the kinetics and energetics of transitions between water repellent and wettable soil conditions: a...

HYDROLOGICAL PROCESSESHydrol. Process. 21, 2248–2254 (2007)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.6747

The kinetics and energetics of transitions between waterrepellent and wettable soil conditions: a linear free energyanalysis of the relationship between WDPT and MED/CST

Peter Douglas,1* Kathryn A. Mainwaring,1 Christopher P. Morley1,2 and Stefan H. Doerr3

1 Chemistry Department, University of Wales Swansea, Singleton Park, Swansea, SA2 8PP, UK2 School of Chemistry, Cardiff University, PO Box 912, Cardiff, CF10 3AT, UK

3 Department of Geography, University of Wales Swansea, Singleton Park, Swansea, SA2 8PP, UK

Abstract:

Soil water repellency is commonly assessed using the Water Droplet Penetration Time (WDPT) and Molarity of Ethanol(MED) or Critical Surface Tension (CST) tests. The former is a kinetic measurement indicating the persistence of waterrepellency; the latter, a thermodynamic measurement, gives the initial severity of water repellency. This study aims to providea theoretical framework to understand (i) what determines differences in persistence and initial severity of soil water repellencyand (ii) correlations between such data. A linear free energy relationship between Solution Droplet Penetration Time (SDPT)and the difference between the surface tension of the droplet solution, �sol, and the Critical Surface Tension of the soil, �cx,has been derived.

ln�1/SDPT� D lnA � [f��sol � �cx�/NkT]

Here: �sol is the surface tension of water (for WDPT) or the solution being used (SDPT); N is the number of adsorbed moleculesper unit area; k the Boltzmann constant; T the temperature; A is 1/SDPT when �cx D �sol; and f is an experimentally determinedparameter. WDPT, SDPT (ethanol/water and propan-1-ol/water), and CST measurements for a group of sandy soils supportthis analysis. The value of f obtained (4Ð2 (š0Ð4)), suggests that the surface free energy contribution to the free energy ofactivation of wetting is given by the difference between the cohesive energy of the molecular film adsorbed on the soil grainsand that of the wetting solution.

In this interpretation: �cx is determined by the cohesive energy of the organic film adsorbed on the soil; WDPT is determinedby the difference in cohesive energies between this adsorbed film and the droplet solution; and soil-to-soil variations in bothWDPT and �cx are due to organic films of different cohesive energies present on the soil particles. For aqueous solutions ofsimple linear alcohols and ketones, SDPT depends on �sol but is independent of the compound used to control �sol. Copyright 2007 John Wiley & Sons, Ltd.

KEY WORDS water repellence; hydrophobic; MED; WDPT; CST

Received 20 January 2006; Accepted 18 January 2007

INTRODUCTION

Water Droplet Penetration Time (WDPT; Watson andLetey, 1970) and Molarity of Ethanol (MED; King, 1981)tests are widely used in studies of soil water repellency.WDPT is a kinetic measurement and gives the persistenceof water repellency (Dekker et al., 1999). MED or,more fundamentally, the Critical Surface Tension, CST,is a thermodynamic measurement and gives the initialseverity of water repellency (Dekker and Ritsema, 1994;Roy and McGill, 2002). There have been a numberof empirical studies of correlations between the dataobtained from these two tests. Crockford et al. (1991)found a close relationship between the WDPT and MEDtest results for most, but not all, samples in a study ofAustralian soils. Harper and Gilkes (1994) found goodcorrelation for WDPTs ranging from 10 to 2000 s and

* Correspondence to: Peter Douglas, Chemistry Department, Universityof Wales Swansea, Singleton Park, Swansea, SA2 8PP, UK.E-mail: [email protected]

computed a conversion formula MED D �1Ð3 C 1Ð1 (logWDPT) in a study of Australian clayey soils. In a study ofmedium textured Portuguese soils with WDPTs rangingfrom <5 s to >5 h Doerr (1998) reported that MEDcorresponded reasonably well to WDPT for highly waterrepellent, but less well for moderately water repellent,soils. However, Dekker and Ritsema (1994), with a muchlarger sample size of Dutch dune sands (WDPTs <5 s to>6 h), found only a limited correlation between the twomethods.

There is currently a lack of a theoretical frameworkto understand (i) what determines differences in persis-tence and initial severity of soil water repellency and(ii) correlations between such data where they are found.Here we explore the relationship between the WDPT andCST tests using a combination of kinetic and thermo-dynamic arguments to provide such a framework. Theapproach used is evaluated using data from a range ofsandy soils examined in a series of previous studies(Llewellyn et al., 2004; Mainwaring, et al., 2004, Doerret al., 2005a; Morley et al., 2005).

Copyright 2007 John Wiley & Sons, Ltd.

KINETICS AND ENERGETICS OF SOIL WATER REPELLENCY 2249

MATERIALS AND METHODS

Soil sampling and sample preparation

Samples of sandy soil were taken from locationswith differing climates and vegetation cover from theUK, Netherlands (NL), Portugal (PT), Greece (GK), andAustralia (AU). The sample sites in the Netherlandsand UK experience an oceanic humid-temperate climatewith rainfall occurring throughout the year. The sitesin Greece are also temperate, but with a summer dryseason. The sites in Portugal and Australia have a warmerMediterranean-type climate with prolonged dry periodsduring the summer. All samples were surface material(<20 cm depth) except at the Netherlands site where theywere taken from successive 10 cm depths up to 40 cm.Sample locations, particle size data, water repellencycharacteristics and total organic carbon are given inTable I.

Samples were dried at 20 °C and passed through a2 mm sieve prior to analysis. Apart from some largerorganic debris all soil components passed the 2 mm sieve.All soils were of medium sand texture (0Ð22–0Ð70 mmdiameter) with a clay content of <0Ð1%. Sub-sampleswere obtained by coning and quartering. For more detailsregarding sampling sites and sample preparation seeDoerr et al. (2005a).

Water droplet penetration time method

Water droplet penetration times for soils at 20 °C�WDPT20� were determined as follows. All sub-sampleswere equilibrated in a controlled atmosphere of 20 °C and45–55% relative humidity for 24 h to avoid any influenceof changing atmospheric conditions on test results (Doerret al., 2002). Samples (¾10 g) were placed in plasticdishes (50 mm diameter ð 10 mm depth) and the surfacesmoothed by gently tapping and shaking the dish onthe bench top. Five drops of distilled water (¾80 µl)from a Nalgene droplet dispenser bottle were placed onthe soil surface from a height of less than 5 mm, andWDPT20 obtained as the time recorded for complete dropinfiltration for each drop. WDPT105 values, for samplesexposed to heating at 105 °C for 24 h as suggested byMa’shum and Farmer (1985) and Dekker et al. (1998),were obtained in the same way.

Critical surface tension ��cx)

Using a series of ethanol/water solutions (or, for someexperiments, propan-1-ol/water solutions) of concentra-tions 0–6 M, to give solutions with surface tensions of33–72Ð8 mN m�1 at increments of �2 mN m�1 (Main-waring, 2004), drops (¾80 µl) of increasing ethanol con-centration (and thus decreasing surface tension) wereplaced on the levelled soil surface of each of three sub-samples (¾30 g) from Nalgene droplet dispenser bottles,from a height of less than 5 mm, until a concentrationwas reached which allowed rapid infiltration �<5 s� ofthe applied drop (Roy and McGill, 2002). �cx was takenas the mean surface tension of the most dilute solution,

-12

-10

-8

-6

-4

-2

00 5 10 15 20 25 30 35 40

ln(1

/WD

PT

)

(γLVw - γcx)/mJ m-2

Figure 1. ln(1/WDPT) against ��LVw � �cx� where x is a water/ethanolmix. Filled diamonds—WDPT20; open squares—WDPT105. Equation ofline of best fit is y D �0Ð23�š0Ð023�x � 1Ð01�š0Ð45�; (R2 D 0Ð81). Onlysamples with WDPT values <18 000 s are included, because at longer

times evaporation caused a significant loss in droplet volume

-10-9-8-7-6-5-4-3-2-10

0 5 10 15 20 25 30 35

ln(1

/SD

PT

)

(γsol - γcx)/mJ m-2

Figure 2. ln(1/SDPT) against (�sol � �cx�, for water/alcohol mixtureson four different soils: NL2—circles water/ethanol, open triangleswater/propan-1-ol; NL3—diamonds water/ethanol, open triangles water/propan-1-ol; AU1—filled triangles water/ethanol; AU3—squares water/ethanol. �cx measured using aqueous solutions of either ethanol orpropan-1-ol as appropriate. Equation of line of best fit is

y D �0Ð19�š0Ð012�x � 3Ð07�š0Ð15�; (R2 D 0Ð80)

which penetrated the surface within 5 s. Error estimatesfor Figures 1 and 2 are one standard deviation.

THEORETICAL APPROACH

Following the results of many previous studies we beginwith the assumption that soil water repellency is causedprimarily by a layer of organic material adsorbed onthe surface of the soil particles (Ma’shum et al., 1988;Bisdom et al., 1993; Franco et al., 2000). We neglectthe influence of any other factors such as particle sizeand size distribution (McHale et al., 2005) and variationin the composition and distribution of soil minerals.We expect our experimental dataset of sandy soils ofsimilar particle size and size distribution to match theseassumptions quite well.

For instantaneous immersion wetting of the soil usingliquid x:

�LVix � �SLi

x � �SVix �1�

where � indicates interfacial tension; LV, SL and SVindicate liquid/vapour, solid/liquid and solid/vapour inter-faces respectively; and superscript i indicates the situationat the instant the drop is added (Jaycock and Parfitt,1981). MED or CST studies provide the critical surface

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 21, 2248–2254 (2007)DOI: 10.1002/hyp

2250 P. DOUGLAS ET AL.

Tabl

eI.

Ori

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2005

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this

stud

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DPT

are

the

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edia

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atio

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em

ean

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sure

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ater

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anol

mix

ture

s;an

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tim

ate

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atth

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ansi

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ty,a

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UK

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alia

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ece,

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tabl

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105

(s)

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105

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m�1

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iro

40°

320 N

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460 W

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316

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40°

190 N

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1800

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496

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040

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Hol

land

51°

480 N

,3°

540 E

Gra

ss&

mos

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–20

0Ð23;

0Ð10

2063

š96

48Ð7

š1Ð5

1800

044

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š0Ð3

NL

3H

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nd51

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0 N,

3°54

0 EG

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300Ð2

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844

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1319

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4063

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452

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0 N,

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76Ð8

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CG

ower

51°

350 N

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060 W

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0Ð39;

0Ð12

072

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72Ð8

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0Ð6A

U1

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36° 2

60 S,14

0°40

0 EPa

ddoc

k0

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608

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49Ð0

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60 S,14

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0 EPa

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140°

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20°

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136

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1526

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947

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K3

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ace

40°

560 N

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KC

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40°

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0–

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72Ð8

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0Ð7



tension, �cx, i.e. the maximum liquid surface tension atwhich instantaneous wetting occurs; and for which

�cx D �SLix � �SVi

x �2�

where x is normally a mixture of ethanol and water. Onlyif �SLw � �SVw ½ �LVw (72Ð8 mN m�1 (or mJ m�2))(where superscript w signifies water) will wet soil instan-taneously. In all other cases instantaneous wetting is notexpected but after a time delay ranging from a few sec-onds to a few hours the soils may be wetted. For this tohappen there must be a convergence such that

�SLw � �SVw D �LVw �3�

This can be achieved by:

(1) Diffusion of surfactant material from soil to water toreduce �LVw.

(2) Adsorption of water at the soil organic interfacebecause of the high water vapour pressure next tothe droplet causing an increase in hydration and anincrease in �SLw � �SVw.

(3) The proximity of water and water vapour causinga change in the surface arrangement of organicmolecules to increase �SLw � �SVw.

(4) Any combination of the above.

Process (1) is unlikely to be important because waterpassed through, and wetting, a column of soil wasshown not to have a shorter WDPT on fresh soil(Llewellyn et al., 2004). It seems likely that the majorchanges with time involve �SLw and �SVw (with �SLw

being the dominant term because of the much lowerconcentration of molecules in a gas). The question is howcan measurement of �cx using ethanol water mixturesbe related to the rate of change of �SLw � �SVw fromthe initial condition to the condition at which the soilbecomes wettable; i.e. how can �cx be related to WDPT?If we make the assumption that the major effect of addingethanol to water is a change in �LV rather than a changein either �SL or �SV; i.e. the major effect of ethanol is todisrupt hydrogen bonding in water rather than changingthe energy of either the surface-liquid or surface-vapourinteractions from those with water alone, then we cansay,

�cx D �SLix � �SVi

x D �SLiw � �SVi

w �4�

and use �cx as a measure of the initial state of the surfacewith respect to water.

From the thermodynamic formulation of transitionstate theory the relationship between the free energy ofactivation, G‡ and the rate is given by Equation (5)(Laidler, 1987; Laidler and Meiser, 1982):

rate / EXP�-G‡/kT� �5�

where T is temperature and k the Boltzmann constant.G‡ can be considered as a sum of terms and for a seriesof reactions in which only one term changes, e.g. for our

case the surface free energy of activation G‡s, we can

separate out the changing term to give Equation (6),

rate D A.EXP�-G‡s/kT� �6�

where A is the rate for the reference process for which,by definition, G‡

s is zero. Since wetting by waterwill occur when �SL � �SV D �LVw then we mightexpect �LVw � �cx, to be related to G‡

s. The simplestapproach is a linear relationship:

G‡s D f��LVw � �cx�/N �7�

where N is the number of molecules per unit area, and f isa constant relating the required change in surface energyto the overall change in energy as the soil shifts fromrepellent to wettable. The rate for the wetting of soil isgiven by 1/WDPT, and with Equations (6) and (7), andtaking logarithms, this gives

ln�1/WDPT� D lnA � [f ��LVw � �cx�/NkT] �8�

Here A is 1/WDPT for our reference process i.e. thatfor which �cx D �LVw which, with our experimental pro-tocols for determining �cx, means complete penetrationwithin 5 s. Relationships between kinetic and thermody-namic parameters are well known in chemistry wherethey are examples of more general Linear Free EnergyRelationships (Gould, 1959; Moore and Pearson, 1981).Such LFERs are usually written as:

log�k/kref� D �� 9�

where k is the kinetic parameter for the system; kref isthe kinetic parameter for a reference system/compound,� is the thermodynamic parameter and � is a constantreflecting the sensitivity of the kinetics to changes inthe thermodynamic parameter. This is of the same formas Equation (8), with k/kref D WDPT0/WDPT (whereWDPT0 is 1/A), � D �LVw � �cx, and � D f/NkT.

RESULTS AND DISCUSSION

Relationship between WDPT and �cx

Figure 1 shows a plot of ln(1/WDPT) against ��LVw ��cx�. The plot shows a good linear correlation (R2 D0Ð81) consistent with Equation (8). There is no systematicvariation in results from soils at 20 °C and those heated to105 °C. The slope of Figure 1 is given by f/NkT. We canestimate N from the number of molecules in a monolayer.Using a molecular area of 2Ð0 ð 10�19 m2 (Moore, 1972;Shaw, 1995) which is a typical value for a close packedlong chain carboxylic acid, then N D 5Ð0 ð 1018 m�2.This gives 1/NkT D 49 J�1 m2 and from the experimen-tal slope of �230 �š23� J�1 m2, f D 4Ð6�š0Ð46�. Thisvalue is interesting because, from a comparison of thenumber of nearest neighbours in bulk and at a surface,the surface free energy is ca 25% the bulk cohesiveenergy (Castellan, 1971), and so it seems that the sur-face free energy contribution to the activation energy for



a given soil is, to a reasonable approximation, the differ-ence between the cohesive energy of the molecular filmadsorbed on the soil and that of water.

Effect of solution composition on penetration time

To establish whether it is �LV or the nature ofthe substance added to reduce the surface tensionwhich determines Solution Droplet Penetration Time(SDPT), we measured SDPT20 for four soils of dif-ferent repellency characteristics (GK3-slightly repellent,GK2-strongly repellent, NL2-extremely repellent, NL3-extremely repellent; see Table I, based on the classifi-cation of Bisdom et al., 1993) using aqueous solutionsof short chain organic compounds all made up to give�LV D 60 mN m�1 (Mainwaring, 2004). For the lin-ear short chain alcohols examined (methanol, ethanol,propan-1-ol, butan-1-ol, pentan-1-ol), increasing chainlength made either no systematic difference to WDPT20

(GK2, NL2, NL3) or possibly decreased it slightly (GK3,GK2). Changing the functional group on a C3 chain froman alcohol (propan-1-ol) to a ketone (acetone) also hadno effect. Furthermore, in studies where the surface ten-sion of the solution droplet was varied by changing theconcentration of the organic component, application ofEquation (8) to data obtained using propan-1-ol gaveresults which are, within experimental error, the sameas those using ethanol (see below and Figure 2). Theseresults suggest that, for simple short chain compoundsat least, the molecular structure of the organic used toreduce �LV is not an important factor. (How far this con-clusion applies to other classes of compounds is uncer-tain. In a previous study (Mainwaring, 2004) it was foundthat a comparable solution of the aromatic compoundphenol gave a much shorter penetration time than thelinear short chain molecules used here.)

Increasing �LV from 72Ð8 (water) to 74Ð4 mN m�1

by addition of 1 mol dm�3 NaCl causes an increase inWDPT for these soils consistent with the increase in�LV. Use of an alkaline solution, 0Ð01 mol dm�3 NaOH,pH 12, �LV D 72Ð8 mN m�1, gave no change in WDPTwhich suggests that either ionisation of long chain acidsdoes not occur at the SL interface, or, if it does, it is notimportant in determining WDPT.

Effect of solution � on penetration time

In our interpretation, Figure 1 reflects the soil-to-soil variation in cohesive energy of the molecular filmadsorbed on the soil compared to that of water. Wecan use an alternative approach to introduce a changein this energy difference by working with a single soilbut varying the solution cohesive energy, i.e. use a rangeof solutions of differing ethanol, or propan-1-ol, contentand hence different LV surface tensions (�sol).

Following the previous argument we expect the rela-tionship between SDPT and �sol to be:

ln�1/SDPT� D ln A � [f��sol � �cx�/NkT] �10�

where �cx is taken as the interpolated solution surfacetension which gives a penetration time of 5 s when usingaqueous solutions of either ethanol or propan-1-ol asappropriate.

Figure 2 gives a plot of ln(1/SDPT) against (�sol ��cx), for penetration times using water/ethanol andwater/propan-1-ol mixtures of varying �sol for four dif-ferent soils: NL2, NL3, AU1 and AU3. The data show areasonably linear relationship with R2 D 0Ð80 for all datapoints, slope of 191 (š12) J�1 m2, and from this f D 3Ð9(š0Ð24). There are, however, differences in linearity fordifferent soils with data for AU1 showing excellent lin-earity while those for AU3 show some curvature, and,when considering the residuals as a whole, the over-all data show some curvature with increasing slope as(�sol � �cx) decreases, particularly at low (�sol � �cx).The reasons for this are not known, but it may be that, atthe relatively high concentrations of organics necessaryfor low �sol (for a solution with �sol D 40 mJ m�2 theethanol concentration is 17% by weight), there is disso-lution of adsorbed organics from the soil by the solutiondrop, or transfer of alcohol to the soil from the dropletvapour.

Interpretation and implications

In this analysis, �cx is determined by the cohesiveenergy of the organic film adsorbed on the soil, with a lowcohesive energy giving a low �cx; WDPT is determinedby the difference in cohesive energies between thisadsorbed film and water via Equation (8), with a largeenergy difference giving a long WDPT; and the soil-to-soil variations in both WDPT and �cx arise from thepresence of organic films of differing cohesive energieson the different soils. Three questions arise from this.

1. What chemical features are important in determiningthe cohesive energy of this film?The total amount of organic material itself is unlikely

to be of major significance unless this brings aboutchanges in intermolecular interactions. We have previ-ously shown that there is little correlation between eitherorganic mass by extraction or organic CH content byFTIR and WDPT for this soil set (Doerr et al., 2005a).

The cohesive energy will depend upon the natureof the compounds present, i.e. the functional groupsthey contain, their orientation, and the nature of theintermolecular forces between them. A low cohesiveenergy would be associated with non-polar materialswhere dispersion forces are the dominant intermolecularinteraction, while a high cohesive energy would beexpected for polar materials which have the possibility ofhydrogen bonding and dipole/dipole and dipole/induced-dipole interactions (Jaycock and Parfitt, 1981). Theintermolecular forces in this organic layer may showa high degree of orientation due to the presence ofthe soil/organic and organic/air interfaces, and somemolecular orientation with respect to these interfaceswould be expected. For example, a compound such as



a long chain acid, which has both polar and non-polarcharacter, would be expected to be orientated with thenon-polar chain extending out into the air interface.In this case, the potential high-energy interactions are‘locked in’ within the organic layer and the hydrocarbonchain is exposed at the surface so the film would beexpected to exhibit a low �cx suggesting a low cohesiveenergy in the interfacial region because what is initiallyexposed to the water droplet is a non-polar surface. Thereal situation is complicated by the presence of manypolar and non-polar compounds. How tightly these packtogether may also be important in determining how easythe layer as a whole can undergo the molecular motions,or hydration processes, necessary to shift from waterrepellent to wettable. It is known from emulsion sciencethat combinations of polar and non-polar compounds cangive strong well packed interfacial films (Shaw, 1995).

CST measures the initial �cx and the history of thesoil may be significant in influencing this. Exposureto organic solvents, water, dry air, or moist air, willall influence the orientation of intermolecular forcesin the organic film because they expose the layer toenvironments of different polarity. Elevated temperaturesand the presence of fluids will influence the rate at whichmolecular reorganisation within the film can occur. Thedegree and duration of exposure to these environmentalinfluences will determine both the equilibrium positionto which the system is moving, and how far the systemcan move towards this equilibrium position (e.g. Doerret al., 2005b) Thus we would expect a soil with anadsorbed organic layer not to have an inherent waterrepellency but rather the potential to display a rangeof repellencies, of narrower or wider range dependingupon composition, with the value of �cx at any one timereflecting the particular kinetically metastable state intowhich the organic materials adsorbed on its surface havebeen locked by the environmental history of the soil.2. What is the mechanism by which the interfacial film

is altered to allow wetting?It seems probable that this involves a change in struc-

ture of the organic film (Franco et al., 2000; Doerr et al.,2005a) to allow exposure of polar, and in particular,hydrogen bonding groups, to water at the surface, andpossibly hydration of polar groups within the film by pen-etrating water molecules. With this possible mechanismin mind the cohesive energy of the film, as determinedhere from �cx, may be regarded as some indicator ofthe combined magnitude and accessibility of polarity andhydrogen bonding potential of the organics present on thesoil. Figure 3 shows the free energy changes involved inwetting by water, together with a tentative reaction coor-dinate for the process.3. How applicable is this work to other soils of different

mineralogical characteristics?Wetting will depend upon the particle size, shape

and packing (McHale et al., 2005; 2007), since thiscontrols the contact area between solution drop and soiland the interstitial dimensions of the soil. It is alsoreasonable to assume that the cohesive energy of an

Figure 3. Reaction free energy profile for wetting by water of a mineralgrain with a coating of organics which include both non-polar andpolar groups, and schematic of suggested mechanism. In this tentativeinterpretation, the initial state, A, has some polar groups bound to thesoil grain surface, some polar groups bound together within the interfacialfilm, and non-polar groups at the organic/air (or water) interface. The stateof highest free energy, the kinetic transition state, B, has some of theinterfacial material in a relatively high energy molecular reorganisationinvolving the break-up of internally bound polar groups and the exposureof polar groups at the surface, and possibly some penetration of watermolecules. By state C, this process has progressed far enough for thechange in �SL to be sufficient to allow wetting of the organic surface.This is followed by penetration of water into both the interfacial materialand the interstices between mineral grains. To what extent the processis controlled by rearrangement of the interfacial material or penetrationof water into the interfacial material is unknown, nor is it known towhat extent water displaces bound polar organics at the soil grain/organicinterface, although wetting with water does not wash organic materialoff these soils. Note that the ‘movement’ of polar groups implied by thisschematic may well be as much an ‘unfolding’ of hydrogen bonding sitesto penetrating water as diffusion of molecular material through the organiclayer. State D shows the final situation in which water has penetrated theorganic later (for simplicity, polar groups other than those adsorbed atthe soil surface are not identified in this diagram since we have no reasonto suggest a specific location for them). The energy difference between

A and D is not known

adsorbed film will depend upon the mineral surface towhich it is adsorbed. However, provided these factorsare kept reasonably constant for a set of soils wewould expect the theory proposed here to be applicable.What is more difficult to assess is the behaviour ofsoils with a heterogeneous mineral composition and/ora wide distribution of particle size and/or adsorbedfilm compositions and/or thicknesses. Here, the wettingcharacteristic of each individual soil grain may differgreatly, and it is not certain that this would lead to someeasily understandable averaged kinetic behaviour.

CONCLUSIONS

A linear free energy relationship between SDPT andthe difference between the surface tension of the solu-tion droplet and CST has been derived. WDPT, SDPT(ethanol/water and propan-1-ol/water), and CST measure-ments for a group of sandy soils support this analysis.The data suggest that the surface free energy contributionto the free energy of activation is given by the differ-ence between the cohesive energy of the molecular filmadsorbed on the soil and that of the wetting solution.



In this interpretation: CST ��cx� is determined by thecohesive energy of the organic film adsorbed on thesoil; WDPT is determined by the difference in cohesiveenergies between this adsorbed film and water, with alarge energy difference giving a long WDPT; and soil-to-soil variations in both WDPT and �cx are due to organicfilms of different cohesive energies on the different soils.For aqueous solutions of short chain linear alcohols andketones, SDPT depends on the surface tension of thesolution used but is independent of the compound usedto control the surface tension.

An important implication arising from this work is thatsoil with an adsorbed organic layer is unlikely to have aninherent water repellency but rather the potential to dis-play a range of repellencies, of narrower or wider rangedepending upon composition, with the value of �cx at anyone time reflecting the particular kinetically metastablestate into which the organic materials adsorbed on itsparticle surfaces have been locked by the environmentalhistory of the soil.

ACKNOWLEDGMENTS

KAM would like to thank the University of WalesSwansea for a Postgraduate Studentship. SHD acknowl-edges funding from the NERC (Advanced FellowshipNER/J/S/2002/00662 and Grant NE/C003985/1). Wewould like to thank Aquatrols Corp. for financial support.

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