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Surfactants in Ink Chemistry.

Somewhere in the evolutionary ladder of human civilization, man's sense of cleanliness would have led him to the adoption of soap-like materials from natural sources such as plants and animals.

These materials which removed dirt and greasy substances were the pioneering prototypes of an important class of chemicals called surfactants [1] (SURFace ACTive AgeNTS).

Surfactants as a class of compounds find application in numerous technologically, industrially, and biologically relevant areas.

It forms a crucial component in commonly used materials such as detergents, tooth paste, cosmetics, health care products, inks, paints, pharmaceutical preparations, etc.

Many technological applications such as enhanced oil recovery, metal purification from ores and minerals, environmental remediation, etc. including high tech areas such as microelectronics, space processing, magnetic recording, nanotechnology, etc. rely on surfactants at several stages. [2,3]

Many emerging and established biologically significant processes such as biotechnology, viral research, gene therapy and other DNA manipulations, [4,5] are dependent on these compounds for their function and performance.

Details on biosurfactants functioning in lungs enhance our understanding of pulmonary mechanical motion that has implications in respiratory distress syndrome found in prematurely born babies, as in the deficiency of Surfactant Protein B.

No wonder, many multi-volume publications like Surfactant Science Series [6] that run into about 90 volumes on a thematic basis, and several general monographs, [7-11] covering the aspects of surfactants, are available today.

This is aside from dozens of journals that publish latest results of fundamental and applied lied research involving surfactants [12] at the interface of surface science.

Surfactants and Inks

An ink chemist's tryst with surfactants commences with the addition of these compounds in an ink formulation. A major use is as dispersants and grinding aids in pigment dispersions, whether the final ink is water-based or solvent-based.

Here the wetting and dispersion stabilizing effects of surfactants are exploited. The are also added in ink formulations [13,14] to combat the foaming problem.

Surfactants also form the components of many polymer emulsions used in inks. Also, ink chemists use these compounds to enhance properties such as print quality, color acceptance and compatibility. Large amounts of surfactants find demand in fountain solutions for printing offset lithography inks.

With the advent of water-based ink formulations, which surfaced from the VOC concerns of regulatory agencies, surfactants have been elevated to a special status in ink chemistry. Solvent-based inks yielded fine print quality, lay smoothness, acceptable print density and color uniformity. But, water-based inks take away most of these fine end properties.

A major reason resides in the high surface tension of water-based inks resulting from the use of water as a main component of these inks. This value is many a time close to the surface tension of water, which is [tilde]72 dynes/cm. Compare this number with the surface energy of many substrates, which lies in the range of 35-40 dynes/cm.

An effective ink coating free of defects can be sustained only if the surface tension of the ink is less than the surface energy of the substrate. Attempts to increase the surface energy of the substrate by corona treatment have only resulted in partial success.

Here comes the use of surfactants with its well known characteristic of bringing down the surface tension of the solvent in which it dissolves.

Surfactants are amphiphilic substances, which means that they contain two opposing parts - hydrophilic and hydrophobic.

In general, the hydrophilic part is a water-loving ionic or polar group and the hydrophobic part is a water-hating hydrocarbon derived alkyl group. When dissolved in water, the former will be readily hydrated and the latter will try to avoid water.

As the concentration of surfactant increases, a remarkable self-organization process occurs in aqueous solutions resulting in the formation of self-assembled aggregates called micelles.

This occurs above a temperature referred to as Krafft point, above which the ionic surfactant solubility enhances dramatically.

In the case of nonionic surfactants, the solubility is maximum below a temperature called cloud point, above which phase separation occurs. The concentration at which the micelle formation commences is designated as critical micelle concentration (CMC).

The relationship among the solubility, concentration, and temperature is depicted schematically as shown in Figure 1. [15]

In fact, micelle formation represents one of the important processes in nature because it is having implications even in the most fundamental biological self-organization processes. It is deeply rooted in the search for the driving force for the formation of first living cell.

The complexities of the self-organization processes are being actively investigated by researchers [16,17].

Above the CMG, the physical properties of the aqueous surfactant solution undergo conspicuous changes at increasing concentrations.

For example, properties such as osmotic pressure and surface tension remain a constant; solubilization (ability of micelles to incorporate large amounts of insoluble substances) and magnetic resonance absorption continue to increase; equivalent conductivity steadily decreases.

In essence, one can observe sharp breaks in the plots of these properties vs. surfactant concentration, and this phenomenon is made use of in the accurate determination of CMC. [18,19] Such a scenario is shown in Figure 2.

Classes of Surfactants

Surfactants are divided into three major classes based on the nature of hydrophilic group -- ionic, non-ionic and zwitterionic.

Ionic surfactants can be either anionic type or cationic type. Sodium Dodecyl Sulfate, SDS, is a well known anionic surfactant

[CH.sub.3][([CH.sub.2]).sub.11][OSO.sub.3](-)Na(+)

Cetyl Trimethyl Ammonium Bromide (CTAB) is a cationic surfactant

[C.sub.16][H.sub.33][([CH.sub.3]).sub.3]N(+)Br(-)

Dodecyl Octaethyleneglycol Monoether is an example for a nonionic surfactant

[CH.sub.3][([CH.sub.2]).sub.11][([OCH.sub.2][CH.sub.2]).sub.8]OH

N-n-Dodecyl-N, N-Dimethyl Betaine is a Zwitterionic surfactant

[CH.sub.3]([CH.sub.2])11N(+)[([CH.sub.3]).sub.2] [CH.sub.2]COO(-)

The surfactant assembly formed after micellization is endowed with many aesthetically pleasing and intellectually stimulating shapes and structures. In the case of ionic surfactants, these aggregates at CMC are spherical in shape in accordance with the large surface area and minimal volume requirements. Figure 3 is a typical representation of a spherical micelle.

Such spherical micelles would have maximum interfacial area and are stabilized by the hydration of polar head groups, interaction between head groups, and the binding of counter-ions, as well as clustering of the alkyl chains through hydrophobic forces. Thus, they have a hydrophobic interior (core) and a hydrophilic peripheral surface (corona).

The cleansing action of surfactants (soaps and detergents) mainly lies in the ability of the hydrophobic core of micelles to entrap insoluble organics like grease and dirt by solubilization. The spherical micellar structures transform into various shapes such as rods and bilayers [20] as the concentration of surfactant escalates (Figure 4).

An analysis of phase diagrams of surfactant systems, which represents the solubility behavior of surfactants as temperature vs. concentration plot, shows that they undergo structural changes at higher concentrations. Some of these phases are having liquid crystalline properties that show anisotropic properties as in the rotation of polarized light. [10]

The spherical surfactant aggregates of SDS contain an average number of molecules designated as aggregation number ([sim]60 for SDS). The CMC of surfactants depend on the molecular structure, especially on the length and branching of the alkyl hydrophobic groups. Ionics have a higher CMC than nonionics. For example, SDS has a CMC of 8.3 x [10.sup.-3] mole/liter and a nonionic one like [[CH.sub.3]([CH.sub.2]).sub.11] [([OCH.sub.2][CH.sub.2]).sub.OH] has a value of 8.7x[10.sup.-5] mole/liter. [18]

As mentioned earlier, the most important property of surfactants as applied to ink chemistry is their ability to bring down the surface tension of water and other solvents. This property arises due to the migration of surfactant molecules to the solvent surface.

The surfactant molecules take up an orientation in which the polar head groups are directed toward the solvent surface, and the hydrocarbon chains protrudes out to air. Such a situation leads to the balancing of forces on the surface and the subsequent reduction in surface tension. The gradual reduction in surface tension will be continued until the CMC is reached, and after that the surface tension remains a constant.

Figure 5 represents the reduction in surface tension (Y-axis) as a function of surfactant concentration (X-axis) [21]. The surfactant molecules pack on the solvent surface until a monolayered structure is formed. The manipulation of these assemblies in thin films that find technological applications is achieved by a method called Langmuir-Blodgett technique. [22]

Surface tension measurements are performed both under static and dynamic conditions, while methods based on capillary action, du Nouy ring, Wilhelmy plate yield static values, bubble pressure method and oscillating jet technique yield dynamic values. Understanding of static and dynamic surface tension phenomena help in the design of efficient inks as the coating process depends, to a large extent, on these parameters. Coating defects such as cratering, crawling, poor salt spray and recoat failure can be attributed to surface tension problems. [23]

Control of foaming-problem in inks is intimately related to the existence of surfactants in them. [14] In general, surfactants tend to stabilize foam. Foam stability is dependent on the interfacial tension operating among various phases involved in the foam. The antifoaming and defoaming effects are modeled based on the interfacial tensions between antifoam and foaming medium, and their surface tensions by the concepts of entering and spreading coefficients as represented by the following two equations:

E = gF + gDF - gD, and S = gF - gDF - gD.

where E is the entering coefficient, S is the spreading coefficient, gF is the surface tension of the foaming medium, gDF is the interfacial tension between the antifoam and foaming medium and gD the surface tension of the antifoam. Polymeric surfactants such as polysiloxanes and polyether-modified siloxanes are found to function more efficiently than conventional mineral oil and hydrophobic particle-based antifoaming agents.

Interactions

Industrially used surfactants rarely exist in pure form. They invariably contain mixtures of surfactants. Above all, several ink formulations contain two or more commercial surfactants.

This leads us to the importance of the interactions among various surfactants in a mixture. Many fine studies are available using pure surfactants and their mixtures in which their antagonistic and synergistic interactions are modeled. The interaction is understood in terms of a b parameter referred to as molecular interaction parameter. For a mixture showing ideal behavior, the value of b parameter will be zero.

In a non-ideal case, b can be either positive or negative. A negative value indicates synergistic interaction and vice versa; in the former case, the CMC will be lower than for an ideal system and in the latter case, higher.

The physical meaning of b parameter points toward the easiness of mixed micelle formation, mixed adsorption layers, and mutual interaction between the two types of molecules. [24]

Another type of interaction of surfactants is with polymeric materials used lavishly in inks. [25] Polymeric materials used in ink can, in many instances, usurp the purported functions of surfactants. For example, surfactants used for stabilizing dispersions may be extracted back from the pigment surface to the polymers. This in turn could affect the rheological and viscosity modifying properties of polymeric resins.

On the other hand, novel molecules fusing the properties of surfactants and polymers are being used to fine tune the physical characteristics of inks. Such compounds are available as polymeric surfactants.

Various types of polyacrylates [26] and other polymers are used in inks in the form of emulsions. Surfactants are essential ingredients of such emulsions. They function by mediating the solubility of two immiscible liquids. It is a common observation that water and oil do not mix. When it is necessary to get a uniform mixture of them (emulsions), surfactants with appropriate structures are added.

In general, two types of emulsions are available: macroemulsions (opaque, particle size [greater than]400 nm, low stability) and microemulsions (optically clear, thermodynamically stable, particle size [less than]100 nm). These emulsions can be either an oil-in-water emulsion (01W) or water-in-oil emulsion (WIO) depending on the preponderance of these components in the mixture. [27] Surfactants that function as stabilizers of these emulsions are known as emulsifiers. An empirical scale to represent the suitability of an emulsifier in a given case is based on the hydrophilic/lipophilic balance concept (HIB).

The HLB scale was first introduced by Griffin[28] for nonionic surfactants of the ethoxylate type. An HLB value of 20 was assigned to a structure composed entirely of ethylene oxide units, and a value of zero was assigned to a completely water insoluble product containing no ethylene oxide fragments. Most of the cases fall between these two numbers. An empirical relation to arrive at BIB numbers is the following:

HLB = % of the hydrophilic group (molar) divided by 5.

Griffin devised a simple methodology to estimate HLB numbers in which a small amount of the surfactant is shaken with water and assigned values according to the appearance of the mixture, i.e., whether it appeared insoluble, turbid, hazy or clear.

Surfactants, with or without polymers, are used in pigment dispersions. They serve to enhance the wetting of the pigment by the solvent by lowering the interfacial tension, facilitate the mechanical process in grinding, and stabilize the dispersion by controlling the flocculation of particles. A basic process in these phenomena is the adsorption of surfactants [29] at the solid-liquid interface. This is very much like the adsorption of surfactants at the liquid-air interface leading to the reduction in surface tension of the liquid. Here, the interfacial tension at the pigment/liquid interface will be affected along with the hydrophobicity and electrical properties of the solid particles. These modifications will regulate the attractive and repulsive forces among particles, necessary to provide stabilizing conditions.

It is obvious that the choice of surfactants in ink chemistry should take into consideration their negative effects on the ink coating. [30,31,32]

In this respect, ionic surfactants which normally lead to foam stabilizing conditions are generally avoided in ink formulations, although sulfosuccinates and phosphate esters are being used to some extent. Nonionic surfactants containing ethoxylates, fluorinated derivatives, and silicones are the natural selection in spite of cost considerations.

The hydrocarbon moiety modified by the acetylene chemistry, as in Surfynols, has also been very popular in water-based ink chemistry. Furthermore, polymeric surfactants composed of ethylene oxide and propylene oxide chains, such as Pluronics, have found access to ink chemistry as an additive.

In short, surfactants are an important constituent of inks due to various reasons. Design of novel surfactants from a molecular understanding [33] of the complex processes involved in ink chemistry can definitely result in fresh solutions to many of the perennial problems faced by an ink chemist.

Joy T. Kunjappu recerved his Ph. D. in organic photochemistry in 1985 and D. Sc. In physical chemistry of surfactants in 1996. Prior to his arrival to the U.S. in 1987, he served as a senior scientific officer with the Department of Atomic Energy of India specializing on many aspects of chemistry. He worked as a post-doctoral research scientist (1987-1989) and associate research scientist (1994-1996) at the Langmuir Center for Colloids and Interfaces and the Chemistry Department of Columbia University, New York.

He has authored about 70 publications which include original research papers, review articles, book chapters, book reviews and symposium proceedings. He also served as the reviewer of technical and scientific papers of eight international publications. In 1989, he edited a special issue of Colloids and Surfaces (Aspects of Interfaces) as a gust editor. His biography is featured in "Marquis Who's Who in Science and Engineering" (1997). Currently, Engineering" (1997). Currently, Dr. Kunjappu is working as a research chemist at Propper M. C., Inc., New York.

References

(1.) J. L. Lynn, Jr. and B. H. Bory "Kirk-Othmer Concise Encyclopedia of Chemical Technology," Fourth Edition, Wiley, New York 1999, p. 1953.

(2.) "Industrial Applications of Surfactants," D. R. Karsa (Ed.), Royal Society of Chemistry Cambridge, U.K., 1999.

(3.) "Reagents in Mineral Technology," P. Somasundaran and B. M. Moudgil (Eds.), Marcel Dekker, New York, 1987.

(4.) Joy T. Kunjappu, S. R. Venkatachalam and C. K. K. Nair, Journal of Scientific and Industrial Research, 54 (1995) 717.

(5.) Joy T. Kunjappu, S. R. Venkatachalam and C. K. K. Nair, Current Science, 66 (1994) 258.

(6.) Surfactant Science Series, Volumes 1-90, Marcel Dekker, New York.

(7.) "Surfactant and Interfacial Phenomena," Second Edition, M. J. Rosen, John Wiley and Sons, New York, 1989.

(8.) "Surfactant Systems; Their Chemistry, Pharmacy and Biology," D. Attwood and A. T. Florence, Chapman and Hall, New York, 1983.

(9.) "Surfactant Aggregation," J. H. Clint, Blackie, London, 1992.

(10.) "Handbook of Surfactants," Second Edition, M. R. Porter, Blackie, London, 1994.

(11.) "The Hydrophobic Effect: Formation of Micelles and Biological Membranes," C. Tanford, John Wiley and Sons, New York, 1973.

(12.) For a recent compilation of essential research publications for surfactant chemists in the domain of surface science, contact the author at jkunjappu@aol.com.

(13.) F. D. C. Gallouedec, A. A. Lamy and S. Brown, American Ink Maker, December, 1999, p.24.

(14.) Joy T. Kunjappu, Ink World, August, 1999, p.44.

(15.) B. Lindman and H. Wennerstrom, in Topics in Current Chemistry, Volume 87, Springer-Verlag, Berlin, 1980.

(16.) G. M. Whitesides and R. F. Ismagilov, Science, 284 (1999) 89.

(17.) Th. Zemb, M. Dubois, B. Deme and Th. Gulik-Krzywicki, Science, 283 (1999) 816.

(18.) "Critical Micelle Concentration of Aqueous Surfactant Systems," P. Mukerjee and K. J. Mysels, U.S. National Bureau of Standards, Washington, 1971.

(19.) "Introduction to Modern Colloid Science," R. J. Hunter, Oxford University Press, U.K., 1993.

(20.) Joy T. Kunjappu and P. Somasundaran, Colloids and Surfaces, 117 (1996) 1.

(21.) R. B. Wettermann, Paints & Coatings Industry, October 1998, p.202.

(22.) K. B. Blodgett and I. Langmuir, Physical Reviews, 51 (1937) 964.

(23.) J. V. Heule, Paints & Coatings Industry, June 1998, p.42.

(24.) J. H. Clint, Journal of Chemical Society, Faraday Transactions, I, 71 (1975) 946.

(25.) Joy T. Kunjappu, American Ink Maker, March, 1999, p.34.

(26.) Joy T. Kunjappu, Ink World, February, 1999, p.40.

(27.) B. K. Mishra, B. S. Valaulikar, Joy T. Kunjappu and C. Manohar, Journal of Colloid and Interface Science, 127(1989) 373.

(28.) W. C. Griffin, J. Soc. Cosm. Chem., 1 (1949) 311.

(29.) Joy T. Kunjappu, Ink World, August, 1998, p.32.

(30.) Sean Crowe (Polytex Environmental Inks), Private Communications.

(31.) D. H. Fishman, American Ink Maker, November, 1998, p.45.

(32.) S. W Medina and F. J. Lee, American Ink Maker, January, 1998, p.49.

(33.) Joy T. Kunjappu, Ink World, December, 1999, p.50.
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Author:Kunjappu, Dr. Joy T.
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Date:Jul 1, 2000
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