AIR WATER PARTITIONING: HENRY S LAW

Transcription

1 AIR WATER PARTITIOIG: ERY S LAW The transfer beteen the atmosphere and bodes of ater s one of the key processes affectn the transport of many oranc compounds n the envronment (1). For neutral compounds, at dlute soluton concentratons n pure ater, the ar-ater dstrbuton rato s referred to as the enry s La constant (or a ). For real aqueous solutons (.e., solutons that contan many other chemcal speces), e use the term ar-ater dstrbuton rato hch, for practcal purposes, e approxmate by the enry s La constant. The enry s La constant can be approxmated as the rato of a compound s abundance n the as phase to that n the aqueous phase at equlbrum. X (aq) X () P atm.l.mol -1 here P s the partal pressure of the as phase of the chemcal and s ts molar concentraton n ater. ote that enry s La constant has been defned n other terms, hch can ntroduce confuson and cauton should be used to check the unts. ( a ) s defned n terms of a untless rato of concentratons. Tabulated values have been reported n unts of Pa M -1 and atm m 3 mol -1. Some texts defne the reacton n the opposte drecton, n hch case the unts appear as M atm -1. Gven that for most oranc solutes of moderate or sparn solublty, the actvty coeffcent at nfnte dluton n ater s rouhly equvalent to that n a urated soluton, γ ~ γ and e can approxmate by. P here P o s for the lqud state (subcooled or superheated for solds and ases, respectvely). Snce, e can also rte; γ 1 o 2O o 2O γ P (L) {pressure volume mols-1 } It ll also be convenent for us to express the enry s La constant as a dmensonless a value. It can be shon that a (dmensonless) RT here the unts used for R the unversal as constant are consstent th the unts for (.e., f s ven n atm L mol -1, then use R L atm mol -1-1 ; f s ven n SI unts of Pa m 3 mol -1, then use R Pa m 3 mol -1-1.

2 Althouh both P o (L) and decrease th molecular sze, the ater solublty s more senstve to sze ncreases due to the reater enery costs assocated th cavty formaton n ater. For a seres of structurally related compounds, the mantude of a ncreases th sze (.e., molar volume or TSA). Ths s true because molecules that have smlar functonal roups ll experence the same type of nter-molecular nteractons. Increasn the sze thn the same functonal roup class, ll result n a systematc ncrease n the dspersve forces. As can be seen n the fure belo, alcohols have a much loer a than alkanes due to the more favourable solute:solvent -bondn nteractons. For molecules of smlar sze, alkenes are more polarzable than alkanes hch results n more favourable solute:solvent nduced dpole dspersve nteractons. Monopolar molecules such as ethers, ester, ketones and aldehdyes experence dpole dpole nteractons and -acceptn nteractons th the solvent, ater. If has a relatvely lare value for a partcular compound, t means t has a reater tendency to escape from the ater phase and enter the atmosphere. onsder the pestcdes DDT and Atrazne both of hch are very nonvolatle th P 1.3 x atm and P 8.9 x atm at 25, respectvely. Althouh Atrazne s about 7 tmes more volatle than DDT, t has much loer tendency to partton nto the atmosphere from ater. The for atrazne s 6.2 x 10-6 atm.m -1 hereas that for DDT s 9.5 x 10-3 atm.m -1 (both at 25 ). Ths s a drect result of the very lo ater solublty of DDT ( 1.4 x 10-8 M) as compared to 1.5 x 10-4 M for Atrazne. As a result, DDT has been observed to be despread n the envronment at least n part due to atmospherc transport. Atrazne, on the other hand, tends to partton from ar to ater and s dstrbuted by ater movement rather than ar currents. l 3 l l l DDT Atrazne

3 Effect of Temperature Snce, and e have; lnp ln o Δ R e ( Δ Δ ) vap Δ R e s + constant T + constant T Δ vap s enry ln + constant + constant R T R T The same result s obtaned for aseous and sold compounds. The temperature dependence of can be predcted th a knolede of Δ vap and Δ e s. In eneral, the heat of vapourzaton of an oranc lqud s postve and ncreases th molecular sze and ncreasn polarty. The excess heat of soluton (Δ e s ) s enerally smaller. ence, the effect of temperature on the mantude of s smlar to that on the vapour pressure of the lqud compound. In other ords, the dependence of the vapour pressure domnates over that of t s ater solublty and enerally decreases th decreasn temperature. The fure belo shos a comparson of the observed temperature dependence of the enry s La constant for trchloroethylene th that predcted from the temperature varatons of vapor pressure and solublty. Salnty The presence of salts n the aqueous phase ll decrease the ater solublty of oranc solutes (ncrease the γ ), but obvously ll have no affect on the vapour pressure of the pure lqud. ence, values ll ncrease th ncreasn salnty. salt s [salt] tot 10 Where s s the salt constant specfc for a ven oranc compound. Recall the effect of salts on the ater solublty; For seaater use [salt] ~ 0.5 M salt 10 - s [salt] tot

4 Methods of Determnn alues Expermental methods of determnn are usually based on analytcal measurements of equlbrated ar-ater samples n a closed contaner. Expressn the fracton of solute n the as phase as the rato; fracton n as phase + here s the volume of the as phase and s the volume of the aqueous phase. Substtutn / yelds; fracton n as phase Thus, f one determnes the fracton of solute n the as phase analytcally, can be calculated th knolede of the volumes of the aqueous and as phases. onversely, f one knos the value of, t s possble to calculate the fracton of solute n the as and aqueous phases. values can be dffcult to measure drectly, especally hen s very lo. onsequently, a number of estmaton technques have been proposed. A number of lnear relatonshps beteen the of a seres of structurally related compounds and ntrnsc propertes such as total surface area or boln ponts have been met th varyn success. Another approach s to sum the contrbutons of structural unts thn a molecule to quanttatvely predct the propertes of the molecule as a hole. One such method consders the contrbuton of each bond type and s surprsnly smply and as ntroduced by ne and Mookerjee. The dea s that a specfc structural unt ll ncrease or decrease a compound s by the same amount reardless of the actual compound. The contrbuton of each bond type s summed over the entre molecule such that; lo Σ a here a s the number of subunts of a partcular type and f s the contrbutn factor for the partcular subunt as refned and summarzed by Meylan and oard n the table belo. f For example; the estmated value of usn ths approach for dethylether s; lo 10(-) + 2(-O) + 2(-) lo 10(0.119) + 2(-1.09) + 2(-0.116) to convert ths to (n atm M -1 ) use; x RT ( L atm mol -1-1 ) (298 ) 1.47 atm M -1 Ths compares reasonably ell th the reported expermental value of 1.62 atm M -1 ven the level of approxmatons nvolved. In eneral, these estmated values are thn a factor of ~ 2 tmes the expermental values.

5 (1) Envronmental Oranc hemstry, 2 nd Ed., R.P. Scharzenbach; P.M. Gschend; D.M. Imoden, J. Wley and Sons, 2004.