Water Repellency and Infiltration Resistance of Organic-Film-Coated Soils1

D. H. FiNK2

ABSTRACTWater repellency and infiltration resistance of porous soils

coated with a variety of thin, organic films were examined.Water repellency was determined by measuring the effectivecontact angle of water resting on the soil surface and infiltra-tion resistance was determined by measuring the breakthroughpressure, i.e., the threshold pressure required to force waterinto the soil pore structure of these water-repellent soils. Theeffective contact angle of water on these treated soils wasfound to be a function of ( i ) the structure of the hydrophobicportion of the organic admolecules, and ( i i ) the proportion ofsoil surface covered with the organic film, but was relativelyindependent of soil chemical and physical properties. Effectiveangles as high as 150 to 160° were recorded for several typesof organic materials. These included several silicone waterrepellents, an amine- and a hydroxy-substituted phenol, and anacetic acid salt of a long-chain fatty amine. The breakthroughpressure was directly affected by the factors that controlled thecontact angle, and was inversely related to the effective poreradii of the porous, water-repellent soils.

Additional Key Words for Indexing: contact angle, break-through pressure, water harvesting.

INTENSIVE utilization of desert areas by man has been pos-sible in the Dast only by importing water supplies. Most

of the rainfall received in these areas soaks into the upper

soil layers, only to be quickly lost and essentially wastedby evaporation. Water-harvesting techniques which preventthis initial shallow infiltration now show promise of per-mitting increased development of many of these relativelydry areas.

Water harvesting is the collection of rainwater from landareas specifically treated to increase precipitation runoff.Several techniques for increasing runoff have been ad-vanced (14), e.g., dispersing the soil surface layer withsalts, altering the vegetation, and waterproofing the soilwith various types of impermeable coverings.

Another promising method is to make the soil waterrepellent by artificially coating the soil surface with thinmolecular layers of hydrophobic organics. A limited fieldtest conducted by Myers and Frasier (15), in which a lightapplication of a water-soluble silicone resin was sprayeddirectly on the soil, resulted in a water-repellent soil sur-face which yielded 94% runoff from a total of only 24.4cm (9.6 in) of rain distributed over 25 storms over a 5-month period. The untreated soil yielded only 41% runoff.

190 SOIL SCI. SOC. AMER. PROC., VOL. 34, 1970

Considerable information on the use and behavior ofhydrophobic substances covering a wide range of scientificand industrial fields has been accumulated: The scienceand art of waterproofing cloth and various building mate-rials is well established (12); numerous cases of naturallyoccurring water-repellent soils, plants, and animals havebeen reported (5, 9, 11); and organic coating materialshave been used to stabilize soils (4, 6). The universal con-cepts of water repellency which pertain to all these ratherunrelated fields have served as guides in the initial develop-mental stages of water harvesting. However, even thoughthe general concepts of water repellency apply universally,many of the specific problems associated with developingwater-repellent soils for water harvesting purposes haveproven to be unique. These problems include the selectionof the organic hydrophobe, how to apply it, and what fac-tors will influence its performance.

The objective of the present phase of the study is toevaluate the effectiveness of waterproofing treatments onsoils, both as a function of the chemical and physical prop-erties of the waterproofing agent and of the soil.

THEORY

Two interfacial phenomena useful for evaluating the water-proofing characteristics of a treated soil are water repellencyand infiltration resistance. Water repellency of a surface is ameasure of its ability to shed water, and may be quantitativelyevaluated by determining the measurable or effective contactangle of a drop of water resting on the solid surface. For aporous solid, which is the case with soils, the term "waterrepellent" normally denotes an effective angle greater than90°, and degree of water repellency is denoted either by theangle itself (ranging from 90 to 180°), or by the absolutevalue of the cosine of the angle (ranging from 0 to 1). If theeffective contact angle is less than 90°, spontaneous "wetting"of the capillary system occurs.

The contact angle of water on the treated soils was deter-mined by using the large, sessile drop method of Poynting andThomas (16). In this method water is added to a flat, levelsurface of the solid being tested until further additions in-crease only the drop diameter but not the height. The contactangle is calculated from this maximum drop height by therelation

COS 6, = 1 —2y

[1]

where 6S is the effective or observed contact angle, p is thewater density, g is the gravitational constant, 7 is surface ten-sion of the water, and / is the maximum drop height.

The infiltration-resistance is a measure of the ability of thewater-repellent, porous surface to resist capillary flow, andmay be quantitatively evaluated by determining the "break-through pressure," which is the pressure just necessary to ini-tiate water flow into the pore structure. The breakthroughpressure is described by the equation

2y cos[2]

where hw is the breakthrough pressure expressed in centime-ters head of water necessary to initiate water flow into the soilpore structure, 9W is the effective contact angle of water onthe water-repellent soil and r is the effective pore radius ofthe soil. Theoretically, 9S and 6W should be equal; the sub-

scripts s and w are used to denote the two methods of evalua-tion.

MATERIALS AND METHODSThe soils used in this study were selected mainly to provide

a texture and surface area distribution range. Several typesalso were included to provide variation in soil chemical prop-erties. All samples were air dried and sieved to less than 2 mmdiameter. The soils and their pertinent physical constants arelisted in Table 1. Texture analyses were carried out using theBouyoucos hydrometer method (3), surface areas using ethyl-ene glycol desorption isotherms (7), and effective pore radiiusing the air-entry permeameter technique of Bouwer (2).

The organic coating materials were selected from commer-cially available stock. The materials were either advertised ashaving water-repellent characteristics, or were chemically re-lated to one or more materials that were water repellent. Thegeneral chemical characteristics of these organic water repel-lents, including molecular structure and concentration, werefurnished by the suppliers. The chemicals used and respectivesuppliers are listed in Table 2.

All the coatings, except one, were added as liquid solutionsor emulsions of various concentrations to the loose, air-drysoils. Uniform coating was obtained by mixing soil and solutionin a cake mixer. The methyltrichlorosilane was added to theair-dry soil as a gas at room temperature. All samples subse-quently were equilibrated for a minimum of 2 weeks in thelaboratory.

Complete details of the techniques used in measuring thecontact angles and the breakthrough pressures already havebeen published (8). Summarizing:

Contact Angle—The contact angle of water on the treatedsoils was determined using the large, sessile drop method ofPoynting and Thomas (16). The flat, level, soil surface re-quired was prepared by pressing the loose soil in a 7.6 cm(3-in) die assembly using a hydraulic press at 80 kg/cm2.Deionized, distilled water was added to the soil surface untila flat-topped drop about 3.8 cm (1.5 in) in diameter wasformed. The relative drop height then was measured with acathetometer, and the contact angle calculated using equa-

Table 1—Particle size distribution, surface area, and poreradii of soil materials

Particle size

Soil

Play sandSalt River bed sandFlushing MeadowsControl sandpachappaGranite Reef

Sand

96.887.390.439.850.351.9

Silt

a0.5

10.75.3

53.341.941.3

Clay

2.72.04.36.97.86.8

Surfacearea

raVg16.423.421.532.222.271.5

Effective poreradii (r)

7.434 .022.861.201.121.33

Table 2—Organic coating materials and suppliers

Silicone resinsbodium methyl silanolateSodium methyl sllanolateAlkoxy silaneAlkoxy silaneMethyltrichlorosllane

Amine acetates fRNH,l+ fOACl"R:90% octylR:90% octadecyl (saturated)R:99% 16 & 18 carbon chains

{46% octadecenyl, unsaturatcd)

Quaternary ammonium saltsMonoalkyl [RNfCH,),] + [Ct] ~

R:93% octadecylDialkyl [RR'(CHj)2l + [C(] ~

R & R':99% 16 & 18 carbon chains

Substituted phenols4 - 1 -bu ty lea tec h ol2-amine ~ 4-t-butylphenol

Fluorochemical

Trade name

R-20Dri-Sil 37None*SS-X-3601None

Armac 8DArmac 18DArmac T

Arquad 18-50

Arquad 2HT-75

NoneNone

FC-134

Supplier

Union Carbide Crop.Midland Silicones, Ltd.Midland Silicones, Ltd.Staffef Chemical Co.Midland Silicones, Ltd.

Armour and Co.Armour and Co.Armour and Co.

Armour and Co.

Armour and Co.

Dow Chemical Co.Dow Chemical Co.

3-M Company

Experimental product designated MAS in report.

FINK: WATER REPELLENCY AND INFILTRATION OF ORGANIC-FILM-COATED SOILS 191

PARAFFIN

STANDPIPE

RECORDER

Fig. 1—Breakthrough pressure measuring apparatus.

tion [1]. Contact angle and breakthrough pressure measure-ments all were made at 20C, and the water density and surfacetension values used in the calculations were those of purewater at that temperature. Four separate drop height measure-ments were taken on each sample, averaged, and the contactangle calculated from this average. The variation in calculatedcontact angle (Os) for any one sample, due to variation inmeasurement of drop height ( / ) , was less than ± 10° for allsamples tested.

Breakthrough Pressure—The breakthrough pressures ofwater into the water-repellent soils were determined with theapparatus shown in Fig. 1. The soil container was a lucitecylinder 5 cm wide by 6 cm deep which had a 30-mm diam-eter, fritted-glass (10-15 /* pore size) immersion tube snuglyfitted through a hole in the center of the base plate. The useof the wide, coarse, fritted-glass surface permitted a more un-impeded entry of water into the soil at the breakthrough pres-sure, improving end point determinations. One hundred gramsof the loose, water-repellent soil was poured around and overthe immersion tube, then packed using a standard procedure.A thin, precoating of paraffin on the side of the immersiontube permitted soil particles to imbed in the soft coating dur-ing the compaction step, absolutely preventing water creepdown the outside of the tube. The sample holder was con-nected to the system as illustrated.

Water was then dripped into the standpipe at a constantrate causing the water to rise in both the standpipe and theimmersion tube until it reached the water-repellent soil. Afterthis point was reached, the rate of rise of water in the stand-pipe became linear (1 cm/min) until the breakthrough pres-sure of the soil was reached. The whole breakthrough pressureprocess was recorded by connecting on a Statham PM 131pressure transducer-recorder setup (Trade names and companynames, when included, are for the benefit of the reader anddo not imply endorsement or preferential treatment of theproduct listed by the USD A.).

RESULTS AND DISCUSSION

Effects of Soil Variation

Variation in degree of quality of waterproofing due toaccompanying variations in the physical and chemicalproperties of soil was studied using only one organic,water-repellent coating on five soils. The coating materialwas a water-soluble, sodium methyl silanolate (R-20).

Contact angle measurements of water resting on thesesoils showed that the angle 0S increased with increasingsurface coverage (Fig. 2)—rapidly at very low coveragesand then leveled out at about 150 ± 10°. A marked de-

160

140

(A

CD120

IOO

90

<90

o „• D

/ D O. I 0*o

- c

-

V

O : GRANITE REEF

A : PACHAPPA

• ! CONTROL SAND

D : SRBS

O t PLAY SAND

^ O

4 6 8 10 12

g R - 2 0 / m 2 SOIL ( x 10s )

Fig. 2—Effective contact angle of water on soil surfaces treatedwith R-20 silicone. Note: The units of the abscissas of Fig.1-7 refer to grams of the organic coating material per unitsurface area of the soil particulate matter.

crease in the rate of increase in contact angle occurred atapproximately the same coverage per unit surface area ofsoil particles for all the soils tested, i.e., 2 X 10~5 gR-20/m2. This point (region) is thought to represent asufficient build-up of the silicone monolayer to effectivelysupport the bulk water phase resting on the normallydirected methyl groups.

The point (i.e., 2 X 10~5 g R-20/m2) can hardly rep-resent the completion of a compact monolayer over theentire particulate surface area as measured with ethyleneglycol; dividing the silicone coverage at this point by areasonable density figure for the silicone at monolayercoverage leads to a completely unreasonable short mono-layer thickness. The R-20 apparently is selectively ad-sorbed on the external surfaces of the soil aggregates, thuseffectively waterproofing the whole, while coating only asmall portion of the total soil surface area. The point,2 X 10~5 g R-20/m2, therefore, probably represents "mono-layer" coverage of the aggregate soil matter.

The relative flatness of the 8S vs coverage plot beyondthis "monolayer" region suggests that additional layers ofthe silicone resin build up, with a molecular orientationsimilar to the first layer, and with no molecular inversionor other type of orientation that would expose the morehydrophilic portions of the silicone molecule.

The relatively small and inconsistent variation in thecontact angle (± 10°) beyond the "monolayer" pointsuggests that at "monolayer" or greater coverages, soil typehas little effect on the contact angle, thus indicating (i)that any variation in surface energy between untreated soilsis effectively masked by the organic coatings, and (ii) thatthe variation in surface roughness (13) of the upper soilsurfaces among samples is small.

It is not particularly surprising that a monolayer coatingof the silicone resin effectively masks any variation in thesoil surface energy between the several soil types tested.Research has shown repeatedly (18) that the surface energyof any solid coated with an organic monolayer is equal tothat of the exposed functional groups of the coating mate-

192 SOIL SCI. SOC. AMER. PROC., VOL. 34, 1970

rial, and is completely independent of the chemical prop-erties of the solid subphase.

This small amount of variation in contact angle betweensoil types coated with the same organic also suggests thatall the soils have about the same degree of roughness. Sur-face roughness (13) contributes to the observed contactangle in the following way:

[3]cos 6" = /t cos 0 — /2

where 6" is the observed contact angle, 6 is the truethermodynamic contact angle based strictly on the surfaceenergy of a smooth surface, and /, and /2 are the rough-ness factors: f1 is the ratio of actual area of a solid surfacecovered by the drop compared to the solid planar surfacecovered (/, > 7), and /2 represents a porosity factorwhich is equal to the air-liquid interface under the dropcompared in this case to the total planar surface under thedrop. If 6 is greater than 90°, both f1 and /2 contributepositively toward increasing the effective, or measuredangle 8"; if 8 is less than 90°, 6" is decreased proportion-ately.

In this report the effective or measured contact anglesare designated by 0sand ^rather than by 0". This separatedesignation is used because the roughness factors were notdetermined per se, and because 0S and 8wfor most caseswere not equal for comparably treated soils.

Even though the variation in roughness between the soilstested seems small, the total contribution of roughness to-ward the measured contact angles (0S) must be quitelarge. Literature reports of true contact angle measure-ments using silicone coating materials with a molecularstructure similar to R-20 on smooth glass surfaces showangles for water ranging from about 90° to a maximum ofonly about 100° (1, 17). If the true contact angle 9 were90°, then in order for measured contact angles greater than140° to exist, at least 75% of the planar area under thedrop must be water-air rather than water-solid interface.Thus, it appears that the porosity factor /2 strongly influ-ences 8S for this silicone treated surface while f1 does not.

The breakthrough pressure curves for these same treatedsoils (Fig. 3) show that at low surface coverages of the

160 \-

~I20ttUl

40

-——-A-

D-

O' GRANITE REEFA I PACHAPPA• I CONTROL SANDD« S.R.B.S.O ' PLAY SAND——D—

-O—

24g R-20/m' OF SOIL (x lO 3 )

Fig. 3—Breakthrough pressure of water on soils treated withR-20 silicone.

R-20, the infiltration resistance also increases markedly forrather small increment increases in surface coverage. This,as already shown, is due to corresponding marked increasesin the effective contact angle of the water phase in contactwith the partially coated soil surfaces. As with the relatedcontact angle measurements on these soils, a distinctmarked decrease in the rate of increase of hw occurred(also at about 2 to 3 X 10~5 g R-20/m2), after which hwquickly leveled off. However, in contrast to the contactangle case where 0S was nearly the same for all soils whencoated with equal amounts of silicone, the correspondingvalues of hw were different for each soil—this of coursebeing predicted from the breakthrough pressure equationwhich states that

h cc-L.w r[4]

The equation predicts that soils with the smallest pores willhave the highest breakthrough pressures. In single-grainstructured soils, comparable to the type studied, the small-est pores are found in the soils with the finest texture.

Even though these coated soils differed tremendouslyin their resistance to water penetration, they were all never-theless capable of shedding water. For water-harvestingpurposes where the coated soils would serve only as watercatchments and not for storage, the soils normally needonly be resistant enough to withstand the penetration forceof the raindrops. The point is simply that given a choice,it is cheaper to waterproof a coarse-textured soil.

Effects of Organic Coating VariationsDegree of quality of waterproofing of treated soils as a

function of the type of organic coating materials also wasstudied using contact angle (0S) and breakthrough pres-sure (hw) measurements. Only one soil (Flushing Meadowssand) was used in order to eliminate variation due to soilproperties.

A group of commercial silicone materials (Table 2) pro-duced similar contact angles for water at equal coveragesof the organic per unit soil area (Fig. 4), thus suggestingthat all these silicones had similar surface energies. Suchsilicones polymerize in the presence of CO2 and H2O toform a resinous film of the type

O

Me—Si—O—Si—OH [5]

O Me

which is thought to physically enmesh the soil aggregatematter. The film also may be chemically bound to the soilthrough the silanol groups or in other ways (1). In anycase, the water-repellency characteristics derive from theexposed alkyl groups (normally methyl) which effectivelycover the soil.

FINK: WATER REPELLENCY AND INFILTRATION OF ORGANIC-FILM-COATED SOILS 193

I6O

140

inCD

120

100

90

<90

—————— 1 —————— i —————— i —————— I —————— I ——— - ' 1 1 1 ——— I ——

F.M. SOIL n

^_ o «//-*' D AT o " o

Ato a

DT A: R-20

a\t

O! DRI-SIL 37

D: SS-X-3601

VI MAS

O: MeSiC l j

D1 L ._! ————————— 1 ————————— 1 ————————— \——J , ————— 1 ————

0 2 4 6 8 1 0 1 2 3 O

g SILICONE/m2 SOIL ( x l O 5 )

Fig. 4—Contact angle of water on Flushing Meadows sandtreated with silicones.

?

F.M. SOIL

cor

g SILICONE /m2 SOIL ( x l O * )Fig. 5—Breakthrough pressure of water on Flushing Meadows

sand treated with silicones.

The methyltrichlorosilane, which was added to the air-dry soil as a gas at its saturated vapor pressure, apparentlyreacted with all the hygroscopic water present and possiblywith some of the exposed silanol groups to form a continu-ous, resinous coating on the soil. In spite of the large cover-age per unit area (30 X IO5 g/m2), which either repre-sents multilayer formation or rather extensive penetrationof the gaseous molecule into the soil aggregates, the contactangle of water on the treated soil was essentially the sameas that for the other silicone materials at "monolayer"coverage.

All these materials gave nearly identical breakthroughpressures at similar coverages of silicone per unit area (Fig.5). These data support the conclusions drawn from thecontact angle (0S) data that all the materials tested havenearly identical surface energies when adsorbed on a solidsubstrate, indicating that type, orientation, and packing ofthe hydrophobic groups must be identical.

Figure 6 shows breakthrough pressure data for severalrelated types of organics having nitrogen present in themolecular structure. All are organic cations, so undoubt-edly bond to the soil primarily through the exchange sites.

The Armac trade name denotes a group of acetic acidsalts of fatty amines. Variation among the three Armacmaterials is in the R group. Armac 8D which did not in-duce water repellency has an alkyl group 8 carbons long;18D which induced repellency has an alkyl group 18 car-bons long; and T was similar to 18D except the R groupof T is highly mono-unsaturated. The C=C group results ina lower breakthrough pressure at the monolayer coveragepoint and seemed to cause a partial inversion of the secondand subsequent adsorbed layers.

The quaternary ammonium salts or Arquads were lesseffective water repellents than the long-chain fatty amines.The 2HT-75 which was water repellent has two long-chainalkyl groups, while the 18-50 which was not water repel-lent has only one. The long-chain aklyl groups for bothtypes of materials are predominantly hexadecyl or octa-decyl, whereas the short-chain groups are methyl.

The general order of water repellency of all these types

30

a: 20uii-

F.M. SOIL

2 4 6 8 IO

g ORGANIC /m2 SOIL (x lO 8 )

ARMACf 1-Lf-

RNH,O

OAC 8D

[RS'N(CH ) ] [ci] 2HT-75 is-soo ' J r - L i O ' _ O L j

Q

HT-1

/?VCH-

Fig. 6—Breakthrough pressure for Flushing Meadows sandtreated with several primary amine and quaternary ammo-nium salts.

of materials (18D > 2HT-75 > 18-50 > 8D) probablyrepresents the degree of isolation afforded to the hydro-philic nitrogen atom of the respective molecules. Contactangle measurements (0S) supported the conclusion drawnfrom the breakthrough pressure measurements.

Breakthrough pressure and contact angle measurementsalso were run on a hydroxy- and an amine-substitutedphenol, 4-t butylcatechol (TBC) and 2-amine-4-t-butyl-phenol, respectively. The hydrophilic portions of thesemolecules reportedly chemically bond to the soil by com-plexing with the exchangeable iron and aluminum (10).The water repellency for both molecules is derived fromthe exposed tertiary butyl group. The breakthrough pres-sure curves for the two materials (Fig. 7), as might beexpected, were nearly identical. The plot shows that the"monolayer" point occurred at about 2 X 10~5 g or-ganic/m2, but also shows that water repellency continued

194 SOIL SCI. SOC. AMER. PROC., VOL. 34, 1970

20

P.M. SOIL

0 2 4 6 8 10 12 14

g ORGANIC /m 2 SOIL ( « I O S )

Fig. 7—Breakthrough pressure for Flushing Meadows sandtreated with phenolic or fluoro-carbon organics.

to improve by increasing the surface coverage. Again, con-tact angle data substantiated these results.

A fluoro-carbon material also was examined. The ratherlow breakthrough for this material (Fig. 7) probably isrelated to the presence of several sulfur and nitrogengroups in the molecule. The exact molecular structure ofthis material, however, was not available.

Even though some of the materials obviously are con-siderably more resistant to water infiltration than others,it must be remembered that any coating material that willprevent capillary flow of water at low pressures shouldpermit water shedding when the water is present on thetreated soil as a thin moving film, e.g., as normally occurson water catchments.

An attempt was made to determine the effective contactangles (9W) controlling the breakthrough pressures, andcompare them to those obtained by the sessile drop method(0S). Since cos 6W is intrinsically related to the effectivepore radius [equation 2], an independent measurement ofpore radius was required. This was attempted by measuringthe air entry value (2) of each of the untreated, com-pacted, water-saturated soils. As in the breakthrough pres-sure measurement, the capillary rise equation [equation 2]pertains. The contact angle between the water-air interfaceand the untreated soil for the air entry determination, how-ever, was ascribed the value zero.

The calculated values of 8W generally were less thanthe comparable, more directly determined contact angles(0S). The lower values for 9W probably are caused by sev-eral factors: (i) the contact angle as determined by thesessile drop method takes only a minute or two to run,while the breakthrough pressure measurement takes con-siderably longer—possibly the water-repellent soil gradu-ally hydrates when in contact with water; and (ii) as pres-sure is increased during the breakthrough pressure mea-surement, water may gradually displace some of the airfrom around the inside edges of the soil grains, thus in-creasing the soil-water interfacial area, which in turn re-duces the porosity factor /2. The lower limit of /2 should

correspond roughly to the real soil porosity—normallyabout 50%. These factors deserve more study. Eventhough in general Bs ¥= Ow, the breakthrough pressureand sessile drop results followed similar trends, and theconclusions drawn from this work were the same usingresults from either method.

There are other factors which must be considered inselecting a water repellent for water harvesting: e.g., thematerial also must stabilize the soil to prevent erosion orelse must be compatible with a separate stabilizing mate-rial; durability against weathering is important, as is easeof application and cost. These and other problems are cur-rently being investigated both in the laboratory and in thefield.