Partitioning of organic chemicals at the air-water interface in

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Environ. Sci. Technol. 1993, 27, 2174-2180

Partitioning of Organic Chemicals at the Air-Water Interface in Environmental Systems John T. Hoff,’vt Donald Mackay,* Robert Gillham,t and Wan Ying Shld

Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario, N2L 3G1 Canada, and Institute for Environmental Studies, Unlversity of Toronto, Toronto, Ontario, M5S 1A4 Canada It is suggested that partitioning of organic chemicals to the air-water interface can be significant and must be quantified in several situations of environmental interest including partitioning of chemicals into small air bubbles in water into small water drops in air (for example, fogs), and between the phases present in relatively dry soils. Experimental data are reported for interface-air partitioning of a variety of chemicals and are consolidated with previous data to show that interfacial partitioning is primarily controlled by the chemical’s hydrophobicity. Correlations are derived for interface-air and interfacewater coefficients from data for 44 polar and nonpolar chemicals. The environmental implications of interfacial partitioning are discussed, and methods are suggested for including the interface as a compartment in mass balance calculations involving air bubbles, water drops, and soils.

Introduction A common environmental calculation is that of the equilibrium partitioning of a chemical contaminant between different phases. Examples include attributing a total concentration in water to dissolved, sorbed, or biotic components; the calculation of gaseous and aerosolassociated concentrations in the atmosphere; and the estimation of chemical partitioning between organic and mineral matter, air, and water in soils. The usual approach is to apply mass balance and thermodynamic equilibrium expressions which relate concentrations in the various phases. In these calculations, quantities in air and water phases are expressed as the products of the respective concentrations and phase volumes. It is not usual to consider the interface as a separate compartment with a significant capacity for the chemical. There is considerable evidence that a significant quantity of the chemical can be present in this interfacial “compartment”, influencing chemical fate and effects. In this paper, we present and discuss evidence in support of the contention which was first advanced by Valsaraj (1, 2) and later Mackay et al. (3) that environmental situations exist in which interfacial partitioning must be considered, and we suggest an approach for correlating the extent of such partitioning. It is first useful to clarify how partitioning calculations can be extended to include interfaces. Mass Balance Calculations. When expressing partitioning between bulk phases, the simplest approach is to define the phases of volume V A (of air), V I , V Z ,..., Vi (m3). The concentrations are C A(in air), C1, Cp,...,ci (mol/

* Corresponding author. + University of Waterloo. 1 University

2174

of Toronto.

Environ. Sci. Technol., Vol. 27, No. 10,1993

m3), then the total amount of chemical M (mol) is

If the equilibrium partition coefficients KiA or C ~ / C A are known, they can be used to eliminate all the Ci values as follows

The concentration in air CA can then be determined. Often it is convenient to express the amount of solid phase as a mass rather than a volume, in which case the concentration is on a mass/mass basis, e.g., mg/kg, and the partition coefficient may have units of litedkilogram. An alternative, but ultimately equivalent, approach is to express equilibrium in terms of fugacity f (Pa), which is related linearly to concentration with a proportionality constant or Z value ( 4 ) .

ci (moi/m3) = zi(mol/rn3pa)f i (Pa)

(3)

The mass balance equation then becomes

M = C V i Z f =fZViZj

(4) from which the single value off and, hence, the various values of Ci can be deduced. These equations ignore any excess accumulation of the chemical at the air-water interface. Since no volume can be meaningfully assigned to the interface, volumetric concentrations cannot be defined. To treat the amount at the interface, it is preferable to assign an area A (m2) and express concentration c on an amount/area basis, i.e., mol/m2,so that the amount cA (mol) can be included with the CV terms. The partition coefficient [designated, for example, kIw (for interface/water)] would then have dimensions such as (mol/m2)/(mol/m3)or m. The physical significance of kIw with this dimension of length is that kIw (m) is the depth of the bulk solvent phase (here water) which contains the same amount (mol) of chemical per unit area as the interface. This follows from the argument that an area of 1m2will contain kIwCw mol of the chemical, which is the same amount present in a volume of water 1m2 in area by kIw (m) deep. A different k can be defined for interface-air partitioning, i.e., k M , which can be interpreted similarly and must be related to kIw by the bulk-phase dimensionless partition coefficient KAW by (5)

In the fugacity formalism, iff is applied to the interface, the 2 value, designated z , has dimensions of mol/(m2.Pa) and c is equated to zf. Instead of calculating V Z , the term Az is used in the mass balance equation. Obviously, interfacial effects will be most pronounced when the chemical exhibits a strong tendency to partition 0013-936X/93/0927-2174$04.00/0

0 1993 American Chemical Society

to the interface and when the interfacial area is large compared to the volume of the air or water phase. This is equivalent to stating that the thickness or depth of the air or water phase is smaller than k u or kIw. Environmental Relevance. Three situations are of obvious relevance in this context. First, when small air bubbles rise through a column of water containing the dissolved chemical,the equilibrium amount at the interface will be comparable to that in the air phase if kIA is comparable to the bubble diameter. Thus, there may be enhanced stripping of the dissolved chemical beyond that expected from Henry’s law. This is the basis of the solvent sublation process (5-8). Also relevant is the “bubble microtome” effect in which small water droplets enriched in surface-accumulating material are generated when bubble skins burst at the surface (9). Second, partitioning of the chemical into water particles as occurs in fogs or mists will be enhanced if kIw is comparable to the droplet diameter. Enhanced partitioning into fog droplets has been observed by Glotfelty et al. (10, 11),Cape1 et al. (12), and Sagebiel(13) and has been discussed by Perona (14). The enhancement effect in fogs is not fully understood and is complicated by the possible presence of dissolved colloidal and particulate organic matter and surface active organic material. Interfacial partitioning has also been noted in headspace analysis by Drozd (15),who has shown that errors may be introduced if the amount at the interface is ignored. Finally, in soils which contain water films of thickness comparable to kIw, there may be appreciable interfacial effects. Pennell et al. (16) have recently shown that p-xylene adsorption at the air-water interface may, under certain conditions, account for up to 50% of the p-xylene present. Clearly, this effect must be included in the partitioning calculations inherent in models which purport to describe chemical fate in soils. Most models, for example, that of Jury et al. (17),assume equilibrium between bulk phases of air, water, and organic and mineral matter and ignore interfacial partitioning. As Pennell et al. (16) have demonstrated, a multimechanistic approach is needed to incorporate all sorption and dissolution mechanisms, especially when the soil is fairly dry and the water films are thin. However, if the water films are too thin, highly sorptive clay mineral sites may become exposed, a t least partially, to the air thereby further enhancing the soil’s sorptive capacity. Interpretation of chemical behavior under these conditions is difficult because of uncertainty about the physical regime which prevails. Laboratory Studies of Interfacial Partitioning. The most convincing evidence that partitioning to airwater interfaces is real is the careful studies of Karger et al. (18, 19) and Hartkopf and Karger (20), who have measured partitioning to the air-water interface by a gas chromatographic retention time method in which interfacial concentrations are measured using a solid support such as Chromosorb coated with water. Similar studies have been undertaken by Okamura and Sawyer (21)with moistened soil and siliceous materials and by Dorris and Gray (22)for normal alkane sorption on macroporous silica. In this study, we describe and report on the acquisition of further experimental data for partitioning of a variety of chemicals at the interface and then consolidate these with previously reported data. Correlations are then sought between the bulk air-interface or bulk water-

interface partition coefficients and readily available physical-chemical properties. The physical significance of the correlations is discussed, and recommendations are made for estimation methods. Finally, the implications of these data are discussed for describing chemical partitioning in the three cases of small water drops in air, small air bubbles in water, and partitioning in the vadose zone. Experimental Section The values of k u and KwA were determined for a variety of compounds by gas chromatographic retention time using the methods of Karger and co-workers (18-20). These references give more complete accounts of the theory. Briefly, it is assumed that organic vapors are retained by a column of water-coated Chromosorb P by partitioning into the bulk water phase and to the air-water interface; other mechanisms being neglected. When the solute is present a t very low concentration, Henry’s law is obeyed, and the net retention volume per gram of sorbent, VN (cm3/g), is given by where AI (cmZ/g)and VW(cm3/g)are the surface area and volume of water per gram of sorbent, respectively, and KWAis the water-air partition coefficient. Mass transfer occurs rapidly in the column (in seconds), so that the measured value of VN is the equilibrium value. Experimentally, VN is equal to KcVM, where Kc is the dimensionless capacity factor and VM(cm3/g) is the volume of gas phase per gram of sorbent. The capacity factor is ( T R - TM)/TM,where TRand TMare respectively the retention times of the solute peak and an unretained tracer such as methane gas. By measuring TR and T M at different water volumes VWfor a variety of chemicals, it is possible to deduce ku and KWA. Mixtures of solute vapors and methane gas were injected with a 10-pLsyringe onto a 1m X 6 mm (i.d.) glass column at 25 f 0.5 “C containing approximately 10 g of watercoated Chromosorb P. The carrier gas was water-saturated nitrogen at 25 cm3/min resulting in a pressure drop of about 20 kPa and a gas residence time of about 1 min. Solute mole fractions at the column outlet were on the order of lo4; thus, near-infinite dilution was achieved. Correspondence with Henry’s law was confirmed by the narrow, symmetrical peaks and the insensitivity of VN to the amount of solute injected. Injections were repeated at least three times for each solute. Net retention volumes were measured for at least three different specific water volumes, Vw, ranging from 0.00654 to 0.320 cm3water per gram of sorbent. The column packings were prepared by adding measured amounts of organic-freewater to calcined Chromosorb P. The packings were stored in sealed glass and Teflon vials for several days to ensure equilibration before packing the column by gentle suction and vibration. VW was also determined by weighing the packed glass column before and after measuring the net retention volumes, and again after the column was dried in the GC oven. No measurable loss of water occurred, and the net retention volumes did not change during the measurement period of about 5 h. The net retention volume was calculated as outlined above using the measured retention times for solute and Environ. Sci. Technol., Vol. 27,

No. 10, 1993 2175

Table I. Experimental Values of h~and KWAat 25 “C, Their Standard Errors (SE), and Number of Determinations ( N ) with Data from Ref 20 for Comparison kM

compound

(pm)

pentane hexane heptane decane isooctane cyclohexane benzene toluene ethyl benzene isopropyl benzene chlorobenzene m-dichlorobenzene dichloromethane trichloromethane tetrachloromethane l,l,l-trichloroethane perchloroethene trichloroethene 1-bromobutane 1,2-dichloroethane ethyl ether methyl formate ethyl acetate acetone

0.052 0.109 0.233 2.26 0.287 0.107 0.443 1.12

2.35 4.12 1.23 2.72 0.186 0.347 0.149 0.292 0.327 0.264 0.850 0.691 5.32 1.93 27.5 18.3

SE

KWA (dim.) SE

0.000

0.000 0.000 0.00

0.001 0.006 0.001 4.17 0.00 3.16 0.01 0.01 0.00 6.49 0.01 8.38 0.001 10.1 0.001 6.28 0.000 0.897 0.004 1.32 0.003 1.55 C.000 2.61 0.002 1.15 0.002 20.6 0.005 0.02 101 0.3 0.1 659

0.12 0.48 0.43 0.92 0.1

0.17 0.036 0.05 0.04 0.07 0.40 0.5 4

34

N

h a KWA’ (rm) (dim.)

5 0.055 5 0.123 7 0.233 7 2.59 4 0.327 4 7 0.550 7 1.23 7 2.68 7 7 1.24

5.19 5.04 4.04 7.12

7 7

0.269 11.0 7 0.429 6.92 7 0.189 0.936 4 4 7 7

7 0.912 18.5 3 3 3 3

Data from ref 20.

methane. VM was calculated in two ways: (i) from the mass and density of the sorbent and the volume of the empty column between the injector and the detector and (ii) from the retention time for methane, the carrier gas flow rate (as measured by soap film meter), and the pressure drop (as measured by a water manometer). The results agreed. The specificsurface area, AI,was calculated from the net retention volume for heptane by assuming that partitioning in bulk water is negligible (as can be demonstrated by using the literature value for KWAand by using the value of ~ I for A 25 “C of 0.233 X lo4 cm calculated from the data of Karger et al.). The specific surface areas ranged from 0.96 to 3.30 m2/gfor the seven specific water volumes. The water film thicknesses thus exceeded those necessary to obtain the thermodynamic properties of the air-water interface. The values of ku and K ~ were A calculated by regressing VRIVW versus AIIVw as suggested by eq 6.

k ~ wvalues obtained by gas chromatography (18-22) generally agree with those obtained by surface tension measurements (23-29). A single value O f k1A was selected for each chemical and, from reported values of KWAor KAW(generally obtained by equilibrium measurements), KIW was deduced. Also included for later correlation purposes are the vapor pressure of the solutes P (Pa) expressed as the corresponding saturation concentration = P / R T (mol/m3)and the saturation in the gas phase concentration in water, Csw, deduced as csA/KAw. cswis thus a deduced solubility or “pseudo-solubility” and is observed to be similar to the reported solubility of the liquid except for the more soluble compounds. Since no solid solutes were treated, no “melting point correction” was needed. Also of interest are the following: the estimated infinite dilution activity coefficient yw of the chemical in water deduced from l/(Cswuw), where uw is the molar volume of water (cm3/mol);the solute molar volume u,; and the solute-air and Bolute-water interfacial tensions, QA and usw (dyne/cm), respectively. Correlation of Interfacial Partition Coefficients. When seeking a correlation for ~ I or A k ~ w it , is useful to calculate a hypothetical solubility or saturation concentration at the interface as kIACSAor klwCSw, which are equal because kIWIku and csA/csW both equal KAW. Inspection of the data, especially of the series of alkanes and aromatics, shows that CS1is unrelated to but is weakly related to CSw.For example, pentane and isopropyl benzene have similar values of CS1and of Csw, but the values differ by a factor of over 100. As the molecular weight of the alkanes increases, CSw decreases by about 0.7 log unit per carbon added, while CS1decreases more slowly by about 0.2 log unit per carbon added. Successful correlation is thus most likely with solubility, activity coefficient in water or with the related octanol-water partition coefficient as discussed by Valsaraj (1, 2). Correlation between partition coefficients must be sought with care because, for example, ku can be viewed as CsI/ CSAand some correlation is inevitable with KWA, which is csW/csA, because of the wide variation in @A.We conclude that CS1 or k ~ wis best correlated with CSw. Figure 1is a plot of log kIw versus log CSw.A regression was obtained, ignoring the obvious outliers indicated, as log ItIw = -8.58 - 0.769 log Csw

(7)

Results

Since k ~ wis CsI/Csw, this can be rewritten as

The experimental ku andKWAvalues and their standard errors are given in Table I. The residual mean square errors for the regressions correspond with those obtained by error propagation calculations using estimated errors for the quantities in eq 6, suggesting that the model provides an adequate representation of the data. The relative errors for KWAare generally larger than those for k u because experimental conditions were designed such that partitioning into bulk water was usually small relative to interfacial adsorption. The experimental KWAvalues for alkanes and several other compounds are not reported, because of the wide confidence limits. Table I shows that A KWA values are generally slightly, the experimental ~ I and i.e., up to 139% ,smaller than those calculated from Karger et ala’sexperimental free energies and enthalpies. Table I1 is a compilation of selected ~ I Adata obtained in this study supplemented by values reported by others.

The implication is that, as the solubility of a substance in water increases, the solubility at the interface also increases, but the relationship is much less than linear. The result is that sparingly soluble chemicals exhibit large kw values primarily because their CSwvalues are low while their CSI values remain fairly large. From a mechanistic viewpoint, we can speculate that the hydrophobic effect (which can be viewed as being induced by forcing water to adopt an unfavored structure around the area of the nonpolar molecule) is also present at the interface, but is reduced in magnitude because only pa.rt of the molecule at the surface is exposed to water. The assertion that hydrophobicity is the key determinant of partitioning at the interface is supported by studies of the surface tension of solutions of homologous series of

2176

Envlron. Sci. Technol., Vol. 27, No. 10, lQQ3

~~

Table 11. Compilation of Data at 25 OC Used To Develop Correlations for k ~ w log Csw (calc.)" log CSW (meas.)/ log u# log C s A USW USA log km a log log (mol/cm3) (cmS/mol) (erg/cm2) (erg/cm2) (mol/cm3) compound case (cm) KAW kIw (cm) (mol/cm3) 1 2

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 e

pentane perfluorohexane hexane heptane octane nonane decane 2-methyl heptane 2,4-dimethyl heptane isooctane cyclohexane cycloheptane cyclooctane benzene toluene ethyl benzene isopropyl benzene fluorobenzene chlorobenzene m-dichlorobenzene dichloromethane trichloromethane tetrachloromethane l,l,l-trichloroethane perchloroethene trichloroethane 1-chlorobutane 1-bromobutane 1,2-dichloroethane ethyl ether n-propyl ether methyl formate ethyl formate ethyl acetate acetone

-5.273 -5.142 -4.935 -4.633 -4.267 -3.971 -3.615 -4.413 -4.414 -4.513 -4.971 -4.564 -4.201 -4.304 -3.930 -3.600 -3.385 -4.259 -3.908 -3.566 -4.643 -4.411 -4.772 -4.535 -4.485 -4.578 -4.347 -4.071 -4.096 -3.274 -2.813 -3.570 -4.231 -2.561 -2.738

1.699

-3.574

1.835 1.966 2.082 2.301 2.468 2.180

-3.100 -2.667 -2.185 -1.670 -1.147 -2.233

2.123 0.860 -0.588 0.616 -0.648 -0.562 -0.447 -0.228 -0.474 -0.829 -0.819 -0.979 -0.815 -0.093 0.053 -0.033 -0.401 -0.140 -0.061 -1.314 -1.280 -0.663 -1.979 -0.548 -2.340 -2.818

-2.390 -4.111 -3.975 -3.585 -4.952 -4.492 -4.047 -3.613 -4.733 -4.737 -4.385 -5.622 -5.226 -4.465 -4.482 -4.519 -4.980 -4.487 -4.132 -5.410 -4.554 -3.476 -5.549 -4.779 -4.901 -5.556

-4.560 -4.928 -5.089 -5.607 -6.159 -6.730 -7.303 -5.959 -5.788 -5.577 -5.281 -5.932 -6.523 -5.291 -5.813 -6.292 -6.607 -5.386 -6.190 -7.009 -4.631 -4.975 -5.213 -5.185 -6.000

-5.398 -5.261 -5.654 -5.349 -4.539 -5.424 -4.499 -4.883 -5.293 -4.907

-6.258

-6.273

-6.924 -7.574 -8.241 -9.031 -9.771 -8.139

-6.958 -7.534 -8.238 -8.766 -9.437 -8.159 -8.603 -7.670 -6.185 -6.515 -7.152 -4.642 -5.253 -5.844 -6.381 -4.798 -5.367 -6.088 -3.808 -4.163 -5.292 -5.262 -6.074 -5.077 -4.925 -5.353 -4.056 -3.089 -4.319 -2.417 -2.798 -3.038 misc.

-7.700 -6.141 -6.520 -7.139 -4.643 -5.250 -5.845 -6.379 -4.912 -5.362 -6.189 -3.652 -4.160 -5.119 -5.237 -5.967 -4.997 -5.121 -5.593 -4.035 -3.259 -4.761 -2.520 -4.335 -2.953 -2.089

2.064 2.305 2.119 2.168 2.211 2.254 2.292 2.214 2.212 2.220 2.036 2.083 2.127 1.951 2.028 2.090 2.147 1.973 2.010 2.059 1.809 1.906 1.987 2.001 2.004 1.955 2.019 2.031 1.899 2.020 2.142 1.788 1.948 1.990 1.866

16.01

49.00

18.40 20.14 21.62 22.85 23.83

51.10 50.20 50.80 50.75 51.20

25.24

50.20

29.84 28.72 28.52 29.29 28.21 27.26 33.59 36.01 27.84 27.32 27.04 25.80 32.33 29.50 23.74 26.46 32.57 17.10 20.51 25.15 23.84 23.97 24.02

35.00 36.10 38.40 37.40 28.30 31.60 45.00 47.50

10.70 17.90 6.80

a Data from Table I and ref 23. Data from Table I and refs 20,43, 48,49, and 53. K m = KUKAW. Data from refs 44,50, and 55-57. CSw = CSA/KAW.f Data from refs 45-47,5+52, 54, 55. gh Data from ref 56. Data from ref 40.

alcohols and acids, in which it is observed that the surface tension reduction is fairly constant per methylene group (30,31).The free energy group contributions for individual parts of molecules,which are so successfullyapplied toward estimation of octanol-water partition coefficient and solubility, also apply to interfacial tension (32). Correlation for Polar Compounds. As illustrated in Figure 1, this simple correlation fails for more polar molecules such as esters and ketones in that k ~ w is larger by an order of magnitude than expected. This is important because many chemicals of environmental concern such as phenols and pesticides are polar. There is extensive literature on the adsorption of polar compounds at the air-water interface in which k ~ wis expressed in terms of the reduction of surface tension induced by the polar solute (33-35). Valsaraj (I, 2 ) developed a correlation for this phenomenon using this general approach. However, his correlation differs from the one developed here. A theoretical treatment as presented by Everett (36, 37) and Eon and Guiochon (38) leads to a suggested ideal expression for k ~ was follows klw = (uw/aw)E(~w/~I) exp(as(awA - asA)/RT) - 11 (9) where uw and aw are the molar volume and surface area of water, as is the molar area of the solute, OWA and USA are the surface tensions of the water and the liquid solute, and yw and 71 are the activity coefficients in the bulk water and at the interface. Using this expression as a basis, taking logarithms, ignoring the term -1 which is

usually negligible, gives a modified expression in which 71 is separated as a fittable parameter: log k,, = log (uwlaw) + log yw + US(UWA - L T ~ A ) / ~ . ~-O log ~R 71T(IO) aw and as can be calculated from the molar volumes assuming the molecules to be spherical, i.e., a is 8.45 X 107 ~ 2 1 3 .The value of UWA is 72 dynlcm or erg/cm2, and the corresponding R is 8.314 X lo7 erg/mol K. The solutewater surface tension data in Table I1 suggest that the polar solutes exhibit large values of IZIW because of their low solute-water interfacial tensions, resulting in a low surface activity coefficient. It thus seems appropriate to correlate 71in terms of ~ w It.is now preferable to express the solute's bulk phase properties in water as an activity coefficient,yw, rather than a solubility, Csw, because many polar solutes are miscible with water. It is found that a satisfactory correlation is obtained by log yI = 1.35asas,/2.303RT The final correlation is then log

= -7.508

(11)

+ log y w + u ~ ( u W A - USA -

1.35~~sw)/2.303RT (12)

Figure 2 shows the fit between observed and correlated k ~ w .Clearly this correlation is preferred for polar solutes which display amphiphilic behavior at the interface, i.e., they are able to adsorb with the polar functional group Environ. Sci. Technol., Vol. 27, No. 10, lSS3 2177

Y 1

pl

-4

35 II

0

28

_I

/ 22

/

-6 -10

-9

-8

-6

-7

Log

CCS,/ m o l

-5

-4

-3

14

34

34

-5

-2

-6

i -5

-4

-3

-2

-1

CALCULATED Log (Klw/cm)

/cm3)

Flgure 1. Plot of log klwversus log PW, the numbers referring to the chemicals In Table I1 and the ilne corresponding to eq 7.

Flgure 2. Plot of log klwexperimental versus log kIwcalculated by eq 12, the numbers referring to the chemicals in Table 11.

oriented toward the water phase and thus have larger kIw values. Equation 11is analogous in form to the regular solution theory model for the bulk activity coefficient (39) where the cohesive energy densities for interaction between like and unlike molecules are replaced with the energies of cohesion and adhesion (40). It is concluded that for polar or amphiphilic chemicals, i.e., those for which the solubility exceeds approximately lo4 mol/cm3 or 0.1 mol/L or the solute-water interfacial tension is less than 30 dyn/cm, eq 12 is preferred. For less soluble, nonpolar chemicals eqs 7 or 8 are adequate and are simpler because no surface tension data are required. When seeking such data, it is useful to note that Girifalco and Good (40) evaluated the interfacial tensions of a large number of water-organic solute systems in terms of the quantity, @, defined as

solute at the interface to that in the air phase at equilibrium is

@ = (as,

+

- asw)/2(asAb&'A)0'5

(13)

The following average @ values were calculated from their data compilation: 0.55, aliphatic hydrocarbons; 0.70, aromatic hydrocarbons; 0.72, halogenated hydrocarbons; 1.0, oxygen-containingcompounds (organicacids, alcohols, aldehydes, ketones, esters, and ethers). It is thus possible to use either measured values of usw in eq 12 or calculated average values for the class of compounds. Environmental Implications

Air Bubbles and Water Drops. For aspherical bubble of air in water of diameter d, the ratio of the quantity of 2178 Envlron. Scl. Technol.. Vol. 27. No. 10, 1993

rd2CI/W3CA/6)= ( ~ / ~ ) ( C I / C=AkIA/(d/6) ) (14) For a chemical such as nonane, k~ is approximately lo4 cm, thus there will be equalquantities at the interface and in the bubble when the diameter is 0.0006 cm. For ethyl acetate the corresponding diameter is 0.016 cm. Smaller bubbles will have larger amounts at the interface. Solutes which display the most pronounced partitioning to the interface from bubbles tend to be those of low vapor pressure and low solubility in water, especially those of polar character. This phenomenon may be of environmental significance in situations when small bubbles are dispersed through water columns, as occurs in breaking waves and in turbulent waters (41).The solvent sublation process referred to earlier is based on this phenomenon. When making measurements of air-water partition coefficient by stripping techniques, it is unwise to create bubbles which are so small that interfacial partitioning becomes appreciable. For a spherical drop of water in air, the corresponding ratio of quantity at the interface to that in the water is is approximately 0.02 cm; thus k1w/(d/6). For nonane k ~ w a droplet of diameter 0.12 cm will have equal amounts in both conditions. The effect is most pronounced for sparingly soluble solutes such as the alkanes. Of particular interest are fog drops, as discussed earlier, in which pesticides have been observed to display high enhancement factors, i.e., overall concentrations which exceed that expected from Henry's law. Recently Sagebiel (13) has determined that solutes of solubility less than about 1 mol/m3 show enhancement in fogs while phenolic wood smoke components which have solubilities in the range of

lo5,

I

One approach is to deduce the amounts (mol) in the soil at a prevailing fugacity f as follows. bulk air phase bulk water phase organic phase dry mineral surface sorption air-water interface

I

IO’

I

I

I

I

I

I

1

The organic phase 2 value 20can be estimated from the organic carbon partition coefficient or the octanol-water partition coefficient Kow. Obviously it is unlikely that bulk water and dry mineral surfaces can coexist. Inspection of these quantities demonstrates that the quantity at the air-water interface is potentially significant when there is a distinct but thin water film on the solid surface (Le. thicker than 5-10 molecules (17) but less than k ~ w )when , the areaA1is large, and especially when the organic carbon content is low. Experimental data which demonstrate this general behavior have been obtained by workers such as Ong and Lyon (42)and Pennell et al. (16).Using this approach it is relatively easy to deduce the partition coefficients. For example a soil to air partition coefficient Kd (mL/g) as measured by Ong and Lyon can be shown to be where Cls has mass units such as microgram per gram, Cs and CA have units of microgram per liter, and p is the density of the wet soil solids (g/mL). The volumetric concentration in the wet soil Cs is given by: f(VwZw + Vozo + AMzM + AIzJ/(Vw + VM) = f(uwZw + uozo + aMzM + ~ 1 ~ 1(19) ) where VMis the volume of soil solids, uw and uo are volume fractions of water and organic matter, and U M and U I are dry mineral surface area and air-water interfacial area per cubic meter of wet soil solids. Since CAis Zd, the final expression for Kd is Kd

(Uwzw + uozo

+ ~ I ~ I ) ( P Z A )(20)

~MZM

The advantage of this approach is that the contributions of each bulk phase and interface are separated and become obvious. Analogous expressions involving partition coefficients become quite complex and more difficult to interpret. The interfacial Zvalues can be calculated simply from the correlations for k ~ w .This paper has treated only equilibrium conditions, but as Mackay et al. (3) have discussed, it is likely that accumulation at the interface will affect mass transport rates. Conclusions The experimental data discussed here lend support to Valsaraj’s pioneering assertion (1, 2 ) that in several situations of environmental relevance, partitioning to the air-water interface can be appreciable and must be taken into account, especially when treating small bubbles of air in water, small droplets of water in air, and relatively dry, low organic content soils. Correlations have been suggested for estimating interface-water and interface-air partition coefficientsand equivalently interfacial Zvalues. Methods of including interfacial partitioning into environmental fate calculations have been suggested, and it is hoped that Environ. Sci. Technoi., Vol. 27, No. 10, 1993 2170

these methods will be exploited when deducing the environmental fate of organic chemicals.

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Received for review January 26, 1993. Revised manuscript received April 27, 1993. Accepted April 27, 1993.