Editor's Note: This is the first of a two-part article.
A surfactant is defined as a linear molecule with a hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail. The molecules tend to clump together in solution and form a surface layer with the heads in solution and the tails in the air. Air bubbles in the solution also act as gathering spots for the tails while the heads remain in the solute (Google).
Properties of Water
Water molecules dissociate into H+ and OH-, allowing the positive ions to equilibrate among protonatable groups, thus making them convenient ions for the creation of electrical potential differences. Hydrogen bonds are fairly strong, so in an air-water mixture, molecules orient themselves with higher bonding potential in the center and lower bonding potential on the edges. To increase the entropy of the system, water minimizes its surface area, resulting in high surface tension.
Figure 1: Amphiphile, also known as a surfactant or soap molecule. (Click on image to enlarge)
Figure 2: Spherical and planer bilayer micelle structures.
Nonpolar molecules will not hydrogen bond; bonding potential of neighboring water molecules is negligible. They do not attract each other; rather they are pushed together when mutually rejected from the water. This is called the hydrophobic effect.
Amphiphiles, Micelles
The amphiphile, also referred to as a surfactant, (surface-active-agent) or soap molecule, has one polar and one nonpolar end, as shown in Figure 1. Most amphiphiles have an affinity for water molecules and nonpolar solutions.
Once an amphiphile stops functioning as a monomer, it starts functioning as a micelle. This point is called the critical micelle concentration (cmc).
Under certain conditions, an amphiphile will form micelles with a particular surface-area-to-volume ratio and, thus, form micelles of a particular size. The average number of amphiphile molecules in a single micelle is described as the aggregate number (m). The cmc and the m together characterize the micelle. A particular amphiphile will form under a given set of conditions. Figure 2 shows both spherical and bilayer micelle structures.
Some generalizations that can be made about micelles are:
As chain length increases, water solubility and cmc decrease;
single micelle chain amphiphiles have a higher cmc than two-hydrocarbon chain micelles;
ionic solutions have greater solubility and higher cmc than nonionic solutions;
and ionic solutions have greater rejection forces between polar groups than nonionic solutions.
Table 1: Micelle Generalizations (Click on image to enlarge)
Figure 3: Visual interfaces of two and three phases.
Table II: Solute Influences on Surface Tension
Figure 4: Water/hydrocarbon mixture with emulsifier.
These observations are charted in Table I.
Nonionic solutions form micelles with smaller surface areas per amphiphile. Increasing ionic strength decreases rejection forces between polar groups of ionic solutions while the m increases.
Amphiphiles demonstrate other properties in addition to surface tension change. They may be identified as an antistat, anti-foaming agent, bacteriastat, corrosion inhibitor, detergent, dispersant, emulsifier, foaming agent, or soap.
Solutions and Emulsions
A phase is a physically distinct and mechanically separable portion of a dispersion or solution. An interface is the boundary between two or more immiscible phases. We can visualize an interface as a distinct boundary, one or more molecules thick. The interface material has a different physical makeup and free energy than does the bulk material, as shown in Figure 3.
The interface material may also carry an electrical potential that impacts the stability of the solution. Phase composition, temperature, and pressure determine the electrical properties of a solution. Ionic materials and pH shift also impact the electrical charge properties, or interfacial surface energy, as measured as surface tension in dynes per centimeter. The addition of solutes may increase or decrease surface tension, as shown in Table II.
A stable mixture of two or more immiscible liquids, held in suspension by a small amount of material called emulsifiers, is called an emulsion, as shown in Figure 4.
Hydrophobic (oil-loving) materials are agnostic to water; they are incapable of dissolving in water. On the other hand, hydrophilic (water-loving) materials have a strong affinity to absorb or bind with water, resulting in swelling and formation of reversible gels. This is characteristic of carbohydrates, vegetable gums, pectins and starches, and complex proteins like gelatin and collagen2.
The balance between the hydrophilicity of the polar moieties and the lipophilicity of the hydrocarbon moieties is referred to as the HLB, influenced by the oil-in-water partition coefficient, a solubility parameter and the hydrogen bonding between the water and other phase materials (on a 0 to 20 arbitrary scale, where insoluble (lipophilic) materials = 0 and water soluble (hydrophilic) materials = 203).
Established calculations allow a formulator to calculate the HLB of a material:
HLB = mol% hydrophilic group/5 or HLB = (E+P)/5 where HLB = (%oxyethylene + % alcohol)/5 (polyhedric alcohol fatty acid ester).
The HLB may or may not be linear. When applied to nonionics, it does show fundamental rationality. It is a tool formulators use in selecting surfactants for their compositions.
Table III: HLB Number vs. Solution Function
The lower the HLB number, the greater its affinity for oil. As the HLB number increases, so does its water stability. Table III relates the HLB number to function. A typical water-in-oil emulsion has an HLB of two to seven while an oil-in-water matrix has an HLB in the seven to 18 range5.
Surfactant Market
Large-scale surfactant production began in the early 20th century. The quantity of surfactants available is extremely large; it was estimated at 40 million tons in 2000 to 2001, not counting polymeric surfactants, and more than 2,000 name brand products were available in the U.S. alone5. Some identical compositions are sold under different names so the actual number of surfactant compounds is less. Other sources, including patent searches and the National Bureau of Standards6, puts the number somewhere between 700 and 6,000.
The U.S. market is expected to grow 6.1% annually through 20067.
The U.S. specialty surfactants industry is a $2.8- plus-billion market that includes cationic, nonionic, anionic, silicone, amphoteric, and fluorosurfactants serving the industrial, personal care, and cleaning products markets. Key suppliers include Dow, Degussa, Cognis, Clariant, Huntsman, Akzo Nobel, Stepan, BASF, Rhodia, and Uniqeme.
Surfactant Classifications
Natural fatty acid soaps are surfactants. There are also four general classifications of synthetic surfactants.
Anionic surfactants: A group of materials that carry a negative charge on the active portion of the molecule.
Cationic surfactants: A group of materials carrying a positive charge on the active portion of the molecule.
Nonionic surfactants: Materials that carry no electrical charge; their water solubility is driven by the presence of polar functionalities capable of hydrogen bonding with water.
Amphoteric surfactants: A group of materials that can be either cationic or anionic, depending on the solution pH. This group includes Zwitteronic types that possess permanent charges of each.
Natural fatty acids include plant or animal fatty acids that have a varying number of carbons linked together in straight chains. These fatty acids are converted to soaps by neutralization, also called saponification, with an organic or inorganic alkali. Often, the resulting water-soluble soap is considered "synthetic," since the saponification process has gotten more sophisticated over the years.
Anionic surfactants make up the largest group of synthetic surfactants, accounting for approximately 50% of the world's production. Included in this category are sulfate esters, fatty alcohol sulfates, sulfated esters, sulfated fats and oils, sulfonic acid salts, aliphatic sulfonates, alkylaryl sulfonates, carboxylate soaps, lignosulfonates, phosphate esters, sulfated fatty acid condensation products, a sulfocarboxylic acids/derivatives, alkyl glyceryl ether aulfonates, and miscellaneous sulfo esters and amides.
Typical products include alkylbenzene sulfonates (detergents), fatty acids (soaps), di-alkyl sulfosuccinates (wetting), laurel sulfate (foaming agent), or lignosulfonates (dispersants).
As a general rule, the above classes of surfactants exhibit superior wetting and emulsifying properties. However, they tend to be higher foaming materials.
Cationic surfactants having a positive electrical charge include imidazoline derivatives, betaines, pyridines, morpholines, and quaternary ammonium compounds. The aforementioned materials have excellent antibacterial properties, provide corrosion protection, and may be used as demulsifiers.
Nonionic surfactants, the second largest surfactant group with approximately 45% of industrial production, offer flexibility to design the degree of solubility by controlling the size of the hydrophilic group8. This group includes polyoxyethylene-based materials, glycerides, block copolymers, alkanolamides, amine oxides, polyglycerides, polyglycerol and other polyol derivatives, glucosides, glycol esters, glycerol esters, polyglycosides, Sorbitan esters (ethoxylates), alcohol ethoxylates, fatty amine ethoxylates, fatty acid ethoxylates, EOPO copolymers, alkylphenol ethoxylates, and sucrose esters.
The sugar-based materials have gained popularity due to their low toxicity.
Amphoterics contain, or can form, both positive and negative functional groups. These include imidazoline derivatives, betaines, amine condensates, sulfobetains, quaternary ammonium compounds, and phosphatides. At low (acid) pH, the above materials function as a cationic surfactant. At a high (alkaline) pH, they are anionic.
Figure 5: Water/hydrocarbon mixture with emulsifier.
Figure 6: The three types of wetting.
In solution, most surfactants are present as monomers. Temperature, pH, and cosolutes all impact the phase the surfactant may be found in. Two general phases are encountered:
Thermotropic liquid crystals (determined by temperature);
and lyotropic liquid crystals (surfactant/solvent interaction). Only natural fatty acid soaps are not lyotropic.
Figure 5 illustrates the three types of liquid crystalline structures: lamellar, hexagonal, and cubic.
Wetting
Three types of wetting mechanisms are illustrated in Figure 6. These include:
Adhesional wetting: A solid is brought from contact with a vapor phase to contact with a liquid phase.
Spreading wetting: A liquid and solid are already in contact.
Immersional wetting: A solid is immersed completely in the liquid phase.
References
Myers, D., "Surfactant Science and Technology," p. 8, VCH Publishers, Inc., New York, NY; 1988.
Lewis, R.J. Sr., "Hawley's Condensed Chemical Dictionary, Revised 13th Edition,"rep p. 593, John Wiley & Sons, Inc., New York.
Griffin, W.C., J. Society of Cosmetic Chemists, 1:311; 1949.
Porter, M.R., "Handbook of Surfactants," p. 127, Blackie and Son Ltd., New York.
"McCutcheon's Emulsifiers and Detergents," North American Edition, Vol. 1, The Manufacturing Confectioner Publishing Co., Glen Rock, N.J.; 2004.
Mukerjee, P. and Mysels, K.J., "Critical Micelle Concentrations of Aqueous Surfactant Systems," National Standards Reference Data Series, Vol. 36: United States National Bureau of Standards, Washington D.C.; 1971.
Branna, T., "Surfactant Market Update," Happi Magazine; September 2003.
Myers, D., "Surfactant Science Technology," p. 68, VCH Publishers, Inc., New York; 1988.
Salager, J-L., "Surfactant Types and Uses," p. 3., Laboratory of Formulation, Interfaces, Rheology and Processes, Universidad De Los Andes, Merida, Venezuela, Version 2; 2002.
Salager, J-L., "Surfactant Types and Uses," p. 5, Laboratory of Formulation, Interfaces, Rheology and Processes, Universidad De Los Andes, Merida, Venezuela, Version 2; 2002.
