Theory and Application of Wet Water
TO PRODUCE A STABLE EMULSION, as we have seen, it is necessary to add a “soap” to the water-oil mixture. Soap may be considered as the salt of a base and an organic acid, similar to other salts, such as sodium chloride or potassium nitrate. The formation of such salts involves the transfer of the metallic ion of the base to the non-metallic ion of the acid. The remaining compound is water, formed from the hydrogen of the acid and the hydroxyl group (OH) of the base. The equation below shows this exchange:
NaOH + HCl_____NaCl + H2O
base + acid_____salt + water
When the acid is a higher organic fatty acid (from 10 to 20 carbon atoms in the molecule), the salt produced is a soap. The process of converting the fatty acids to soaps is called “saponification.” The equation below shows the formation of a typical soap:
NaOH + C17H35COOH_____C7H35COONa + H2O
base + stearic acid_____soap + water
Soaps, like other salts, are ionic in character and will dissociate in water solution, and because they are also colloidal, they are called “colloidal electrolytes.” Upon ionization, the sodium or metallic atom will take on a positive charge and become the positive ion, or “cation.” The rest of the molecule, containing the hydrocarbon chain and the organic acid group—COO, acquires a negative charge and becomes the “anion,” and is that part which gives the compound its surface active properties. It has a hydrophobic (water repelling) group at one end of the long chain and a hydrophilic (water-attracting) group at the other. A typical soap molecule is shown in Fig. 12. Such compounds with a negative ion attached to their active groups are called, “Anionic surfactants,” and include soaps, almost all of the detergents, and the more important wetting agents.
Balance in the surface
Soaps are characterized by long, pencil-like molecules, consisting, as we have seen, of 10 or more carbon atoms in the chain. This structure imparts special properties to the molecules in regard to water. Thus we find that the hydrocarbon end of the molecule, being hydrophobic in nature, attempts to keep the entire molecule from going into solution. The other end containing the acidic group is hydrophilic and it tends to drag the molecule into solution. The result is that the molecule concentrates in the surface with its hydrophobic end sticking up into the air and its hydrophilic end down into the body of the water. It acts as both a polar and a non-polar substance at once. Orientation of soap molecules in the surface is shown in Fig. 13.
It is necessary, therefore, that a balance between polar and non-polar ends be reached so that the molecule is neither pulled entirely above the surface nor entirely below. This is called the “hydrophobic-hydrophilic balance.” It has been found that hydrocarbon chains of between 10 to 20 carbon atoms seem to produce substances with surface active properties.
Practically all chemical substances may be divided into two classes: those soluble in water, and those insoluble. Surfactants, however, do not belong to either of these two classes. They comprise a third group in which the substances have properties unlike those in the other classes, most important of which is concentration in the surface, neither dissolving nor separating. Such properties are peculiar only to this class.
Because this is so, a soap may, for example, surround each oil particle in an oil-water mixture so that the long molecule is oriented, somewhat like those in Fig. 14, the polar end sticking into the water and the non-polar end into the oil drop. This collection of oriented molecules at the oil-water interface keeps oil from coalescing and settling out because, in effect, it is surrounded by a negative electric charge which is repelled by similar charges on the other oil drops. This process helps soap clean as it does because it enables oils and greases to be emulsified and carried away in the rinsing water. If solids are also present, the process is similar. The same orientation of molecules takes place and solid particles are dispersed throughout the liquid and carried away. In the case of solids, the mixture is called a “suspension” rather than an emulsion.
† The first and second parts of this article appeared in the August and December 1957, issues.
This process of removing dirt, grease, oils and solids, by a combination of emulsifying, solubilizing, decreasing surface tension, suspending solids, preventing coalescing, and rinsing away the mixture is called “detergency” and the substance is called a “detergent.” Soap, of course, is the oldest and best known detergent.
Practically all soaps, except those of sodium and potassium, are insoluble in water. This causes ordinary soap to have many drawbacks when used in waters of varying composition. In “hard water” containing calcium, magnesium and other metallic ions, a chemical reaction occurs in which sodium ancl potassium atoms of the soap are replaced by the other metallic atoms and the insoluble soaps formed are precipitated as a curd. The organic acid portion of the molecule, called the “carboxyl acid group,” is the weak point in such reactions because of its affinity for the other metallic ions, and the fact that resulting compounds are insoluble. In a somewhat similar way, salt water as well as acidic water will cause formation of insoluble fatty acids.
For ages, substitutes for soap which would overcome these faults have been sought and only in recent years has progress been made. As a result of this search, a new group of compounds has been synthesized which outdo soap in many of its qualities and have none of its faults. In these compounds the organic acid group is so altered that it cannot precipitate as an insoluble soap or free acid. There are three ways to accomplish this. One way is to “block out” the organic acid group by tying it up in a chemical compound to prevent further reaction. Such a compound consists of a short hydrocarbon chain having a reactive group at either end. One end ties itself to the acidic group and the other end ionizes to carry a negative charge. Sulfonates are examples of this type and a typical “blocked carboxyl” surfactant is shown in Fig. 15.
A second method is to convert the acidic group to a hydroxyl (OH) group, which is characteristic of the class of compounds known as alcohols. The long chain alcohols are also called “fatty alcohols” corresponding with fatty acids. When the fatty alcohol is treated with sulfuric acid, the hydrogen of the alcohol is replaced by a sulfonate group, which is strongly hydrophilic. Fig. 16 illustrates a typical fatty alcohol sulfonate.
The third method is to attach a long hydrocarbon chain to a “cyclic hydrocarbon like benzene and naphthalene. These molecules are in the shape of a hexagon and as such they lack the long chain structure necessary to give proper H-H balance. However, by attaching a hydrocarbon chain of about 12 carbon atoms to part of the ring and then sulfonating, a new compound is formed that possesses surface active properties. The hydrocarbon chain added to the ring structure is called a “side chain.” The new compounds are called “alkyl aryl sulfonates” and are probably the most numerous surfactants on the market. Fig. 17 shows a typical alkyl aryl sulfonate.
The three types of surfactants just considered are all anion active surfactants and are the most commonly used as detergents and wetting agents. Another group of compounds in which the active portion carries a positive charge when ionized is also important, although possessing poor properties of detergency and wetting. In these compounds the chlorine atom carries the negative charge, while the long chain end carries the positive charge. Their chief uses are as agents to prevent accumulation of static charges, to disperse solids in liquids, and in germicidal preparations. They have little application in the fire service. A typical cationic surfactant is shown in Fig. 18.
We have considered so far two types of surfactants, both of which ionize in water. A third group coming into prominence, the members of which do not ionize at all, are called “non-ionic” surfactants. They are made up of a long chain hydrocarbon portion attached to a second chain consisting of units of two carbon atoms and four hydrogen atoms joined by oxygen atoms. This type of bond is called an “ether linkage” because it is characteristic of organic compounds known as ethers. Ordinary ethyl other is an important member of this class.
The surfactants of this class do not ionize, but they derive their surface active properties from the fact that the ether linkages are weakly hydrophilic. When a number of these linkages are combined in a compound a proper balance between solubility and insolubility is attained, and the compounds show excellent wetting properties. They also possess some dictinct advantages over the others because they do not ionize. Thus they can be used freely in hard water and they will not react with calcium or magnesium ions. In addition, they may also be used in acidic, salt or alkaline waters. Fig. 19 shows a typical non-ionic surfactant.
The primary action of these substances we have just considered is in the surface and from what we have said about detergents one may expect them to be good wetting agents. This is true, and it is a fact that soap and some other older detergents have indeed been so utilized in industry for many years, to aid in such processes as dyeing, dry cleaning, metal cleaning, and incorporating solids in liquids. The fire service, as we have indicated, has become aware of the usefulness of these compounds in making water a more efficient extinguishing agent, and as we continue in our study we shall see how much more effective it becomes when this knowledge is applied to the problems of our profession.
The surfactants discussed possess to some degree all of the properties described previously. They are selected for use on the basis of excelling in one property more than in the others. Thus, we find that a good emulsifying agent may also to some extent be a wetting agent, while a good wetting agent may also have some emulsifying properties. These facts are important in our use and selection of surfactants because the substances can not and do not exert their effects in one manner only. There is always a combination of actions with one dominating the others.
The important characteristic of wetting agents is lowering the angle of contact between water and hydrophobic surfaces. This is brought about because of the strong attraction between the hydrocarbon end and the surface to be wetted. As each chain anchors itself to the surface, its polar end pulls water molecules along with it, laying down a covering which is strongly hydrophilic. The type of molecular structure which gives detergent properties is like that of soap, in which a straight long chain hydrocarbon is coupled with a polar group at its other end. Superior whetting properties, however are possessed by that type of structure in which a side chain or branched compound has its polar end near the middle of the molecule.
Surface tension reduced
Of lesser importance is the reduction in surface tension of the water. Aside from causing water to have greater mobility and easier flowing qualities, surface tension reduction of itself has little effect on wetting ability. In fact, some very good wetting agent solutions have a higher surface tension than some of the poorer ones. Reducing surface tension alone without a reduction in the angle of contact may actually reduce the penetration of water into capillary pores.
To understand this better, reference is made to Part II, in which the mechanism of capillary penetration was explained. It will be recalled that movement of water in a capillary is the result of wetting the glass around the edges of the water surfaces, causing it to creep up the walls. This wetting of the glass surface causes the water to spread out as it attempts to cover the glass surface. Surface tension then operates in an effort to reduce the surface area, but because adhesion between water molecules and glass molecules is greater than cohesion between molecules of water, the pull across the surface results in a net force acting upward. The extent to which water is drawn up the tube against the pull of gravity depends upon the strength of the surface film in supporting the weight of the water column. Now, suppose the surface tension were reduced without affecting the angle of contact. The water molecules around the edges will still creep up the walls as before. However, because the strength of the surface film is now less than before, the height of the column will be lower. The reduced surface tension is able to support only a small weight of water as compared with that of the stronger film.
Reduction in surface tension alone, then, does not materially increase the wetting qualities. It is important that adhesion between water molecules and the surface to be wetted be increased, or, in other words, that the angle of contact be lowered.
(To be continued)