J ’66 CHEMISTRY AND INDUSTRY Dco. 19, 152.C

COLLOIDAL PROTECTIONS By JEROME ALEXANDER

I~zfellectztal inertia is, pcrhnps, thc mofit nppropriatc t c m to apply to that attitude of mind which resists tlic acccptnnco of truths inconsistcnt, or apparently inconsistcnt, with prcviously-ncccptcd or “ classicalJ’ idcns of science. I n most cascs in industry, prnctical incn nrc fnr inorc numerous thnn chemists nnd nrc continnnlly in close contact with actual operating conditions ; nnd in addition to the nicrc nunicricnl ndrantngc in opportunity for observation tlius given , thc prncticnl ninn is usually not hnndicnppcd by prior knowlcdgc of d i n t hc must scc, nnd what it is unorthodox to obscrrc.

It is for this rcnson that technical prncticc frc- qucntly is far in ndvancc of tlicorctical or scientific knowlcdgc. Anotlier rcnson is that ninny tcnchcrs of chemistry look oshncc at cspcrinicntal data wliicli do not fit into tho alrcndy sufficicntly nunicrous nnd coinplicntcd laws and principlcs that tlicy must mnkc plnin to tho coming gcncmtion. And, finally, -dim non-conformist facts stubbornly press thcir way fonvarcl, tho attempt is gcncrally niadc to force thcm into coinplinncc m-ith existing theories. Tliiis tho purple of Cnssius was regardcd by Bcmclius to 110 a dcfinitc chcmicnl compound, for on prccipi- tation nnd re-solution it actcd ns a uni t ; and for many years it T V ~ S SO acccptccl. Ahout tn-cnty ycnrs ngo, lion-ever, Zsigniondy proved it to bc n n adsorption co~n~~ormtl by sliowiiig that it could be sjm thcsiscd by simply mixing colloiclnl gold and colloidnl stnnnic ncid ; and just rcccntlg A. Rulicr (Plqpsikal. Zcit., 25,46 (1026)) confirmed Zsigmondy’s proof by X-ray analysis, which showccl that nono of tho gold is clicmicnlly combined with tho tin.

Thc tccliiiicnl usc of wh:it wc now call ‘‘ protcctirc colloids ” goes bncl; into tho dim mists of nntiqiiitv. Thc CIiinesc used glac in tho niniiufacture of th& ink to dcflocculnto tho Ininp-blnck, just ns tlicy also used sIiccpsJ tliyroids to aid morons-both nror1;cd. And tho nncicnt Egjrptinns uscd gum (probably ncncin) in thcir inks for tho same rcnson, ant1 mnclc tlicir clay ns worlinblc ns that of Babylonin by tho iisc of infusions of straw. Tlic nlcllen~ists of tho 3fiddlc Ages produced tlicir azirztnz polabile by rcducing solutions of gold in tlic prcscnco of stabilising cthcrenl oils; and a t least ns cnrly as 1704 silk wns dycd with colloidal gold. Bcnelius, in his LehrDt1ch (1844), gircs rccipcs for producing scvcral shnclcs of colloidal gold, and long prior to this, isinglnss, cgg nlbunicn, and starch were uscd for thc sainc. purposc. In 1807, just boforo %ignondy (lSgS), unnwnrc

of Fnraduy’s cnrlicr (1856) work with ‘‘ jolly ” ns n protector for colloidal gold, lind inclcpcnclcntly rcdis- covcred tho phcnomcnn , von RIcycr and Lotterluoscr (J. pral;. Client., 56, 241) rccogniscd tho protcctivo action. T h y drcm attcntion to tho fnct thnt albumin stnbiliscs siliw sols-a fact well Icnoa~n to practical photogrnphere. In his book “ Anorganisclio ICol- loidc,” p. 60 (1001), Lottcrmoscr suid: “ On tho addition of very stablc colloids, such ns albuniin,

*Papor road boforo tho Anioricnn Scction of tho Socioty of Dhomical Industry on AIny 10, 1034.

gelatin, agar, or gum nmbic, to a silver sol, no precipitation is caused by clcctrolytes until the stnblc colloid is congulntcd. Tlic lcss stnblc silver sol is thus protcctcd against thc clcctrolytc by tho inorc stnblc colloid ; it hccomcs niorc liltc i t in its. ldinviour.J’

To put i t tcrscly, n protector is n substance that;. opposes tho nggregation of niolcculcs or particles into largcr groups. It is nntaral to cxpcct that substnnccs will vary widcly in thcir protcctirc power, if , indccd, tlicy sbon- nny protcctivc nction a t all ; and also that a substance which is a protcctor in ono sct of conditions, may, under other conditions, shorn n different dcgrcc of protcctiro action, or nonc a t nu. To establish sonic niensurc of protcctirc actiont Zsigniondy dctcrniincd thc nuinbcr of niilligrnms of protcctirc substnnco ~vhicli would just fail to prcvcnt 10 C.C. of purc colloidnl gold solution from chnnging its colour from rcd to violct upon thc addition of 1 C.C. of 10 pcr ccnt. sodium chloridc solution. ‘Illlcso ruby-rcd gold SOLS coutniii 0.005- O*OOG per ccnt,. of gold, and arc cscccdingly scnsitiro to trnccs of clcctrolytcs. Tlic suhjoincd table gives such “ gold numbcrs ” for n mricty of substances , tnkcn iroin tlic rrsiilts of Zsigniontly cintl of S c l i r p x :-

Gold nuI11bcr 0.005 t o 0.01 0.008 t o 0.01 0.0 1 to 0.02

Substnneo Gclntin . . . . .. .. .. nussinn gluo . . . . .. .. Isinglass . . . . .. .. .. Cnsein (in nnirnonin) . . .. . . 0.01 Egg-globulin . . . . . . . . 0.02 to 0.05 . . 0.04 Lo 0.0s Ovornucoid .. .. .. Glycoprotcin .. .. . . . . 0.05 to 0.1 Amorplioiis cgg-nlbiiiih .. . . 0.03 to O.OG Crystnllisetl epg-ellniiiiin .. . . 2.0 to s.0 Fresh ogpwliito . . .. .. . . 0.06 to 0.1G Cum nrnbic Cum trngncnntli . . .. . . . . PO* Dcstrin .. .. .. .. . . G.0 to 20.0 Wlicnt stnrcli . . .. .. .. 5,0* POtflfO Stflrdl .. .. .. .. ”*Of Sodiurn olcntc . . . . .. .. 0.4 t o 1.0 Sodium stcnrnto nt 100 degrees . . . . 0.01 Sodium stenrntc nt 60 dcgrccs . . . . 10.0 Do11 tero-nlbuniosu .. .. .. v)

Cnnc sugar .. .. .. .. v)

Urcn .. .. .. .. .. VJ Stnnnic ncid sol (old) . . .. .. 0

.. .. .. . . 0.5 t o 4.0

Tlic rcsults witli sodiuiii stcnrntc iiidicnto that the dcgrce of dispersion of tlic protector is it vcry iuatcrial factor in its action, just ns it is in tho dctcrgcnt \v~luc of soup, nnd lends support to tho gcncrally-ncccptcd ricw of Bcclihold that protcct,irc action is conscqucnt upon tho ndsorption of a laycr of t h protcctor a t tho intcrfncc bctwccn tlio dispciwxl and tho dispcising pliasc. Tho tliinncss of the layor scciiis obvious froiu tlio fact tlint Zsigmondy could sco no dituinution of the Urownian motion cnused by it, nor nny ultramicroscopic cvidcncc of i ts csistcnco. Sinco protectors nro substnnccs linving considcritblc attraction for wntcr and, as n rulc, u grcnt swelling capacity, i t is not unli1;cly tliat aftcr ordinary dcsiccatioii t h y may swcll cnougli to forco tho constitucnt particles of tlic protcotcd siibstniico far cnougli apnrt so that tlioy arc boyond tho range of iuolcciilnr attmctivc forccs which is ordinarily sstiiunted to bo about 50 pp. But it is not intended

to go here into the mazc of theoretical discussion- t,he experimental facis of protection are interesting and sugpestive.

A capital instance of the effect of colloidal pro- tection is seen in thc case of milk, whcrc the casein, an uilstablc colloid, is protcctcd or stabiliscd by lactalbnmin. -In the older books this relation of tlic two proteins was obscurcd by the practicc of reporting their sum as “ total protcids ” ; but the following table, based on the averages of Koenig, nt oncc sliom t.he great differences betmeon milks, as well ns the import,nnce of the cnscin-lactdbumin ratio in dctcrmining thcsc differences :-

.4rcragc Composition of Prrrioits J l i t h

COW . . 3.02 . . 0.53 . . 3.64 . . 4.6s Kind Casoin Lnctnltnunin Fnt Lac1o.w

Womnn . . 1-03 .. 1.26 .. 3.78 . . 0.21 Gont . . 3-20 .. 1.00 .. 4.78 .. 4 4 0 Ewe . . 4.97 .. 1-55 .. 0.80 .. 4.91 Xnro . . 1-24 . . 0.76 . . 1.21. . . . 5.07 ASS .. 0.07 .. 1.55 .. 1.04 .. 6.90

Esqmimcnt show that CO\FB’ milk is much more readily coagulntcd by ncid and hy rennin than is motlicm’ milk, and that tlic congiln of nioflicrs’ milk, if formcd, nrc much fincr. Thc nddition of any protector-proteins like gclntin or nlbuniin, cnrbohydrntcs lilic ilcxtrin, Irish moss, or plm nrnbic, or crcn snlts lilrc sodium citrnte-will mnkc corn' milk behaw more liltc mothers’ milk, and will make it morc digestible for the infant. Regarding thc highly protcctcd asses milk, so cmincnt nn authority as A. Jncobi stated as fnr 1)ncI; ns 1S88 thnt it is a rcfugc to which tlic p1i;ysician flics when ncitlicr corn’ niilk nor evcn mothcrs’ milk is tolcratcd.

I n taliis conncxion it mmns probablc that any crime which dcd)cascs tlic proportion of lactnlbumin to cnscin in mothers’ milk-such ns ncrvoiis sliocl; or nn,.omin--wilI tend t o register itsclf in n digcstivc upsct in tlic nursing iiifnnt. This rntio iindoubtcdly rarics with tlic pcriodof lactation aswcll nsin ~liffercnt~ cows, so that tlic 111illi of n Iicrd is much niorc iiniform thnn that of onc cow.

Dee. 10, 1024 CHEMISTRY AND INDUSTRY 3 3 7

PLURAL PROTECTION An outcoluc of thcse obserrntions on niilk wns tho

developmcnt of the idcn of do?lble or plural protccfioit . TI10 nttempt to producc a n imitation of cnscin by forming n prccipitnte of cnlciuni pliosphatc in tlie prcscnce of protectors (gelatin, gum arabic) w a s not fruitful ivhen thc protcctor was addcd to tho pliospliatc solution or to the cnlciuni solution. On rcflection, however, i t wns evidcnt tlint in the orgnnisin, bot?b the reacting solutions contain protectors, and on protcctiirg both tlio sodium phosphate am1 the calcium chloridc, there was obtained a colloidal calcium phosphate which could bo prccipitntcd by acid or by rcnnin. Olivc oil was emulsified in this, and a stablo imitation milk was obtained.

Scnm-tl instnnccs csist in tlic litcrnture wlicrc this principlo has been uiiwittingly uscd to produce ultra- tine dispersions. Thus in the gmialess cniulsioii of Lippruailn (so cnlled bccausc of tlic fact that i t is transparelit), tlic recipe calls for tho addition of gclntin to each of tho rcncting solutions of potassium bromide and siivcr nitrntc. Caroy h a protlucecl sonic

of his hest colloidal silver by adding the protcctive Rocliclle salt to the silvcr nitrate and to the ferrous sulphate. Lobry dc Bruyn (Rcc. Iiau. clbim., 19, 236, 1 W ) produced cxtrcmely fine precipitntea or even colloidal solutions by adding gelatin to each of his reacting solutions.

Tlic value of this principlc in tccluiology is obvious, and in medicine and in bioclicmistry it must be of. cxcccclingly grcnt importancc, for in thc organism all rencting solutions are protcctccl. ‘J!hc work of Augustc Lumi&rc nnd otlicrs has slioivn tlic disastrous conscqucnccs tha t follow the formntion of flocculates of sufficient sizc t o cause ncrvc irritntion, capillary cmbolism, and t.he midc vnricty of symptoms tha t may follow more or less clircctly or indirectly from thc nppcarancc in tlic blood strcani of pnrticlcs of imusual dimensions. In addition, protectors very gencrcllly lead to tlie formntion of crypto-cr~.stallino structures, obviatiag sharp crptallino deposits;

AUTOPROTE~TIOX Sincc cvcry substnncc, in the coursc of nggrcgnting

from ntomic or molcculnr dispcrsion into visiblc mosses, niust of ncccssity linvc in i t nt fioine time pnrticlcs of colloidnl dimensions, we must fnco the condition Iinoun ns iso-colloidism, wliercin tlic colloidnl dispersion of tlic substnncc is in contact with n non-colloidnl plinsc of tlic snmc substnncc. If the colloiclnl plinse has tlic property of being adsorbed or of nhorbing, ITC mny hnvc airtoprofcclioiz, whcrc tlic siil~stnncc delays or prcvcnts its own crystallisation. Thc work of Quincke, 0. Lelnm-mi, and others indicates that crystal formation is prc- ccdcd by the formntion of tiny globulitcs ; and by rapid cnqoration of n film of a solution of sulphur in carbon disulpliidc contniningn little Canada bnlsam to slow down tlic nggrcgation process, thcsc globulitcs and thcir various stngcs of nggrcgntioii nmy bc sccn (J. Alcsnnder, Colloid Sy~nposium Nonograph, Univ. of Wisconsin, 1023). An nnulogous nilsorption sccnis t o dclny tlic transforinntion of y-iron into a-iron (allocolloidism), nnd it is cspccinlly obserrablc in ammonium salts and in sonps, q., in olcates.

Jn the case of solutions, wlicrc tlic viscosity is not too great, tho nutoprotcctirc stagc is of limitcd life. Thus tlic gels foraicd by cooling solutions of ~-dimcthyl~minon~iilino-~,~-diphcny1cyclo-l, 2-dionc in organic fiolrcnts (W. 13. Hardy, Proc. Roy. SOC. Loizd., 8 7 ~ , 29, 1913) ; by dibcnzoyl-1-cystine (Gorber and Hoffniaiin, J. Am. Clrent. SOC., 43, 2109, 1931) ; by compliorylpl~cnylthioscmicarba~idc (Forster and Jnckson, J. C/bcnz. SGC., 91, 1881 (1000), dissolved in organic solvcnts-all bccomc crystnlliiio mithin a comparativcly sh0i.t timc, which mny run into W C C ~ ~ S .

Cuxur,xrw~ P~~OTEOTION Aij-thing that cliruigcs the nature or dcgrcc of

amgrcgution of a protcctor will influciice tho stability oP t ~ i o aspersion stabi~iscd by it. Tliis may bc brought about by hcnt (albumin), coagulants (e.g., ferric chloride, by flocculating gum nrabic, “ brenlis ” eniulsions mnde with tho gum), cnzyincs (rennin coiivcrts lnctnlbumiii into a congulniit in ndditioii to clcstroying its protective action in milk (J. Alcs-

DcC.'lO, 102.1 1268 CRENISTRY AND INDUSTRY

andcr, 8th Inter. Cong. AppI. Chem., 1912). We may, thcreforc, havc protectors for protectors, through a wholc series.

This conception may lead us to understand some of the pcculiar phcnorncnn obser\?d in miscures. Thus tlic individual silicates in glass tcnd to crys- tallisc, but in adniisturc thcy forni n mntuallg pro- tective chain, being so choscn by thc glass tcchnolo- gist. If thc inisturc is not right, or if i t has bccn alhvcd to remain too hot for too long n time, tho c h i n brcalcs down, and the glass suffers dcvitrificn- tion. In alloys, cspecially in complcs alloys, mc may in this manner conceivc how cvcn sinall pcrccntagcs of some of thc constitucnts may cscrcisc a potcnt influoicc.

In biological mistiircs sucli as tlic gluten of wlieat or ryc flour tlic principlc of cumulative protection is of conscqucnce, and i t inny be tllnt imny of tlic proteins (albumin, gelatin, globulin, casein) may be adsorption complcses of siniplcr cliciuical compounds, which do not brcalc up under the conditioris to which tlicy arc ordinarily subjected. This vicw is coicjoiiaiit with the fact that., although sonic protcins arc not deh i t c chcmical individuals, tlicy arc approximately uniform in composition.

RANCIDITY AND THE KREIS TEST By T. W. JONES. B.Sc.

Thc rancidity induced in fats and oils by dctc- rioration is important ccononiically if for no other reason. IZsactly what i t is i n thc fat or oil that constitutes rancidity is still one of tlic numerous incognita of tlic chcmistry of oils aiid fats; i t is, thcrcfore, nLuattcr for soinc surprise that the.litera- turc upon the subject is not uiorc cxtciisivc, for really very little attention appears to liarc bcen given to the problem. Scrcral thcorics hare bccn arlranccd to account for thc plicnonicna, aiid a ccrtaiii amount of work has been donc upon the conditioiis giving rise t o rancidity, with thc result that, with reasonable care, the possibly licavy losses that would occur through deterioration of stored fats can be avoided. Esposure to light, lieat and the ntmosplicrc causes the ranaidity of fats ; moisturo appcars from recent work to be but a secondary factor, whilst bacteria thrive onIy \vhcii nitrogcnous material is present, such as animal tissue and tlic marc of fruit remaining from crudc mcthods of cstrnction.

It was a t onc time supposcd that the acidity of an oil was thc stline t,liiiig as its mncidity, but this was disposed of by cs~~crimcntal investigation, as was also tlic conversc theory that rancidity could set in without the libcration of frec fatty a d s . It is now gciicrnlly acccptcrl that if an oil is rancid thcn i t contains a fair proportion of fatty acids. Aiiotlicr theory that was quashed was that the oldcliydic bodies alwtiys found in a rniicitl oil or fa t wcrc re- spon'siblc for tlic rancidity; rccent work, howcvc.r, may ham tlic cffcct of rcriring this theory, as i t uppears that rancidity may be directly concerned with thc prescnco of iwrolcin.

With this confusion regarding the nature of ran- cidity it is no wonder that a satisfactory test has not yct been evolved for its detection. Though ran- cidity can scarccly be ignored when well developed in any oil or fat, yet in its initial stages, especially if masked by a strong natural odour of the oil, its dc- tcction is sometimes a matter of difficulty, and on these occasions a positivc chcmical tcst is to be desired. It is not difficult to mis a slightly " off " oil with D. fresh onc that has a strong natural smcll in such a way as to disguise its deterioration efIec- t i d y for the comparatively short period it is on tlic market. It is to detect such adultcrations that i t is impcrativc to have a trustworthy tcst. Of thoso proposed tho riiost successful so far and most ge~icrally used is thc I h i s tcst, a modification of Bishop's test for sesauiiim oil. 'I'tiis test consists in shaking the suspcctcd fat or oil with strong hydrochloric acid and then with an ethereal solution of yhloroglucin. A rancid fat yields a rctl or pink colour. Krcis ascribed the reaction to the presencc of aldehydes hnd kctones i n the rancid fat, as a similar colour results when tlic test is applied to such cornpourids as \*anillin, cugcnol and so 011.

This test has becn widely used of recent years aiid consequently its valuc has been very thorouglily investigated. The result has Lccii that a ccrtaiii amount of doubt has arisen regarding its efficacy for tlic purpose of detecting rancidity. \\'inkcl,1 for csaniplc, objectcd to i t on the grounds that (n) it is iiot specific and gives the same coloration with aldehydes and kctoiles not found in nincid oils, ( I ) thc colour is not proportional to the dcgrcc of ran- cidity, and (c) lastly, the test is too delicate, and if oils were to be condcmncd on tlie strength of the reaction many that werc perfectly fit for use would havc to bc discarded. A few years ago the \Vashing- ton AIe:it lnspcction Laboratory? madc an cshausti\~e btudy of the tcst which confirmed IYinkcl's objcctioiis, and i t was found, in addition, that certain oils, par- ticularly crudc and freshly rcfincd cottonseed oil even whcii pcrfcctly fresh and swect, gave tlie colour reaction. Tlic Laboratory did, however, find the test of value for confirmatory purposes and for samples whcrc the natural odour niaskcd that of rancidity; also it was concludcd that its extreme sensitiveness was not wholly a drawback, as i t was found to give evidence of an incipient rancidity which in cvcry instance dcveloped latcr. The test, thcrcforc, is of usc in giving warning of tlic onset of rancidity somc time bcforc thc cliangcs ham pro- gressed to such a point as to bc cridcnt to tlic scnscs. Similar conclusions were rcaclied by J. F. Be\is3 : " Under most conditions thc interval of timc bctwccn thc first appcaruncc of the Krcis test and tho sensiblo nppefiraiice of rancidity is sufticicnt to perinit tho conscrvation of tho product by immcdiatc, USC."~ 'I'hc Krcis tcst as modified by IqcrrJ is carrictl out as follows :-

To 10 C.C. of tlic suspcctcd oil or iuclted fat is added 10 C.C. of hyrlrochloric acid (sp. gr., 1.19) in t~ largc test tube (8x 1 in.). This is then stoppcrcd and shaken vigorously for approximately YO seconds. 10 C.C. of a 0.1 pcr cciit. solution of phloroglucin in ether is then added: irnd tho whole well shakcn and