TWO-DIMENSIONAL FORM OF FLOCCULATION 0INTERCHEMICAL CORPORATION, NEW YORK 19, N. Y
W
HEN particle size measurements are taken with a micro-
scope, the investigator must be able t o distinguish betwecn single particles and particle groups such as flocculates and aggregates, Sometimes this is easy to do; a t other times it becomes a d i 5 c u l t problem. T h e present paper deals with a type of flocculate t h a t can readily be mistaken for a single large particle. Before this type of flocculate is discussed, i t should be made clear t h a t there are many cases where particles form firm aggregates. If these aggregates are not orushed but retain their original size when subjected t o the various grinding and milling operations required, then the aggregate itself constitutes the practical ultimate particle and can be measured as such. However, if interest is centered in the size of the actual ultimate particle, then the investigator must isolate t h e particle or else resort to some means of measurement other than t h a t employed by the microscope. T h e term Liflocculate” is used here t o designate a group of particles loosely held together. T h e group is not held firmly because a t some time prior t o its formation each particle is separate and in complete contact with the vehicle in which i t is suspended. Under such conditions hard aggregates do not form, unless some chemical reaction takes place t h a t produces a cementing agent. If there is a decrease in the free energy of the system when two separate particles collide, they will adhere and form a group. This group is easily dispersed upon stirring but is also easily ref o y e d when the currents thus produced subside. T h e group is not likely to be held together strongly because the particle surfaces are too uneven t o form large surface contacts. T h a t is, the
I n making particle size measurements with the microscope (either white light or electron), the investigator is often confronted with the problem o f deciding just what constitutes the ultimate particle. This article presents photographs of a certain type of flocculate that can b e mistaken readily for a single particle. This type of flocculation has been called “two-dimensional” or “sheet” flocculation. During mounting for microscopical observation, the sheets are broken down (unintentionally) and the individual pieces look like single particles. Nothing can be gained by measuring them, for their size is artificially created b y the microscopist. Several tests have been described here for detecting sheet flocculation.
actual particle surface lost when two particles touch is.prol)al)ly of molecular magnitude only. The drop in free energy, thcn, would be relatively small in comparison with what it would be if the entire pigment surface was involved. Only a slight force would be necessary to deflooculate such a group mechanically. As a rule, deflocculation can be carried out temporarily by pres.ing on the cover glass under which the material is in suspension. The technique for studying the structure of a pigment) vchicle suspension is simple. A very small drop (1 or 2 mg.) of thc: material is placed on a microscope slide. This drop is covcrcd with some of the vehicle from which the suspension is.made. A cover glass is placed on top and pressed down rather firmly. T h e original drop of the suspension will flow out into a flat disk. Microscopical examination of the edge of this disk will show the state of flocculation in which the pigment exists. Figure 1 illustrates this point. There are cases where pressing on the cover glass causcs no visible deflocculation but instead simply squeezes the material out into a thin sheet. This sheet ‘might tear, break, or wrinkle without showing any signs of deflocculation. , I t is obvious that such a particle group is not a hard aggregate, for under the cir-
x 600 Figure
x150
I. Edye of Drop of Gas Black Suspension Showing Deflocculation 679
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Figure 9 .
Photomicrographs of Copper Powder
Figure 3.
Vol. 38, No. 7
Photomicrographs of a Blue Pigment
( T o p )Two-didrnsional flocculate ( X 300)
( T o p )Wrinkle test showing individual particles ( X 800)
( C e n t e r ) Wrinkle lest (X 10)
( C e n t e r ) Wrinkle I d ( X 150)
( B o t t o m ) Tear lest (X 10)
( B o t t o m ) Tesr lest(X 150)
luly; 1946
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INDUSTRIAL AND ENGINEERING CHEMISTRY
cumstances i t would be crushed into many individual pieces. T h e fact, t h a t i t flows and, in the main, retains its identity as a group indicates t h a t i t is a form of flocculate. It will be called “two-dimensional” or “sheet” flocculation. Sheet flocculation can be produced in ways other than squeezing. Whenever a precipitate is formed in a liquid interface such flocculation might occur there. When two liquid solutions that precipitate each other come into contact, a two-dimensional flocculate is formed in the intwface. If these liquids are continuously stirred, the flocculate breaks u p into fine pieces which settle out while new flocculation is taking place in the new liquid interface t h a t is constantly being formed. In this way B pigment is produced which is composed only of finely broken picccs of sheet flocculate. The Inexperienced microscopist could easily be misled into believing t h a t these broken picccs arc thc ultimatc indi-
.vidual particles. I n appearance they often resemble very thin rectangular crystals. However, t o the more experienced mjcroscopist, these particles do not look quite like real crystals, First, t h e opposite angles of the rectangular pieces (which are obviously not hemimorphic) are never exactly equal. Second, the edges of the particle are not quite so straight as they would be in a real crystal. These facts are brought out clearly in electron micrographs. Third, the particle has a n unnatural thinness. There is still another b u t rather unusual way t o form a twodimensional flocculate. Copper powder which has been given a water-repellent coating will float on t h e surface of water and form a tenacious sheet of particles. T h e individual particles are large enough in this case t o be seen under low. magnification (Figure2, upper photomicrograph). There are two tests for sheet florrulation. .\ i;hect ran be made t o wrinkle or it can be torn.
Figure 5 .
Electron Micrograph of Barium Lithol Toner (X 13,000), Showing Large Sheets
The white oval spaces are holes in Ihe nitrocellulose supporting film
Figure 4.
Electron Micrographs of Naphthol Blue
( T o p ) Two-dimensional tlocculalion individual particles iust visible (X 3600). (Center) Wrinkle lest (X 5000). ( B o t t o r n ) T e a r t e s t ( X 7000)
Figure 6.
Electron Micrograph of Sodium Lithol Toner
(X 13,000)
Sheets broken u p into particlelike piecesduring dispersion and mounting on supporting film
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Figure 3 also shows the. sheet of copper parti(b1w \vhich \vrinklod after it had been pu*lictl against a I)arrier, as well as the same sheet t h a t had been torn. Sornial throe-tlimensional flocculation shows little or no evidence of w c h roactions. As long as the individual particles are large enough t o be vi.sihIc, there is no question of confusing the flocculate with t h e ultimate partirle. Only when t h e ultimate particle is microscopically invi)r making tlie photomicrographs and clcctron niicrographs. before t h e Twelfth .innun1 Chemical Engineering Symposium ( N e a s u r e m e n t a n d Creation of Parti‘,le Size) held under the auspiees of t h e Division of 1ndustrial.and Engineering Chemistry, A V E R I C A S C H E M I C A L S n c x m Y , a t Polytechnic I n s t i t u t e of Uroohl? n , N . Y. I’RESESTED
SIZE AND SHAPE OF MACROMOLECULES IN SOLUTION Polymer molecules are chain- or threadlike structures of very small cross-sectional area. Consequently, although the coiled molecule may extend over distances of several thousand Angstrom units, the determination of its size, shape, and weight is not susceptible to most of the direct methods used for particle size determinations of more compact particles. Indirect methods of determining the size of polymer molecules are reviewed here with the hope that they may b e applicable to the determination of particle size of other substances where the ordinary direct methods fail or need corroboration. Methods based on the ultracentrifuge, on osmotic pressure, on light scattering, and on streaming birefringence are discussed.
M””‘
natural and syrithet,ic high polymers :ire soluble, nnd extensive investigation of their dilutc solutions has contributed considerably to our knowledge of the size and shape of the dissolved macromolecules. Conditions, however, are i n genernl more complicated than those prevailing during the investigations of suspensions or dispersions. Before enumerating t h e various methods and reviewing some of the most significant results obtninetl with them, it may be useful t o point out those factors n.hich distinguish polymer solutions from other, simpler dispersed systems.
1. hlany polymer molecules have the character of a long, thin. more or less flexible chain. A cellulose acetate chain ivitli a molecular vxigiit of xhout 300,009 lia,q,if fully extended, a length of aboui 5000 A . (500 nip or 0.5 p ) , \vlicwas its iliametor is only 6 or 7 A . I n the course of its intcmial and estcrnal Brownian motion, such molecule exhibits a concidcxrablo space reqiiirement and ha.;, therc>fore,ample opportunity to collitlc! \vith other molecules and become nionicntarily attarhed t o them. During these transient contacts, the t\vo polymer mo1eculc.s do not a c t as completely independent kinetic units and would riot be rcxyistered a s such by the methods to be deacribed hrre. .\ggregntion a n d other factors do, in such ci~sui,falsify the result of the analysis in the sense that the niolccular ivciglir ,Ir, better, particle weight as observed experimentally, is different from the “true”
P O L Y T E C H N I C INSTITUTE OF B R O O K L Y N ,
N. Y .
molrcular weight-namely, the weight of that particle in which all monomers are held together by strong chemical bonds (55). I n order t o ensure obtaining the correct molecular weight, it is advisable to perform osmotic pressure measurements in a t least two difiorent solvents and t o carry out the measurements a t a s low conccxntrationh as possible. I n extreme cases it may be necessary t o carry out the measurements a t a higher temperature in order to eliminate agglomeration. 2. .ill polynicr samples, as available synthetically or from natural produc!s by certain purification procedures, are mixtures of macroniolrcules of different size anti, in many cases, even of different internal structure. There is good experimental evidence that cellulose in ivood pulp con\ists of a mixture of chains, each of w1iic.h is, in principle, built up by glucoside-bound @-d-glucose residuw. The number uf mononicr units, however, differs from chain t o chain, and it may &vel1 be that the degree of polymerizntion of the shortcit chains in the Pample is only around 40 or 20. ivhc~rc~a* that of the 1arge.t nioleculw amounts to several t l i o u ~ i i d . I n addition, it is known that in sonie of the glucose units the primary hydroxyl group is oxidized to a carboxyl group : ~ n dthat in wnie others carbon atom 6 is missing altogethrr. \ye must tlic,rtsfiJre recognize the existence of two differences between individual chains: the degree of polymerizat.ion or chain lt3ngth arid the prescnce of groups of different chemical character. In other casrs, such as polystyrene or polyvinyl chloride, there are tlirce (or morel significant differences between individual cliaiiii: their length, thc degree to which they are branched, n n d t he presence of chemically diffcrent groups. Thrreforc, it must i n general be recognized that a molecular weight distribution rurve, as obtained by various fractionation procedures, rc~jlly i c p r ~ w w t sa separation of the various species of molecules in thc .-ample in wgard to their solubility characteristics rather than i o tlic>irmolecular weight alone. The latter does, of course, influrnc,c the solubility of the different species in the various mixi ure. of solvc,rit-i)rccipitant, hut the above-mentioned structural ilutail- cannot IJC a priori neglected. It may be that in cc~rtain ( ~ a s w >ac.li , as uniformly saponified cellulose acetate or poly.tyi‘t’ne (polyiiierizcd in solution up to a lo^ tlogrrt, of ronvctt,.sioI1), t lit‘ “chemical” diff(,rcmcc= bctwcrn the intiividuiil cliaiiis arc of
