Colloidal Systems Jerry Sarquis Miami University-Middletown Middletown. OH 45042 Most of us are aware at least in a general fashion of the existence of colloids and their importance to our everyday lives. We are familiar with examples of colloids, i.e., milk, mayonnaise, inks, latex, and blood,'ewn if we may not rewgnhe [hem to be colloidal. The uniaue provertie.; of colloids are responsible for their utility in ;myriad of diverse applications. This article explores the nature of colloids while studying the properties of the colloidal systems of paints and clay drilling muds.
What are Colloids? Colloidal svstems are mixtures of two or more components. Colloidal pa;ticles exist in a dispersed state and &e intermediate in size between molecules and the smallest particles visible under an optical microscope. A common characteristic of colloid.; is that at least one dimension is in the IOK-2000K (1-2WnmJ ranxe. Thus, fibers and filmscan ben>lloidsa5well as solid particles. Practically all substances, whether solids. liquids, or gases, can exist in a colloidal state. Colloidal dispenions are metastable mixtures. On standing, they tend to form stahle separate phases. This, however, can be a slow process-gold sols prepared by Faraday in 1857 remain disoersed more than 100 vears after thev were prepared. colloidai particles are small enough that nor& filgation will not s e ~ a r a t the e mixture. Techniques such as membrane filtration can separate the components of a colloidal mixture.
Types of Colloids A colloidal system or dispersion contains particles of colloidal size, referred to as the dispersed substance, spread throuehout a disoersine medium. Virtuallv anv substance can exist rn the col1o;dal state. Both the dispe&edsubstance and disversine medium can he in the solid, liquid, or pas phase. ~ o i l o i d a l ~ ~ s t eare m susually classified a s t o the siatLof the dispersed substance and the dispersing medium (see table). Most colloidal systems can be classified as lyophobic or lyophilic. These classifications are most useful when making disiinctions hetween the~ different ~ ~ ~ ~ ~ ~ - tvnes ~ -.~ of~ sols. aerosols. and emulsions. "Lyo" refers to the continuous dispersant phase, "nhobic" means fearing. ...and "ohilic" lovine. If the dispersant phase in water, the terms hydruphopic and hydrophilic are often used. The term lyophohic implies that there is a repulsion between the dispersed parriclrs and the dispersing medium. This is misleading becausr Iv~~phohic colloids certainly - . are "wet" by the dispersing medium. This means that arelativelv strong attraction usuallv exists between the colloid and a t least 1or 2 monolayers ofthe suspending medium. Lyophobic systems are, however, thermodynamically unstable and represent a suspension of colloidal size aggregatesof individual atoms or molecules. Lyophobic colloidal systems are often difficult to DreDare because of their instabilitv. Lyophilic colloids represent true solutions of particles, such as macromolecules, that happen to have colloidal dimensions. Lyophilic colloids spontaneously form colloidal solutions and have a very strong attraction for the solvent. Proteins and nucleic acids are only two examples of biologically important lvoohilic colloids. Svnthetic macromolecules can also form Gophilic colloidal systems. A third class of colloids, association colloids, is often classified separately from lyophilic colloids. Association colloids, such as soaps and detergents, have very strong attractions for the dispersing medium. They, however, represent aggregates 602 1 Journal of Chemical Education ~~~
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Colloidal Systems Dispersed Substance
Dispersing Medium
liquid solid
gas gas liquid
liquid aerosol soiid aerosol foam
gas
liquid liquid solid
emulsion so1 solid foam
liquid
solid
solid emulsion
solid
solid
solid so1
as liquid solid
Common Name
Examples fog. spray smoke, dust whipped cream. egg meringue, beer "head" milk, mayonnaise paints, milk of magnesia marshmallow, pumice. IVO@ soap cheese (butter fat in casein) opal (H20 in SiOl) some alloys, colored gems
of small molecules (micelles) rather than large individual molecules. Micelles appear only after a minimum concentration of the dissolved molecules is achieved. Not all colloids fit into one of these classifications, and some colloids have characteristics common with more than one of these classes. How Colloidal Systems are Formed The lyophilic and association colloids form spontaneously and are relatively stable. Lyophohic colloids, on the other hand, may he difficult to prepare and are relatively unstable. Lvovhohic colloids can be reo oared bv two methods: disand condensotion ~ispeisionis [he process by which laree ~aniclesare mechanicallv eruund w colloidal dimensions or i n khich coarse suspensioncare further broken down by high speed mixing or blending. With clays, for example, erosion and weathering have produced particles that are usually relatively easily dispersed in water by agitation. In other cases, such as a gold or sulfur sols, smaller particles are condensed until they are large enough to he considered colloids. If growth continues, the particles can get too large and form a precipitate. What Stabllires Colloids? Some colloids are stahle by their nature, i.e., gels, alloys, and solid foams. Gelatin and jellies are two common examples of a gel. The solid and liquid phases in a gel are interdispersed with both phases being continuous. In most svstems. the maior factor influencing the stabilitv is the char& on the colloi~alparticles. If a pakicular ion is preferentially adsorbed on the surface of the particles, the particles in suspension will repel each other, thereby preventine the formation of aeereeates that are lareer than colloidal &nensions. The i o n G W b eeither positive or negative depending on the particular colloidal system, i.e., air bubbles accumulate negative ions, sulfur particles have a net negative charge in a sulfur sol, and the particles in a metal hydroxide sol are positively charged. Accumulation of charge on a surface is not an unusual phenomenon-dust is attracted to furniture surfaces by electrostatic forces. When salts are added to lyophobic colloidal systems the colloidal particles begin to form larger aggregates and a sediment forms as they settle. This phenomenon is called flocculation, and the suspension can be referred to as flocculated, or colloidally unstable. If the salt is removed, the suspension can usually be restored to its original state; this process is called deflocculation or peptization. The original and restored colloidal systems are called deflocculated, peptized, or stable sols.
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Why does a small amount of salt have such a dramatic effect on the stability of a lyophobic colloidal system? The answer lies in an understanding of the attractive and repulsive forces that exist between colloidal particles. Van der Waals forces are responsible for the attractions, while the repulsive forces are due to the surface charge on the particles. In a stable colloid, the repulsive forces are of greater magnitude than the attractive forces. The magnitude of the electrical repulsion is diminished by addition of ionized salt, which allows the dispersed particles to aggregate and flocculate. River deltas provide an example of this behavior. A delta is formed a t the mouth of a river because the colloidal clay particles are flocculated when the freshwater mixes with the salt water of the ocean. Paints: Stable versus Flocculated Suspensions Paints consist of a pigment suspended in a liquid medium. For oil base paints, the liquid medium is oil, as the name implies. Latex paints are dispersions of oil and pigments in water. The pigment in paint will settle slowly on standing; therefore, paints must be stirred shortly before use.
Figure 1. Illustrationof settling In (a) a stable and (b) a flocculated suspension. (a) Smbb suspension gives a dense sediment of individual particles. (b) Flocculated suspension gives a voluminous sediment of aggregated particles.
What Properties of the Suspension will Give the Paint Desirable Characteristics
A stable suspension consists of individual repelling particles while a flocculated suspension contains groups of attracting particles. At first glance, it would appear desirable that a paint have the characteristics of a stable suspension. However, even in a stable suspension, the pigment will settle. If the pigment particles settle individually, as they would in a stable suspension, a very compact dense layer will form a t the bottom of the paint can (Fig. 1).In contrast, a flocculated suspension consists of aggregates of the pigment. These aggregates have irregular shapes, and when they settle, a more voluminous, less dense sediment results. To ready the paint for application, the pigment must be resuspended. If the original suspension was stable, it would he difficult to reform the suspension, because of the dense, compact nature of the sediment. A loosely packed sediment, which would result from a flocculated suspension, would he much easier to resuspend. This resuspension, or homogenization can usually be accomplished by simple stirring. An important consideration in the formulation of a paint is therefore the ease with which it can he homogenized. This can be controlled by making adjustments to the composition of the suspension. Thixotropy of Paints
Some colloidal systems flow more readily when a stress is applied, hut recover their original viscosity over a period of time after the stress is removed. This property is called thixotropy. Wben a paint is either brushed or sprayed, it will be subjected to shearing forces. If it is thixotropic, the paint will thin as it is applied. This thinning enhances the ease of application of the paint. A good paint will, however, "level" after i t isapplied; that is, the hrush marks or spray droplets in the applied layer of paint will form a smooth uniform coat over the painted surface. I t is therefore advantageous for paint to flow until it has leveled but then recover its resistance to flow. If it did not, it would drip. This recovery of the original consistency, a result of the thixotropic nature of the paint, is aided by the use of volatile solvents. As the solvent evaporates, the concentration of the colloidal pigment increases, and the flow of the paint is reduced. Subtle adjustments in the degree of flocculation of the pigment suspension can be made during the preparation of the paint so that the characteristics of application, leveling, and nondripping are optimized. The phenomenon of thixotropy is also observed as the paint is stirred. Stirring imparts a stress which thins the paint. Wben stirring is stopped, the stress is removed and the paint thickens. This thickening tends to retard the rate of sedimentation so the paint does not need to be continuously stirred to remain relatively homogeneous during application.
Drilling Technology Colloidal dispersions, both sols and suspensions, of solid clay particles in water have been studied extensively and are important in a variety of applications. One such application is their use in drilling technology. Necessary Properties of Drilling Muds
In rotarv drilline of oil wells. a drilline bit is attached to a hollow sectioned d h l pipe. The outer Lameter of the bit is larger than the outer diameter of the drill r~ir~e. As the hole is drilled the cuttings formed must he removeb,*andthe bit must be cooled and lubricated. This is accomplished by pumping drilling fluid down the well inside the hollow drill pipe. The drilling mud comes out of holes in the hit at the end of the pipe and iscirculated back up to the surface in the space between the wall of the hole and the outer diameter of the drill pipe. During this process, the cuttings become suspended in this drilling fluid. On the surface, the drilling fluid containing the cuttings is returned to a reservoir which~mayhe either amud pit or mud tank. The cuttings are then separated by sieving and settline. In this wav drilline mud can he recvcled and recirculated ihrough thebure hoie as drilling c o n c h e s . The drilling fluid or drilling mud is a colloidal clay svstem. Its consisteniy is critical. 1t must he fluid enough-to be pumped: yet it must be thick enough to keep the cuttines . suspended. In order tomaintain the &sure and prevent the loss of fluid, i t is necessary to minimize the amount of fluid entermg the porous fmmatiuns which are encountered as the oil well is drilled. The colloidal clay in drilling muds acts as a f i l m cake. When a susnension of a finelv divided ~rerinitate is filtered, the filtration is slow hecause the particles pack tiehtlv hv a coarser - . on the filter. The filter cake denosited . precipitate of larger particles is less dense and more p(,rous. If the susoended clay oartirles in the drilling mud can denosit a dense impenetrabie'filter cake on the wali of the bore 'hole, fluid loss will he minimized. This process is called olasterim. The characteristics of the filter cake formed upoh filtration of these suspensions depends on the degree of ~eptizationor flocculation-of the susp&sion. Stable (peptized) Huspensions form dense, compact sediments, while flocculated suspensions form more vol~lmin(ussediments. The filter cake formed from a stable suspension will br. dense and rt.latively impen~trat~le in comoariion to that formed from a flocculated k s ~ e n s i o n . Thus a stable suspension has more effective plastering characteristics. Therefore drilling muds are treated with . oe~tizine chemicals to enhance their fluidity and improve their plastering properties. The plastering property of the drilling mud depends upon the type of clay used. Often local clays must he
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edge
+ -- - - - - - - - - (b)
(a 1 Figure 2.
" W s e of cards"sbudue of clays. (a) Schematic clay particle showing edge (positivecharge)and face (negativecharge) surface^. (b) Edgeface i*aniis. (c) ''House of cards" ~tructure.
would lead to the voluminous sediments that are characteristic blended with clays having better plastering properties to optimize the mud characteristics. of an unstable, flocculated clay suspension. In concentrated clay suspensions, such as the drilling muds, the house of cards When the drilling operation is interrupted, for example to add a new section of drill pipe or to change a bit, it is necessary structure is continuous throughout the total volume of the for the cuttines in the drilline mud to resist settline. The suspension. When peptizing chemicals are added to break drilling mud &ould therefore exhihit thixotropic stiffening, down this gel structure, a free flowing colloidal system rei.e.. it should become more viscous when the flow is s t o ~ ~ e d . sults. ~ o k e v e rin , the mud reservoir it is necessary for the cuttings Characteristics of Peptization to settle so the clay suspension can again IFpumped down the The mechanism of action of peptizing agents is related to h e hole. A rlwrtllntcd (unstahle~suspension will exhihit a their ahility to adsorb on the edges of the clay particles. greater degreeof thixotropic stiffening than a peptized susPeptizing chemicals are anions and thus they adsorb on the pension, and when the mud pit is stirred, the cuttings will positively charged edges. When this adsorption occurs, the settle. You will recall, however that a stable suspension is card house structure which depends upon the edge-face atneeded to achieve effective plastering. Thus, a compromise tractions is no lonaer possible; edges as well as faces are now is required to achieve the optimal conditions of the drilling negatively chargd, and the clay s.kpension is peptized. The mud.^ peptizinr chenlicals arc r m l y adsorbed on the edws which have As the drilling proceeds, adjustments to the drilling fluid 'mall surface area relative to the total clay pirticle surface, must he made for a variety of reasons. For example, the pepand thus only small amounts are required to facilitate peptitizing chemicals con decompose, or clay from the clay strata zation. Thus, even a large quantity of a clay suspension can that the hole passes through may he mixed with the drilling be treated economically. mud and thereby changeits composition and properties. Anions with a large negative charge are effective peptizing When the producing zone of an oil well is reached, plastering agents. The polymetaphosphate anion, (POa),-" may have is no longer desired because as it may diminish the flow of oil. a charge of -30 or more. Alkalies are also used as peptizing Thus, throughout the drilling process the properties of drilling agents but are not as effective as the polymetaphosphate ion mud must he routinely monitored so that the necessary adin reducing the stiffness of drilling mud. They do, however, justments can be made. find use if the clay is acidic. The bentonite clavs. derived from the mineral montmorilOrganic tunnares are about equal to inorganic peptizing lonite, A I ~ O T ~ S ~ O ~ . Hare ; O ,ideal for use either as drilling agent9 in their detloccdnting ahility. Clay suspensions are muds or as blending materials for local clays. Bentonite clays uiually prptizcd hy the addition of only u few tenths of one consist of layers of alumina between silica sheets and are found percent of n tmnate. The most widely used peptizing tannate in the midwestern United States and Canada. is r h e r d c ~ h r c dtannin extracted from thr South American Many types of chemicals can he used to peptize drilling Quehracho tree. I t has the monomer formula: muds. These include inorganic salts, alkalies. and organic compounds. Addition of only a few tenths of one percint of OH a peptizing chemical will change a stiff, viscous mud into a relatively free flowing liquid. What is i t about the structure of clays that causes such a dramatic change in the flow properties when small amounts of peptizing chemicals are added? Naturally-occurring tannates are polycondensation products House of Cards of the monomer. They are also characterized by large negative Clay sols are negative sols, i.e., u,hen placed in an electric charges like the polymetaphosphates. iield the partidc.s of clav move t w a r d the positi\.e electrde. Similar in structure are synthetic alizarin dyes. They also 'l'his surface charge tends t c ~kwp the clay parriclrs from copeptize clays. alewing. Even thotigh there is a net nrgsti\.e charge on the n nu surincc tdu clav" ~artirle. it is thourht that reeiuns oirsni~ive . " surface charge also exist. The clay particles can associate due to the attractions between their negative and positive regions. Clay particles can exist as thin, flat plates with the large face surfaces havina the neeative charee and the edee surfaces having the charge. The edg~snrfacearea iHvery small Although their expense prohibits their routine use in drilling com~aredto the face surface area, and thus the articles have muds, they have potential applications as tracers since they net rwgatire charge. ( ' l n y s can aggrcgnrc in '.house of cards" impart their intense color to the clay suspensions. tvve structurra. Fiwre 2, with poiitivr edges heine: attracted Drilling fluids peptized with the quehracho tannates and to the negative faEes. This type of struciural arrangement ~~
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a slight excess of base are characteristically red; thus, the name red muds is appropriate. A desirable property of these red muds is that they inhibit the dispersal of clay from the wall of the bore hole. They also prevent the disintegration of lumps of clay in the drill cuttings. Although the tannates peptize the drilling mud, they do not peptize these formation clays, because tannates do not diffuse into the interior of the clay lumps. Red muds, however, stiffen a t high temperature, so they are limited to use in shallow wells. This stiffening is possibly a result of flocculating products that form from the reaction of the clay with the alkali. Addition of calcium hydroxide (lime) will lessen the stiffening most probably hecause
it precipitates the reaction products that are otherwise responsible for the flocculation. References Kennan, Charles W.. Klcinhiter. Donald C. and W d .Jease H.,"Generd College Chem. -67. ietry."6th Ed.. Hsrpcr& Row, New York, 1980. PP. 3fC Malijevit, Egon, CHEMTECH, 3.656 (1913). Nebeqdl, WiUlam H., Schmidf Frederick C.. and Holtrclnar Henry% "Gcnud Chernlstry." 5th ~ d . D. , C. Heath and Company, Lexington, MA, 1 9 1 7 , ~317-21. ~. o'connor. Rod, "Fundamentals of Chemistry." 2nd Ed., Harper & Row, New Yark, 1917, pp " d W ,
Slal.au;n \Ieudrli H. and Pam.md.Thcron D..' General Chem#urf.":
