T H E ELASTICITY O F GELATIK I S RELATION TO PH AND SWELLISG * B Y G . W. SCARTH
The method here followed in investigating the relation of the elasticity of a gelatin jelly to H-ion concentration is such that the results bear directly on two theoretical questions. The first is physical and concerns the theory of swelling. Does the force which opposes and limits imbibition in gelatin, namely. its cohesion, measured by the modulus of bulk elasticity, remain a constant throughout the range of H-ion concentration as assumed for example in the Procter-SVilson-Loeb theory of swelling and viscosity; or if not, how will its variations affect these other properties? The second question is physiological and deals with the mechanism of protoplasmic contraction. Do any changes in elasticity occur in a gelatin jelly when subjected to acid, comparable to those occurring in contractile protoplasm during the liberation of acid, which is probably not confined t o muscle merely as the chemical cause of contraction? If so, which phase of the response in gelatin. that of shrinkage up to pH 4.7. or of swelling beyond that point corresponds as regards elasticity, to the contraction phase in protoplasm. Previous work on the relation of H-ion concentration to the elasticity of gelatin does riot allow us to draw conclusions as to these questions. The only systematic investigations are those of Sheppard and Sweet ( 1 9 2 2 , 1924). In their experiments sols of various concentrations both of gelatin and of acid or alkali were allowed to set to jellies and the elasticity of the latter was measured in air or in oil. But in the case of both the specific probleins inentioned above, the gels concerned are surrounded by a watery medium and free to imbibe water until they are at equilibrium with that medium whatever H-ion concentration it may a s u m e . Elasticity as well as volume is affected by imbibition. Moreover, in all experiments on physical swelling and shrinkage, and in some cases at least of protoplasmic contraction also, the acid acts upon a preformed gel and not on its formation from the sol. The distinction is important in view of the profound influence that mode of formation exerts on the physical properties of gels. In the following research therefore the variations in elasticity were measured in preformed and originally uniform jellies when brought to approvimate equilibrium with media of varied H-ion concentration.
I. Relation to pH Effect in General Isotropic gelatin. Gelatin melted and cast into cylinders is designated isotropic, s~velling being equal in all directions. Sheets of comn~ercial‘leaf’ gelatin, which swell *RePearch asisted by a Grant from the Honorary .Advisory C.o:incil for c’cicntific and Industrial Research of Canada.
Ioro
G . W. SCARTH
unequally, are signified when the term ‘aeolotropzc’ is used. Sheets of Coignet’s “gelatin A” were washed for 48 hours in 3 changes of ice cold water, acidified to a pH of 4.6 originally, falling to 4.9 eventually. The gelatin was then melted below 60’ C in its own water of imbibition and kept warm but below 40’ for several hours. A small quantity of toluene was added t o the above and to all subsequent solutionq. The concentration of gelatin at this stage was 1 2 . 7 5 . The solution was poured into a series of test tubes of I cm. diameter. Before it set two celluloid discs, each pierced by a bcnt pin, were inserted, one sinking to the bottom. the other buoyed near the top. After two days the tubes were slightly heated, and gently broken, and the pieces stripped off leaving smooth cylinders of gelatin. These w r e trimmed at each end so that a portion of the bent pin emerged as a means of attachment. They were suspended i.tl a medium of pH 4.8 + . I , temperature about rj°C. for 18 hours (or longer) a t the end of which time they had imbibed water sufficient to reduce the concentration of gelatin t o about IIC;. The average volume of the cylinders was 8.5 C.C.the average length from disc to disc 7 cm. and the diameter of each approximated 1.1 cm. At the end of 18 hours a t the isoelectric point, volume is nearly at an equilihriuni, and extensibility will remain constant if temperature does not vary. T701iiinewas determined by displacement. The measurements of extensibility were made by a system of amplifying levers permitting great delicacy of reading. These levers were used also for applying the stretching force. the equivalent of r 4 grams weight in the first set of experiments. The cylinders of gelatin having been standardized as regards dimensions and extensibility, were transferred to various concentrations of acid and alkali for 24 hours. RIeasurements were repeated and the pH of the medium at the end determined electrometrically. 24 hours was found necessary to approximate maximum change in estenqibility with small additions of acid and alkali. In high concentrations a longer period cause5 excessive disintegration clue to uneven swelling. Pieces flake off with conchoidal fracture. This was partially obviated by a graded increase in the concentration of acid or alkali. But beyond the pH limits of the curves in Fig. I , no reliable measurements could be made, n-hile the extreme readings even there are probably rather lower than they would he but for fracture. Even slight fluctuations in temperature may have a greater effect on elasticity than a considerable change in p H (the temperature coefficient being negative). All the measurements recorded below nere made at the same temperature ( r j”C) but this does not obviate the periistent effect of fluctuations in the interval. The latter were not eliminate(! entirely but their resultant effect was found from variation in the control a t pH 4.8,the extensibility of which, as shown by trial. would at constant temperature have remained constant. The neceqsary correction ( I to 7C;) i* niade in the results given.
ELASTICITY O F G E L A T I S
1011
Results: I n Fig. I are plotted, in percentages, the variations in volume, resistance to stretch (reciprocal of stretch) and Young's Modulus of stretch. The actual value of Young's Modulus (E) a t pH 4.7 was about 6 grams per square millimeter, slightly higher a t pH 7. It was calculated from the formula below-a clcse enough approximation for small stretches. 1 E = -Stress =--- P Strain S ' L where P = the force applied S = the cross section (stretched) 1 = the increase in length L = the length (stretched) S varies as L? in the swelling of this isotropic gelatin.
~1 I
A
G
8
10
1 J
FIG.I Elasticity and Swelling of isotropic cylinders of gelatin in rclation to pH of medium. E =Toung's 3Iodulus. I ext =reciprocal of extension under a constant stress.
Cnder the conditions of the above experiment when we think of elasticity in terms of Young's Modulus we regard each combination of gelatin, water and acid or alkali as a new substance of which we wish to know the modulus of elasticity, i.e. calculated for unit length and croSs section. When we consider how the concentration of gelatin varies (namely inversely as the volume) the uniformity of Young's llodulus is remarkable. A discussion of the results is deferred until other experiments have been recorded.
d eolotropic gelntin. More extensive experiments were performed on strips simply cut from sheets of the same commercial gelatin as was employed above. In virtue of their thinness they come to equilibrium much more speedily, but, since swelling is greater at right angles to their greater extension than in the same plane, the cross section increases a t a relatively greater rate than the length of the strip; consequently resistanre to stretch, as we might expect, increases
IO12
G . W. SCARTH
more rapidly in proportion to swelling than it does in isotropic gelatin. The relative difference in resistance to stretch falls far short, however, of the difference in cross section in the two cases. With an increase in cross section (at pH 3.1 as compared with pH 1 . 7 ) five times greater in aeolotropic than in isotropic gelatin ( 3 0 0 ~v.~ 6 0 7 ) the increase in resistance to stretch was barely doubled ( 6 j 7 c v. 35%). Strips of Coipnet's Gelatin A ( 7 X z cm. more or less) were clamped lcosely at each end by a cork clamp and suspended vertically in a fluid medium; the lower end, being weighted, rested on the bottom of the vessel, and the other
FIG.z Elasticity and STv-clljng of aeolotropic1 s t r i p of gelatin in relation t o pH of thc mccliuni.
was attached to an ausonometer as brforc. In this case. ahout z grams weight was the stretching force used. Extensibility, dimensions, arid pH were first determined in distilled water after 24 hours exposure. Subsequently acid or alkali was added t o the same medium and on the elapse of four to five hours the measurements were repeated. Tcmperature was not recorded. It probably varied but little in the few hour- that matter. Each plotted point was determined on a separate strip and is expressed as a percentage of the value in distilled water. In thiq Get of experinients lactic was the acid used in order to strengthen the analogy to the conditions in active muscle. The results are the same with HC1 over the greater part of the curve.
Results: In Fig. z elasticity curves corresponding t o those in Fig. I are given for aeolotropic commercial gelatin. Below is the approximate volume at the time of measurement of elasticity, and also a portion of the volume curve as given by Loeb for well washed gelatin. The variations in the curve of resistance to stretch are exaggerated as compared with those of Fig. I , and demonstrate more strikingly the correlation with swelling. The Young's Modulus curve falls away more rapidly in acid and alkali owing to the fact mentioned above that resistance to stretch in these aeolotropic strips does not increase p a r i pnssic with cross section area. The percentages are only approximate since thickness was measured not on the strips
1013
ELASTICITY O F G E L h T I S
tested but on similar pieces kept in the same solution. The measurements were made by an adaptation of the apparatus used t o measure extension. In another set of esperiineiitb actual valuw of E were determined. Some averages are given 11elow. That in distilled water is the same as for the isotropic gelatin of Fig. I n-hich also had a similar amount of imbibed mater.
TABLE I Young's Modulus in grams per ii7ni2 for strips of commercial (aeolotropic) gelatin at varied pH. Ktnni~ilt
ac1d
Lac t I(' sr:d
Initial conc.
hI -
hl -
2 00
j00
Lactic.
Appros. 3 . 0 final pH.
Gelatin conc. cc
3
E. of av. strip. 3 5.
3.6
5.5
6.3
9.0
1
I3
I2
8
3.6
10.0
>
6.4
R e l a t i o n s h z p o j t l i p Cuwes. Both in Figs. I and 2 the outstanding difference between the resistance to stretch and z,olume curves respectively is that in the case of the first the msxirniiin is higher on the alkali qitle and in the case of the second on the acid side. Correspondingly Young's 1Iodulus curve rises on the alkali side 81'0. Another asynlmetry common to all the curve' is a flattening hetween pH 4.; and p H 8.0. In this connection it may be recalled that Kil.on and Kearn i1922) obtained under certain conditions a second ~ninimurnof swelling at pH 7 . ; ; Davis and Oakes (1922) found the miniinuni viscosity above 3o°C. to be at the same point, while Sheppard and Sweet's elaqticity curves are highest in this region al'o. To e.cplain these and other (optical) phenomena the existence of two forms of gelatin has been postulated.^ On this hypothesis if each form had its maximum cohesion at it5 "isoelectric point," viz. 4.; and 7.7 respectively, the general shape of the compound curves could be explained. But n-hile elasticity antl'volunie are evidently correlated there can be no simple causal relation in one direction only. for the interaction i. mutual. Before theorising on thiq subject, however, we must further investigate the separate factor4 which affect elasticity. *This view has however been criticisetl for example by 1Jitc.t cock (1924) and Iiraemer (1925).
1014
G . W. SC.4RTH
11. Relation to the Separate Factors in the pH Effect Two methods were employed to distinguish the individual influence of the various factors involved in a change of pH, ( I ) separating them by critical time studies and ( 2 ) varying them independently. (I)
TIMESTUDIES
I first quote an experiment on the progressive uptake of acid and loss of material into solution when acid is added to isoelectric gelatin. Ezperinzenf: Five sheets of gelatin, each I gram, were kept a t approximately the isoelectric point (ca. pH 4.7) for 24 hours (each in 2 j o c.c HC1 6 X IO-: K (pH 3 . 2 ) at start, becoming pH 4.9 to 5.0 at end of period). Thereafter they were transferred to S 2 0 0 0 HCl, (each 2 jo c.c.),the various pieces removed a t intervals and the following determinations made :-
Pieces
I
to
Piece
j
TABLEI1 of isoelectric gelatin put’ into N/zooo HCl, pH 3.31.
Time
pH of medium
Mllimols acid taken
o hrs.
3.31
__
, 9 1 8 grms.
2.
I
3. 4.
4 26 48
3.51 3.64 3.81 3.86
.043 ,064 .08 I . os2
,901 ,899 ,886 ,880
NO.
“D I.
5.
” l’
” ”
Dry weight of gelatin. Origi n d wt. I grm.
l’
)’ ” ”
:,(
of isoelectric wt. 100.0
98.3 98. I 96.8 96.2
The amount of acid taken up would be equivalent to about one molecule to each molecule of gelatin-if its molecular weight were 1 2 , 0 0 0 . As a maximum about I j times that amount of acid can be absorbed. Other time studies were made over several days of the corresponding variation in volume and resistance to stretch of aeolotropic gelatin. The results for a 2 2 hour period of both sets of studies are combined in Fig. 3. The curves start with the addition of S 2 0 0 0 HC1 after 24 hours a t about pH 4.7. The t\ime curve of resistance t o stretch is seen to be compounded of three different curves, represented respectively by I ) A slight drop, corresponding to the period of maximum combination with reagent which is half completed in less than one hour. (If swelling is rapid this part of the curve is represented merely by a lag in rising). 2 ) A steep and large rise, corresponding t o the period of maximum swelling which lags behind the absorption of acid and is half completed (taking three days swelling as the “total”) only after about 8 hours. 3) X gradual fall thereafter, the cause for which is not apparent from the figure, It is not loss of material into solution, that factor being shown by the dry weight curve to be quite insignificant. This slow loss of elasticity explains perhaps the absence of a definite swelling limit.
ELASTICITY O F GELATIN
IO1j
The time curve of resistance to stretch in alkali differs from that in acid in showing no initial drop, as might have been anticipated from the corresponding values of Young’s Modulus. d rapid rise followed by a gradual fall are exhibited just as in acid. Of the 3 factors revealed in Figs. 3 the combination with reagent* and the imbibition of mater are reversible when the pH is reversed. The third factor is only partially reversible as is shown by Fig. 3 . After 1 2 hours exposure to S ’2000 HCI the strip was transferred to a medium of pH 6 which it gradualljchanged to about 4.8. T’olume falls rapidly, and with it, resistance t o stretch for about 3 hours. As the volume curve flattens out elasticity begins to recover. This recovery, however, never restoreq elasticity to its former value; nor does volume revert entirely to the original. This is equally true of the result of swelling in alkali, The inference is that a persistent structural change is brought about by swelling even in dilute solutions of acid and Further indication of the persistence qtructural modification is indicated by the result of a repetition of the cycle under exactly the same conTIC 3 ditions. T’olume increases much more Time study relating rrsiqtanre t o stretch in the second Or succeetling t o t h e separate factors involrrd in acidificntion of isoelertric gelatin The experimmt cycles than it does in the first, until starts n i t h the addition of S ; 2 o ~ o H C after l the first swelling limit is approached, 24 hours a t the iroelectric point. after which the rate is the same as before, The modulus of bulk elasticity must have suffered loss along with that of stretch to allow of the more rapid swelling. As regards resistance to stretch the second rise is also much quicker and greater than the first-the result obviously of the more rapid imbibition. But the second maximum still falls short of the first becauee there has been a permanent loss of elasticity. The most instructive feature, however, of the second cycle is that if the swelling is not too prolonged there is little further loss of elasticity. The second niinimum is little lower than the first. This points to an actual mechanical damage resulting from swelling, which, once done, does not require to be repeated by a similar distension. (2).
I X D E P E X D E X T T’ARIATIOS O F
FACTORS
(a) THECHEiiIcu FACTOR. Variation of pH with swelling partially suppressed. By the addition of a neutral salt the swelling in acid or alkali can be largely inhibited, while the amount of acid or alkali taken up by the gelatin is changed but little. *The statement requires slight modification for reaction with strong a!kali cf. Lloyd f19201, IVilson (1qz3l. Fairbrother (19241.
1016
G. W. SCARTH
The result in the case of acid and salt i R wen in Curve B, Fig. j . There is a considerable and protracted depression of the resistance to stretch indicating clearly that if smelling could be entirely suppressed, combination with acid would reduce elasticity. In t,he case of alkali and salt, however (Curve B, Fig. 6) there is no fall in the resistance to stretch, but merely a diminished rise correqponding to the relatively diminished swelling. (b) THEIMBIBITION FACTOR. RIodificatioIl of swelling with H-ion concentration unchanged. h comparison of the result when swelling is allowed free play as in Curves *A, figs. 5 and 6, and when it is partially suppressed (Curves B) demonstrates
FIG.
1
Time study. T h e effect of successively acidifying isoelectric gelatin with S ; x c o HC1 a n d restoring to t h e isoelrctric state. The first swlling; causes a permanent loss of plasticity, t h e second has little fuither effcct.
the powerful effect of the imbibition factor in increasing the resistance to stretch. The following experiments illustrate the same action of sTvelling when the pH is maintained at the isoelectric point. Imbibition per se. It was noted that the cylinder; of isoelectric and isotropic gelatin used in the first experiment, when imbibing water-as they did for some time in the pH 4.7 solution-became gradually less extensible, maintaining in fact, in spite of dilution, an almost constant modulus of elasticity. Salt e f e c t nt the isoelectric point. The statement frequently made that neutral salts repress swelling at every pH is incorrect. I n the pH zone of low swelling the effect of moderate concentrations of salt is to markedly increase the volume, higher concentrations bringing about a decrease. The maximum of course is much inferior to that induced by acid or alkali and in pH regions of high swelling only the depressing effect of salts remains conspicuous. But
:or:
ELASTICITT O F G E L A T I S
in the range including the isoelectric and neutral points the general effect of salts resemble.. that of acids or alkalis. Salts with a polyvalent ion are most active. Experiment. To a cylinder of the above gelatin a t constant equilibrium with a pure acid niediuiii of pH 3 . ; successively greater concentrations of h l S 0 3 ) were 3 added a t two clay intervals keeping the same pH.
120
TABLE I11 Effect of L a ( S 0 J 3 at the i.oelectric point. Reagent
7 cmp C
pH
rj0 3 .liter 4 days in HC'I alone (ca. S 2 0 , 0 0 0 ) p1u.i 2 days in HCI and 31 1000La(x03)3 1 6 ~ 3 4 plus 2 days in HCl and 11 2 0 0 La ( S O , ) < 13' plus z days in HCI and 11 I O La(l;03)3 13' 3
1-01
I
Lxt
I,
IOO
roo 98
7 ; 7
roo 130 Ijj
106 116
7
135
60
IOI
73
1018
G . W. hCARTH
T-olunie and resistance to htretch increase to a point antl fall thereafter. The concentration of L a ( S 0 3 ) 3at which the maxima lie is apparently of the same order of magnitude as the corresponding concentration of HC1, viz: between IO-* and I 0 c 3 molar. Toung’s Modulus remains perfectly constant until a high concentration of salt is reached.
(e) THEFACTOR or STRUCTL-RAL C‘H~UGI:. I t s relation to the other t w o fnctors. Figs. 5 and 6 illustrate what happens nhen a strip (A) which has undergone high swelling in pure acid or alkali. is transferred t o the same concentration of acid or alkali with neutral salt added. The volume falls and gradually approaches that of strip (13) which has remained for the same period in the mixed medium. But the point is that the resistance to stretch and the modulus of elasticity of d fall far below that of B. This relative loss of elasticity can only be ascribed to -1’q greater snelling. On the other hand, B with only moderate swelling lc>es clasticity al-o. It is not fea4ble entirely t o suppress swelling in presence of acid or alkali in order that we may find out if any portion of the total loss of elasticity iq directly due to the action of the reagents, but the indications are that it is; for cohesion is most rapidly weakened in high concentrat ions of acid and alkali beyond the points of maximum swelling. Conclusions and Theoretical Considerations T-ariation in pH of the hurrountling medium affects the elasticity of a piece of gelatin in at least three different ways:-^) directly through chemical (or physical) combination or tlecombination : z ) indirectly through swelling or shrinkage, and 3) through slow. iupposedly qtructural. changes induced by factors I ) and 2 ) . I ) The direct effect of acid on isoelectric gelatin, apart from iecondary swelling effects, is to cause a clecreabe in the reiistance to stretch. Tensile strength is also markedly diniinished though no quantitative measurements were made. It is probable therefore that molecular cohesivenev is decreased. This is what one might expect nith an electrically charged a< coinpared with a neutral state of the colloidal particle.. . On the other hand there i q no evidence that low concentration of alkali causes loss of cohesion. I n this connection however the povibility that part of the gelatin is “isoelectric” at pH 7 . 7 muit be kept in mind (see antea). On that assumption as the pH is shifted from 4.7 to 7 . 7 part of the gelatin i3 departing from and part approaching it? iwelectric point. Hence loss of cohesion in the first case may be balanced by gain in the second. 2 ) TT-ater of imbibition reinforces the elnqticity of gelatin t o w c h an extent as to compensate for diminishctl concentration of solid matter as long as structure is unimpaired. It \\-a\ qualitatiwly evident in performing the experiments and has been proved by Sheppard antl Sweet. that elasticity t o torsional and bending stresses Yaries n ith that of stretch. Tensile strength however tends on the contrary to decrca*e with water of imbibition. That i i to say, although, within the limits of elahticity, extensibilit\y ia decreased,
ELASTI CITY O F GELATIS
1019
yet, if me increase the load, the breaking point is earlier attained in the more swollen gel. As to the mechanism through which the water acts, fortified resistanceby increase of internal pressure-to a decrease of volume resulting from deformation is apparently ruled out since the volume of gelatin is unaffected by stress. Since the evidence points to a tn-o-phase structure in the gel, it may be suggested that the more solid phase imbibes water at the expense of the more liquid. leaving less room for relative displacement of the strands of the mesh. By this hypothesis elasticity and rigidity in the gel become correlated with viscosity in the sol. 3) Structural changes may be induced bp the above factors as is demonstrated by the persistent effect on elasticity and on subsequent swelling. It is recognized that properties of a gelatin jelly, which can only be ascribed to structure, da,te largely froin the “last n-arming” as Bogue expresses it. Swelling capacity in relation to concentration at that moment has been most studied. But elasticity is equally influenced. A jelly set from a 14~:sol has an elastic modulus greater by one half than that from an I I C ~ sol. , But if allowed to imbibe water till its concentration falls to II~;, its modulus, being scarcely diminished, is still one half greater than that of the ,jelly which was originally formed a t that concentration. The “last swelling” in acid or alkali must also be given a place in determining the structure of a formed jelly. The effect, and probably the mechanism, is the same as that of gentle warming. Presumably dispersion is increased, material passing from the gel to the sol phase and weakening the structure. On reversing the pH, as on cooling, re-aggregation occurs. but it begins in a more dilute gel and the reintegrated structure is thus less dense with a lower elastic coefficient than the original. A p p l i c a t i o n io T h e m e s of Srzclling. If gelatin were a homogeneous substance the moduluc of bulk elasticity would be directly related to that of stretch. But in a diphasic system structural inotlification can influence rigidity without greatly affecting bulk clasticity when volume change iq unaccompanied by change of shape. I t is doubtful therefore if the resistance to swelling can be calciilated directly from Yoling’s ;\Iodulus of stretch. We have seen evidence however that they do in some degree vary together; consequently the rather wrprising constancy of Young’s 1Iotlulus (in the isotropic gelatin of which alone we need take account) justifies in large meawre the assumption which J. -4. and IT, H. Wilson (1918)made for their hypothetical niialogue of gelatin that the bulk modulus is a constant. The deviations from constancy, viz: a fall in acid and a slight rise in alkali up to a point are only huch as to explain the asymmetry exhibited by the curve of swelling. I t is clear a t leact that the larger undulations of this curve must be euplained by variations in the force of imbibition, whatever its nature, rather than bv variation? in the oppoiiiig force,
A4pplicatiojz to [ h e Theor!/
01Pi,oloplr!.stjiic Contraction.
Muscle pl:isii!3. is neutral or slightly alkaline, n-hile the proteins of muscle have a decidedly acid isoelectric point. That of myosin is giT.-en as p H 3.9 (Granstroin), the average of the inyoprotein granules as between pH j arid 4..6~Quagliarielloi. The first effect of lactic acid piotluction must therefore be to render the alkaline proteins 1norc1 nearly isoelectric. The contractile protoplasmic mhstaiicc is aeolotropic. Son-,in an aeolot,ropic gelatin jelly a corresponding change of pH procliicw r11.orc or less uniaxial shrinkage (i.e. cont'raction) and, as i w have shonn, an increased cxtensiliility. Correspondingly incwasctl estc.nsiijility. a~ is gmerally stated. is a feature of contracted imiclc also. So. too, g w t t e r flesihility antl flaccidity chnracterise a ciliuin a t tlie end of its effective stroke, which, tieing the position of minimum potential energy. corresponds to the phase of iiia~iiniinicont,raction in muscle (Gray). Similarly a fa811in viscosity ton.arrl the isoelectric point is shown by gelatin in the sol or sol-gel transition state (Bogiie). Siich a change is associated with a reversible contraction of the chloroplasts of Spirogyra isearth, 1 9 2 2 , 1 9 2 4 ) while in dnioeba the coi1tr::ction of t?ie posterior portion of the ectopla~iiiic t'ube is acccm.panied hy its liquefaction into ciitlopla.~iiiin tlie same region. Thus far the physical changes ~vhicliwe have noted in protoplamiic contraction agree Iyit'h those occiiri,irig in gelatin \dicn it is Zci(lifiec1 t o the isoelectric point. GeIat'in however is not coni pletcly rcprrwntative of all protein gels in its physical behavioiir nlien reudered isoelectric. The removal of electric charge froin lyophobe colloitlal particles teiitli to alloir- of their aggregation. The same tendency is in-pnrtetl t o l~-ol~liilc particle:: with the :tdditional tendency to lose some of the water \r-hich the;\- lioltl. The two l)rocesse~may h v e an opposite effect ozi phyficnl properti and tlic result will depend on which dominates. Only when c!ehydrat8ion prccioniinatcs-as in gelatin--r:bould viscocit,y antl rc4stcncp to stretching fall. If the prcdoniinating result is the a g g r e g a h n ant1 li111;ing ~ i of p particlw: inicellae or inoleciilc~--as in blood plasine,-visco~ity mid elasticity inust riw. Even in gelatin, it is at, the isoelectric point that we find niasini~initurbidity antl iiiaximuin syneresis antl increase of viscosity on lapse of tinic-aggregation phenomena. The recent, work of Gaswr aii(1 Hill ( I ( s ~ : , ) ,therefore, pointing to a decided increase both in viscosity and elastic iiiodiilus when a iiiuscle ib stiinulatetl t o contract is not oppccwl t o the general theory of a shift to the isoelectric point, though the physical changes are not well typified by gelatin. X colloid more easily aggregated would form a het'ter nioclel. If, fundamentally, the process is the 3ame in all cases, yet the difference in the effect on viseosit,y and elasticity in muscle, as compared with the more rudimentary structures mentioned, points to a corresponding clifferenee in the nature of the colloidal substance on which tlie chemical factor acts.
ELASTICITY O F G E L A T I S
I02I
Summary The relation of the elasticity of gelatin to pH is determined by the method of transferring preformed gels from a medium of one pH to one of another and comparing the extensibility under a given itre- after volume change is more or less completed. The reciprocal of the extencion (resistance to stretch) has a minimum value a t pH 4.7 and maxima about pH 3 and pH I I , the alkali maximum being the higher. Young's RIodulus in the case of isotropic gelatin varies very little between pH 4.; and pH I I ; it falls slightly on the acid side of the isoelectric point. d salt with a trivalent ii.e. colloidally active) cation has the same general effect on gelatin a t a uniforii~.pH of 4. j , as has the addition of acid or alkali to isoelectric gelatin. T-olunie and resistance to stretch increase up t o a certain concentration of salt (betlyeen Io-?ancl I O - ~ M )and , fall in higher concentration. The modulus of elasticity remains constant up to a certain limit. Three separate factorc: affecting elasticitp are involved in pH change:I ) The direct action of the reagent in combining chemically or by adsorption with the gelatin. In the ease of acid the effect is to reduce elasticity; in the case of alkali, in low concentration to incrcaqe it slightly. in high to decrease it. The change i i reversible. 2 ) The modification of imbibition requiting froin such combination. Swelling increases the total reqistance to .tretch and maintains the modulus of elasticity a t a constant value in spite of dilution of the pel. The change is again reversible. 3) Structural change induced mainly by 2 ) by partly by I ) . This entails a loss of elasticity, partly reversible and partly not. Theoreti cul Concliisi ons.
Gelatin gels have a definite hetcrogeneous structure nioclifiahle hy welling agents 2s well as by heat. Tariations in the degree of swelling with chanpt. of pH arc not eyplicable as the result of contrary variations in bulk elaqticity except a s regards illinor features such as the greater swelling in acid than in alkali, The major variations must be due to changes in a positivc forcc attrscting nater. The elasticity and viscosity changes attending protoplawiic contraction are analogous t o thoFe attending the shrinkage of protein gels TI hen acidified from a neutral or alkali condition t o the iboplectric point. References Boguc: "The Chemistry 2nd Technology of (;clatin and C;lur". C . E. Davis and E. T. Onkcs: J. ;Im. C'hem. Soc.. 44. 464 i19221 F . Fnirhrothcr: Biochem. ,J., 18, 647 (1924). H . S.Gasspr :inti A . V. Hill: Proc. Roy. Soc.. 96 B. 398 f r9z$. I
