THE USE OF A MONOMOLECULAR FILM TO RETARDEVAPORATION FROM FREE WATER SURFACES*

by

T. L. CoulthardMember C.S.A.E.

Department of Agricultural Mechanics,University of British Columbia, Vancouver,

British Columbia

alcohols, esters, and acids was recognized by Rosano and La Mer (7, 8)to control evaporation from free watersurfaces. Of over approximately 100evaporation retardants tested by theU.S. Bureau of Reclamation, hexadecanol and octadecanol showed thegreatest promise (3). In Canada thereis a need to evaluate the availabledata and to test successful methodsunder our domestic conditions.

Evaporation is defined by Dalton'sLaw: E=C (pw—pa)

where —

E=evaporation rate;

pw=maximum vapour pressurecorresponding to the temperature of water;

ap=vapour pressure in air;

C=coefficient depending uponwind velocity, barometric pressure, etc.;

(pw—pa)=pressure gradient.

Several types of organic molecules(fatty acids, fatty amides, fatty al

cohols) when applied to water surfaces will form a film one moleculethick over the surface of the water.These molecules possess a polar groupsuch as OH, C=0, and NH,. The

-OH

polar groups are strongly attracted towater molecules which possess a permanent dipole (2). When packed together, the molecules stand on endand form a film that helps resistevaporation of the water thus covered(1). An excess of film forming ma-terial is necessary at all times tokeep the film tightly packed and torepair it if it should be broken. Therate of spreading of the monmole-cular film depends on the surfacetemperature of water and the characteristics of the chemical compound.The average rate of spreading foralcohols recommended for water conservation is approximately twenty tothirty centimeters per second at 20°C.(6).

The hexadecanol film is about twomillionths of a millimeter thick andis not visible to the naked eye; however, since it reduces ripples on the

K. PohjalcasMember C.S.A.E.

Experimental Farm, Swift Current, Saskatchewan

INTRODUCTION

Future increases in human and livestock populations will create increasing demands for fresh water. In manyareas, including semi-arid and aridregions of Western Canada, the available water supply is limited. In orderto utilize the water resources to theirmaximum efficiency, water conservation becomes a necessity.

Water is lost through the processof evaporation from free water surfaces, evaporation from moist soil,and through deep percolation intoground water. The elimination of unnecessary waste is the first step inthe water conservation program. Thelosses from soil and water surfacesthrough evaporation perform littleuseful function except replenishingthe air vapour storage with moisturenecessary to complete the hydrlogiccycle. In semi-arid and arid regionsthe potential evaporation losses exceed the total precipitation.

There are several methods of reducing evaporation from water surfaces. The concentration of water intolarge deep reservoirs with a minimumsurface area helps to reduce evaporation. Windbreaks designed to reducethe wind velocity and consequentlythe evaporation rate have been recommended. Even roofs and floatingcovers have been proposed in certainareas. The use of monomolecularfilms is one of the methods employedto reduce evaporation from free watersurfaces. This paper presents data ofevaporation control with monomolecular films formed by hexadecanol.

REVIEW OF LITERATURE

The first successful use of mono-molecular films to retard evaporationwas reported in 1927 by Langmuir(4) who found that hexadecanolfilms reduced evaporation by fiftyper cent. Since that time many experiments, mainly laboratory studies, havebeen concerned with the ability ofvarious chemicals to form monolayers.Mansfield (5) reported that hexadecanol significantly reduced evaporation from pans and larger bodies ofwater. Hexadecanol along with other

J. D. Beaton

Research and Development Division, ConsolidatedMining and Smelting Co. of Canada, Ltd.,

Trail, British Columbia

water surface the extent of spreadingcan be observed. As the spreadingproceeds within the reservoir the surface pressure increases and the rate ofspreading decreases until an equilibrium pressure is reached. A surfacefilm must meet the following requirements: (1) to form an unbrokensurface film over the entire watersurface; (2) to be impervious to watervapour; (3) to be able to regeneratethe film when broken; (4) to beodorless and non-toxic; (5) to be easily generated by using chemicals ofreasonable cost; and (6) it must notseriously retard the diffusion of oxygen and carbon dioxide in water. Areduction in the rate of movement ofsuch gases would seriously upset thebiological balance of bodies of waterso treated.

MATERIALS AND METHODS

Four separated 4-foot diametertanks as shown in Fig. 1 were installed in the ground within the enclosure of the Summerland ResearchStation Weather Station. The tankswere of cylindrical design, 24 inchesdeep with an inside diameter of 471/2inches. The tanks extended 2 inchesabove the ground surface.

For each tank a stilling well wasmade and the fixed point was installed exactly two inches below theupper edge of the tank, which corresponded to the ground level around

RESULTS

The addition of hexadecanol inflake form was found to be unsatisfactory. For this reason, in all subsequent studies, it was dissolved in gasoline and applied to the surface bymeans of an eyedropper. Non-leadedgasoline was preferred since it is morevolatile than ordinary motor fuelwhich contains lead compounds.

It was found in a short trial periodthat the spreading pressure of thehexadecanol film should be main-

crease in effectiveness of the film wasobserved. Film pressures in this investigation were thus maintained between these two pressures.

In general, increased rates of hexadecanol caused progressive reductionsin evaporation rates (Fig. 2). Applications of less than 2V3 pounds peracre produced erratic results wherethe reduction in evaporation rangedfrom 10 to 20 percent. When V/zpounds were applied the reductionin evaporation was between 20 and

the tank. The amount of water evaporated every day was determined byadding water from a graduated plasticgauge. One division of the plastic

4 gauge was equivalent to a hundredthof an inch of evaporation. The readings were taken to the nearest hundredth of an inch.

The tanks were filled daily between7:00 a.m. and 7:30 a.m. P.S.T., exceptfrom July 24 to August 2 when diurnal rates of evaporation were studiedand tanks were filled at 7:00 a.m. and7:00 p.m. P.S.T. At this time floatingforeign matter was removed from thesurface except when the influence ofsuch impurities on the effectivenessof the film was being studied. Theperiod of sunshine after 7:00 p.m.was negligible mostly due to theshading effect of the mountains inthe southerly and westerly direction.Other meteorological observationssuch as evaporation rates and watertemperature in Class A Land Pan,hours of sunshine, solar radiation,relative humidity, wind speed, andambient air temperatures were alsorecorded at this time. Barometricpressures, however, were obtainedfrom the Penticton Airport WeatherStation approximately 6 miles distant.

Evaporation from each of the fourtanks was recorded for two weeks before hexadecanol was added. Following this period, hexadecanol in flakeform was then placed in wire cagesand set in two tanks. This applicationmethod was found to be unsatisfactory. Next, different quantities ofhexadecanol were dissolved in gasoline and applied with an eyedropperto the surface of the tanks. For therest of the experiment, hexadecanolsolution in gasoline was used to formmonomolecular films.

The amounts applied in this manner were 0.11, 0.263, 0.655, 1.09, 2.34and 4.68 pounds per acre. The effectof these applications on the rate ofevaporation was studied for 5 dayswith the two lowest rates, 6 days for0.655 pounds per acre, 2 days with1.09 pounds per acre, 34 days at the2.34 addition, and 4 days for the highest rate. The effect of impurities onhexadecanol films was studied fromJuly 12 to 16. During this intervalthe water surfaces were not cleanedof foreign matter as was usually done.

Two indicator oils, oleic acid andcastor oil, with spreading pressures of30 and 17 dynes per square centimeter, respectively, were used to obtain an estimate of the spreadingpressure of the hexadecanol monolayer.

THE EFFECT OF HEXADECANOL ON EVAPORATION

z<

<XId

O UJD Q-O

f as

aoQ.<>

*

FIG. 2

tained between 17 and 30 dynes persquare centimeter. Below these pressures evaporation was not markedlyreduced, while above them no in-

DIURNAL VARIATION

T

0.20 0.30 0.40

EVAPORATION FROM FREE WATER SURFACESIN INCHES PER DAY

Figure 2. Application rates versus reduction in evaporation.

30 percent. Further reductions resulted when the dosage was increasedto 42/3 pounds. On occasion these reductions reached 46 per cent.

IN THE EVAPORATION RATES

D» EVAPORATION IN INCHES FROM 7 A.M. TO 7 P.M.7 P.M. TO 7 A.fc

SURffcCE TREATED WITH 2.34 LM./aCRE

FIG. 3Figure 3. Diurnal differences in evaporation rates.

The results illustrated in Fig. 3indicate that greater diurnal differences occurred in the evaporationrates from surfaces covered with amonomolecular film than those from

a free water surface. During the dayevaporation was markedly lowerthrough a monomolecular film thanfrom an untreated surface. At nightthe rate of evaporation from both wasapproximately the same with the lossfrom free water surfaces being slightlyhigher.

The effectiveness of the hexadecanol film diminished quickly unless itwas renewed daily. Figure 3 presentsthe reduced effect of the film betweenJuly 29 and August 3 when the filmpressure remained above 17 dynes persquare centimeter without daily renewal of the film. On August 3rd thepressure dropped below 17 dynes anddaily applications were resumed.

The rate of evaporation which depends upon the vapour pressure gradient reached its maximum after sunset when water temperature was highest and when the air temperature decreased. The efficiency of the hexadecanol monolayer probably decreasedwith an increase in temperature ofthe water since the interaction between permanent dipoles is known todecrease with a rise in temperature(2). The cooling process itself which

involves the movement of warm watermolecules to the surface may interrupt the continuity of the film as wellas contribute to the effect of temperature on the interaction. This tendency for the warmest molecules toappear at the surface will maintainthe vapour pressure gradient at amaximum. It is suggested that forthese reasons evaporation reached amaximum after sunset.

The vapour pressure gradient during the sunlight hours was usually lessthan at night. Following sunrise theair was normally warmed up quicklyand approached or exceeded the temperature of the water in the tanks.As a result the vapour pressure gradient was low or even assumed a negative value (see Dalton's Law). Sinceevaporation is a function of vapourpressure gradient, a low gradient isassociated with low evaporation rates.During the warming process evaporation was not enhanced as in the cooling process. Apparently the surfacewater molecules that were warmed,because of their decreased density, remained at the surface. The dipole interaction, which increases with decreasing temperature, became strongerbecause of the cooling effect of evaporation. A stronger interaction wouldtend to increase the effectiveness ofthe film and, therefore, reduce evaporation.

Fig. 4 shows that the effectivenessof the monomolecular film on theretardation of evaporation was greatlyreduced by the accumulation of floating foreign material on the surface.Where the surfaces of the water covered with the hexadecanol monolayerwere not cleaned daily a rapid increase in the evaporation rate occurred. At the end of three days

diurnal differences occurred in therates of evaporation from hexadeca-nol-treated water as compared to theuntreated water surfaces.

REFERENCES1. Beale, B. W., and R. R. Cruise.

Water Conservation Through Control of Evaporation. J. of American Water Works Assoc. Vol. 49,No. 4 1947.

0.4

0.0

Cetyl alcohol added surfacecleaned daily.

No alcohol added

surface not cleanedCetyl alcohol addedsurface not cleaned.

Cetyl alcohol addedsurface cleaned daily.

-10

6 8 9 10 II

Days : 3rd to 20™Figure 4. The effect of impurities

without cleaning the evaporation rateof the monolayer-treated waters hadapproached that of the free watersurfaces. The amount of pollen, dust,and other airborne materials was believed to be unsually high becauseof the dry conditions prevailing atthis time.

It was observed that the surface contaminants accumulated more rapidlywhere hexadecanol had been added tothe water. Here the clusters of hexadecanol and foreign material whichformed seemed to interrupt the continuity of the film as they were movedabout the surface by the wind. Theimpurities became heavily coated withhexadecanol molecules, which mayhave resulted in the formation of anincomplete film. The addition of excess amounts of hexadecanol to thewater when impurities were presentdid not appreciably reduce evaporation. This was unexpected since halfan ounce of hexadecanol is sufficientto form a monomolecualr layer on anacre of water (6).

CONCLUSIONS

Evaporation was reduced between20 and 30 per cent through hexadecanol monolayer when 2^3 poundsof hexadecanol per acre were used.On occasion reductions of 46 per centwere observed with application of4% pounds. Foreign matter accumulating on the water surface markedlyreduced the effectiveness of the monomolecular film. Daily additions ofhexadecanol were necessary to suppress evaporation effectively. Greater

free water surface

water covered with

monomolecular film.

12 13 14 15

July 1958

on monomolecular film.

16 17 18 19

2. Glasstone, S. Handbook of Physical Chemistry. D. Van NostrandCompany, New York. pp. 546-551.

3. Harbeck, E. Can EvaporationLosses Be Reduced? Address byE. Harbeck, Jr., U.S. GeologicalSurvey, presented at the 3rd International Commission of Irrigation and Drainage, San Francisco,Calif. April 29, 1957.

4. Langmuir, I. and Langmuir, D.B. The Effect of MonomolecularFilms on The Evaporation ofEther Solutions. J. of PhysicalChemistry, pp. 1719. 1927.

5. Mansfield, W. W. Influence ofMonolayers on the Natural Rateof Evaporation of Water. Nature,pp. 175-245. 1955.

6. Price (Bromborough) Ltd. Tech.Bull. No. 1. Fatty Alcohols forWater Conservation.

7. Rosano, H. L. and V. K. LaMer.The Rate of Evaporation of WaterThrough Monolayers of Esters,Acids and Alcohols. ColumbiaUniv. p. 23. August, 1955.

8. Rosano, H. L. and V. K. LaMer.The Rate ofEvaporation ThroughMonolayers of Esters, Acids andAlcohols. The Journal of Physical Chemistry. Vol. 59 and 60.

ACKNOWLEDGEMENTThe authors are grateful to Dr. J.

C. Wilcox, Head of the Departmentof Irrigation and Plant Nutrition atthe Summerland Research Station forhis assistance and criticism during thecourse of this study.