Document 5LkjZyQ01qqR5rN1V5q4GeL6e
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AR226-2237
Solubility of C-8 in Water and General Effect of pH
T h e solubility of C -8 in water h a s b e e n studied an d published in the open literature by Shinoda (Shinoda, K., et al, J. Phys. Chem ., V 76, No. 6 (1972)). D a ta h ave b e e n collected at low tem peratures (< 10C ), w hich extend slightly a b o ve the Krafft point. T h e Krafft point is defined a s the tem perature at w hich the solubility of an ionic surfactant b ecom es equal to the Critical M icelle Concentration (C M C ), and is equal to 2 .5 C for C -8 . T h e C M C is eq ual to 0 .0 3 3 mole/liter. T h e s e d a ta a re shown below in tabular and graphical form.
0.6 1.58 3.4 4.24 5.01 6.73 8.38 9.9
0.0326 0.0339 0.0367 0.0438 0.0716 0.106 0.199 0.319
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14.0506 14.6109 15.8177 18.8778 30.8596 45.686 85.769 137.489
160.0
140.0
120.0
CO
36 100.0
>. 80.0
5 5Wo
60.0 40.0
20.0
0.0
Solubility of C-8 in H20 Temperature (C)
Points in the a b o ve graph represent Shin o d a d ata and the curve is sim ply a "best-fit" of th o se data.
It is important to note that C -8 in a q u e o u s solution will exist primarily a s ions according to the electrolyte dissociation reaction
NH4PFO = NH4+ + PFO'
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w here P F O rep resents the perfluoro-octanoate portion of the C -8 molecule. T h is dissociation will affect the volatility of C - 8 from a q u e o u s solutions.
Ions are not volatile from aqueous solutions within normal temperature ranges (at tem peratures le s s than the critical point of w ater). T herefo re, if the com ponent in question exists a s a n ion in solution, the com ponent will still be volatile, but the m ole fraction of com ponent able to volatilize will be the m ole fraction of the un dissociated m olecular sp e cie s in solution, not the total concentration. T h e mole fraction of un-dissociated compound can be calculated from the dissociation constant, and for the general reaction
Sait = Cation+ + Anion'
is expressed as
[C a tion *]* [Anion ~]
Ki ~
[Salt]
K, is a thermodynam ic property which is a function of the com ponents and
temperature.
F o r C-8 K) = 457 0 , so up to the point of saturation (w here a seco nd p h a se of un
dissociated C -8 e xists) only a very sm all fraction of the C -8 in a q u eo u s solution will be available for volatilization. F o r exam ple, according to the Shin o d a data at 1 ,58C, the solubility of C -8 is 0.0339 gmole/iiter. At this concentration, only
0 .0 0 0 7 % of the C -8 exists in m olecular form (NH4PFO) cap ab le of volatilization.
pH will also h ave an effect on the e x iste n c e of C -8 (a s the ammonium form of the compound) in aq u e o u s solution. If the pH d e c r e a s e s to le v e ls approximately <2.2, the formation of perfluoro-octanoic a d d will o c cu r according to the reaction
PFO' + H+= HPFO.
T h is reaction will prevent formation of the am m onium form of the com pound by removing P F O ' from the C -8 equilibrium reaction. L ik ew ise, if the pH in crease to le v els approxim ately >9.2, am monium io n s will de-protonate according to the reaction
NH4+ = NH3 + H+.
T h is reaction will prevent formation of the ammonium form of the compound by removing NH4+from the C -8 equilibrium reaction.
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