic force but may be attributed to hytions. in which the primary dration-that is, to the compression of a shell of liquid The apparatus shown in valence forces are satuabout the particles. This results in a contraction of Figure 1 was designed for rated still exhibit the ability the system which can be measured by means of a this purpose. It is essentially to combine with or attach dilatometer and may be considered a relative measure a simple dilatometer, consistmolecules of w a t e r . T h i s of hydration. A portion of the total swelling at other ing of a bottle with a groundphenomenon is described by than isoelectric conditions must also be attributed to glass stopper, through which the term hydration, and we hydration. is sealed a calibrated capilmay distinguish two types of The variation of this contraction with changes in lary tube. The ground-glass hydration, as the water may temperature and in pH value is measured and is conconnection is lubricated with be combined in stoichiometric trasted with the effect of the same conditions upon stopcock grease and will hold ratio to form a definite hyosmotic and total swelling. It is shown that hydration without leaking, even under drate or it may be l o o s e l y and swelling are not parallel effects. a considerable hydrostatic attached as an “atmosphere” The application of this method to the soaking of pressure. However, heavy or “shell” of water of varianimal skins is indicated and the probable relation rubber bands attached to able density and thickness. of plumping and swelling is suggested. the neck of the bottle are The first type is illusWhen fully hydrated hide or gelatin is placed in solupulled over the cork cushion trated by the crystalline hytions of electrolytes dehydration occurs and an exto insure a tight joint. drates, in which the number pansion of the system results. The measurement of The volume of the bottle and arrangement of the water this expansion may serve as a method of determining is 150 cc. and in most of molecules that are associated the relative astringency of various solutions. the experiments 20 grams of with a molecule of the anhygelatin in flake form was used. drous substancesareprobably Within the range of 5 to determined bv the limits of accommodation in the particular crystal lattice. The identity 20 grams of gelatin, however, the contraction observed is of hydration in the isomorphous series, such as the alums directly proportional to the quantity of gelatin present. and the vitriols, lends support to this view. This type of hy- Because of the simplicity and compactness of the apparadrate does not exist as such in solution. I n the class of tus, a number of them were used concurrently in the same definite hydrates may be included also the products formed thermostat in order to obtain checks or to observe the when anhydrides combine with water to form acids and effects with different solutions under identical external bases. I n these products the water molecules are not conditions. The apparatus and all matecombined as such, but a partial redistribution of electrons rials were always brought to a occurs, so that hydrogen or hydroxyl ions result. The second type of hydration, sometimes called solvation, u n i f o r m t e m p e r a t u r e i n a is exhibited by ions in solution and by hydrophilic colloids. thermostat before each experiAs postulated by Kohlrausch, the difference in the migra- ment. The materials were then tion velocities of, for example, the alkali ions may be attrib- quickly transferred to the bottle uted to differences in hydration, the smaller ions being more part of the apparatus and any highly hydrated and hence less mobile than the larger ions. air bubbles were removed. When This relationship, known as the lyotropic series, is apparent the stopper was inserted the in many phenomena. This is the type of hydration which liquid rose in the capillary to obtains in the case of gelatin and similar substances and a height which served as the which we have attempted to measure relatively under vary- origin for subsequent readings. ing conditions. The apparatus was kept immersed in the thermostat during Apparatus and Method the experiment. The position Pauli (4)considers the hydration of colloids to result in of the liquid in the calibrated the formation of a highly compressed liquid shell about the c a p i l l a r y was n o t e d a t inparticles. Snch a phenomenon must be accompanied by tervals, which gave a direct an increase in the density of the system, which indicates measure of the changes in volFigure 1-Dilatometer a compression of the medium (water) or a compression of ume of the system. A similar apparatus was described by Neville and Jones the particle by its water layer. If this is the correct view, it follows that a contraction of the system when gelatin is (3) for the study of the hydration and setting of plaster of soaked or dissolved in water must indicate hydration, and Paris and Portland cement. The analogy of these processes the measurement of this contraction under varying con- to the hydration of gelatin was pointed out by those investiditions must give a relative measure of the hydration of this gators. Svedberg ( 6 ) attempted to show from density measureReceived October 2, 1929. Presented before the Division of Leather ments that the degree of hydration of gelatin in solution and Gelatin Chemistry a t the 78th Meeting of the American Chemical Society, Minneapolis, Minn., September 8 t o 13, 1929. parallels the viscosity changes observed by Loeb, giving a
V
I
1
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
58
minimum a t pH 4.7 and a maximum at about pH 3. This relation between viscosity and hydration is not necessarily true, as the viscosity of gelatin solutions depends upon the electroviscous effect, as well as upon the degree of hydration of the particles. The method of calculation used by Svedberg was shown to be erroneous by Brown (I). Svedberg later published some data obtained by the volume-change method ( 7 ) . These results, so far as they go, are in agreement with ours. Svedberg found that the presence of acids, bases, and salts decreases the contraction of the system; that there is no change in hydration when a gelatin sol sets to a gel; and that hydration increases with decreasing temperahre.
"
0.18 0.18
::
ip
0.18
44.8300 grams water X 100782 = 5.0000 grams gelatin i1.356 = 49.8300 grams mixture =
=
Average density of mixture
45.18057 cc. water 3.68730 cc. gelatin 48.86787 CC. mixture
49.8300
=
48.86787
1.0197
-
Density increase of system due to hydration = (1.0214 1.0197) = 0.0017 Calculated volume of mixture (cc.)....................... 48.86787 Volume of sol calculated from weight and density 48.78598 _ (cc.). .
... ---............. 0.08189
Contraction of system due to hydration (cc.) Contraction per gram of gelatin 0.01638 cc. at 4O0 C.
-
-
26.8'
2 0.20 3 0
flask and solution were then weighed. It was found that the water added weighed 44.8300 grams. The density of this solution was determined at 40" C. in the pycnometer. It was found to be 1.0214. The average density of gelatin and water in this proportion, considering the solution as a simple mechanical mixture, may be calculated as follows:
-
0.OOC
r;
Vol. 22, No. 1
-
16.00
-
0.32'' 0 a0
10.8
'
'
4Q 60
I
80
'
100
"
120 1 40
"
180
180
0
1.00 .
'
zoo
T m IR KINUTgS
Figure 2-Contraction
of S y s t e m Gelatin-Water Solution in Equilib-
at Various Temperatures. r i u m , p H = 6.3
Results
Typical curves which represent the volume decrease of the system gelatin-water with time are shown in Figure 2. These results were obtained on 20 grams of bone gelatin in flake form in freshly boiled distilled water. The pH value of the supernatant liquid at equilibrium was 8.3. Each curve represents the volume change at the temperature indicated. Two features are evident. First, the final contraction of the system, and hence the degree of hydration, increases as the temperature is decreased. Second, the rate at which equilibrium is attained is much greater at the higher temperatures. Thus, a t 38.3" C. very little change is observed after the first 30 minutes, while a t the lowest temperature, 1" C., 3 hours are required to reach equilibrium. Both of t'hese effects are characteristic of adsorption phenomena. The change in the degree of hydration of this gelatin with temperature is illustrated in Figure 3, where the equilibrium contraction of the system gelatin-water is shown as a function of temperature. The regularity of the curve in Figure 3 supports the statement of Svedberg that there is no observable change in hydration between the gel and sol states of gelatin. The increased density of the system resulting from hydration when gelatin is dissolved in water may also be measured ,directly by comparing at the same temperature the density of a gelatin solution with the calculated density of a simple mixture of gelatin and water in the proportion of the solution. This was done for a 10 per cent solution of Difco gelatin at 40" C. The following data were obtained: Weight of water in pycnometer at 4 0 DC. (grams). . . . . . . . . . . . . 26.3478 1.00782 Volume occupied b y 1 gram of water a t 40° C. (cc.). . . . . . . . . . . 26.53384 Volume in pycnometer at 40' C. (cc.). ....................... Weight of xylene in pycnometer at 40° C. (grams). . . . . . . . . . . . . 22.4980 0.8472 Density of xylene at 40° C . . ............................... 2.9803 Weight of xylene displaced by 4.7714 grams of gelatin (grams). 1.356 Density of gelatin (1.601 X 0.8472). ........................
.
Exactly 5 grams of gelatin was placed in a weighed flask, approximately 45 cc. of water was added, and the flask and contents were heated to 40" C. to dissolve the gelatin. The
By repeating these measurements and calculations with solutions of varying pH value the increase in density or degree of hydration could be obtained over any desired range. However, the dilatometric method is more convenient. Hence it was used to measure the variation in the contraction of the system with changing pH value. We have measured by this means the contraction which occurs when flake gelatin is permitted to soak in solutions of hydrochloric acid and sodium hydroxide over a considerable range of pH values. The data represented in Figure 4 are the contractions observed after 3 hours, at which time equilibrium has been attained. The pH value in each case is that of the solution in equilibrium and is obtained by pouring off the supernatant liquid and mixing it. A sample of this was taken and the pH value determined by means of the Youden quinhydrone electrode. Salts such as sodium chloride likewise decrease the contraction, giving much smaller volume changes than would correspond to the point on the curve in Figure 4 if pH value were the only responsible factor.
d
50.0
I
TNPERATURE
Figure 3-Variation
I
--
1
I
I
OC.
in C o n e a c t i o n of the System, or Hydration,
with Temperature
If gelatin exhibits maximum hydration a t or near its isoelectric point, it should be possible to effect dehydration by placing fully hydrated gelatin in a solution which is considerably more acid or basic. Dehydration may be pictured as a decrease in the compression of the water envelope of the particles. This should result in an expansion of the system and a rise in the capillary of our apparatus. This is exactly what is observed when blocks of a 10 per cent gelatin jelly are placed in solutions of electrolytes in this apparatus. When such pieces of gelatin jelly are placed in distilled water there is no observable change in the volume of the system. This indicates that the gelatin in such condition is fully hydrated. The results with solutions of hydrochloric acid and sodium hydroxide are represented in Figure 5. The striking similarity of the curves in Figures 4 and 5 show that we are dealing with an equilibrium condition which may be approached from either side.
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1930
59
Relation of Hydration to Swelling
to explain swelling. It is neglected, for example, in the Procter-Wilson theory, as the observed contraction of the The terms hydration and swelling have been largely con- system cannot be explained as due to the transfer of solfused, or used synonymously, with reference to the effects vent by osmotic force. I n other words, the study of swelling which obtain when gelatin, hide substances, or similar ma- has been limited to a consideration of the changes in volume terials are permitted to soak in water or aqueous solutions. or weight of the solid phase. Swelling is eraluated by measuring the final increase in volume If we may assume that the swelling of isoelectric gelatin or weight of a definite quantity of a material when it is im- in an isoelectric solution is due solely to hydration, then we mersed in a liquid. The effect of variation in hydrogen- may superimpose, as in Figure 6, the swelling curve of Loeb ion concentration upon the magnitude of the swelling of gela- upon the hydration curve shown in Figures 4 and 5, thus tin was investigated by Loeb, who obtained the now familiar contrasting the influence of pH value upon each. Many investigators have stated that swelling is always accompanied j: 0 . 2 1 by a decrease in the volume of the system. That this is not necessarily true can be shown by a simple experiment and is illustrated in Figure 6,. If we permit gelatin to hydrate and swell a t pH 4.7 and then transfer this swollen and hydrated material to a solution of pH 3, the material will swell further, as can be measured by its increase in weight or volume, but in this case the volume of the system as a whole will also increase owing to the dehydrating effect of the higher acid concentration. That is, swelling will follow the upper curve from the isoelectric point in Figure 6 representing the volume or weight change of the solid phase alone, , , , , , , , , ,
30*ml 0
1
2
3
4
pa
6
6
7
e
8
1
0
U
VALUE
Figure 4-Variation i n Contraction of System, or Hydration, with pH Values
swelling curve showing a minimum swelling a t the isoelectric point of the gelatin (pH 4.7). It has been assumed by most investigators that hydration parallels swelling and that hydration therefore will likewise exhibit a minimum a t pH 4.7. By means of an application of the Donnan theory of membrane equilibrium, Procter and Wilson ( 5 ) have furnished a very satisfactory theory of the mechanism of swelling. This theory accounts for the effect of variation in p H upon the degree of swelling as represented in the curve obtained by Loeb. According to the Procter-Wilson theory, the entrance of water, and hence the increase in volume of a solid block of gelatin, is caused by osmotic force resulting from a difference in total concentrations of diffusible ions inside and outside the gelatin. Hgwever, as pointed out by Loeb, a t the isoelectric point gelatin is practically not ionized and there can be no Donnan equilibrium. Yet when dry grains of isoelectric gelatin are placed in water of pH 4.7 a marked swelling occurs. With regard to this Loeb ( 2 ) concludes: The swelling (at the isoelectric point) must be determined by forces different from those set up by the Donnan equilibrium. In the first place, there are those forces of chemical attraction between the molecules of water and certain of the groups of the gelatin molecule which cause the solution of gelatin in water when the forces of cohesion between the gelatin molecules forming the gel can be overcome. The absorption of water by dry grains of isoelectric gelatin a t pH 4.7 is, therefore, primarily but in all probability not exclusively due to the residual valency forces, and the swelling of solid isoelectric gelatin granules is primarily a phenomenon of solid solution.
This factor to which the swelling a t the isoelectric point is attributed must also be present and active to some extent a t other pH values and must be responsible for a part of the total swelling obtained under these other conditions. Figures 4 and 5 represent the variation with p H value of this absorption of water or hydration if the contraction of the system may be considered a correct measure of this factor. It has been noted in many investigations of swelling that, although the material which is swelling increases in volume, the system as a whole undergoes a contraction. This fact has not been taken into consideration in the various attempts
.o.w
0 e 0.01
-.
.
0.09
p
0.0s
0.04
E
0.08
B5
0*07 0.08
?
,
,
,
,
,
,
,
=
26.8
, ,
,
,
,
Figure 5-Expan~ion of System, or Dehydration, Whea Gelatin with Maximum Hydration Is Placed i n Solutions of Different pH Values
pn
Figure 6-Relation
vuut
of Hydration to Total Swelling
while the change in volum; of the system as a whole follows the lower curve, the expansion resulting from a condition of decreased compression of the water shell. Here we have a case of swelling accompanied by an increase in the volume of the system. As swelling may thus be accompanied by either an increase or a decrease in the total volume, it seems probable that the two effects are largely independent and that the volume change of the system is due to other factors, an important one of which is the compression of the medium by the adsorptive forces of the material. While we have assumed, as in Figure 6, that at or near the isoelectric point hydration is the only factor causing a
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
60
contraction in the volume of the system, there probably are, particularly at lower and higher pH values, other influences which have not yet been evaluated but which may affect the volume of the system. The increase in the final contraction shown in Figure 4 for p H values less than 1.5 must be due to such factors. At this high concentration of acid the original nature of the gelatin may be changed by hydrolysis or other chemical action. Such action may be accompanied by a volume change or a modified gelatin with a different hydration capacity may be produced. ZP
r
316
9
E si2
1
z 3 0
:: 2
T
4
I
26.8’
C
0
T I U I N YINUTES
Figure 7-Cured Skin Soaked 24 hours Limed 5 Days, Placed in’Astringent Solutions
The contrast in Figure 6 between the influence of variation in pH value upon hydration and upon swelling is also reflected in the influence of other factors upon these two phenomena. The decrease in hydration with increasing temperature is shown in Figures 2 and 3. On the other hand, as osmotic pressure is proportional to absolute temperature, the osmotic force, which, according to the Procter-Wilson theory, causes swelling, must increase with increasing temperature. Hence, swelling increases with temperature and, in the case of gelatin, when the osmotic force causing the entrance of water into the gelatin structure exceeds the cohesive strength of the gelatin, the gelatin goes into solution. This usually occurs a t temperatures above 30’ C. As already stated, it was found by Svedberg and in our experiments that the presence of electrolytes decreases the degree of hydration of gelatin. On the other hand, electrolytes may either increase or decrease the degree of swelling, depending upon whether they increase or decrease the disparity in total concentration of diffusible ions inside and outside the gelatin structure. With regard to the soaking of hides, leather technologists distinguish two effects which they call “swelling” and “plumping.” It is suggested that swelling corresponds approxi-
Vol. 22, No. 1
mately to what we have called hydration, that water taken up by the osmotic swelling is mechanically held and is largely lost when the hide goes through the wringers, but water of hydration cannot be removed by the application of pressure, as its addition is accompanied by a contraction of the system. The volume-change method here described for the study of gelatin has also been applied by the writers to animal skin in the form of corium and cured hide. It has been shown (8) that the soaking and liming processes can be controlled conveniently by this method. Degenerative changes, presumably due to bacterial action and resulting in destruction of leather substance, show u p with our apparatus as an expansion following a latent bacterial period during which the normal contraction of the system occurs. I n the case of poorly cured domestic hides this expansion begins after 6 hours when the hide is soaked in water or very dilute acids, while for South American hides a longer soaking period, in some cases 36 hours, may be used before bacterial action sets in. It was suggested that liming should be started on any batch of hides a t that point of maximum hydration before bacterial action becomes dominant. The results shown in Figure 5 , which indicate the expansion of the system occurring when a fully hydrated material is placed in electrolytes, suggested that the action of substances considered astringent might be studied by this means. In Figure 7 are shown the results obtained when 20-gram samples of cured hide which had been soaked 24 hours and limed 5 days were placed in solutions of recognized astringent action. Similar results may be obtained by using pieces of a 10 per cent gelatin jelly, as was done in obtaining the data shown in Figure 5 . As there is no convenient standard method of evaluating the astringency of various solutions, this volume-expansion method may prove of value in giving a relative measure of this property and in setting up definite standards. On the basis of the results discussed in this paper, it would appear that the action of an astringent is the dehydration of tissues, causing them to contract and thereby also forcing out some of the mechanically held water of swelling. Literature Cited (1) Brown, J . Am. Chem. Soc., 46, 1207 (1924). (2) Loeb, “Proteins and Theory of Colloidal Behavior,” p. 193, McGrawHill, New York, 1922. (3) Neville and Jones, Colloid Symposium Monograph, Vol. VI, 309 (1928). (4) Pauli, “Colloid Chemistry of the Proteins,” p. 10, Blakiston’s, Philadelphia, 1922. ( 6 ) Procter and Wilson, J . Chem. Soc., 109, 307 (1916). (6) Svedberg, J . Am. Chem. Soc., 46, 2613 (1923). (7) Svedberg, Ibid., 46, 2673 (1924). (8) Theis and Neville, IND.ENG.CHEM.,21, 377 (1929).
11-A Method for Determining Transition Temperatures of Gels and Sols1 *
Harvey A. Neville, Edwin R. Theis, and C. T. Oswald
WM. H. CHANDLER CHEMICAL LABORATORY, LEHIQHUNIVERSITY, BETHLEHEM, PA.
T
HE gelation temperature of a given gelatin sol and the
melting temperature of the gel apparently are not characteristic constants but depend upon the rate of cooling or heating, previous treatment of the material, and other factors. However, if all other conditions are maintained as nearly constant as is possible, the determination of these temperatures for different gelatins, or for the same gelatin when varying one condition, may give relative values of considerable importance. 1
Received October 2, 1929.
Two general methods have been available for such measurements. By one method (2) gelatin solutions are placed in test tubes of the same size and these are immersed in a water bath provided with a stirrer. The temperature of the water is gradually changed. The temperature a t which the content of each tube solidifies is noted. This is determined as that temperature a t which the gelatin can no longer be poured from the tube. The reverse process is followed to determine melting points. By the second method (3) air bubbles are permitted to rise through cooling gelatin sols. The
January, 1930
INDUSTRIAL AND ENGINEERING CHEMISTRY
temperature a t which the bubble ceases to rise is taken as the gelation temperature. The melting point is determined as that temperature a t which the bubbles again begin to rise when the gel is warmed. It seemed desirable to devise a method which would give continuous readings, in which the gel structure would not be disturbed while it was being formed, and which would give results independent of the judgment of the individual o bserrer . Apparatus and Method
61
of the spiral prevents the thermal contraction of the larger portion of the gelatin sol from registering in the oil manometer. A break in the cooling curve of the solution is thus obtained for the temperature a t which the resistance to flow of the gelatin in the spiral becomes appreciable. This is essentially the temperature a t which the viscous flow of the sol changes to the plastic flow of the gel. This point can be determined with considerable accuracy by taking careful and frequent readings in this range. After the system has been cooled below the gelation temperature it may be warmed again and the temperature a t which the gel melts will register on the capillary as a sudden rise.
As pointed out in Part I there is no indication of discontinuity in the volume change of a gelatin sol when it sets Results and Discussion to a gel. That is, although the degree of hydration of gelatin A typical curve, showing the gelation and melting points increases with decreasing temperature, there is no observable difference in hydration between the sol and gel states of gela- of a 10 per cent gelatin solution, is represented in Figure tin at the transition temperature. The volume change 3, The curve obtained when warming the gel does not follow the cooling curve and the with variation in tempera-I melting temperature is alture of a 10 per cent soh- l, w a y s considerably higher tion of Difco- g e l a t i n a s A modified dilatometer, by means of which the gelathan the gelation temperacompared with the volume tion and melting temperatures of gelatin solutions can t u r e . T h e difference bechange of the same quantity be determined under carefully controlled conditions, tween these values is termed of water, using the dilatomeis described. By this method continuous readings the hysteresis of the gel. ter described in Part I, is are obtained and the gel structure is not disturbed That portion of the coolshown in Figure 2. The during the process. ing curve above the gelaincrease in hydration with Typical results are given for gelatins of different tion temperature, in all the decreasing temperature is Bloom strengths and the effect of repeated gelation experiments we have pershown in this curve by its and melting is shown. f o r m e d , c o n s i s t s of two increased deviation from the A sudden change in slope of all cooling curves of gelap r a c t i c a l l y straight lines water curve as the temperatin solutions is noted at about 36" C. It is suggested having a point of intersecture is lowered. This curve that the explanation of this is the transformation of tion, which in Figure 3 occ o n t i n u e s a s a straight sol form A into gel form B at this temperature. curs a t about 37" C. This line through the g e l a t i o n I point is also apparent in Figtemperature ( b e yo n d t h e I' ure 2. The cooling curve range of this chart) giving no indication of the change from sol to gel. The point for water shown in Figure 2 is not a straight line and should of inflection of this curve a t about 39" C. will be discussed not be, as the coefficient of expansion of water varies with temperature. However, the water curve shows no indication later. A modified dilatometer, by means of which the gelation of a sudden change in slope when obtained by means of the and melting temperatures of gelatin can be determined, is shown in Figure 1. This apparatus consists of a piece of Pyrex glass tubing drawn into a thin tube and coiled into a spiral. The bore of this spiral tube is about 2 mm. in diameter. Below the coil is attached a larger tube having a volume of about 15 cc. and bent in a U-shape. At the other end of this tube is a stopcock and a wide mouth to be used in filling the apparatus with the gelatin solution. The apparatus and gelatin solution are brought to a constant temperature of, for example, 45" C. in a vessel of water. The apparatus is then filled with the solution to somewhat above the top of the spiral and the stopcock is closed. Mineral oil is poured into the open tube above the gelatin solution and a rubber stopper bearing a capillary tube is inserted. Figure I-Modified Dilatometer The oil rises in the capillary tube, where its position is followed by reading a millimeter scale placed behind the capillary. The large vessel of water in which the apparatus is dilatometer shown in Figure 1 under the identical condiimmersed is provided with a stirrer and with a means of heat- tions used for the gelatin solutions. The only explanation ing and cooling at a fairly constant rate. I n these experi- we can suggest for the point of inflection in the gelatin curves ments this was done by means of regulated streams of warm is that it involves the transition of the two forms of gelatin. and cold water. The existence of two forms of gelatin which are reversibly After the apparatus and contents are in thermal equilib- transformable has been discussed by Smith (4). H is ' rerium with the bath, and the oil in the capillary has attained sults are based upon the specific rotation of gelatin a t varia constant position, the system is cooled a t a constant rate ous temperatures. Smith found that a t temperatures above and readings are taken a t short intervals. The spiral por- 33" to 35" C. the specific rotation is practically constant tion of the apparatus has a very thin wall and the bore is a t about -123O, while a t temperatures below 15" C. the small, so that the gelatin solution in this follows very readily specific rotation has a constant value of -266". Hence, the temperature changes in the baths. When the gelatin he concludes that gelatin in aqueous solution exists in two in the spiral begins to form a gel its adhesion to the wall modifications, the one stable a t temperatures above 35 "
INDUSTRIAL A N D ENGINEERING CHEMISTRY 32
53 34
Vol. 22, No. 1
-
-
g 35 -
g 36 :: -
e
67
u?
2
38
-
3
39
-
3
41 42 -
8
1 Y
40
20
40
la
80 100
60
180
220
260
CAPILLARY DROP IN MILLIMETERS
0
Figure Z-Comparieon of t h e Cooling Curve8 of Water and of a Gelatin Sol
C., which he calls sol form A, and the other stable below 15" C. which he denotes gel form B. Between these temperatures a condition of equilibrium exists and the mutarotation observed seems to be due to the transformation of one form into the other. Smith also found that a smaller quantity of alcohol was required to coagulate gelatin sols a t temperatures below 35" C. than at higher temperatures. This would indicate that the degree of hydration of the gel form B is less than that of the sol form A, or, more probably, that the degree of dispersion of the gel form B is less, owing to the formation of B units from two or more A units. This result is consistent with the direction of the change in slope noted in our curves. The inflection indicates a decrease in the hydration of the form stable at the lower temperatures and is probably due to a decrease in the degree of dispersion of the gelatin a t the lower temperatures.
Bloom Strength : 1 2 5 .
45
I
I
I
10
35
x)
I X6
TOYPSFATWRR I n DEW
I
I
10
15
CEBPMLAW
Figure (-Changes in a 10 Per Cent Gelatin Solution upon Gelation, Melting Repeatedly I-After 6rst cooling held in get condition for 48 hours before heating. Arrows pointing up indicate cooling; arrows pointing down indicate heating.
perature above this a decrease occurred until the viscosity equaled that of an unseeded portion at the same temperature. It was attempted to determine by means of our apparatus if there is any relationship between the Bloom strength of gelatin and the gelation, melting, or transition temperatures. The data obtained (Table I) show that there is no apparent regular relation, although, in general, the melting point is higher the higher the Bloom test. between Bloom Strength and Temperature of Transition, Gelation, and Melting BLOOM STRENGTHTRANSITIONGELATION MELTING HYSTERESIS O c. O c . c. O c. 125 37.6 22.2 27.2 5.0 150 37.1 26.0 * 29.4 3.4 175 34.8 26.8 29.4 2.6 200 36.8 25.0 29.0 4.0 225 33.6 26.8 31.2 4.4 250 36.4 26.8 31.4 4.6
Table I-Relation
I n Figure 4 are shown the results of the repeated alternate gelation and melting of a 10 per cent gelatin solution. The gelatin was left in the gel condition for 48 hours and this resulted in increasing the hysteresis to about 10" C. The apparent effect of repeated melting and gelation is to bring these two temperatures closer together, that is, to decrease the hysteresis. The dependence of the observed gelation and melting points upon the previous history of the gelatin is also illustrated in these curves. Literature Cited
f
