PLASTICITY AND STRUCTURE I?; GELATIS SYSTEMS’
J. Phys. Chem. 1924.29:1233-1238. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 09/10/15. For personal use only.
BY ROBERT H . BOGUE
The viscosity of gelatin systems has received attention at frequent intervals for a great many years. The concept of plasticity is a more recent development, formerly not separated at all from the concept of viscosity in studies on the flow of gelatin. I t is one of those studies which have evolved in an effort t o fix more definitely in our minds the structural characteristics of matter. For by a recognition of the lams governing the flow of matter, we come closer t o an understanding of the internal complexity which determines the flow. I n fact, studies designed specifically t o solve some of the mysteries of structure have constituted practically the entire work that has been done on the plasticity of gelatin. Plasticity has served as the tool for attacking the larger and more important problem. A paper? on “The Sol-Gel Equilibrium in Protein Systems” by the writer in 1 9 2 2 was the first attempt that had been made to utilize the principle of plasticity in an application t o the structure of gelatin. Certain other studies however ha-,,e furnished data which bear indirectly on this field. These will be referred t o in their bearing on the problem in this paper. The controversy on the existence of a definite transition temperature between the sol and the gel form of gelatin brings up prominently the question of difference in structure between the two forms. And siiice the properties of the tn-o forins are so different, one is justified in expecting that a decided dissimilarity in internal structure exists. Furthermore, since the gel takes the form of a solid, and the sol the form of a liquid. the natural sequence of ideas would lead t o the inquiry; are the tvio forms solid and liquid, respectively, in the classical sense of the terms. One of the criteria of solid crystalline matter is the existence of a welldefined melting point temperature. Another, niore recently introduced, is the requirement of a finite shearing stress to produce flow. All attempts t o obtain a definite melting point for gelatins have failed. Of course values have been given but they have little significance and are not absolute. d s Sheppard3 has said, “both the ‘melting point’ and the ‘setting point’ are more or less arbitrary conceptions, and their cletermination depends iiiainly upon standardized experimental conventions”. As far as we are able t o determine, the transition from the hydrosol t o the hydrogel condition in these systems is continuous. This would seem t o indicate that if the gel js strictly a solid material, and the sol a liquid material, then the change from solid to liquid is a gradual and continuous process, and not an abrupt and discontinuous process, in these systems. The bearing of this conclusion on the significance of plasticity measurements will be referred to later. Paper presented at the Plasticity Symposium Lafayette College, Oct. 1 7 (1924). * R . H. Bogue: J. Am. Chem. Poc.. 44, 1313 (1922). 3 J. Ind. Eng. Chem. 13, 423 ( 1 9 2 1 ) .
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I234
ROBERT H . BOGUF:
An att,ack iipoii tlie conclitioiis of transition from sol t o gel has h e n made from another angle by (.'. R. Smith]. He reported that at temperatures ahove 33' t o 3j0 the specific rotation of g l a t i n is practically constant at about - I Z ~ " ,while at teinperatures lielow 15' the specific rotation is practically constant at about - 266'. -itall tenipcratures intermediate between 35' and 15' the rotat>ionvaries hetween these two limits. Smith arrives at t8he conclusion that gelatin in aqueous solution exists in t v o modifications ; the one stable at temperatures ahove 33"-.35' which he denotes as dol form A , the other stable at t'emperatures below 'j3 which he denotes as Gel form B. "Between t,hese teinperat#uresa condition of equilibrium between the two forms exists and the mutarotatmionobserved seems t o be due to the transformation of one form into the ot'her hy a reaction which is reversible wit'h temperature". This work constitut.es therefore a second line of evidence indicating that the transition from gel to sol is a gradual anti continuous process. ,J. A. IYilson? has reported p i n & of mininiuni sn-elling of pure ash-free gelatin at pH 4.7 and ;.i,an(l lIathews3 has shorr-n that the ahsorption spectrum of gelatin has iiiiniriiuni values for the wave length of inaxiiiium absorpt,ion in t'he ultraviolet a t pH 4.69 and 7.65. IYilson tentatively interprets these findings to indicate that the gel forin has an isoelectric point' at pH 4.7 but that between pH 4.7 and 7 . 7 the gel passes into t'he sol form and that the latter has an isoelect,ric point at pH 7 . 7 . He concludes .that both temperature and hydrogen ion concentration may h a r e the capacity t>lierefore of transforming, reversibly, the gel to the sol form. -4 check on this suggestion was obtained by a study on tlie influence of hydrogen ion concentration on the optical rotation. If Smith is correct in his conclusion that t,he gel has a specific rotation of -266" nhilc t h e sol has a specific rotat,ion of - 1 2 3 ' ) then t8hevariation in this property with pH, at a given temperature, should indicate n transition. if this occurs. This problem was studied hy AI. T. O'('onnt~l1and the using an isoelectric ash-free gelatin obtained from the East8iiiaii Iiodak Company thru the courtesy of Dr. S.E. Sheppard. Lita constant teniperature of 3 0 3 , which is the onlj- one yet investigated, t'he specific ro-tation of a 2 percent solution was found t o very with the pH in a regular iiiariner, rising from - 90' a t pH 0.3 t o -134.5' at pI-1 2.8, dropping to 04' at pH 4.j , rising to -139"at pH 7.0 and again dropping to 69" at pH 13.4. In no case however was t'liere found any indication of a value of the order of -266" which was the value given by Sniit8hfor the pel form. And there was no indication of depression in the c u r w in t,he neighborhood of pH 7 . 7 , tlie 1:oint' suggested by Kilson as possibly represent,ative of thc isoelectric point of t h c sol form. It would seem from these studies that whereas temperature could effect the transition, snd does eff'ect a gradual and coiltinous transition from gel J. Am. Chem. SOC.,4 1 146 (1919), J. Inti. Erig. Chem.. 12, 2878 i19201.
* J. d m . Chem. a
SOC.,44, 2633 (1922): 45, 3139 11923). J. .Am. Chem. SOC., 46, 8g2 (1924). J. Ani. ('hem. Soc. 47, 1694 ( I ( J ~ < ) ,
J. Phys. Chem. 1924.29:1233-1238. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 09/10/15. For personal use only.
to sol, yet that hydrogel] ion concentration is unable to do so. a t least at the tcniperature of the experiment, 3 0 3 . To be conclusive however, t,he work must be repeat,ed a t a low temperature. Tlie work thus far referred t o in this paper has S ~ O T V I Iin every case a gradual and continuous transition from gel t>osol, and soenis to indicate therefore a transition of a similar character froin the solid to the liquid state. Iienieinhering Ringhaiii's criterion for the solicl vs. the liquid state. liasctl 011 plastic vs. viscous flew, we would expect. if thc ahove conclnsions were suhstantiated, that the gel would exhibit plastic flow ancl show a yield value, and that the sol nould show only viscous flow. This was suhstantiatetl cornliletely i n the work reported liy t'he writer1 in 1 9 2 2 . Tlie angulai*deflection produced by the use of a 3lac3lichael viscometer was nieasuretl, using gelatin solutions of I O , 2 0 , ant1 2 j percent concentrations, a t varying shearing stress (revolutions per minute of the cup from j to 1 0 0 1 for teiiipcrat'ures froin z j oto 60'. The angular deflection was plotted on the abscissa against the shearing stress (R. P. 31.) on the ordinates. The curves were t'hen estrapolated to the intersection with the axes. It was found that in all cases where the temperaturc vas above 34'. the estrapolated curves passed t hru the origin of the axes, while at teiiiperatures l~elow34' the extrapolat'ed curves in general intersected the ahscissa. a t a distance froin the origin, this distance increasing as the temperature was i,cdiicetl. This iiieans that in those cases where t h e intercept lies on t'he ahscissa an infinitely small shear will result in a deflection of finite iiiagnitude. That is, t,he gelatin under these conditions offers a permanent arid fisetl resistance t o cleforination. I t is an elastic bod!-; it possesses a iiieasurable tlegree of rigidity, and cleforination may riot occur riiitil after a certain ininiiiiuiii of pressure, exerted against it, has been esceetlecl. These are the attriliuted of a plastic suhstance. A i h o ~ -ae certain temperature, however, (at any given concentration) thc curves folloa- the laws of viscous flow; they converge, when extrapolated t,o the axes, a t the origin. That is, at high teiiiperatures the gelatin appears to be :t viscous liquid; a t low teiiiperatures. a plastic solid. -1point of considerable interest, however, is the fixiiig of the temperaturf. of transition, and our datu are convincing in this respect. In 2 j percent solut'ions. the highest tempwature at which evidence of plastic flow was observed was ut aliout 3 + 3 . Rut iii 2 0 percent solutioiis it was ,3,3°3 while in I O percent ,solutions it was ' g o . Ahthe concentration of gelatin in t,lie solution is tlecrcasetl. the Iii:txiiiiuiii teniperature a t which plastic flow is observed is tlwreased. Thus, if we may speak of this t'eiiiperaturc ad the transition t,eiiiperatureq then it 1)ecoiiiPs otivious that the transition temperature is riot L: fiscd point. iiivariabl~for a11 concentrations. h i t tbat it is a concentration variable. The writer has previously ventured the suggestion that gelatin sols appear t.0 consist of molecules which, upon a lowering of the teiiiperatiue, cohere R.H. Bogur: .J. =\in ( ' I i c ~ n i . S o ( ... 44. 1 3 1 3 ( ~ g z r ) .
J. Phys. Chem. 1924.29:1233-1238. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 09/10/15. For personal use only.
1236
KOBERI' H. B O G T E
into aggregates i n the ioriu of catenary threads of niore heavily hydratecl niolecules. The resiliency or elasticity of the ,jelly probably depends upon the length of these filaments. It seems also that, in the case of gelatin, elasticity in the gel qtate is synonymous with plasticity in the liquid state, for on account of the amicroscopic or ultrzmicroscopic size of these particles and the short filaments characteristic of the so1 state, any tlisplacenient of them in the fluid would meet with so great a frictional resistance that the propertly of plastic flow woiild be imparted to the n-hole mass. This is what is observed when the curve of viscous flow changes to one of plaqtic flon-. If it is true that plasticity is an expression of interfibrillar elasticity, and that the elasticity is deterniind hy the length of the fibrils, i t must follow that the actual measurement of this property will depend upon the ccncentration of fibrils in the solution. and this will tie proportional t o the concentration of gelatin in the solution at any given temperature. Rut from all the evidence thus far presented, the transition is gradual and continusus, n-hich mean5 that the increase in number. size antl hydration of the fibrils procedes continuously over a n ide interval of temperature. And no instrument has been observed n hich will indicate the monient of beginning or of ending of the transition. That is, the exact temperature of transition observed in any given case will depend upon thP sensitivity of the instrument used. So while TW can say definitely that plastic flow is manifest, for CYample, in a '5 percent solution of gelatin at 34'. w e are not permitted t o say t h a t it does not exist at any temperature above this point. The greater the qensitivity of our instruments, the higher will be the temperature at which plasticity will be manifect. There is no fixed point of transition.
The only work which has postulated a definite antl exact teniperature of transition was containctl in a paper by Davis antl Oakes' in 1 9 2 2 . They reported that at a temperature of 38.03" gelatin sol and gel could exist in equilibrium, while this was not true for any other temperature. They arrived at this conclusion by noting that a "seeded" qolution (one t o which a little gelatin gel has been added) qhowed no change in viscosity with time at this temperature. whereas at any temperature below 38.03' a regular increase in viscosity with time was observed, and at any temperature above this a decrease occurred until the viscosity equaled that of a similar iinseedetl portion at the same temperature. Sheppard? had found some\\ hat siiiiilar result?, hut Loeb3 had reported that a t any teinperature above 35' the viqcosity (of a 2 percent solution of gelatin of pH 2.7) dcoeaserl on standing. The writeri designed experiments to hear upon this problem antl found a decrease in viscosity with time at 35' J. Chem. Sor.. 44, 464 (19221. Discussion itt 62nd.Meeting. =\meric*:in Chcmiirnl Society. SCT l - o r k , deptcmkier 6I O . (1921). ,i J. Gen. Physiol.. 4. IC: (1921). I
