Chapter 15
Photochemistry in Aqueous Surface Layers: 1-Naphthol 1,2
1
Richard A. Larson and Stewart A. Rounds 1
Institute for Environmental Studies, University of Illinois, Urbana, IL 61801 Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, GA 30613
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2
i-Naphthol was reactive toward direct photolysis in buffered aqueous solutions (at pH 7, half-life in sunlight was about 90 min) and in cyclohexane (half -life about 15 min). The reaction rate in water increased with pH. The mechanisms of the principal photolysis pathways in both solvents did not involve singlet oxygen. In aqueous solution, the major ether-extractable product, lawsone, may have been formed by a radical process involving 1,2-naphthoquinone and superoxide as intermediates. In cyclo hexane, most of the photolysis products observed were very polar. Several oxygen-containing compounds derived from the solvent were also observed. In a two-phase system including cyclohexane and pH 7 buffer, the photolysis appeared to proceed by independent mechanisms in both phases, but intermediate polar products formed in the organic layer diffused into the aqueous layer. In a system containing a surface-active material, SDS, the rate of photolysis decreased; the photoproducts were typical of those observed in cyclohexane rather than water.
The f a c t t h a t t h e a i r - w a t e r i n t e r f a c e i s a n a r e a of u n u s u a l p r o p e r t i e s r e l a t i v e t o t h e u n d e r l y i n g w a t e r column has been recognized f o r many y e a r s . The s u r f a c e t e n s i o n o f p u r e w a t e r . f o r example, i s much g r e a t e r t h a n t h a t o f n a t u r a l w a t e r s c o n t a i n i n g o r g a n i c m a t t e r , even t h o u g h t h e bulk c o n c e n t r a t i o n of such m a t e r i a l s may be v e r y low (1-10 mg/1). I n f a c t t h e s u r f a c e t e n s i o n of many n a t u r a l w a t e r s i s v e r y close t o t h a t o f weakly a s s o c i a t e d l i q u i d s such as benzene. T h i s suggests t h a t much o f t h e o r g a n i c m a t t e r n o m i n a l l y d i s s o l v e d i n w a t e r must h a v e c o n s i d e r a b l e s u r f a c e - a c t i v e p r o p e r t i e s . M o l e c u l e s showing such a c t i v i t y a r e those where t h e r e i s a s t r o n g s e p a r a t i o n between p o l a r a n d n o n p o l a r r e g i o n s w i t h i n t h e molecular s t r u c t u r e ; a 0097-6156/87/0327-0206$06.00/0 © 1987 American Chemical Society
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Downloaded by COLUMBIA UNIV on March 8, 2013 | http://pubs.acs.org Publication Date: December 8, 1987 | doi: 10.1021/bk-1987-0327.ch015
15.
LARSON AND ROUNDS
Photochemistry in Aqueous Surface Layers
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simple example would be a long-chain f a t t y a c i d or a d e t e r g e n t s u l f o n a t e . Two t y p e s of s u r f a c e - a c t i v e molecules, wet and dry s u r f a c t a n t s , h a v e been d i s t i n g u i s h e d (1). V e t s u r f a c t a n t s are t h o s e i n w h i c h a l a r g e p o r t i o n of t h e molecule i s p h y s i c a l l y l o c a t e d i n t h e s u b s u r f a c e l a y e r and only a small f r a c t i o n of t h e s u r f a c t a n t p r o j e c t s above t h e s u r f a c e i n t o t h e a i r . Such molecules a r e n o r m a l l y of q u i t e l a r g e size and i n c l u d e p r o t e i n s , w h i c h t y p i c a l l y have a few h y d r o p h o b i c c h a i n s and a l a r g e r e g i o n s of h y d r o p h i l i c s t r u c t u r e . D r y s u r f a c t a n t s a r e those such as d e t e r g e n t s , i n w h i c h most of t h e molecule p r o j e c t s out of t h e w a t e r and only a small p o r t i o n i s a s s o c i a t e d w i t h i t . A l t h o u g h i n e a r l y s t u d i e s of s u r f a c e m i c r o l a y e r s , most a t t e n t i o n was g i v e n t o t h e d e t e r m i n a t i o n of d r y s u r f a c t a n t s , r e c e n t work has c o n f i r m e d t h a t most of t h e m a t e r i a l c o n s i s t s of polymeric, wet surfact a n t s . F o r example, r e c e n t work on s u r f a c e - a c t i v e m a t e r i a l s i n f r e s h w a t e r s i n d i c a t e s t h a t humic s u b s t a n c e s were t h e p r e d o m i n a n t s u r f a c e - a c t i v e m a t e r i a l s i n n e a r l y a l l samples. Only i n a few e x c e p t i o n a l cases was t h e s u r f a c e a c t i v i t y due p a r t l y t o t h e p r e s e n c e of d e t e r g e n t s (2). A n o t h e r e v e n t w h i c h w i l l lead t o a s u r f a c e f i l m on w a t e r i s a s p i l l of a h y d r o p h o b i c m a t e r i a l s u c h as petroleum. In t h i s case, i t is likely, at least i n a f r e s h spill, t h a t the surface layer w i l l be dominated by i n s o l u b l e m a t e r i a l ( a l i p h a t i c and a r o m a t i c hydrocarbons) r a t h e r t h a n s u r f a c e - a c t i v e material. M a n y h y d r o p h o b i c p o l l u t a n t s may t e n d t o become more concentrated m n a t u r a l l y o c c u r r i n g m i c r o l a y e r s , b u t because t h e y a r e t y p i c a l l y t h i n (
