Matter exists in various forms such as gas, liquid and solid. Arrangement of atoms in these forms gives them their characteristic physical properties like density, shape and color. If we subdivide the solid state into fine particles until we reach molecular dimension, we observe a spectrum of particles, say, ranging from millimeter to nanometer size. Colloids are concerned with the properties of matter involving characteristic length scales that range from 1 to 1000 nm (1nm = 10-9 meters).
Graham in 1837 coined the term colloid to distinguish between gelatin-like materials that did not pass through a membrane and small molecules that passed through it. He named gelatin type materials as colloids from KOLLA, which in Greek (κολλα) means glue, to distinguish them from substances that formed true solutions which were called crystalloids.
Colloids exist as fine particles distributed in one of the phases of matter. The bulk phase in which the fine particles are distributed is known as dispersion medium, and the fine particles are known as disperse phase. For example, in smoke the disperse phase is solid carbon and the dispersion medium is gaseous air (technically categorized as aerosol). Similarly, slurry is a colloidal solution of a solid in a liquid and foam is a colloid of a gas in a liquid. In fact all combinations of states are possible in which the gas/gas system does not have a meaningful existence.
Colloidal science has always been in focus owing to an impressive array of applications it provided in technologically and biologically relevant areas.[1,2.3] Many practical applications such as inks, paints, paper making, emulsions, gels, pharmaceutical preparations, food stuffs like ice cream, drinks such as beer and wine, cosmetic and agricultural products, photographic films, lubricants, and foams derive a lot from colloidal materials and principles. In biology, colloids manifest in the form of biopolymers (nucleic acids, proteins and polysaccharides), cell membranes and other cell components: A simplified view of blood treats it as a colloidal dispersion of red corpuscles in a liquid. Milk is a biocolloid. The synovial fluids that lubricate joints in our body are colloidal in nature.
Materials such as polymers and surfactants and subjects pertaining to them did not have a separate identity in the beginning since they were considered as a part of colloidal science. Polymers, which usually have length scales of hundreds of nanometers and surfactant micelles that have colloidal size range were considered as colloids. Later on the science of surfactants and polymers gained independent existence in view of their proven and emerging applications.[4,5]
Ink chemistry is the confluence of many colloidal principles in action. Primarily, the pigments, colored or not, that form an important constituent of inks exist in the colloidal state in the solvent, aqueous or non-aqueous. They are dispersed into the colloidal size by grinding in the presence of aids called dispersants. Many of the dispersants are either surfactants or polymers. These materials themselves, in solution, behave as colloids. In addition, polymers find a place in inks as rheology controllers and tuners of film property. Surfactants are also present as wetting agents, emulsifiers and defoamers. Emulsion products are colloids of one liquid distributed in another liquid. Foams, an inevitable follower in ink making, are also an example of colloids. Needless to say, understanding and manipulation of colloidal properties are of top priority to ink chemists.
Colloids may be divided into two major classes: the type that readily goes into solution is called a lyophilic (solvent-loving) colloid (hydrophilic if the dispersion medium is water), and the type prepared from sparingly soluble substances is called a lyophobic (solvent-hating, hydrophobic in the case of water) colloid. Surfactant micelles and polymers are examples for the former category, and metals and pigments for the latter. Lyophilic colloids are thermodynamically stable and lyophobic colloids are thermodynamically unstable, which will be reflected in their settling behavior. Thus, suitable methods and conditions should be adopted in making lyophobic colloids, and they were important concerns of colloid chemists.
Lyophobic colloids (sols) are prepared either by breaking the material into smaller particles (dispersion methods) or by building up the colloidal particles from the molecular size (condensation methods). In dispersion methods, the colloidal range is attained by grinding or milling the bulk material in the solvent. In pigment dispersion methods, larger aggregates of pigment particles are dispersed by grinding or by shearing in a high-speed mixer. The inherent thermodynamic instability leading to reaggregation is circumvented by suitable dispersing agents. Other methods involving ultrasonication and electric arc are also available.
Condensation methods are more versatile: simple dissolution and reprecipitation can result in a sol, as by pouring a solution of paraffin in alcohol to boiling water to yield paraffin in the colloidal state. Chemical methods are the most useful one. The precipitation of colloidal sulfur in acid solutions of hydrogen sulfide is a common experience in qualitative inorganic analysis. Other reactions such as the double decomposition between silver nitrate and alkyl halides, and hydrolysis of ferric chloride in acid solutions can respectively yield the silver halide and ferric oxide sols. In the absence of an added specific stabilizer these sols are stabilized by one of the ions present in the preparation step giving them a charge. Here, the like charge repulsion causes the stability. Chemical methods can often produce colloids with a very narrow particle size range (monodisperse) by regulating the growth of nucleation sites.
The stability problem of lyophobic colloids has been the subject of numerous experimental and theoretical investigations. The colloidal particles are under the influence of attractive and repulsive forces. The magnitude of these forces dictates whether a colloidal dispersion aggregates together or remains as a stable dispersion. Theoretical treatments consider equations relating the potential energy of these particles at a certain distance of separation between them.
The colloidal particles have a charge gained through surface dissociation or preferential adsorption of ions. This primary charge of one type on the particle will be compensated by the opposite charge in the bulk continuous phase. Thus an electrical double layer builds up around the particle surface. Different attempts were made to model this double layer by Helmholtz, Gouy, Chapman and Stern. When Helmholtz considered the double layer to be two fixed charged layers as in a parallel plate condenser, Gouy and Chapman pinpointed the need for a diffuse layer of the opposite charge extending to the bulk phase. This was to justify the diffusion of some of the counterions into the solution due to thermal agitation. However, this model failed with surfaces with high charge densities since it neglected the finite size of ions, by treating them as point charges. Stern modified the theory by considering the double layer as consisting of a fixed layer and a diffuse layer. Figure 1 represents schematically the distribution of charges as envisaged in the Stern theory and the variation of electric potential as a function of distance from the charged surface.
The colloidal particles aggregate under the influence of interfering charged ions whose charge number and concentration are important in the aggregation process. A manifestation of these effects is the background for the rule of Schulze and Hardy that deals with the dependence of charge and concentration of added electrolytes in the coagulation of hydrophobic colloids. This is the reason for the muddy appearance of estuarine waters since the colloidal clay particles in the river coagulate on contact with the salty waters (containing ionic compounds) of the ocean. The practice of adding alum to clarify the muddy water to make it potable is another example. This principle is implied in the process of delta formation. (Nile delta has the shape of the Greek letter delta – Δ – which is triangular in shape, and that prompted the Greek historian Herodotus to coin such a term.)
The theory concerning the stability of lyophobic colloids was developed by Derjaguin and Landau, and Verwey and Overbeek independently and is referred to as DLVO theory. It treats the potential energy of interaction between colloidal particles as being the sum of van der Waals attractive force and double layers’ repulsive force as explained above. Preponderance of one of these will result in stabilization or aggregation of colloids.
Another type of repulsion called steric repulsion comes into play when the surface of the colloid adsorbs large molecules like surfactants and polymers. This effect is prominently observed in the case of polymers. Adsorption of molecules on solids provides a coating of the adsorbate on the particle surface very much similar to the coating of paint on a wall. When surfactants and polymers are coated on colloids, molecular chains protrude into the solution. Of course, some portion of the molecule would be anchored on the surface. The protruding chains extend to the liquid phase and interact with each other. Two effects are recognized: osmotic effect and entropic effect. In osmotic or mixing effect, a high concentration of polymer chain elements is observed in the region of overlap between the particles. Both solvent-chain and chain-chain interactions happen; the supremacy of the former results in stabilization and that of the latter leads to coagulation. Entropic stabilization operates when particles come close by and the adsorbed layers interpenetrate. Under these conditions, some of the solvent molecules are excluded to the bulk dispersion medium. This results in a decrease in the entropy of mixing of the chains with the solvent molecules providing stability. Under ideal conditions, colloids could be indefinitely stable: a gold sol made in 1857 by Faraday is exhibited in London museum, which does not show any sign of settling.
The significance of attractive and repulsive forces may be seen from Figure 2 in which the individual and sum of these energies are plotted against the distance of particle separation for the case of electrostatic repulsion. At very small separations, the total energy tends to negative infinity and it is not possible to separate the particles at these ultrashort separation distances. This is known as primary minimum from which dispersion is very difficult. At large separation distance the total energy is again negative, but its value is only a fraction of kT (secondary minimum). Particles in this shallow well also aggregate, but their separation is easy. Even simple agitation can deaggregate them. This type of aggregation is termed as flocculation and the former type is known as coagulation. But the particles corresponding to the maximum in the curve which is about 15 to 20 kT would provide a stable suspension, where the repulsive energy overtakes, which is sufficient enough to keep the particles separate. Similar potential energy curves may be drawn for steric stabilization.
These concepts are important in dispersing pigments. In fact, polymer stabilization was used in stabilizing inks as early as 2500 B.C. The first ink (incidentally, the first man-made colloid), which was black in color, was made from carbon from lamp soot as the pigment. Carbon black was mixed with natural biopolymers such as casein from milk, egg albumin or gum arabic, and molded into pencil like sticks. This generated an instant ink on mixing with water since the polymer coating spontaneously redispersed the pigment to yield the ink.
The optical and electrical properties of colloids are the main tools to investigate and exploit them. The colloidal particles are in continuous random motion in the dispersion medium, a phenomenon demonstrated by Robert Brown known as Brownian motion. This effect can be observed under a microscope. The optical properties of colloids are much more spectacular. If a beam of light is passed through a true solution, the path of light will not be visible in the solution. But in a colloid, the light path would be clearly distinguishable. This is known as Tyndall effect. The path of a stray light beam entering a room would be clearly visible if the room is dusty exposing the presence of colloidal dust in air. A simple revealing demonstration would be to illuminate a beaker containing sodiumthiosulfate solution with a flash lamp during the addition of dilute acid. In the beginning the light path would not be visible in the beaker. But as and when thiosulfate produces colloidal sulfur, the light path would be visible.
Tyndall effect is the manifestation of an optical effect called light scattering by particles. Consider the visible green light with a wavelength of 500 nm passing through a solution where the particles are about 0.5 nm in diameter. The particles here are too small to interact with this light. But, if the particle size is around the same wavelength range as this light (hundreds of nanometers), scattering is significant.
A layman’s exposure to the light scattering phenomenon commences with the familiar explanation of the blue color of the sky based on Rayleigh scattering in which the scattering intensity is inversely proportional to the fourth power of the wavelength. This can be applied to small particles in the range of a few nanometers where the scattering intensity is proportional to the square of the volume of the particles. This relation can be linked to the molecular weight of the particles since the scattering per unit volume would be proportional to the mass. For larger particle size, the scattering pattern would be less symmetric and the angular dependence of total intensity more complex. This has been treated by Mie scattering theory. Even the simple turbidity of colloids which is a manifestation of scattering by light may be used to generate information on the molecular weight and size of the particles.
As mentioned earlier, the electrical charge on the colloid is one of their stabilizing factors. This charge manifests itself in electrokinetic phenomena, the movement of colloids in an applied external electric field. The electrophoretic mobility of colloids has many practical analytical and preparative applications. Many complex protein mixtures can be separated by this method, and running an electrophoretic gel is a routine procedure in molecular biology laboratories. This phenomenon is also used in the electrodeposition of paint suspensions onto metallic substrates.
The rheology of colloids, the study of the relation between the applied stress and the resulting deformation, is crucial in many applications. The ideal liquid-like behavior called Newtonian behavior is shown by fluids where the applied shearing stress is directly proportional to the rate of shear. But complex fluids like inks often show non-Newtonian behavior, although most of the raw materials are Newtonian in behavior. This will be reflected in the viscosity measurements in weak and strong shearing fields. Colloids can show both shear thinning (viscosity decreases with increase in shear stress) and shear thickening (viscosity increases with increase in shear stress). The former effect is known as pseudoplasticity and the latter as dilatancy. Most of the inks show pseudoplasticity. In dilatancy, the particles cannot move fast enough past one another and the system rigidifies. A ring of dry sand around foot steps seen while walking through wet sandy beach is caused by dilatancy of the wet sand.
Another rheology related property of colloids is thixotropy. It is the flow behavior in which viscosity is reduced by agitation or stirring. Thixotropic systems can form gels on standing. A gel is a colloidal solid having a network structure such that both solid and liquid components are highly interdispersed. These days sol-gel processing is becoming popular in ceramic industry where a colloidal sol is transformed into a gel chemically by changing parameters such as concentration and pH. The opposite phenomenon of thixotropy is rheopexy where under a steady shear rate the viscosity goes up before reaching a maximum value.
The surfaces of colloidal particles show great affinity for many molecules to adhere onto them by adsorption. Thus polymers and surfactants adsorb on pigment particles in inks giving them the necessary stability to remain as a dispersion. Colloids in the adsorbed state are known as solloids. The colloidal implications in foam and their occurrence in inks have been explained elsewhere.
In summary, understanding of colloidal principles is essential in the efficient design of ink systems. Practicing ink chemists very well know the importance of pigment charge, rheology of pigment dispersion and final ink, stabilization of dispersions by surfactants and polymers, the nuisance created by foams, and the optical color effects brought by the colloidal state. No wonder colloidal science is still pursued as a fertile area by both basic and applied scientists.
1. “Colloid Science,” H. R. Kruyt, Elsevier, Amsterdam, 1952.
2. “Introduction to Modern Colloid Science,” R. J. Hunter, Oxford University Press, New York, 1993.
3. “Colloid and Interface Chemistry,” R. D. Vold and M. J. Vold, Addison-Wesley Publishing Co,, Ontario, 1983.
4. Joy T. Kunjappu, “The Emergence of Polyacrylates in Ink Chemistry,” Ink World, February, 1999.
5. Joy T. Kunjappu, “Surfactants in Ink Chemistry,” Ink World, July, 2000.
6. “Surfactant and Interfacial Phenomena,” Second Edition, M. J. Rosen, John Wiley and Sons, New York, 1989.
7. B. Derjaguin and L. Landau, Acta Physicochim, 14 (1941) 633.
8. “Theory of Stability of Lyophobic Colloids,” E. Verwey and J. Th. G.Overbeek, Elsevier, Amsterdam, 1948.
9. Th. F. Tadros, Colloids and Surfaces, 18 (1986) 137.
10. Joy T. Kunjappu, “The Complex World of Ink Chemistry,” Ink World,August 1998, p.32.
11. The term SOLLOID was coined in this paper. P. Somasundaran and Joy T. Kunjappu, Colloids and Surfaces, 37 (1989) 245.
12. Joy T. Kunjappu, “Yes, We Have a Foaming Problem,” Ink World, August 1999, p.44. n