CVEN 5424 Environmental Organic Chemistry Lecture 7 Aqueous Solubility and Air-Water Exchange Equilibrium

Announcements Reading  Chapter 5, Aqueous Solubility  Chapter 6, Air-Water Partition Problem sets PS 3 due Thursday Office hours – semester Tuesday 11:30 am-1:00 pm (?; see updates from Joe) Wednesday 9-10 am

Aqueous Solubility  Effect of temperature  Effect of salinity  Effect of co-solvents  Effect of organic liquid mixtures Na +

Aqueous Solubility  Temperature dependence  liquid  small  w H E  small temperature effect  solid  small  w H E, large  fus H  large temperature effect  gas  small  w H E, large  cond H  large temperature effect

Aqueous Solubility  Gas  C w sat increases with decreasing temperature  Liquid  not much   Solid  C w sat decreases with decreasing temperature

Aqueous Solubility  Liquid  slope is  w H E /R over T range of interest  slope very small  slope not constant   w H E is very small, sometimes +, sometimes – (see Table 5.3)

Aqueous Solubility  Solid  slope is  w H/R = (  fus H +  w H E )/R  Usually, enthalpy of fusion is greater than the excess enthalpy of solution:  fus H >  w H E

Aqueous Solubility  Gas  slope is  w H/R = (-  vap H +  w H E )/R  Usually, enthalpy of condensation (-  vap H) is greater than the excess enthalpy of solution: -  vap H >  w H E

Aqueous Solubility  Salt in water  “electrostriction”  water forms hydration shells around ions  less water available for compound to dissolve into  “Salting out”  decreases solubility of nonpolar organic compounds Na +

Aqueous Solubility  Setschenow equation  Setschenow constant K S  compound-specific (increases with decreasing C w sat )  salt-specific (increases with increasing ion hydration)  total molar salt concentration [salt] t  single salt or mixture of salts (e.g., seawater) (Eqn. 5-27)

Aqueous Solubility  Example: pyrene “salting out” of seawater  C w sat (25  C) = M  K S = 0.30 (Table 5.7 for seawater)  [salt] T  0.5 M  {[cations (M)] + [anions (M)]}/2

Aqueous Solubility  Example: pyrene  seawater reduces solubility by about 29%

Aqueous Solubility  Co-solvents in water  high solubility organic compounds  replace water in cavity around nonpolar organic compound  Increase solubility  more favorable interactions with nonpolar organic compound pyrene in pure water pyrene in 20% methanol solution

Aqueous Solubility  Estimate solubility in co-solvent mixture   il activity coefficient in co-solvent/water mix   iw activity coefficient in pure water   i c “co-solvency power” (depends on f v )  fractional volume of co-solvent in water f v (Eqn. 5-33)

Aqueous Solubility  Example (see Illustrative Example 5.5)  What is the solubility of naphthalene in a 20% acetone/water solution at 298 K?  activity coefficient of naphthalene (solid)

Aqueous Solubility  Example  activity coefficient of naphthalene (solid)  C w sat = M  V w = L mol -1  vapor pressure ratio

Aqueous Solubility  Example  activity coefficient of naphthalene (solid)  vapor pressure ratio   = 0 (no SP3, no SP2, 1 RING)   = 4 (2 planes of rotational symmetry)  T m = 80.2  C = K

Aqueous Solubility  Example  activity coefficient of naphthalene (solid)   i c = 6.5 (Table 5.8)  f v =  increase in solubility

Aqueous Solubility  Example  solubility: naphthalene in 20% acetone/water

Aqueous Solubility  Organic liquid mixtures  petroleum – gasoline, oil, kerosene  coal tar  PCBs – Arochlor

Aqueous Solubility  Organic liquid mixtures  petroleum – gasoline, oil, kerosene  coal tar  PCBs – Arochlor

Aqueous Solubility  Solubility of an organic liquid  assumptions  x L  1 (essentially no water in organic phase)   L = 1 (pure liquid; ideal interactions) L w

Aqueous Solubility  Solubility of an organic liquid mixture  assumptions  x L is the mole fraction of the compound of interest   L  1 (not a pure liquid, but nearly ideal interactions) L w

Aqueous Solubility  Solubility of an organic liquid mixture  assumptions  x L is the mole fraction of the compound of interest   L  1 (not a pure liquid, but nearly ideal interactions) L w

 Coal tar  BTEX  PAHs

Aqueous Solubility  Organic liquid mixtures   org mix  1 (to 5)  x org mix  need average mw of organic liquid mixture  e.g., coal tar 150 g mol -1  no melting costs  compound is already in liquid phase in organic mixture

Aqueous Solubility  Organic liquid mixtures example: What concentration of benzene should we find in water in equilibrium with gasoline containing benzene at a concentration of 1 vol%?

Aqueous Solubility  Benzene in water in equilibrium with gasoline containing 1 vol% benzene?  Need estimates for and

Aqueous Solubility  Benzene in water in equilibrium with gasoline containing 1 vol% benzene?  activity coefficient of benzene in gasoline,  mole fraction of benzene in gasoline,

Aqueous Solubility  Benzene in water in equilibrium with gasoline containing 1 vol% benzene?  activity coefficient of benzene in water,

Aqueous Solubility  Benzene in water in equilibrium with gasoline containing 1 vol% benzene? benzene MCL: 5  g L -1

Next Lecture  Henry’s Law Constant Air-Water Exchange Kinetics  Read Chapters 6 and 18, 19, and 20