Theory and Application Of Wet Water Part 3 (continued)†
Another outstanding feature of wetting agents, as well as soaps and detergents in general, is the power to produce a mass of stable bubbles called a “foam,” when the solution is agitated. This property is of considerable importance in the use of soaps and detergents because it aids in the cleaning process. In wetting agents, however, this property is of minor importance, except in a few specialized cases.
Pure liquids of simple structure do not produce foams, while water solutions of simple salts produce foams of a temporary nature. In order to produce a stable foam it is necessary to add other substances which in combination with the original liquid will aid in its formation and stabilization. These substances are called “foaming agents” and among the best known are soaps, detergents and wetting agents.
The formation of a stable foam or bubble is a function of surface activity. The molecules of surface active substances collect in the air-liquid interface of the film and are oriented somewhat like those in Fig. 20, with their hydrophobic ends sticking out into the air and their ionized ends in the water layer.
Of the many factors necessary to form and stabilize a foam, there are three of considerable importance. They are: Low surface tension, low volatility of the liquid phase, and high viscosity of the surfactant molecules.
Lowering the surface tension of water is important in producing a stable foam. Because a bubble can be blown up to large proportions and because it persists for so long a time, it is customary to think that a high surface tension is necessary in its formation. The opposite is true, however, and only those liquids with low surface tension can produce the largest and most stable bubbles.
Low volatility of the liquid is also important because preservation of liquid in the film must be maintained or the bubbles will collapse.
Low mobility of the surfactant molecules is important because too much movement in the surface causes collapse of the bubbles.
Certain substances, such as finely divided solids and proteins, while not necessary in the formation of a foam, add mechanical strength to it by collecting in the surface.
Agitating the solution will produce large quantities of foam. Fire streams containing wetting agents can do this when directed on a fire by the impact of the water against solid surfaces. Pulsations from pumps can agitate a solution to form a foam, and fog streams may cause enough agitation to produce the same result.
The formation of foam, then, may attend the use of wetting solutions unless they are specifically treated to prevent this. The presence of a foam blanket may or may not be objectionable or reduce the fire extinguishing efficiency of water depending upon conditions and the type of material burning.
Destroying foam blankets
If the presence of a foam covering is undesirable, means for removing or destroying the blanket must be considered. This can be done by attacking the active substance in the film. Excess acidity will usually break down a foam by destroying the surfactant, and converting part of the molecule to an insoluble fatty acid. This can also happen in the presence of cationic substances which react with the anion of the surfactant and render it insoluble.
An excess of finely divided solids will destroy a foam, and it is possible to break down a blanket with deliberate overagitation by directing water streams against it. An excess of water causes collapse of a foam covering by dilution to an extent that the surfactant concentration becomes too low to support a stable foam. Heat, too, destroys foams, as will jets of air or steam directed on them.
As mentioned earlier, except in those few instances where presence of foam is definitely objectionable, the deposit can be disregarded, because after a while it will eventually collapse and leave only a slight covering of surfactant ingredient to clean up.
Acidity, alkalinity, buffering
In our discussion thus far we have spoken of the effect of water of varying composition on the efficiency of surfactants. It has been pointed out that acids destroy the surfactant properties of the substances with which we are concerned.
Acid or basic properties are the result of dissociation or ionization of compounds which produce ions, electrically charged atoms, in solution. Acids always dissociate to release hydrogen ions, H+, while bases always dissociate to form hydroxyl ions, OH–. These substances are called weak or strong according to the number of ions they release in water solution; the greater the number or concentration of the ions, the stronger the acid or base, and the smaller the number, the weaker they will be. An acid which releases twice as many hydrogen ions as another, will be twice as strong as the second. Similarly, a base which releases twice as many hydroxyl ions as another will also be twice as strong by comparison. Strong acids and bases dissociate completely in water and exist entirely as ions. Weak acids and bases, on the other hand, do not dissociate completely so that a number of ions exist in solution together with undissociated molecules of the substance which are neutral in action. It is possible, too, that some of the undissociated molecules are insoluble, such as those of some organic fatty acids.
In order to distinguish between the strength of acid and basic substances, some other substance which is neutral must be used for comparison. Pure water is such a substance and is taken as the standard of measurement. Any deviation in the number of hydrogen ions or hydroxyl ions, upward or downward, from the number of such ions in pure water, is a measure of the acidity or alkalinity of the substance measured.
Pure water itself, ionizes to a slight extent, and releases both hydrogen and hydroxyl ions. The number of both ions, however, are equal and water acts as a neutral substance. Hydrogen ion and hydroxyl ion concentration is measured in weight per unit volume of water, and in pure water each is equal to a concentration of 0.0000001, or 1 X 10-7. In pure water an increase or decrease in concentration of one ion will always result in a corresponding increase or decrease of the other, so that the product always equals 1 X10-14. This provides a very simple and convenient method of expressing acidity and alkalinity in terms of one ion. The hydrogen ion is chosen as the standard and the strength of both acids and bases may be expressed in terms of the concentration of hydrogen ions.
Under this plan, an index numbered from 1 to 14 is used to measure hydrogen ion concentration, and is called the “pH scale.” The expression, pH, is the logarithm of the reciprocal of the hydrogen ion concentration. Therefore, the pH of
pure water is 7 (pH = Log 1/10-7 = Log 107
= 7).
All concentrations falling below 7 are acid; all above 7 are basic. This scale is shown in Fig. 21.
Soaps, detergents and wetting agents work most effectively in a narrow pH range, generally between pH 10 and 11, which is a basic region. Acids destroy the properties of these substances because they introduce an excess of hydrogen ions which cannot exist together with those already present. The excess ions seek to force the other hydrogen ions out of the solution in what may be described as a “common ion effect,” and causes them to react with the carboxyl group to form free fatty acids, which are insoluble in water and settle out.
This action continues until all of the excess hydrogen ions introduced by the acid react with the carboxyl groups of the surfactant molecules continually destroying their effectiveness. It is necessary, therefore, to use considerably more surfactant in acid solution to overcome this action.
In a basic solution, however, excess hydroxyl ions, may actually aid the surfactant because they bring about the formation of a strong base, sodium or potassium hydroxide. Both of these bases are, in turn, completely ionized in solution. so that no precipitate can form. In addition to this, the bases can react with oils and greases on hydrophobic surfaces to form soluble soaps.
Because there is an optimum range in which the surfactants are most effective and economical, it is desirable to add to the mixture substances called “buffers” or builders which resist changes in the pH of the solutions and thereby maintain the alkalinity needed. Buffers and builders usually contain a salt of a weak acid and a strong base. Such a salt will dissociate in water to a slight extent. The ions in turn react with water to form a weak, slightly dissociated acid, and a strong, completely dissociated base. It is the strong base which helps maintain alkalinity by neutralizing any acid and saponifying oils and greases. As the base is used up, water acts on the undissociated buffer molecules and causes them to release additional basic ions. They aid further by reacting with and tying up metallic ions in hard water presenting them from combining with the surfactant to form insoluble soaps.
Buffers and builders, then, are important ingredients in a surfactant mixture and have been discussed here, along with the pH scale, in an attempt to conx’ey some idea of the complex nature of the problem of formulating and using an agent for fire protection purposes. Wetting agents used in the fire service must be selected on the basis of effectiveness under all conditions, as well as on the basis of economy.
Mechanism of an action
Before proceeding it is well to pause for a moment to apply some of the knowledge we have gained in order to explain a certain series of events.
This reference, of course, is to the duck whose complacency was shattered when a wetting agent was added to its watery element and which suddenly ceased supporting him. We find now that an understanding has been gained as to why this happened. Primarily, the surfactant permitted a reduction in the angle of contact between water and the surface of the feathers, because one end of the oriented molecules had a strong attraction for this surface, while at the same time, the ionic end remained anchored in the water. This lowering of contact angle forced air out of the feathers and replaced it with water. While this action was taking place, it was aided by the emulsifying action of the agent on the oils. Some of the oil was also solubilized. Some of it was also saponified, adding the product of the action to further aid the wetting action. Lowering of surface tension permitted water to spread and flow about more easily. Some detergent action was exerted in suspending and carrying away greasy dirt and other solid particles.
The net result was that the feathers no longer contained air, but water, and the duck sank.
Further significance of these facts will be appreciated as we progress through our study to the next part in this series which will cover the application and action of wet water on the materials xve are most likely to encounter at fires.
BIBLIOGRAPHY Part III
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