Flocculation, Dispersion, and Settling of Pigments in Relation to Adsorption L. W. RUN., Titaninrn Pigment Co., h e . , Rrooklyn, N. Y., W. 11. I%.~RKINY, AND D. M. CANS,University of Chicago, Chicago, Ill.
S
IKCE a paiilt is ti suspension of crystallinr: solid particles of varying dimensions in a more or less complex liquid, it is evident that important results concerning the sta-
hility and otlicr cliara,cterist.icsof paints would be obtained if the interfacial eriergy and otlier relatioxis corrccrnirig the region of contact between tlie solid and liqiiiii could he fully understood. The importance of the interface is brought out by Bragg, the great worker oil crystal structure ( I ) : We must hear in mind also that there is something emn more important thm the solid itself, namely the bmiindary hetmen solid and liquid or cas. It is here that a vast number of changes
TOTAL ~ . U t ; I t O YOF
Since what is often called tlie "iwttahility" of tile pigment by the vehicle 1ia.squite gcnerally been considered as the most imporbant characteristic associated v.itll t l i e staliility of the paint suspension, t,his was the first property to be studied. The total or the free energy of immersion might have been takcii as a measure of wettability. Since, Iiowcvcr, there is no known inethod for the determination of the true free energy, the total energy was found by the use of an extremely sensitive calorimetric system. The rc.;uli,s as obtained by Harkins and Dahlstrom are given in Table I. TAXLE
I.
TOTAL
CILOXZBB PEPl 0~~~
The orientatioii of the molecules of the liquid and possildy also of the moleeoles of the solid must greatly iriilucrice t.lie surface reactions at the interface, and these surface reactions in turn niust influence or determine the stability a i d otlier charact,eristics of a paint. An investigation to determine soine of tlie fundainciital ~ arid organic liquids relations hetween p o m i e ~(pigments) (vehicles) i n s startcd a few years ago iii order t.o produce, if possible, a clearer picture than had been ohtained u p to that time of the complcx systenr called a lpaint. It was hoped to arrive iiit,iniateiy at a more sound basis for paint formulation, and to prevent dry or hard settling. It ,%-asobvious t,liat to start with tile comjiles paint itself or with such comples consbituents as liiiseod oil, for exampic, vould be impracticalile. Therefore, pure liquids, each a representative of wrtain types found in paints, liave been used with different pignieiits and powders. The relations betxeen these simple compounds and pigments xitliout doubt indicate the relations 1,etween the more complex liquids and pigmrnts.
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interest i n d import.ance.
I1.lXEfW(JS OF ~ I G S I E N T s
ST*NNIC
oxroe 0.685
1.15
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ZINC
oxrm
SILIOA
1.18
0.903
Ikiireiie (dry) 0.06% raw lingeod oil 99.94% benrerie boilcd linseed (d i-99:9% beiimne ke!tle.hodied h?aeod 011 99.9% bciirene 0.85% boilrl I!naeod o!l 09.15% benzene 0.10% blown !inseed 011 99.9% benrene
+ + +
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fimeoCes
0.00 1.61
3.36 8.08 15.60 y
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0.897
0.39 0.63 0.71 0.74 0.76 0.7s
c o i nsr*'""& 0 , 390 0.593
0.762
0.998 1.120
0 The &moan1of heat iibersted b , tire ~ ?ulufions of liriseed oil iocreaaes xvith the extent of oridstion of the 0?1, and increases Fith the smorint of oil mti! the sdsorbed layer heeomas s tightly wuked r n o n ~ m ~ l e ~ ufilm. lar
When suspended in a liquid containing a polar- and the volume of the suspension large-for example, nor~polarcompound, pigments adsorb on their sur- 1G cc. per cc. of solid with a certain pigment. I n case 2 the flocculation disappears if enough polurfaces u rnonomoleculur film of the polar-nonpolur compound. This adsorption has a marked in- nonpolur molecules are present lo give a monofluence on tlie flocculation, dispersion, and settling molecular adsorbed film on tlte grains of powder, of the pigments. Experiments on the settling of und the tulumefulls to about 5 cc. I n case 3 water various pigments in dry pure organic liquids, und added to ( 2 ) restores the flocculation arid high volume also in organic mixtures, show renzarkable differ- of suspension characteristic of (1). The powders ences between ilte extent of the settling when (1) the are dried in a high uucuum a1 temperatures from pure liquid or solution consists entirely of nonpolar 350" to 500' C., and the liquids are also dried with molecules, (2) either all, or a small fruclion. of the care. Experimerds have been carried out with molecules are of the polur-rwnpolar type, and (a) several white pigiwnts in a great many organic liquids, and in solutions in benzerte o j metallic water is also present. I n case 1 the suspension is highly jlocculated soaps arid various organic compounds.
About half the data given in Table I were presented by cross oii tlre butyric acid curve represents the adsurpt.ion of Harkins and Dahlstrorn (5) in a prelinrinary paper. The a monoinulecular film. The energy of irnmersion of thcse powders (pigments) in additional data were not publislied. It is found that, if a dry clean pigment is immersed i n a organic liquids whose molecules contain polar groups is much liquid, an amount of lieat is liberated, the quantity of which higher than tlie energy of immersion of Tvater in the same increases vitli increase of polarity of the liquid, or wit11 in- liquids. This indicates that the pigments attract the polar crease of polarity of certain groups in the molecules of the groups much more strongly and adsorb them to a ~nuchgreater liquid. Thus the energy of immersion of any of the powders extent than water. In either ease the maximum adsorption, (pigments) in vatcr (a highly polar liquid) is from three to a.nd very nearly the maximum heat development, is attained four times higher than if benzene or any other nonpolar or- when the adsorbed film around tlre salid particle, or around the drop of water, becomes as lriglrly concentrated as tlie space ganic liquid is used. The energy of immersion of a pigment in a noripolar liquid relations permit-that is, when a tightly packed monomois greatly increased by the presence of minute quantities of lecular film is formed. This indicates that, when a pigment is immersed in a polara polar liquid. For example, although vater is commonly supposed to be insoluble in benzene, the amount of watcr nonpolar liquid, tlie molecules of the liquid at the interface contained in ordinary, supposedly dry benzene, is often suf- orient themselves in such a way that the polar group is toward ficient to increase the energy of immersion from its low value the surface of the pigment particles and the nonpolar group toward tlrc oil. This fact also indicates that pigment parin perfectly dry benzene, to three or four times this valuethat is, to what would he obtained if water alone were present. ticles adsorh on their surfaces a monomolecular film of the se contain a polar group. I t is remarkable that, if the concentration of ivater or butyric substance ~ ~ l r o molecules I n other words, all the data of this earlier work indicate acid in benzene is such as to give 13 of either of these molet h a t a pigment i m m e r s e d in cules to 100.000 of benzene. oil h a properties similar to an then i t is foubd that thc energy enrulsion of water in oil. The of immersion is more than 95 per principal difference is that the cent as high as in pure water f o r c e of attraction a t the inor pure b u t y r i c acid, respect.erface is much stronger than tively. Figure 2 shorn on the the force of attraction at the iny-axis the amount of heat deterface between water and oil. veloped when 1 gram of powder There ha.s been considerable (titanium oxide) is i m m e r s e d difference of opinion concerning in 20 ec. of liquid whose inibial the thickness of tlie interfacial concentration is g i v e n on the film betmen oil and an aqueous z-axis. At an initial concentrasoap s o l u t i o n . Thus at one tion of 0.016 inole per kg. of time McBain considered tlresoap solvent (I mole of solute to 800 film to be polymolecular, whereas of solvent) the heat dereloped FIGURE 1. 1 i A R o OR D R Y SETTLED I'lGHEXr CAUSED &ifin ( 3 ) , \,an d e r M e n l e n is almost as great as with pure BY IDIPHOPER I'*I\IT V,iH,CLS and R i e m a n (7), and Ilarkins water or p u r e b u t y r i c a c i d , and Beeman (4)determined it resnectirelv. The arrom a t the rigit indicate the values for thesc two pure liquids. At this to be monomolecular; liowever, the concent.ration in the concentration either the water or the butyric acid forms a film \vas not obtained with much exactness. More recently Fischer and Harkins (&) definitely estabnearly condensed monomolecular film on tlie surface of the crystals. Since nearly all of tlie solute is adsorbed, this lished that a stable emulsion of water in oil produced by a initial eoncentration is reduced to about 1 inolecule of water soap, contains a t the interface a condensed monomolecular or butyric acid to 8000 of the solvent (benzene) in the solution film of the soap, $lie minimum area per soap molecule being which is in equilibrium with this nronomolecular film. It is about 26 square Angstrom units. I t is interesting to note that the energies of immersion of obvious tha.t it is this final concentration whicli is uniquely determined by the activity of the adsorbed solute. The titanium oxide, stannic oxide, silica, barium sulfatr, and zinc
1290
l N D U S T K l A L A N D E N G 1 N E E R I N G C k1 E M I S T R Y
oxide are all of the same urder US magnitude, except when a definite chemical action is knoivn to occur. Thus, when zinc oxide is imniersed in a liquid with wlrich it rcacts chemically, the energy increases greatly, owing to the chemical reaction. All these powders and pigments (some inert and at least one very chemically reactive) apparently exhibit practically the same adsorptive capacities. MONOMOLECULAR l h ~ AT s S o i . i i i - L i ~INTEISFACE ~~ In the second part of the investigation it \\'as determined that monomolecular films a t the solid-liquid interface have an extremely great, , , , , , , , , , effect upon tile scttliiig of pigments in organic liquids. The results of this work I / I aere eivcrr i n a naoer
,
A dry pigment (titanium oxide) inimersed in pure dry beneene (a poor wetting, nonpolar liquid) remains as a some-
FIGURE2. HEATDEVELOPED WHEN TITANIUM OXIDE IS IMMERSED 1A SOLWIONS IN BENZENE
Vol. 24, No. 11
' I hpigments iised iserc titanium oxide, Titmux-13,' Titanm-C,' zinc oxide, arid siirc: sulfidc. The pure organic liquids used were beiiaeiie, carbon tetr:ichloride, methanol, benzaldehyde, ethyl acetate, oleic acid, and, for a part of the work, a great inany oilier liquids. The rnetallic soaps used were precipitated and fused lead linoleate, precipitated and fused cobalt linoleate, precipitated aluminum stcarate, precipitated titanium stearate, arid i n some experiments precipitated and fused lead tungate.
Tlie flocculntion and the extent to which a pigment settles i n solution are highly dependent on the amount of water present; so it is necessary to dry thoroughly both the pigment and the liquids which are to be used in work of this type. €'riEP.uuTioN OF PIGMENTS. From 8 to 200 grams of pigrirent were placed in a thick-walled tube of I'yrex glass and this was sealed to a niercury condensation pump. A trap cooled with Dry-Ice was always provided betreen tlie pornl~ and tlie tube which conbained the pigment. The tubc aiid its contents were heated to us high a. temperature as pussiide &bout incurring any risk (if increasing the size of tlie grains, and the tube was evacuated at this high temperature for a period of 24 houn or more; depending upon the amount of pigment which i t contained. The tenigerature chosen for titanium oxide, for example, was d m u t 460" C., and for zinc oxide and zinc sulfide was almut. 340° C. Even if the sioe of tlie grain shoiild be somerrliut modified, the method of procedure was such as to liavc 110 influence upoil t,he results obtained, since they relate silrrply to the effects produced by various suirstances and since every sample of a particular pigment wihi: subjected to exactly tlie same treatment. PREPARATION ~ ~ S U ~ P E N SThe I ~ mcthods N S . for the preparation of the pignient suspensions may be described as: (1) a method in which excessive care was taken to prevent the adsorption of moisture: (2) R method which was carried out
contains enougli oleic acid (a p o l a r - n o n polar liquid) to form a in o n omo 1et: u I a r film aliout each particle of tlie pigment, lretter dispersion is attained and the pigment set.tles to a smaller volume tlian that occupied in tlie pure benzene. q7hen a small amount of water (a highly polar liquid) in which tho Iliginent exhibits an even hieher merev o" of .. irnmprsion than in oleic acid, is added to the titanium oxide in the benzcnc-oleic acid system, a high degree of f l o c c u l a t i o n appears and the total volume occupied by the pigment becomes greater than that occupied by tlie pigment in pure benzene. Thus i t was shown t h a t t h e e x t e n t of flocculation and of settling is not B simple fuuction of the wettahility of the pigment by the liquid. 'I'itanium oxide exhibits a high energy of immersion in both oleic acid and water, and a low energy in bensene, yet, oleic acid largely deflocciilates the suspension and the subsequent addition of water causes a much higher degree of flocculation than that in pure benzene. An adsorption method for tlie d e t e r m i n e .e. . tion of the area of the surface of a p i g m e n t A. Flocdnted n. DenoOCuiated was also described (6). The method involves a F l G U R E 3. TImNruM OXIDE ( X 1000) determination by chemical analysis of the amount of oleic (or stearic, ctc.) acid adsorbed by a givciit w i x l t t or with great care b u t which is more suited for a larger-scale a dry pigment from pure dry benzene, the area rier iiiolivvl
