THE EFFECT O F LIGHT OK T H E ELECTRICXL CI3,IRGE OF SCSPESDED P-ARTICLES BY
s. n-.
YOI'SG
;\SD L. K . PISGREE'
TYith the extension of the theory of contact potential differences to suspended particles. the measurement of such potential differences and the de termination of the various factors which afYect them are becomiqg matters of increasing importance. This is perhaps especially true because the phenoniena have in all probability a more or less intimate connection n-ith many biochemical processes. Quite a considerable number of investigations has-e had for their purpose the determination of the character and value of the charge upon bacteria anti upon the 1-arious toxic and antitoxic bodies de\-eloped under infection. This is in continuation of Eordet's idea that many. if not all, of the antigenantibody reactions are explainable as colloidal reactions of one sort or another. The determination of the charge on a suspended particle is n o simple matter. I t is usually done by subjecting the suspension to a gi\-en potential gradient that is. by placing electrodes of knon-n electromotil-e force a t either end of a tube or trough containing the suspension i and notiiix the rate of tnigration in the one direction or the other. 'l'he rat? of migration hon-ever depends upon a considerable number of factors, some of \\-hic.h are difficult t ( J coiitrol. ?'has, uiiless the suspension is eleetrolyte--free,changes in the character a n t i concentration a t the electrodes occiir. and this causes a change in the value of the charge upon the suspended particle. The 1-alue of the charge also depends upon the 4ze of the particles, t h a t is. upon the deg-ee of dispersion. This in turn depends upon a number of factors, that most difficult to control being the age of the suspension. -1s \-an l!enimelen has shoi\-n. the colloidal ,3r suspension state is one of no rest, changes in a g g r e g a h i arid dispersion coatinually occurring. 1
-~
' -1bstracted I,. I!.
from a thcsis for thc Dcgrcc of Mastcr o f A r t s . p~-csciitc.tiI,? Pingrce to the 1:aculty oi Staniord Yni\-ersit>-,31
The case i h a t a suspension mairitaiiis the same degree of dispersion over any coiisiderable length of time is exceptioiial. I n addition to these and other difficulties is the fact t h a t the tube or other \-esse1 carryinq the suspension is charged 17 ith respect to the medium, j u i t as the suspended particles are This results, in the potential gradient, in a current of medium don-ing along the sides of the vessel, whose direction will be n i t h or against the direction of migration of the suspended particlei, according to the character of the charges of the yessel and of the particles n i t h respect to the medium. Thus if the \-esse1 and the particles both take o n charges of the same sign, the current of the medium \vi11 flow counter t o the direction of miqration of the particles I n two recent papers, Risdale Ellis' has developed a method whereby the effect of the counter-current is eliminated. The method consists in measuring the rate of migration under the microscope The suspension is placed on a special slide, and is contained between the slide and the cover glass which are adjusted a t a known distance from one another. By focussing the microscope a t different depths within this layer of suspension, the rates of migration a t the surface of the glass and a t different distances from i t are directly measured. From these data, u i t h the aid of an integration formula. the true rate is determined. The folloiving investigation n-as undertaken in order t o determine the limitations of the simpler method of measuring migration rates directly in U-tubes This method has been considerably used, b u t is condemned by Ellis as unreliable. It would seem probable, hon-ever, t h a t b y increasing the diameter of the tube, a size would be reached m-here the effect of the flow of medium along the walls would be insignificant I n order t o determine Ivhether this were true, and if so what size of tube would be necessary, a set of U-tubes was made, all of uniform length, b u t of varying diameters. The diamZeit. phys. Chcm., 78,
321 (1911); 80, 5 9 j (1912).
Efleect o j Light
011
Suspended Particles
659
eters v-ere, respectively, 3 . 4) j , 6. and 6 .j mm. The potential fall in the tubes was always I I O volts. The first suspension subjected t o investigation n-as one of colloidal arsenic sulphide, containing I gram to the liter o f arsenic sulphide. This was prepared by the usual method of adding slowly to a solution kept saturated n-ith hydrogen sulphide, a moderately dilute solution of arsenious acid. The excess of hydrogen sulphide was removed by bubbling pure hydrogen through, and the whole diluted to the above strength. The rate of migration of arsenic sulphide suspensions depends very considerably on the details of the method of preparation. The presence of a n excess of hydrogen sulphide noticeably accelerates, while an excess of arsenious acid greatly retards. f Unpublished results obtained in this laboratory b y X r . R. C . l’ollock, and confirmed by JIr. Pingree.) F o r this reason a coiisiderable stock of the reagent !vas made u p and carefully kept in the dark and in tightly stoppered bottles. The first set of results obtained are sholi-n iii Table I. The total length of liquid columri was in a11 cases 2 I .jcm. Cnder D is the distance wandered in centimeters;. Under ’r is the time in minutes used in wandering the distance indicated under D. The tn’o columns for each tube of diflerent diameter represent parallels obtained in entirely independent experiments. If one examines these results. it \vi11 be seen that they are in the first place very erratic. There is a general tendency t o slon. don-n after 3 or 4 cm have been traversed, Ti-hich is in all probability due to slight accumulation and difiusion of acid around the cathode (from n-hich the sulphide n-anders) n-hereb y the charge upon the particles as well as their degree of dispersion is somen-hat reduced. This effect should, however, be more or less constant in all experiments and should not cause such wide variations as are shown. Temperature variations suggested themselves as a possibility in producing erratic results. Experiments were then undertaken t o deter-
660
I
3
'I
:E
a
1 0
k
-
F: c
'?
73
rl
Q
Eject
OJ
Light
OH
Suspmded Particles
66 I
mine the magnitude of the temperature coefficient of the migration. Measurements were made under otherwise like conditions ( a s was then supposed, at temperatures of 2 , 3j, and ;oo C, respecti\-ely. The results are sh0n.n in Table 11.
T.II
11
1)
IO 0
20 0
-
'Total timc
0
-~
,- >,
0
'
3 -
>>
->
I O 0 IO j 11
5
_ _
0j o
-1s to the itifluence ot' the diameter of the tubes on the rate of migration. the results in the dark shon- nothing more than fluctuations to he expected irom the present degree of experimental accuracy. The results in the light might a t first be taken to indicate that under conditions of illumination. there is a distinct tendency ton-ard slower migration in the tubes of smaller diameters. T h a t this is to be definitely ascribed to flon. of medium along the glass seems, however, rather doubtful. If the current is counter to the direction of migration of the particles, as has been shown by Ellis' to be the case for glass vessels carrying negatively charged suspensions in water, the effect of such counter-current \Till consist of two factors \vorking in opposition to one another. I n so far as the current is one of medium only, it will give a n LOC.cit.
Ejtct
0-i
Light
OH
Suspeuded Particles
66 j
apparent acceleration to the migration of the suspended particles. I n so far ;is the counter-current carries suspended particles with it, it TT-ill tend ton-ard an apparent negatix.e acceleration of the particles. It will be difficult, therefore, to tell in any gix-en case n-hat the efiect 11-illbe. Ellis's measurements were all made on coluniiis of liquid about six-tenths of I mrn in thickness, and consequently thron- little light on the conditions in much thicker colurnns. The frequent occurrence of a curved surface of migration, noted by various investigators, appears at first sight to lend support to the theory of a large counter-current effect. From our observations, hon-ever, this is not necessarily the case. lire have frequently observed this phenomenon, especially n-ith bacterial emulsions. In some cases such surfaces Trill develop to a very marked degree, so much so occasionally as to form a hollon- cone I? cni or more in depth, ~vhilein what should be a perfect parallel experiment, the migration surface will remain quite Aa t . lyhile there can be nil doubt t h a t the counter-currcnt effect is a real one, Ire are rather inclined to think that quite a great portion of the reduction of migration rate in small tubes (Table 111) is due to :Ldifferent cause. If one considers that IT-hen a bundle of rays of light normally strikes the surface of a cylinder, these rays are refracted to a. focal line or axis, it n-ill immediately be seen that the location of the focal line \!-ill depend tipoii the refractix-e iiides of the material of the cylinder, and upon it:; diameter. In larger cylinders it \vi11 lie Ivithout the cylinder, Ii-hile in snialier ones (of shorter radius; it may lie within. I n any ex-ent, the smaller the radius of cur\-ature of the cylinder. the stronger ivill be the convergence of light within it. Thus with tubes of small diameter the intensity of illuniinatioii \vi11 be greater than it1 those of greater diameter, under the same source of illuinination, IT-hence a greater retardation \vi11 he found in small than in large tubes. This seems especially plausible since this effect is only to be found when illumination occurs. The rates in the dark are roughly independent of the diameters of the tubes used. In order to settle these matters satis-
factorily, it would seem wise to Ivork 11-ith flat-sided tubes, in order to avoid uneven distribution of light, and plans for such an investigation are under way. I n the meantime i t seemed advisable t o extend the work to other colloids and to other factors which might throw more light on the general phenomena of light sensitiveness. Before leaving arsenic sulphide, however, the results of measurements to determine the effect of small amounts of acid and alkali upon the rate of n-andering will be given. These are most instructive when plotted as curves, which has beell done in Fig. I . Ordinates are distances in centimeters,
Fig. I .-Plot
I)(
Migration in Arc-light Illumination
Curve ( I ) .lsLSiof concentration Curve ( 2 ) ;\s,S, of concentration Curvc ( 3 ) .Isis, of concentration
I I I
gr. per liter gr. per liter, K,/IOOOwith N a O H gr. per liter. N , 1000 with H,SO,
11 hile abscissae are total times in minutes. It n-ill be noticed t h a t in the thousandth normal acid there is a t first a considerable acceleration which, however, 50011 falls off. I t appears most probable that the initial acceleration is perfectly normal, and that the subsequent falling ofi‘ is due to a
E-@ec-tof Light
oia
Suspeuded Particles
667
slow increase in the aggregation of the particles. In the case of the alkali, the first effect is a reduction in the rate, followed later by an increase. This is explainable in the same \my, t h a t is, as due to a subsequent gradual increase in the degree of dispersion of the particles. If this reasoning is correct, it seems safe to say that the eft-ect of acid is t o accelerate and the effect of alkali is to reduce the rate of migration, but t h a t both produce subsequent changes in the degree of dispersion of the suspended substance, ivhich more or less obscure the earlier and normal effects Experiments with Colloidal Ferric Hydroxide Colloidal ferric hydroxide \vas prepared by the ~ i s ~ i d method of hydrolysis. Ferric chloride was used and the hydrolysis carried on for ten days or more. - i t the end of this time the hydroxide \!-as found to be in a sornen-hat flocculated condition. JIigratioi: esperiments ir-ere carried oii iritli this material as \vel1 as t v i t l i samples of it to ~ r h i c h minute amounts of dilute acetic acid had been adtlecl, Tollo\verl b?-boiling. in order to increase the degree o f tlispei sioii. Taljle \- gives the results of these rneaiuremeiits. Instead of folloiving the niigration centimeter by centimeter, lthe total distalice imridered after a certain time 1va5 cieteriiiiiied. This ~1-2sthought to be sufficient, :is only coniparatil-e resu1t.s Ivere needed a t i.his time. I n Table 1. the total time allo\re:l \\'as 2 7 tiiiuutes. Sample I {vas the original dialyzed material. Sample 2 contained a \-cry miall ani01.111t o-t' acetic acid, and resuits are gi\-en in duplicate. Saniplc 3 coiitailied a somen.liat qreater arnount of acetic :icitl. The carhon arc ]vas ilsed for i l l ~ i m i i i a t i ~ ~ i . ~ l ~ . ~ l ~\ .l , l ~ r *
I h e of migration, 2 7 minu t e i
S. I.V. I7oui?g a
668
d
L. TI'. PiTzgree
Thus there is a n invariable reduction in the migration. rate of ferric hydroxide under the influence of illumination. X far greater number of measurements were made than are reported here, always n-ith the same result. Thus, while the effect is a relatively small one, amounting to only 6 or S percent, there is no reason to doubt t h a t the phenomenon is a genuine one. The results are of especial interest at this point, as the ferric hydroxide is a positively charged colloid, whereas arsenic sulphide is negati\-ely charged.
Gum Mastie Emulsions of gum mastic \\-ere prepared by dissoh-ing the gum in absolute alcohol. and adding a fen- drops of the solution to a coiisiderable \-olunie of pure If-ater. Three emulsions of different concentrations were prepared sample I being the most concentrated, m d sample j the most dilate. The results are given in Table 1.1. ~l..\l~l.l:
T.1
1 inie of inigrx\ioii, 7 I miiiute..
I *
*irc li$t
~iied
Here it is seeii Lhat t.he a.cceleratioii untier tlie infliience of light is positiye and o f 1-ery considerable magnitude. reaching in case of sample I a value (if I O O percent. ?'he rapid increase in migration r a t e 11ith iiicreasing dilution. that is, i\-ith increasiny t i e g e e of dispersion, is I-ery clearly shon-11.
Common Rosin Emulsions of coninion rosin \\-ere prepared by the same method as \\-as ased ii-ith the gum mastic, and their migration rates measured both in the dark and under arc-light illumination. The results \\-ere precisely like those with mastic emulsions, except that the light sensitis-eness vas somen.hat
E f e c t o j Light
011
Suspeizdod Particles
669
less, amounting in some cases, however, to as much as 30 or 40 percent. Chlorophyll -4 quantity of chlorophyll was prepared by extracting green grass with alcohol, evaporating a t room temperature to dryness, and again extracting with a small amount of absolute alcohol. There was thus produced a highly concentrated solution which showed reddish brown by reflected, and deep olive-green by transmitted light. Emulsions \vue prepared by adding a fen. drops of this solution to considerable XTolumes of water, and measurements of migration rates in the dark a n d under arc-light illumination were carried out. The results of a fen- of these measurements are gilren in Table 1-11,
T . ~ H L\ E7 1 1 - - - C LL ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Time of migration as specified Xrc.light illumination used -~~
Samplc
Time Min
The chlorophyll showed itself to be negatively charged like mastic and rosin. -After wandering for a fen. centimeters, the migration surface became wavy and uncertain, so t h a t the times of migration are all rather short. .Again a positive acceleration is invariably shown, whose \-slue is, ho\ve\.er, smaller than for mastic or rosin.
Experiments with Bacteria I t is a \\-ell-known fact that bacteria, when suspended in aqueous solutions, are in general negatively charged J\-ith respect to the medium, and consequently, when placed in a potential gradient, they wander ton-ard the anode, j u s t as do arsenic sulphide, mastic and the like. I t was thought to be of interest a t this 'point. to determine whether or not they. too,
were influenced in this respect by light Cultures of several species a-ere started, and kept on hand b y transplanting on agar slants, by the usual method The species mostly used were Sarcina flava, Sarcina rosea, and Bacillus prodigiosus. Emulsions of these bacteria 11ere prepared as follon s, using isotonic glucose solution as the dispersing medium in order to avoid electrolytes : Into the slant in which n-as a n-elldeveloped culture, a couple of cubic centimeters of the glucose solution n-ere placed. then with a sterile platinum loop the bacteria were scraped loose from the agar and rubbed u p Jvith the glucose to a smooth, thick emulsion, care being taken not t o disturb the agar bed of the slant. This emulsion iias then poured ofi, the tube rinsed with another small portion of glucose solution. which \vas added to the first. The n-hole of this emulsion was then diluted to I j or 2 0 cc with glucose solution, which gave from one culture sufficient material for a number of measurements. In Table \-I11 is qiven a set of the results thus obtained 'I'WLZ
1-111
Time of migration, 20 minute, -1rc-light illumination Specics
DD
DL ~
S Hava
j 0
-40
S rosea B prodi,'~ 1 0 > U S
4 2
3 2 3 0
3 s
In Table I S are the results oi measurements on B. prodigiosus to determine approximately the relative effects of sun and arc light T.~BLE IS Bacillus prodigioius. Time of migration, 2 0 iiiinutes
Ejfeci o j Light
071
Sfispciidcd
P i l i tic-1c.s
6j r
Thus bacteria, like arsenic sulphide, experience a considerable reduction in their rate of wandering under the influence of illumination, and sunlight and the carbon x-c are roughly equal in their effects. -\ large number of results have been obtained, b u t only a fen- are gii-en here, for the reason that the conduct of bacteria a t times seems \-ery coinplex. For example, the migration rate for a gii-en species seems to x-ary in different cultures. Furthermore, the measurement of the rate of Trandering in the light is often complicated by the formation of most curious migration surfaces. \*cry deep conical surfaces are sometimes formed, 50 t h a t the bacteria near the surface of the glass of the tube 11-here illumination is most direct 1 seem scarcely to Irander a t all 11-hile those in the axis (or near i t ) of the tube wander quite lrell. Curiously enough i t , very often happens t h a t the axis of the conical surface in coiisiderably displaced, giving rise to such surfaces as are shon-n in Fig. 2 , the source of
II
7592-
illumination being considered a t the right I t i i belie\ ecl t h a t these phenomena are all to be explained as due. in the main, to unequal distribution of light in the tube, arid it is hoped t h a t experiments ivith flat-sided tubes 11 ill settle this matter. considerable number of experiments have been carried out for the purpose of determining the differences in conduct of dead and living bacteria, if such exist \T7hile this ~ \ o r k is by no means complete. such results as hai-e been obtained seem to indicate quite clearly that the rate of nanderinq of dead bacteria is considerably less than t h a t of live ones, and
t h a t their light sensitiveness is also somewhat less, although i t does not seem to disappear entirely. The work has been greatly hampered by the formation of irregular migration surfaces in the light The influence of the presence of small amounts of acid and alkali on the migration rate of B prodigiosus. both in the dark and in the light, was also determined The results are sho\\-n in Table X Similar determinations n e r e also made using Sarcina rosea, and ga\-e perfectly similar results, except t h a t the taking of readings in the light \vas very difficult, owing to the irregular migration surfaces I t has been our experience in general t h a t , for some reason, B prodigiosus gil-es more even surfaces in the light than do the Sarcinae. I . i i j i , E 1--B
I .
PIIOI)ILIC)SI 5
Time of migration, 30 minute-, Soltition
I)1 j
DJ
Thus the migration rate is accelerated measurably by small amounts of acid and retarded by like amounts of alkali, and the efiect is shon-n both in the light and in the dark. Larger amounts of acid are kno\\m to retard the rate of wandering very materially, in fact, bacteria may be made to \\-ander in the opposite direction, toivard the cathode, by sufficient acid concentration. In small amounts there is hon.e\-er a n acceleration and this completely in keeping \\-ith the conduct of arsenic sulphide.
Discussion of Results In the foregoing i t has been shoivn t h a t all suspensions. emulsions and colloidal solutions investigated shon- a x-ery distinct light-sensiti\-eness ivith respect to their rates of
E j e c t o j Light on Suspended Particles
673
migration in the electrical potential gradient. In some cases the effect is a positive, and in other cases a negative acceleration. This effect is readily explainable in either of two ways: I ) The influence of light is to affect the degree of dispersion of the suspended matter, increasing the dispersion in cases of positive acceleration and reducing it in cases of negative acceleration. ' 2 ) The influence of the light is a direct one upon the static charge carried by the suspended particles, increasing this charge in case of positive accelerations, and reducing it in the case of negative accelerations. If it r\-ere not for the fact t h a t bacteria, ivhose degree of dispersion cannot alter in any ordinary sense of the word, are also affected by light as are other suspensions, it might be dificcilt to choose betn-een the tlvo hypotheses. E u t since bacteria cannot well change their degree of dispersion, it seems certain that, a t least in their case, the effect must be upon the value of the normal static electrical charge which they carry. I t is also highly probable t h a t in other cases also a t least a considerable portion of the effect is to be ascribed to the same cause, although changes in the degree of dispersion may also occur. I n fact such changes in the degree of dispersion would in all probability naturally follon- as a result of the change in the value of the charge. since it is usually considered that the degree of dispersion or aggregation is, to a great extent, dependent upon the charges carried by the suspended particles. TI-hile it is not true that coagulation or agglutination occurs always a t the isoelectric point, that is, a t the point rvhere the potentials of the particles and medium become equal, it is ne\-ertheless quite certain that in a general ivay the approach ton-ard the isoelectric point favors such a result. I t seems most plausible a t the present time to consider the above-described effect as primarily a photo-electric one, and any changes in dispersion which may occur as resultant and secondary. In this connection it is interesting to note that Freundlich and Schucht' have recently noticed that the '
Freundlich and S c h u c h t : Zeit. phys. Chem., 80, , j f i ~ I
O I ~ )
reaction between colloidal arsenic sulphide and certain cobaltammonium salts is light-sensitive to a high degree. Bovie' finds t h a t albumen and other proteins are not only coagulable by ultraviolet light a t 1011- temperatures, b u t t h a t many reactions of decomposition, similar to those occurring on heating, also take place. The coagulation is readily explained as due to a reduction of the electrical charge, and since, as Freundlich and Schucht's if-ork shows, chemical reactions may be accelerated between a colloid particle and a substance dissolved in the medium and probably n-ith the medium itself when the system is exposed to light, it is quite possible t h a t Bovie's decompositions are to be explained as fundamentally photo-electric effects. The experimental results reported in this paper lead quite naturally to an interesting theory of free oxidation, especially in heterogeneous systems, as lye11 as to a simple photo-electric theory of photochemical action in general. This theory will be discussed in a subsequent paper. The results obtained suggest many lines of further in\-estigation, some of n-hich are already under way, and others !vi11 be started as soon as possible. -1mercury quartz arc and quartz migration tubes are also being provided, in order t h a t the investigations may be extended into the ultraviolet field. \\-here, i t is natural to suppose, much larger effects ivill be found. Stliiijiwti l.uPLcrsiij,, i'cil.. .\Iaj, 29. r9r.3
13o\-ie: Science. 35,
21 [
1913)
