Theory and Application of Wet Water

PART 2t

IT WAS SAID EARLIER, that only those molecules possessing greater than average energy could remain in the surface against the pull of the molecules in the liquid below. The surface molecules are in a state of stress due to the unbalanced forces acting on them. They possess a greater amount of potential energy because of their more or less restrained positions in the surface. With this extra energy, the surface would soon become unstable and, as we have mentioned before, in order to bring the system into a stable form, some of the molecules are forced back into the liquid. The surface molecules are continually attracted to the body of the liquid resulting in a minimum surface area which is a stable condition, once the surface has attained this smaller area.

Any attempt to force the surface molecules apart in order to increase the area will require the expenditure of energy in the form of work. Heat energy added to the liquid will impart a greater amount of kinetic energy to the molecules. As a result, more and more of them gain energy beyond the average. Having gained this additional energy, these molecules are able to push their way up into the surface, separating the molecules already there. The surface area becomes larger and there is a tendency for some of the more energetic molecules to fly off into the space above it. All the while new molecules are pushing into the surface; it is becoming a reservoir of super-energized molecules, and the tension that existed among the surface molecules is being gradually reduced. A point will be finally reached at which the surface tension disappears and the surface molecules at once seem to erupt from their positions and fly off into the space above to take on a new identity as individual gas molecules. This point we call the “boiling point.” We see, then, that added energy in the form of heat will reduce the surface tension of liquids. The effect of heat on the surface tension of water is illustrated in the graph.

The boding point of liquids, as well as the flash point, depends on the surface tension. Water has a high boding point pardy because it has a high surface tension. Conversely, one of the reasons organic liquids like gasoline have low boiling points is because they have low surface tension. Other liquids, such as mercury, fused solids and molten metals, have extremely high boiling points because they have extremely high surface tension.

Surface tension, then, may have a significance in fire protection other than that which we are considering in this study. A lower surface tension in a flammable liquid means a higher hazard due to its lower flash point and boiling point.

TABLE I—How Surface Pressure Affects Boiling Point and Flash Point

The energy gained by the surface when more molecules are forced into it by heating gives it some added reactive properties. The liquid is enabled to adhere more strongly to surfaces than it could under its former condition of high surface tension and minimum surface energy potential.

Surface active agents

We have considered up to this point, heat energy as a means of increasing the energy level of the surface to make it more reactive toward other surfaces. There are certain substances, however, which are able to bring about these changes in a more marked way and at lower temperatures. Such substances are commonly called “surface active agents,” or simply, “surfactants,” because they impart greater activity to specific surfaces. Surfactants commonly used in fire protection work reduce the surface tension of water as much as 70 per cent at ordinary temperatures.

Interfacial energy

In our discussion of surface effects thus far we have considered surface tension and surface energy as though they were properties of liquid alone. This is not strictly correct, for while we speak of surface molecules as being acted upon by these molecules below it, we have disregarded completely the effect of any molecules above the surface.

A liquid surface, as we have seen, separates two material phases—liquid from gas. In an open container, the gas phase above the surface consists of air molecules as well as some of the evaporated liquid molecules. In their random and disordered flight, some of these gaseous molecules will approach the surface, collide with some of the surface molecules and either bounce off or be held by them. As more and more gaseous molecules are trapped in the surface, their energy is transferred through collision to the others, until one or more molecules gain enough energy to evaporate itself. Thus there is a continual flight of molecules back and forth between the gaseous and liquid phases. At any temperature, a point will be reached in a closed system, at which the number of molecules evaporating equals the number condensing, and a condition called “dynamic equilibrium” is reached. At this point the vapor pressure will be a constant for the temperature.

A liquid surface in contact with its own vapor and air will have a lower surface tension than it would have if it were in a vacuum or an open container where the evaporated molecules were being continually removed. This is so because some of the gaseous molecules do exert some attraction on the surface molecules and tend to balance somewhat the pull of those below.

The surface may also separate two like phases, such as one liquid from another, in which it does not mix, like water and oil; or the surface may separate a liquid from a solid; or the separation may include all phases, gaseous, liquid and solid, as when a drop of water rests on a wood surface in air. In this case there are a variety of forces at work at the junction. There is the separation of water and air, that of water and wood, and wood and air. Each has a characteristic surface reaction.

The energy characteristics and play of forces at each separation result in a net effect of the whole. This effect may be that water stands up in a drop on the wood or that it will flatten out and sink completely into the wood surface. If it does the latter we call the effect “wetting.” More will be said of this effect.

Interfacial tensions between liquids and solids, or liquid and liquid, are always much less than the surface tension of the liquid in air. This is due to the fact that more molecules are within the sphere of influence above the surface because of the greater density of the solid and liquid as compared with air.

Surface tension, then, is an inaccurate term which tells us nothing of the conditions under which it is measured. A more descriptive and accurate term is “interfacial tension” and the term “interfacial energy” to denote the energy, in which the phases are named along with the dimensions of the forces. For example, we speak of the interfacial tension of water in contact with benzene, as being 35 dynes per centimeter. Water in contact with air, on the other hand, is 72.8 dynes. The term “dynes” is a measure of force, or pull, needed to impart a certain acceleration to a standard mass. It is equal to the force needed to cause a mass of 1 gram to accelerate one centimeter per second per second.

Having made this distinction between surfaces, we shall use the more descriptive term “interface” when we speak of a specific surface separating phase, and “interfacial tension” or “interfacial energy” to mean the forces acting in this surface.

Interfacial tension between water and another liquid or a solid is always less than that between water and air. The second liquid or the solid being more dense than air, exerts a pull on the interface molecules which counteracts to a degree the pull of molecules in the liquid below.

Contact angle

If we examine the junction where tire interfaces meet between water and air at the surface of a glass container, we find that the water level at that point rises and climbs up the wall for a short distance. This is shown in Fig. 4(a), where 1 is the air phase, 2 is the water phase, and 3 is the glass phase. For this system, the angle at which the liquid meets the solid surface will be a constant. This angle, (a), in Fig. 4, is called the “angle of contact.”

Close examination will show that this line is not straight, but is in the shape of a long curve, which is shown in truer form in Fig. 4(b).

The surface takes this shape because of the forces acting on it. Adhesion of glass for water molecules causes the water to creep up the wall against the pull of gravity. Cohesion causes other water molecules to be pulled along. As the distance from the glass becomes greater, the force of adhesion decreases and gravity causes the surface to assume a curved shape until it reaches its own level.

In capillary tubes, the force of adhesion pulls water up the walls, while tension in the surface acts at the same time to decrease the surface area, with a net resultant force acting upward, which causes the surface level to rise. The column will rise until the surface forces just support an equivalent weight of water against the pull of gravity.

The angle of contact will indicate the relative values of the forces of adhesion and cohesion. If adhesion is large as compared with cohesion, the liquid wets the walls and there is a tendency for it to flatten out and cover all the wall surface. The contact angle in this case is said to be 0 degrees. If the force of cohesion, on tlie other hand, is large and that of adhesion is small, as with mercury and glass, then the liquid will tend to collect together and attempt to leave as much of tlie solid surface uncovered as possible [see Fig. 4(c)], The contact angle in this case is 140 degrees. In the one case, water is said to wet glass, and in the other, mercury is said to not wet glass.

A contact angle of 0 degrees indicates perfect wetting, while an angle of 180 degrees indicates almost perfect resistance to wetting. Between these extremes lie the values of the common materials with which we are concerned. For practical purposes, we may consider any liquid with a contact angle of 90 degrees or less when in contact with a solid, as wetting that solid. A liquid with a contact angle of over 90 degrees we will consider as not wetting the solid.

On flat surfaces, the effect may be shown in another way. Water, on an oily or dusty surface, will stand in drops, as in Fig. 5(a). It does not wet tlie surface and the interfacial tension will cause it to assume the shape of a slightly flattened sphere. In Fig. 5(b), the drop wets the surface and therefore flattens out and tends to completely cover the surface.

Cause of surface forces

Up to this point we have discussed the part played by adhesion and cohesion in the action of surfaces, without reference to the actual causes of these forces. This was necessary because of the many concepts anti theories that had to be introduced before considering this next phase of our study.

In an earlier paragraph in Part I, reference was made to the electrical nature of matter, in which the flight of electrons about the nucleus of atoms has been likened to the movement of the planets around the sun. This, of course, is a highly idealized picture of atomic structure in which the electrons describe regular orbits around the nucleus. Actually, the paths taken by the electrons are extremely irregular and complicated, at times elliptical and at others, circular. As a result, even the simplest atom we know, hydrogen, with one electron revolving about the nucleus, becomes a confused picture with its one electron constantly changing orbits and positions, so that it appears to be in all places at once, seemingly filling up the entire space. In effect, the one electron acts as though it were a cloud of electrons. The same thing is true of all the other atoms, except that they become more and more complicated as the number of electrons increases.

The electron cloud may align itself so that it is denser at a point distant from the nucleus. As a result, the atom becomes similar to a magnet, with the end containing the nucleus carrying a positive charge, while the end with the electron cloud is negative. In chemical reactions these electrical effects give atoms the property of uniting at definite distances apart and at definite angles. In the case of water, we see that instead of having a straight-line structure as in Fig. 6(a), it actually has the structure shown in Fig. 6(b). The two hydrogen atoms combine with the oxygen atom in a special configuration 105 degrees apart.

The water molecule formed is almost spherical in shape, and because it has a separation of electron clouds and hydrogen nuclei, the result is that one side (hydrogen) is positive and the other (oxygen) is negative. This separation of unlike electrical charges in a body is called an “electrical dipole,” or simply, “dipole.” A water dipole is illustrated in Fig. 7(a).

Each water dipole will align itself so that its negative end attracts the positive end of another dipole. This continues throughout the entire bulk of the liquid, forming associated groups of water molecules into giant molecules containing hundreds of H₂O molecules. At the surface, the molecules may be aligned as shown in Fig. 7(b). This lining up of dipoles, with negative ends attracting positive ends is called “orientation.” Orientation in surfaces greatly affects the properties of the surfaces. Fig. 8 shows this orientation on a molecular scale.

Because of certain structural features, the hydrogen nuclei exerts a powerful force on other negatively charged bodies and is so important that the name “hydrogen bond” is given to it. It is this force which gives water some of its unusual properties and causes it to behave as it does toward other substances

Polar and non-polar substances

Because of the electrical effects of its dipoles, water is able to dissolve certain substances and free their atoms in a charged condition, which we call “ions,” in a process called “ionization,” giving a solution which conducts an electrical current. Such a liquid is called a “polar liquid” and substances which dissolve in it, to give electric conducting solutions, are called “polar substances.”

All salts (substances formed from an acid and a base), most inorganic substances, acids, bases, and such liquids as hydrogen peroxide, are polar.

Those liquids dissolving substances which do not form electrically conducting solutions (and the substances themselves) are called “non-polar.” These include most organic substances such as oils, waxes, benzene, gasoline and carbon tetrachloride.

Liquids tend to dissolve those substances which are similar to them in structure. Thus polar liquids, like water, will dissolve polar substances like salt, ethyl alcohol and sodium bicarbonate, but not non-polar substances, like charcoal, benzene, gasoline and oil. Nonpolar liquids, such as benzene, will dissolve non-polar substances, like rubber.

As a rule, the same principles apply to the ease of wetting. Water, for example, will more easily wet inorganic substances, while organic liquids will wet organic substances better than inorganic.

Properties of surface dipoles

When water contacts another substance which it wets, the dipole molecules along the edge of the drops will be attracted to the dipoles in the substance itself, negative ends being attracted to positive, and vice versa. As the dipoles attach themselves to the other surface, they spread out and form a layer or covering, upon which other water molecules can spread easily by cohesion. The tendency is for water dipoles to completely cover the surface with a layer about one molecule thick, but other forces under normal conditions prevent such perfect wetting. Under these conditions the wetting layer may be many molecules thick, depending upon whether the force of adhesion or cohesion is greater. If adhesion is greater, then the wetting layer is thinner. If cohesion is greater, then the wetting layer is thicker. F’igure 9 illustrates this. Figure 10 illustrates how water dipoles behave on a surface it does not wet.

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WET WATER APPLICATION

Continued from page 1210

The foregoing wetting action is possible in substances which themselves have a permanent dipole structure. It is also possible for water dipoles to induce temporary dipoles in substances which do not have this structure, and thus enable water molecules to stick to the surface, although the adhesion is not strong.

In those substances, like oil, in which dipoles do not exist or in which they cannot be induced, wetting cannot ordinarily be accomplished. However, these substances are wet by liquids similar to it in structure. It is possible to combine two substances in one molecular structure, one end of which is attracted to water and the other to oil, so that wetting can be brought about.

The end which is attracted to water is called “hydrophilic,” or literally, “water loving,” while the other end, which has no attraction for water is called “hydrophobic,” or “water hating.” When it is intended to indicate whether one end is attracted or not to an oil, wax or grease, the names given are “lipophilic” and “lipophobic,” respectively. Similarly, ends which are attracted to organic substances or not are given the names “organophilic” and “organo-phobic,” respectively.

Water-oil mixtures

Substances, such as oils, waxes, fats, greases and resins do not dissolve in water. They are hydrophobic in nature, but do tend to dissolve in each other. If one tries to dissolve a small amount of kerosene in water, for example, by pouring them together and shaking vigorously, the resulting liquid will be seen to consist of a myriad of small drops of kerosene floating throughout the water, giving a milky appearance. On standing for a short time, the liquids separate into two layers, with kerosene, the lighter one, on top.

The shaking caused both liquids to break down and become thoroughly mixed. Because kerosene does not dissolve in water, it remained as drops scattered in the water. The interfacial tension of water is higher than that of kerosene, and the water attempts to decrease its area by squeezing out the kerosene drops. As the kerosene drops are squeezed out they seek to combine and produce a larger and larger body of liquid in a process called “coalescence.” At the same time water drops are coalescing and eventually the liquids revert to their original condition, the kerosene atop the water layer, with a minimum of interface.

The temporary mixture, which persisted for a very short time, is called an “emulsion.” The kerosene, since it was the smaller in quantity was scattered or dispersed through the water. The kerosene is called the “internal phase,” and water the “external phase.”

This mixture of two liquids which are insoluble in each other we have called an emulsion, and the one described is an “unstable emulsion” because the two liquids tend to separate quickly into layers.

If this same mixture is shaken after adding a small amount of common soap, the resulting emulsion will be more stable and the kerosene drops remain suspended in the water without any tendency to coalesce. A stable emulsion has been formed by the addition of a new substance, soap, which we call an “emulsifying agent” because it has the power to produce a stable emulsion.

This orientation of soap molecules around a water particle is an emulsion is shown in Figure 11.

Such an emulsion or suspension differs from a true solution in the size of the particles. In solutions the particles approach the dimensions of single molecules, while in emulsions they are of the order of large collections of molecules. A true solution appears clear and transparent, while an emulsion appears cloudy and a beam of light passed through it will be sharply defined, due to reflection of light rays from the larger particles. The characteristic cone of light associated with emulsions is called the “Tyndall cone,” after its discoverer. The state in which the range in size of the particles is greater than molecules in solution, but smaller than those which settle out of mixtures, is called the “colloidal state,” and the particles are called “colloids.” It is considered that the study of the colloidal state is the study of giant molecules, and is one of the most important branches of chemistry. Most of the common materials with which we are concerned belong to the class of colloids. Wood, textiles, plant and animal material, as well as thousands of chemical products are really colloids.

One outstanding feature of colloids is the great increase in surface area over that in the liquid or solid state. Another feature is that colloids often carry electrical charges.

Some emulsifying agents have the power to produce suspensions of such small size particles, that the resulting mixture has the clarity of a solution. The internal phase is then said to be “solubilized” and tlie substance that caused it is said to be a “solubilizing agent.” The mixture, though clear, is still colloidal in nature, and can be so proven by passing a beam of light through it, when the characteristic Tyndall cone will appear.

Both emulsification and solubilization will take place to some extent whenever an emulsion is formed.

BIBLIOGRAPHY—Part 2

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