IV. THE HEXADECANOL METHOD

Hexadecanol is an industrial chemical used in cosmetics, detergents, and emulsifiers. It is a white, waxy, crystallinelike solid, generally available in flake or powder form. It is relatively tasteless and odorless. Hexadecanol is derived from tallow, sperm oil, or coconut oil. Cetyl alcohol is another name for this material.

Hexadecanol's effectiveness in reducing evaporation has been demonstrated in numerous laboratory and field tests. In laboratory studies up to 65 percent of evaporation losses can be eliminated by a hexadecanol film (35). In experimental field tests, savings are generally in the order of 20 to over 30 percent (22, 24). Australian workers consider that 25-percent reduction of evaporation can be secured by farmers using the hexadecanol method on small ponds (6), and on a large reservoir test there a 37-percent saving is reported (12). Results of tests vary widely, and large reservoir control operations are dependent on additional research work. Variation in test results may be due in part to differences in the conditions of the tests, or they may be due to differences in the methods used for applying the film; probably also some of the variation in results is due to difficulties in accurate measurement of water losses. As an example of variations in the results of tests associated with seasonal changes, Roberts reports (26) that in a 100,000-gallon tank experiment in Illinois, the 33 percent reduction secured in early September declined to 11 percent reduction by mid-October.

Australian Government authorities recommend the hexadecanol method as a desirable water-saving practice for farmers and stockmen in the water-short areas of that country (6). Preliminary research in this country confirms in general the Australian experience on small ponds, and widespread tests and demonstrations can now round out the experimental work. Such extensive tests would bring about the needed adaptations to actual farm and ranch conditions, and they would also provide reliable data for determining water savings and costs.

Use of hexadecanol on storage reservoirs is dependent on development of suitable techniques for establishing and maintaining the film. The dispensers used on small ponds are effective for only about 1 acre each, and this spacing is, of course, impractical on large reservoirs. There are, furthermore, additional problems on large water surfaces that are not present on small ponds. Wind and wave action, and disturbance by boats, or other moving objects, may be critical in maintaining film coverage of large surfaces. Other problems may also develop from reservoir releases or spills that would remove the film. In order to determine the extent of practical control operations on large reservoirs new methods must be worked out to overcome these and similar factors, and cost data must be secured.

RESEARCH ACTIVITIES

The problems of devising evaporation control methods for large reservoirs, along with improvement of methods for small ponds, currently are being investigated in many countries. An intensive program in Australia is sponsored by the Commonwealth Scientific and Industrial Research Organization, an Australian Government agency. Elsewhere abroad, work is reported by the East African Meteorological Department, and from Brazil, Israel, and Sweden. The International Commission on Irrigation and Drainage, a UNESCO affiliate, sponsors international coordination of evaporation control research.

Most of the work on evaporation control in this country to date has been done by the Bureau of Reclamation and the Southwest Research Institute. The Bureau of Reclamation is concerned primarily in developing practical methods for control of evaporation from the water resources projects for which it has responsibility in the Western States. SRI is a private research organization located in Texas, and in these investigations it does the technical studies for a private association of water districts and industries organized as the Southwest Cooperative Project for Control of Evaporation From Reservoirs. A number of State agencies also are engaged in evaporation control research. These include the State agricultural colleges and experiment stations of Arizona, Colorado, Oklahoma, and Texas, and the Illinois State Water Survey. Several city water departments are cooperating, notably those of Denver and Oklahoma City. The Salt River Valley Water Users Association in Arizona also is initiating a research program cooperatively with the United States Geological Survey. The United States Public Health Service and the Geological Survey are actively cooperating with a number of organizations in their specialized fields.

Current work falls into three main lines of investigation. One line of work is in the nature of basic research in physical chemistry even though it is done largely in the field rather than in the labolatory. These investigations are concerned with how hexadecanol workshow the film is formed, how it heals itself when broken, and similar problems.

A second line of investigations deals with the practical problems of evaporation control methods that is, how to place the hexadecanol on the water surface, how to maintain the film, in what forms the material may be used advantageously, and what quantities are required under various conditions of climate and surface disturbance. Determinations of costs in terms of water savings is an important part of these studies.

Testing and measuring the effectiveness of the film involves so many novel and difficult problems that the development of these techniques is properly considered a third line of investigations. This includes methods for detecting the presence and extent of hexadecanol films, and it includes also methods for measuring the amount of water that is saved.

BASIC RESEARCH

The use of hexadecanol for evaporation control grew out of laboratory research in fundamental problems of physical chemistry, and, perhaps for this reason, basic problems have been examined somewhat more fully than often is the case for new control operations.

Intensive research on hexadecanol was started about 1925 by Sir Eric Rideal in England, and in this country by Dr. Irving Langmuir and his associates, particularly Dr. Victor K. LaMer, at Columbia University. These investigations, concerned with theoretical problems of physical chemistry, recognized evaporation suppression by hexadecanol as a laboratory phenomenon useful in exploration of these theoretical problems. This research provided an understanding of the nature of the evaporation control process, and it established the maximum evaporation reduction effect that can be secured under laboratory conditions.

In 1953, Mr. W. W. Mansfield, an Australian physical chemist, moved the laboratory problems into the field in order to investigate practical methods for controlling evaporation losses from stored water. Mansfield's work is done under the auspices of the Commonwealth Scientific and Industrial Research Organization. The Bureau of Reclamation began its investigations at about the same time, and shortly afterward investigations were started by the East African Meteorological Department, the Southwest Research Institute, and by the other organizations, all of whose work has been utilized for this report.

How hexadecanol works

The present understanding of the physical chemistry processes of monomolecular films provides a working basis for the improvement of practical evaporation control methods. This understanding has, in the main, been supplied by the research at Columbia University and the work of CSIRO, the Bureau of Reclamation, and other research workers. Continuing research will clarify some of the unresolved questions and this will undoubtedly aid in the design of control methods.

Hexadecanol forms an evaporation-retardant film over the water surface that can be thought of as though it were a roof impervious to escaping water vapor. The process by which this occurs is, however, a far more complicated matter that involves energy relationships of the water molecules and the film molecules (18). The film, only one molecule deep, measures only about six ten-millionths of an inch in thickness. In spite of this extreme thinness, a hexadecanol film that has been suitably formed effectively suppresses the evaporation process that would, in the absence of the film, vaporize the exposed water and dissipate it into the atmosphere. Fortunately, the film that prevents loss of water does not prevent the entrance of rain. Apparently this happens because the raindrops break holes in the film through which they enter, and then the holes heal themselves and close up after the storm.

The extreme thinness of the hexadecanol film seems to be related to its durability. Being thin and flexible, the film undulates with the water surface instead of breaking up, as do the heavy oils that form thick films. Another important property of hexadecanol films is that the molecules of which they are composed stand on end like the bristles of a brush or, according to one description, "as paper matches in a matchbook" (35). The molecules assume this position because one end of the long, spindle-shaped molecule is attracted to water, and the other end is repelled by water.

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Film pressure

The third important property of the hexadecanol film is that, with sufficient hexadecanol available, the individual molecules jam up against each other very tightly. As a result of this tight packing of the bristlelike molecules, water vapor is unable to escape between them. This is the property that makes the films evaporation resistant. It is fortunate that the tight packing together of the hexadecanol molecules, although it prevents the escape of water vapor, is nevertheless porous to the passage of oxygen and carbon dioxide. As a result, hexadecanol films do not interfere with normal gas exchange that is essential to maintenance of aquatic life and sanitation.

Film pressure is the technical term used for the tightness with which the molecules are packed together. The term is defined as the difference between the surface tension of water and the surface tension of water covered with a hexadecanol layer (31). This difference is readily observable by mixing a small quantity of powdered tale with the hexadecanol as it spreads across the untreated water surface. The greater the hexadecanol film pressure, the more rapidly the talc is carried across the surface. Film pressure is measured in dynes per centimeter, which is a standard term of physical chemistry for measurements of force. The more closely the molecules are packed, the higher is the force of the film pressure in dynes per centimeter.

It is important to have the hexadecanol film at a pressure in the order of 40 dynes per centimeter because evaporation reduction is greatest in that range (23), and substantially decreased effectiveness results from lower film pressures. Laboratory observation indicates that marked lowering of the film pressure destroys the ability to restrict evaporation. Maintenance of optimum film pressure is complicated by temperature effects on it, thus indicating that films might have to be fortified with additional material as the temperature changes.

Another finding of basic research that has important application in the design of control methods relates to the manner in which the film is generated. Hexadecanol molecules leave the source of supply one molecule at a time. This separation takes place along the plane of contact between the surface of the water and the hexadecanol mass. As a result, the rate of film formation depends, among other factors, on the size of this contact plane. This means that formation of the film at an adequately rapid rate requires that a large area of hexadecanol material be in contract with the water surface (23). In designing control methods, therefore, hexadecanol is used in a form that will have large surface area in relation to the weight of material used. This is the basis for tests using the material in flake form.

Practical control methods also benefited from theoretical analysis relative to the spacing of film generators. As it is generated from each individual hexadecanol supply source, the film moves outward to cover an ever-increasing water surface area. Because of this radial spread of the film, more molecules are required for each foot of linear extension. The computations of basic research are supported by actual trials in determining that, in order to secure satisfactory film formation, the generators should generally be spaced not farther apart than one generator per acre of water surface (6, 23).