Phenomena in Mixed Surfactant Systems - ACS Publications

9 Monolayer Properties of Octadecyldimethylamine Oxide and Sodium Alkyl Sulfate 1

Downloaded by STANFORD UNIV GREEN LIBR on February 15, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0311.ch009

David L . Chang and Henri L . Rosano Department of Chemistry, City College, The City University of New York, New York, NY 10031

Monolayer properties of octadecyldimethylamine oxide alone and in combination wth sodium alkyl sulfate on aqueous substrate have been investigated. Nonionised amine oxide produces more expanded film than the ionised species; minimum film expansion and highest surface potential are obtained at half ionisation. Due to hydrogen ion competition, sodium octadecyl sulfate exhibits film expansion with increasing acidity of the substrate. Mixed films of these two compounds show marked contraction atallpHinvestigated. The sequence of film condensation of C18amine oxide/C SO Na association follows the order 1:1 (pH 10.9) < 1:1 (pH 2.2) < 2:1 (pH 5.5). C18amine oxide/C SO Na mixed monolayers s t i l l show film condensation at both pH 10.9 and pH 5.5, the former corresponds to an ion-dipole interaction, and the latter is an ion-ion interaction. Due to chain incompatibility, C18amine oxide/C SO Na form less tightly bound associations than C18amine oxide/C SO Na. Correlation between surface and bulk properties of these two classes of compounds are discussed. 18

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I n a p r e v i o u s p u b l i c a t i o n (V), r e s u l t s were p r e s e n t e d on t h e m i c e l l a r p r o p e r t i e s o f b i n a r y mixtures o f s u r f a c t a n t s o l u t i o n s cons i s t i n g o f a l k y l d i m e t h y l a m i n e o x i d e (C12 t o C i a a l k y l c h a i n s ) and sodium d o d e c y l s u l f a t e . I t was r e p o r t e d t h a t upon m i x i n g , s t r i k i n g a l t e r a t i o n i n p h y s i c a l p r o p e r t i e s was o b s e r v e d , most n o t a b l y i n t h e v i s c o s i t y , s u r f a c e t e n s i o n , and b u l k pH v a l u e s . These changes were a t t r i b u t e d t o 1) f o r m a t i o n o f e l o n g a t e d s t r u c t u r e s , 2) p r o t o n a t i o n o f amine o x i d e m o l e c u l e s , and 3) a d s o r p t i o n o f hydronium i o n s o n t h e mixed m i c e l l e s u r f a c e . In a d d i t i o n , p o s s i b l e s o l u b i l i s a t i o n o f a l e s s s o l u b l e 1:1 complex, form between t h e p r o t o n a t e d amine o x i d e and t h e l o n g c h a i n s u l f a t e was a l s o c o n s i d e r e d . 1

Current address: The Gillette Company, Gillette Park 6F-1, Boston, M A 02106. 0097-6156/86/0311-0116S06.00/0 © 1986 American Chemical Society

In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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Monolayer Properties

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In the p r e s e n t s t u d y , t h e monolayer t e c h n i q u e i s used t o i n v e s t i g a t e the s u r f a c e p r o p e r t i e s o f o c t a d e c y l d i m e t h y l a m i n e o x i d e and sodium o c t a d e c y l s u l f a t e , as s i n g l e component f i l m s and i n combination. The i n t e r p r e t a t i o n o f the r e s u l t s p r o v i d e s a d i r e c t u n d e r s t a n d i n g o f t h e mechanism o f i n t e r a c t i o n between t h e s e two surface a c t i v e agents. Goddard and Kung (2) s t u d i e d t h e s u r f a c e c h a r a c t e r i s t i c s of d o c o s y l d i m e t h y l a m i n e o x i d e a l o n e and i n mixed f i l m s w i t h n o n a d e c y l benzene s u l f o n a t e . The amine o x i d e s i n g l e component f i l m showed l a r g e v a r i a t i o n s w i t h the pH o f the s u b s t r a t e , but t h e mixed f i l m s d i d not r e v e a l e v i d e n c e of pronounced i n t e r a c t i o n . However, t h e s e a u t h o r s i n d i c a t e d t h a t a d i f f e r e n t i a l t h e r m a l a n a l y s i s s t u d y o f the s h o r t e r c h a i n homologs, d o d e c y l d i m e t h y l a m i n e o x i d e and sodium dodecylbenzene s u l f o n a t e , d i d show i n t e r a c t i o n between t h e s e components, i n a c c o r d w i t h r e s u l t s o b t a i n e d from s t u d i e s of aqueous s o l u t i o n c o n t a i n i n g s i m i l a r m a t e r i a l s (_3). They s u g g e s t e d t h a t t h e c h o i c e o f more s u i t a b l e s p r e a d i n g s o l v e n t s , t o e f f e c t more complete m i x i n g o f the components would e n a b l e the d e t e c t o n o f i n t e r a c t i o n between t h e s e two s p e c i e s . Experimental O c t a d e c y l d i m e t h y l a m i n e o x i d e (CieDAO) was a commercial sample from Onyx C h e m i c a l Company, J e r s e y C i t y , N. J . (25% a c t i v e ) . After e v a p o r a t i n g the s o l v e n t i n a r o t a r y e v a p o r a t o r under reduced p r e s s u r e , the crude p r o d u c t was r e c r y s t a l l i s e d s e v e r a l times from e t h y l acetate. The f i n a l p r o d u c t was d r i e d and s t o r e d i n vacuo over P2O5 Sodium o c t a d e c y l s u l f a t e (SODS) was a sample p r e p a r e d i n t h i s l a b o r a t o r y p r e v i o u s l y , and was r e c r y s t a l l i s e d from e t h a n o l b e f o r e use. Sodium d o d e c y l s u l f a t e (SDS) was o b t a i n e d from A l d r i c h C h e m i c a l Company, and was o f 98% p u r i t y . I t was f u r t h e r p u r i f i e d by r e p e a t e d c r y s t a l l i s a t i o n from e t h a n o l f o l l o w e d by e t h e r e x t r a c t i o n . Benzene and methanol were g o l d - l a b e l r e a g e n t grade, purchased from A l d r i c h C h e m i c a l Company (Metuchen, N. J . ) . The s p r e a d i n g s o l u t i o n s were p r e p a r e d as f o l l o w s : CieDAO was d i s s o l v e d i n 45/5, t h e SODS i n 25/25, t h e SDS i n 40/10, benzene/ methanoImmixture. The c o n c e n t r a t i o n s o f t h e s e s o l u t i o n s were 1.157xl0~ M. F o r mixed monolayers, m i x t u r e s a t d i f f e r e n t volume r a t i o were made from s t o c k s o l u t i o n s p r i o r t o s p r e a d i n g . The aqueous s u b s t r a t e s used were u n b u f f e r e d , and the experiments were performed i n the p r e s e n c e o f a t m o s p h e r i c C 0 . Deionised-distilled water was used t o p r e p a r e t h e s u b s t r a t e . The e x p e r i m e n t a l a p p a r a t u s employed was d e s c r i b e d elsewhere (4). B r i e f l y , s u r f a c e p r e s s u r e s were determined c o n t i n u o u s l y from s u r f a c e t e n s i o n measurements u s i n g a sand b l a s t e d p l a t i n u m b l a d e , suspended from a t r a n s d u c e r - a m p l i f i e r (Sandborn, model 311A). The t r a n s d u c e r s i g n a l was f e d t o a X-Y r e c o r d e r . S u r f a c e p o t e n t i a l was measured w i t h an e l e c t r o m e t e r ( K e i t h l y , model 61OB) u s i n g an a i r e l e c t r o d e coated with R a and an Ag/AgCl e l e c t r o d e i n the subsolution. The s u r f a c e o f the s u b s o l u t i o n was c l e a n e d by f i r s t d u s t i n g i t w i t h T a l c powder and t h e n sweeping t h e s u r f a c e by moving a T e f l o n s l i d e from one s i d e of the t r o u g h t o the o t h e r . The T a l c powder a l o n g w i t h t h e i m p u r i t i e s ( i f any) were t h e n removed u s i n g 3

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s u c t i o n from an a s p i r a t o r . T h i s was done s e v e r a l times to make s u r e no i m p u r i t i e s remained on the s u r f a c e . The monolayers were s p r e a d e v e n l y on the s u r f a c e u s i n g an A g l a m i c r o s y r i n g e . A time i n t e r v a l o f t h r e e minutes was a l l o w e d f o r s p r e a d i n g s o l v e n t to e v a p o r a t e from the monolayers. Three t o f i v e monolayers of each s o l u t i o n were made, and the r e s u l t s r e p o r t e d a r e the average s u r f a c e p r e s s u r e f o r each a r e a p l o t t e d .


Results S i n g l e Component Systems. S u r f a c e p r e s s u r e - a r e a (ττ-Α) and s u r f a c e p o t e n t i a l - a r e a (Δν-Α) i s o t h e r m s o f CieDAO on 0.01M NaCl s u b s o l u t i o n a t v a r i o u s pH v a l u e s a r e shown i n F i g u r e 1, (the NA0H/HC1 was p o s t added. At the h i g h e s t pH i n v e s t i g a t e d , v i z . ; pH 10.9, the amine o x i d e i s i n the n o n i o n i c form and g i v e s r i s e t o the most expanded curve as w e l l as a lower c o l l a p s e p r e s s u r e . R e d u c t i o n i n b u l k pH r e s u l t s i n c o n t r a c t i o n s of the f i l m . At pH 2.2, p r o t o n a t e d amine o x i d e predominates and r e s u l t s i n a l e s s expanded c u r v e . However, the l e a s t expanded f i l m i s o b t a i n e d a t an i n t e r m e d i a t e b u l k pH v a l u e , v i z . ; pH 5.5. The f i l m i s e x p e c t e d t o be composed of b o t h n o n i o n i c and c a t i o n i c forms of the amine o x i d e m o l e c u l e s a t a p p r o x i m a t e l y a 1:1 mole r a t i o , as suggested by the a c i d - b a s e t i t r a t i o n b e h a v i o u r of i t s C 4 homolog (I). These r e s u l t s a r e i n agreement w i t h the mono l a y e r p r o p e r t i e s o f d o c o s y l d i m e t h y l a m i n e o x i d e r e p o r t e d by Goddard and Kung ( 2 ) ; however i n t h e i r case the amine o x i d e a l s o shows a phase t r a n s i t i o n from expanded t o the condensed s t a t e s , and t h a t the f i l m c o n d e n s a t i o n upon i o n i s a t i o n i s more pronounced than i n the case o f CieDAO. The s u r f a c e p o t e n t i a l s of amine o x i d e f i l m s v a r y s t r o n g l y w i t h the b u l k pH o f the s u b s t r a t e , and hence the c a t i o n i c / n o n i o n i c r a t i o of the c o n s t i t u e n t m o l e c u l e s i n the f i l m . With a f o r m a l p o s i t i v e charge on the n i t r o g e n i n the c a t i o n i c s p e c i e s , i t i s expected t h a t the c a t i o n i c f i l m has the h i g h e s t p o t e n t i a l . How e v e r , the mixed f i l m o f c a t i o n i c and n o n i o n i c amine o x i d e s e x h i b i t s an even h i g h e r p o t e n t i a l t h a n the c o m p l e t e l y i o n i s e d f i l m , w h i l e the n o n i o n i c s p e c i e s (pH 10.9) shows the lowest p o t e n t i a l . This b e h a v i o u r i s a l s o seen w i t h the C homolog which show a maximum i n s u r f a c e p o t e n t i a l i n the v i c i n i t y of pH 5.6 under i d e n t i c a l e x p e r i mental c o n d i t i o n s . 2 2

In F i g u r e 2 the π-A and AV-A p l o t s f o r SODS on 0.01M NaCl subs o l u t i o n s h a v i n g d i f f e r e n t pH v a l u e s a r e shown. In a l l c a s e s , phase t r a n s i t i o n s from l i q u i d - e x p a n d e d to l i q u i d - c o n d e n s e d s t a t e a r e e v i d e n t ( 5 ) . A c i d i f i c a t i o n of the s u b s o l u t i o n i n c r e a s e s the t r a n s i t i o n p r e s s u r e but the t r a n s i t i o n i s l e s s pronounced a t the lowest pH s t u d i e d . T h i s i s a l s o accompanied by an e x p a n s i o n of the condensed p a r t of the c u r v e . Small negative surface p o t e n t i a l s are observed over most a r e a s . The h i g h e s t p o t e n t i a l i s o b t a i n e d f o r f i l m spread on the pH 2.2 s u b s o l u t i o n . F o r s m a l l a r e a s , the s u r face p o t e n t i a l a t t a i n s a p o s i t i v e value. T h i s may be r e l a t e d t o r e o r i e n t a t i o n of the d i p o l e moments of the m o l e c u l e s which o c c u r once a t h r e s h o l d s u r f a c e c o n c e n t r a t i o n i s exceeded (6). Mingins and P e t h i c a Ç7) s t u d i e d the monolayer p r o p e r t i e s of SODS on v a r i o u s sodium c h l o r i d e s o l u t i o n s (0.1, 0.01 and 0.001M) a t 9.5°C, and they showed t h a t the monolayer i s o n l y s t a b l e on the more c o n c e n t r a t e d s a l t s o l u t i o n s (0.1 and 0.01M). In t h i s work, no n o t i c e a b l e




CHANG A N D ROSANO

F i g u r e 1. E f f e c t o f pH on π-Α and AV-A i s o t h e r m s o f o c t a d e c y l dimethylamine o x i d e (CieDAO) a t 25°C, 0.01M NaCl s u b s o l u t i o n s (NaOH o r H C l / N a C l ) . S u b s t r a t e pH:1-10.9, 2-5.5, 3-2.2.




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F i g u r e 2. E f f e c t o f pH on ττ-A and Δν-Α i s o t h e r m s o f sodium o c t a d e c y l s u l f a t e (SODS) a t 25°C, 0.01M NaCl s u b s o l u t i o n s (NaOH o r HCl/NaCl). S u b s t r a t e pH: 1-10.9, 2-5.5, 3-2.2.


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d i f f e r e n c e i n b o t h the π and Δν d i f f e r e n t experimental runs.

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r e s u l t s were o b s e r v e d

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Mixed Component Systems. C^gDAO/SODS. These systems were examined at t h r e e s u b s o l u t i o n pH v a l u e s , namely 10.9, 5.5, and 2.2 (0.01M N a C l ) , and v a r i o u s c o m p o s i t i o n s . The compression i s o t h e r m s a r e shown i n F i g u e r s 3-5, r e s p e c t i v e l y , and f o r comparison, i s o t h e r m o f t h e CieDAO i s a l s o p l o t t e d . At h i g h pH v a l u e , i n t e r a c t i o n between the two components i s q u i t e n o t i c e a b l e . A d d i t i o n o f SODS produces a condensing e f f e c t on the f i l m ( w i t h r e s p e c t t o C D A 0 f i l m i n a l l c a s e s , and i n most c a s e s w i t h r e s p e c t t o SODS f i l m ) . The phase t r a n s i t i o n which i s c h a r a c t e r i s t i c o f the SODS c u r v e , i s s t i l l e v i dent a t a l l m i x i n g r a t i o s . Reduction i n t r a n s i t i o n pressure i s o b s e r v e d f o r the mixed f i l m s w i t h i n c r e a s i n g SODS c o n t e n t up t o the e q u i m o l a r m i x t u r e , which shows t h e l o w e s t t r a n s i t i o n p r e s s u r e , t h e r e a f t e r the v a l u e i n c r e a s e s when t h e amount of SODS i n the f i l m becomes e x c e s s . In terms o f t h e Ci DA0/S0DS molar r a t i o , the 4:1 and 3:1 mixed f i l m s have h i g h e r c o l l a p s e p r e s s u r e s t h a n the C-^gDAO f i l m , but lower t h a n t h a t o f t h e SODS f i l m . The 2:1 and 1:1 mix t u r e s form s o l i d f i l m s , and t h e i s o t h e r m s become i d e n t i c a l a t h i g h p r e s s u r e and s m a l l a r e a , b u t t h e e q u i m o l a r m i x t u r e shows the most condensed f i l m f o r l a r g e a r e a s . Mixed f i l m s c o n t a i n i n g e x c e s s SODS show t h e h i g h e s t c o l l a p s e p r e s s u r e s , s u g g e s t i n g t h e s e f i l m s a r e quite stable. S u r f a c e p o t e n t i a l s o f the mixed f i l m s g e n e r a l l y f a l l between the v a l u e s o f t h o s e o f the pure component f i l m s . However, f o r s m a l l a r e a s , m i x t u r e s c o n t a i n i n g e x c e s s amine o x i d e show an even h i g h e r p o t e n t i a l than t h e pure C^gDAO f i l m , p r o b a b l y due t o c l o s e r p a c k i n g o f the m o l e c u l e s .


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Mixed monolayers on t h e pH 5.5 s u b s o l u t i o n ( F i g u r e 4) show pronounced c o n d e n s a t i o n , i n d i c a t i n g v e r y s t r o n g i n t e r a c t i o n among t h e components; t h e s e i n c l u d e b o t h the c a t i o n i c and n o n i o n i c forms of amine o x i d e , as w e l l as t h e l o n g c h a i n s u l f a t e . The most con densed c u r v e i s observed f o r the 2:1 amine oxide/SODS m i x t u r e , c l o s e l y f o l l o w e d by the e q u i m o l a r m i x t u r e , and a t h i g h p r e s s u r e s t h e two i s o t h e r m s become i d e n t i c a l . U n l i k e the c a s e a t h i g h pH s u b s o l u t i o n , no phase t r a n s i t i o n was d e t e c t e d i n t h e p r e s e n t c a s e . S u r f a c e p o t e n t i a l s of the mixed f i l m s f a l l between t h o s e of the amine o x i d e and SODS a t c o n s t a n t a r e a . However, t h e 4:1 and 3:1 f i l m s have s u r f a c e p o t e n t i a l v a l u e s t h a t a r e s l i g h t l y h i g h e r than the pure amine o x i d e f i l m a t s m a l l a r e a s . A t the l o w e s t pH v a l u e o f the s u b s o l u t i o n i n v e s t i g a t e d , v i z . ; pH 2.2, the amine o x i d e m o l e c u l e s a r e p r o t o n a t e d , t h e r e f o r e l a r g e i n t e r a c t i o n i s e x p e c t e d w i t h an a n i o n i c s u b s t a n c e , such as the l o n g chain sulfate. Such an i n t e r a c t i o n i s r e f l e c t e d i n t h e monolayer b e h a v i o u r o f the mixed f i l m , as shown i n F i g u r e 5. The mixed f i l m s a r e a l l more condensed t h a n t h e pure component f i l m s , w i t h the e q u i m o l a r m i x t u r e e x h i b i t i n g the most condensed c u r v e . Phase t r a n s i t i o n s a r e s t i l l d e t e c t e d i n some c a s e s when t h e SODS c o n t e n t i s i n e x c e s s , but w i t h lower t r a n s i t i o n p r e s s u r e t h a n the SODS s i n g l e component f i l m , and t h e t r a n s i t i o n p r e s s u r e i n c r e a s e s w i t h i n c r e a s i n g amount of the a n i o n i c c o n s t i t u e n t . A w e l l d e f i n e d f i l m c o l l a p s e i s o b s e r v e d o n l y when the amine o x i d e i s i n l a r g e e x c e s s (4:1 and 3:1), a l l o t h e r s become i n c o m p r e s s i b l e a t h i g h p r e s s u r e s . The s u r f a c e p o t e n t i a l s a r e s i m i l a r t o t h o s e d i s c u s s e d p r e v i o u s l y .




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F i g u r e 3. π-Α and AV-A i s o t h e r m s o f mixed f i l m s o f CieDAO and SODS on pH 10.9 (NaOH), 0.01M NaCl s u b s o l u t i o n a t 25°C. CieDAO/SDS r a t i o : 1-4:1, 2-3:1, 4-1:1, 5-1:2, 6-1:3, 7-1:4.



F i g u r e 4. ïï-A and AV-A i s o t h e r m s o f mixed f i l m s o f QeDAO and SODS on pH 5.5 (HC1), 0.01M NaCl s u b s o l u t i o n a t 25°C. CieDAO/SODS r a t i o : 1-4:1, 2-3:1, 3-2:1, 4-1:1, 5-1:2, 6-1:3, 7-1:4.



F i g u r e 5. π-Α and Δν-Α i s o t h e r m s o f mixed f i l m s o f C i D A O and SODS on pH 2.2 (HC1, 0.01M NaCl s u b s o l u t i o n a t 25°C. Ci DAO/SODS r a t i o : 1-4:1, 2-3:1, 3-2:1, 4-1:1, 5-1:2, 6-1:3, 7-1:4. 8

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Cj DAO/DSD. M i x t u r e s o f CieDAO/SDS were spread on s u b s o l u t i o n s of pH 10.9 (NaOH) and pH 5.5 (HC1), and 0.01M NaCl. S i n c e SDS i s h i g h l y s o l u b l e i n water, and hence has a h i g h d e s o r p t i o n r a t e , compressions were made a t a f a s t e r speed. The r e s u l t i n g c u r v e s were f i t t e d such t h a t the v e r t i c a l p o r t i o n of each curve c o r r e sponds t o 20 â / m o l e c u l e , which a p p r o x i m a t e s the l i m i t i n g v a l u e of an a l k y l c h a i n , i f the o r i g i n a l c u r v e showed a v a l u e s m a l l e r than that. F i g u r e s 6 and 7 show the f i t t e d i s o t h e r m s f o r pH 5.5 and pH 10.9, r e s p e c t i v e l y . At low pH, i t a p p e a r s t h a t the a d d i t i o n o f SDS causes c o n t r a c t i o n o f the f i l m , and t h a t t r a n s i t i o n i n f i l m type i s evident f o r a l l mixtures. I t a l s o a p p e a r s t h a t the r e l a t i v e degree o f f i l m c o n t r a c t i o n i n c r e a s e s w i t h i n c r e a s i n g amount o f SDS i n the mixed f i l m . However, w i t h e x c e s s SDS i n the m i x t u r e , the a p p a r e n t i n c r e a s e i s p r o b a b l y caused by the d e s o r p t i o n o f SDS. At h i g h pH, t h e mixed i s o t h e r m s a r e s i m i l a r t o t h o s e o b t a i n e d a t low pH, except t h a t the t r a n s i t i o n i n f i l m type i s e v i d e n t o n l y when the amount o f SDS i s e q u a l t o o r g r e a t e r t h a n the amount o f amine o x i d e , and t h a t t h e r e seems t o be a more s y s t e m a t i c i n c r e a s e i n f i l m c o n t r a c t i o n w i t h i n c r e a s i n g amount of SDS i n the mixed f i l m . I n t e r a c t i o n between the two components i s d e f i n i t e , s i n c e SDS a l o n e d i d not r e v e a l a r e a s o n a b l e c o m p r e s s i o n p a t t e r n under the p r e s e n t e x p e r i mental c o n d i t i o n s . a


2

Discussion Ci DA0 Films. The u n u s u a l f e a t u r e o f a I k y I d i m e t h y l a m i n e o x i d e i s t h a t the i o n i s e d form y i e l d s a l e s s expanded f i l m t h a t the noni o n i s e d s p e c i e s , c o n t r a r y t o the e x p e c t a t i o n t h a t the p r e s e n c e o f charged head groups s h o u l d r e s u l t i n a more expanded f i l m , due t o e l e c t r i c a l r e p u l s i o n among the c h a r g e s . This p a r t i c u l a r characteri s t i c o f amine o x i d e f i l m s has been a t t r i b u t e d (2) t o the a b i l i t y o f the hydroxy group i n i o n i s e d amine o x i d e t o hydrogen bond. This l e a d s t o a r e d u c t i o n i n r e p u l s i v e f o r c e s among the head groups, g i v i n g r i s e t o the c o n d e n s i n g e f f e c t upon i o n i s a t i o n . At an i n t e r mediate pH, v i z . ; pH 5.5, the appearance o f c a t i o n i c s p e c i e s can cause the c o n d e n s i n g e f f e c t by a s s o c i a t i n g w i t h the n o n i o n i c s p e c i e s , s i n c e the i o n i s a b l e p r o t o n i s c a p a b l e o f hydrogen bonding t o the n e i g h b o u r i n g oxygen t h e r e b y d e c r e a s i n g the average d i s t a n c e between a d j a c e n t head groups e f f e c t i v e l y . In the b u l k s o l u t i o n , t h i s e f f e c t m a n i f e s t s i t s e l f i n an i n c r e a s e i n the v i s c o s i t y i f the s o l u t i o n c o n c e n t r a t i o n i s s u f f i c i e n t l y h i g h . From the a c i d base t i t r a t i o n s t u d i e s o f the C i and C i * homologs ( 1 ) , i t was r e p o r t e d t h a t the appearance o f c a t i o n i c s p e c i e s l e a d s to a subs t a n t i a l i n c r e a s e i n the s o l u t i o n v i s c o s i t y , r e a c h i n g a maximum value at h a l f i o n i s a t i o n . T h i s change i n v i s c o s i t y was e x p l a i n e d i n terms o f r e d u c t i o n o f r e p u l s i o n , and the f o r m a t i o n o f e l o n g a t e d micelles. B

2

The s u r f a c e p o t e n t i a l o f the n o n i o n i s e d s p e c i e s i s r e l a t i v e l y high f o r a nonionic s u r f a c t a n t . This i n d i c a t e s that there i s a s t r o n g d i p o l e i n the head group, which i s the p r i m a r y cause of exp a n s i o n o f the f i l m . A m i x t u r e o f c a t i o n i c and n o n i o n i c s p e c i e s has an even h i g h e r s u r f a c e p o t e n t i a l v a l u e , t h i s may be due t o , a t l e a s t i n p a r t , the c l o s e r p a c k i n g o f the m o l e c u l a r assembly i n t h e f i l m , consequent o f the d i m i n i s h i n g r e p u l s i o n i n the head group region.




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F i g u r e 6. π-Α i s o t h e r m s o f mixed f i l m s o f CieDAO and sodium d o d e c y l s u l f a t e (SDS) on pH 5.5 (HC1), 0.01M NaCl s u b s o l u t i o n s a t 25°C. Each c u r v e i s f i t t e d such t h a t the h i g h p r e s s u r e r e g i o n c o r r e s p o n d s 2 t o 20a i f the e x p e r i m e n t a l v a l u e preceded t h i s number. CieDAO/SDS r a t i o : 1-4:1, 2-3:1, 3-2:1, 4-1:1, 5-1:2, 6-1:3, 7-1:4.



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F i g u r e 7. ïï-A i s o t h e r m s o f mixed f i l m s o f CieDAO and SDS on pH 10.9 (NaOH). 0.01M NaCl s u b s o l u t i o n a t 25°C. Each c u r v e i s f i t t e d such t h a t t h e h i g h p r e s s u r e r e g i o n c o r r e s p o n d s t o 20a* i f t h e e x p e r i m e n t a l v a l u e p r e c e d e s t h i s number. CieDAO/SDS r a t i o : 1-4:1,

2-3:1, 3-2:1, 4-1:1, 5-1:2, 6-1:3, 7-1:4.



128


I t has been shown (8) t h a t the c a t i o n i c form o f amine o x i d e i s more s o l u b l e i n water t h a n the n o n i o n i s e d s p e c i e s , i t has a h i g h e r CMC v a l u e . More i m p o r t a n t l y , a C-13 nmr study of t h i s c l a s s o f compound (8) i n d i c a t e d t h a t t h e degree o f p e n e t r a t i o n o f water m o l e c u l e s i n t o the m i c e l l a r c o r e i n c r e a s e s w i t h i n c r e a s i n g c a t i o n i c c h a r a c t e r o f the m i c e l l e (up t o t h r e e carbons o f a f u l l y i o n i s e d m i c e l l e ) , r e s u l t i n g i n an u p f i e l d c h e m i c a l s h i f t of c a r b o n atoms near the head group r e g i o n w i t h i n c r e a s i n g degree o f p r o t o n a t i o n o f the m i c e l l e s u r f a c e . The c o n d e n s i n g e f f e c t on the monolayer upon i o n i s a t i o n can t h e r e f o r e be c o n s i d e r e d as a consequence o f the i n c r e a s e d s o l v a t i o n ; the m o l e c u l e s p e n e t r a t e i n t o the b u l k aqueous substrate. At an i n t e r m e d i a t e s u b s t r a t e pH, the p r o t o n a t e d molec u l e s a r e more s o l u b l e , t h e r e f o r e more a r e embedded i n the s u b s o l u t i o n , w h i l e the n o n i o n i s e d f r a c t i o n remains h i g h e r i n the s u r f a c e i n order to minimise p a r a f f i n - w a t e r i n t e r f a c i a l area. Under t h e s e c o n d i t i o n s , the monolayer can assume the l e a s t expanded c o n f i g u r a t i o n , due t o p o s s i b l e " s t a g g e r i n g " of the s u r f a c t a n t m o l e c u l e s , and t h i s s p a t i a l arrangement i s b e s t seen a t h a l f i o n i s a t i o n . Further a c i d i f i c a t i o n o f the aqueous s u b s t r a t e i n c r e a s e s the f r a c t i o n o f m o l e c u l e s i o n i s e d , but the c o n d e n s i n g e f f e c t due t o i n c r e a s e d s o l v a t i o n i s o f f s e t by an i n c r e a s e d e l e c t r i c a l r e p u l s i o n between n e i g h b o u r i n g charged head g r o u p s , r e s u l t i n g i n a s l i g h t e x p a n s i o n o f the monolayer. T h i s i n t e r p r e t a t i o n i s most c o n s i s t e n t w i t h b o t h the C-13 nmr and the monolayer r e s u l t s . SODS F i l m s . F o r an i o n i s e d monolayer, a r e l a t i v e l y l a r g e s u r f a c e p o t e n t i a l i s expected, negative i n s i g n f o r a long chain s u l f a t e monolayer. However, the p r e s e n t r e s u l t s show t h a t the s u r f a c e p o t e n t i a l i s s m a l l , i n d i c a t i n g t h a t the s u r f a c e i s p r o b a b l y not c o m p l e t e l y i o n i s e d , o r t h a t the c o u n t e r i o n s , a l t h o u g h d i s s o c i a t e d , a r e w i t h i n a v e r y c l o s e v i c i n i t y t o the p l a n e o f the n e g a t i v e l y charged i n t e r f a c e , as was s u g g e s t e d by the e l e c t r o m o t i v e f o r c e d e t e r m i n a t i o n o f the a p p a r e n t b i n d i n g o f c o u n t e r i o n s (Na i o n s ) on the sodium d o d e c y l s u l f a t e m i c e l l e s u r f a c e ( 9 ) , and on sodium t e t r a d e c y l s u l f a t e m i c e l l e s u r f a c e f r o m an e l e c t r o c h e m i c a l s t u d y ( 1 0 ) . +

The e x p a n s i o n o f the f i l m w i t h i n c r e a s i n g a c i d i t y of the subs t r a t e may be due t o the c o m p e t i t i o n o f c o u n t e r i o n s a t the i n t e r face. The swamping amount of H i o n s i n low pH s u b s o l u t i o n competes with N a i o n s a t the n e g a t i v e l y c h a r g e d i n t e r f a c e . Such comp e t i t i o n has been shown t o e x i s t between H* i o n s and K i o n s a t the n e g a t i v e l y charged m i c e l l e - s o l u t i o n i n t e r f a c e (11). S t u d i e s on t h e c o u n t e r i o n e f f e c t s i n sodium d o c o s y l s u l f a t e monolayers (6,12) have shown t h a t the f i l m e x p a n s i o n f o l l o w s the sequence L i > Na^~~> K+. It follows that H s h o u l d g i v e r i s e t o the most expanded f i l m . +

+

+

+

Mixed F i l m s . Isotherm d a t a from two-component monolayers a r e f r e q u e n t l y r e p r e s e n t e d by p l o t t i n g the mean m o l e c u l a r a r e a as a f u n c t i o n of f i l m composition a t constant s u r f a c e pressure. A linear r e l a t i o n s h i p i s u s u a l l y o b t a i n e d when the two components a r e immisc i b l e o r when they form an i d e a l t w o - d i m e n s i o n a l s o l u t i o n . For m i s c i b l e components, d e v i a t i o n s from i d e a l i t y r e s u l t i n a nonl i n e a r i t y i n the p l o t . P o s i t i v e d e v i a t i o n s i n d i c a t e an i n c r e a s e i n the a r e a o c c u p i e d by e i t h e r one o r b o t h components, p r o b a b l y due to



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129

a more r e p u l s i v e i n t e r a c t i o n i n n a t u r e , whereas n e g a t i v e d e v i a t i o n s are i n d i c a t i v e of condensation. F i g u r e 8 r e p r e s e n t s such a p l o t f o r the system CieDAO/SDS s t u d i e d on the pH 5.5 s u b s o l u t i o n . C l e a r l y t h e s e two components i n t e r a c t f a v o u r a b l y so as t o produce c o n d e n s a t i o n o f r e l a t i v e l y l a r g e magnitude. M i x t u r e s s t u d i e d a t pH 10.9 and pH 2.2 e x h i b i t t h e same f e a t u r e s . At l e a s t two f a c t o r s a r e i n v o l v e d i n the p r o c e s s : 1) c h a i n l e n g t h c o m p a t i b i l i t y and 2) head group i n t e r a c t i o n . The former i s r e s p o n s i b l e f o r the h y d r o p h o b i c i n t e r a c t i o n between the c h a i n s : c o m p a t i b i l e c h a i n s r e s u l t i n maximum c o h e s i v e i n t e r a c t i o n s between the a l k y l groups w h i l e i n c o m p a t i b l e c h a i n s have a d i s r u p t i v e e f f e c t on the p a c k i n g o f m o l e c u l e s i n t h e monolayer. S h i b a t a e t a l . (13) have shown t h a t t h e most pronounced c o n d e n s i n g e f f e c t i s u s u a l l y o b s e r v e d when the two components have e q u a l c h a i n l e n g t h , f o r a f i x e d t o t a l number of c a r b o n atoms. T h i s c o m p a t i b i l i t y i s i m p o r t a n t i n d e t e r m i n i n g the p r o p e r t i e s o f mixed m i c e l l a r s o l u t i o n s ( 1 ) , and i n the f o r m a t i o n o f m i c r o e m u l s i o n systems (14-16). The second f a c t o r , namely the head group i n t e r a c t i o n , can a l s o i n f l u e n c e t h e s u r f a c e p r o p e r t i e s o f mixed s u r f a c t a n t markedly. In p a r t i c u l a r , a n i o n i c / c a t i o n i c s u r f a c t a n t m i x t u r e s e x h i b i t the l a r g e s t e f f e c t (17,18). In n o n i o n i c / a n i o n i c s u r f a c t a n t m i x t u r e s , s y n e r g i s t i c e f f e c t s can s t i l l t a k e p l a c e t o a s i g n i f i c a n t e x t e n t , as r e v e a l e d i n F i g u r e 3 (pH 10.9, n o n i o n i c amine o x i d e w i t h a n i o n i c long c h a i n s u l f a t e ) , s i n c e i n s e r t i o n of n o n i o n i c s u r f a c t a n t m o l e c u l e s i n t o an i o n i c s u r f a c t a n t m o l e c u l a r assembly m i n i m i s e s e l e c t r o s t a t i c r e p u l s i o n (19). Except i n a v e r y a c i d i c medium i n which the amine o x i d e molec u l e s a r e p r o t o n a t e d by the e x c e s s p r o t o n s a l r e a d y p r e s e n t i n the b u l k s o l u t i o n , i t i s a l s o p o s s i b l e t h a t the s t r o n g i n t e r a c t i o n s between t h e s e two s p e c i e s i s a r e f l e x i o n of an i n d u c e d a c i d - b a s e e q u i l i b r i u m s h i f t o f the amine o x i d e i n t h e aqueous medium, brought about by the a d d i t i o n o f l o n g c h a i n s u l f a t e . As i s known from the Gouy-Chapman t h e o r y (20,21), a d i f f u s e l a y e r o f c o u n t e r - c a t i o n s b u i l d s up and a n i o n d e p l e t i o n i s e s t a b l i s h e d near a n e g a t i v e l y c h a r g e d s u r f a c e . One a d d i t i o n a l consequence o f t h i s n e g a t i v e s u r f a c e p o t e n t i a l i s t h e a c c u m u l a t i o n o f p r o t o n s a t t h e s u r f a c e and hence the s u r f a c e pH i s lower than t h e b u l k v a l u e . The p r e s e n c e of a l o n g c h a i n s u l f a t e i n an amine o x i d e m o l e c u l a r assembly can t h e r e f o r e s i g n i f i c a n t l y m o d i f y the a c i d - b a s e e q u i l i b r i u m s t a t e of amine o x i d e ; p r o d u c t i o n o f t h e p r o t o n a t e d s p e c i e s i s enhanced. T h i s i n t e r p r e t a t i o n i s c o n f i r m e d by the s o l u t i o n b e h a v i o u r o f C18DA0/SDS m i x t u r e s . S o l u t i o n s o f t h e s e two components have been shown t o be t u r b i d and biréfringent, and the a d d i t i o n o f SDS t o CieDAO r e s u l t s i n t h e p r o d u c t i o n o f f i l a m e n t - l i k e s t r u c t u r e s and an i n c r e a s e i n t h e b u l k pH v a l u e , s u g g e s t i n g the f o r m a t i o n of a new s p e c i e s between p r o t o n a t e d CieDAO and SDS, which i s a l s o r e s p o n s i b l e f o r the s u r f a c e t e n s i o n l o w e r i n g ( 1 ) . The i n c r e a s e i n b u l k pH v a l u e i s t h e n a consequence o f the consumption o f hydrogen i o n s i n t h e p r o d u c t i o n o f c a t i o n i c amine o x i d e , and the p r o t o n a t e d amine o x i d e and the l o n g c h a i n s u l f a t e p r e c i p i t a t e out s t o i c h i o m e t r i c a l l y . M i x t u r e s o f c o m p a t i b l e c h a i n systems, v i z . ; C12DA0/SDS and CmDAO/SDS, l e a d t o the f o r m a t i o n o f mixed m i c e l l e s which i s a l s o accompanied by an i n c r e a s e i n the pH o f the s o l u t i o n . However, the i n c r e a s e i n pH i n t h e s e two c a s e s may n o t be caused by the p r o t o n a -




130

F i g u r e 8. Mean m o l e c u l a r a r e a v s . c o m p o s i t i o n p l o t s f o r CieDAO/SDS mixed f i l m s s p r e a d on 0.01M N a C l , pH 5.5 (HC1) s u b s o l u t i o n a t 25°C.



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Monolayer

Properties

131

t i o n process. P r o t o n a t i o n i n i t s u s u a l sense i m p l i e s an a c t u a l bond f o r m a t i o n . I f the amine o x i d e m o l e c u l e s a r e p r o t o n a t e d , t h e n an i o n - i o n i n t e r a c t i o n w i t h the s u l f a t e w i l l be r e f l e c t e d i n t h e b u l k by t h e f o r m a t i o n o f an i n s o l u b l e 1:1 d o u b l e l o n g c h a i n s a l t ; t h e absence of p r e c i p i t a t i o n would t h e n be i n d i c a t i v e of the ab sence o f any s u b s t a n t i a l i o n - i o n i n t e r a c t i o n . In the c a s e s o f Ci2DAO/SDS and CiifDAO/SDS m i x t u r e s , s i n c e no p r e c i p i t a t i o n o c c u r s f o r a l l mixing r a t i o s i n v e s t i g a t e d , the observed i n c r e a s e i n b u l k pH i s p r o b a b l y due t o the a d s o r p t i o n o f H3 0*" i o n s on the m i c e l l e s u r f a c e , r a t h e r than a c t u a l p r o t o n a t i o n o f amine o x i d e m o l e c u l e s , o n l y when t h e r e a r e s u f f i c i e n t p r o t o n s a v a i l a b l e , such as w i t h the a d d i t i o n o f an a c i d , w i l l p r o t o n a t i o n o f t h e amine o x i d e and t h e f o r m a t i o n of t h e double c h a i n complex t a k e p l a c e . T h i s a l t e r n a t i v e i n t e r p r e t a t i o n o f the i n c r e a s e d pH v a l u e has been c o n s i d e r e d p r e viously. T h e r e f o r e i t i s proposed t h a t t h e r e a r e two d i f f e r e n t mechanisms, b o t h o f which a r e needed i n o r d e r t o e x p l a i n the i n t e r a c t i o n s between a l k y l d i m e t h y l a m i n e o x i d e and a l k y l s u l f a t e s a t i s factorily. When the a l k y l c h a i n s match i n l e n g t h , p a r a f f i n p a r a f f i n i n t e r a c t i o n i s maximised, t h e f o r m a t i o n o f mixed m i c e l l e i s f a v o u r e d and i s accompanied by t h e a d s o r p t i o n o f hydronium i o n s on t h e s u r f a c e . When t h e a l k y l c h a i n s do not match, head group i n t e r a c t i o n p r e d o m i n a t e s , and p r o t o n a t i o n of amine o x i d e i s f a v o u r e d , r e s u l t i n g i n the f o r m a t i o n o f a 1:1 i n s o l u b l e complex. Goddard and Kung (2) have i n v e s t i g a t e d t h e mixed monolayer p r o p e r t i e s o f d o c o s y l d i m e t h y l a m i n d e o x i d e and n o n a d e c y l benzene s u l f o n a t e under c o n d i t i o n s i d e n t i c a l t o t h i s s t u d y . The mean mole c u l a r a r e a v e r s u s c o m p o s i t i o n p l o t s show s m a l l p o s i t i v e d e v i a t i o n s from i d e a l i t y , and t h e ττ-Α c u r v e s o f t h e mixed f i l m s a r e o f s i m i l a r shape. T h i s i s p r o b a b l y due t o the p r e s e n c e o f benzene r i n g i n the c h a i n ; the b u l k y group has a n e g a t i v e s t e r i c e f f e c t on t h e p a c k i n g o f the a l k y l c h a i n s . N e v e r t h e l e s s , f a v o u r a b l e i n t e r a c t i o n s can s t i l l be e x p e c t e d . Kolp e t ^ a l . (3) have i n v e s t i g a t e d the s o l u t i o n b e h a v i o u r s of t h e s h o r t e r c h a i n homologs, d o d e c y l d i m e t h y l a m i n e o x i d e and dodecylbenzene s u l f o n a t e . They found t h a t the p r o t o n a t e d amine o x i d e m o l e c u l e s and t h e l o n g c h a i n s u l f o n a t e p r e c i p i t a t e metathetically. Removal o f t h e b u l k y group o p t i m i s e s the p a c k i n g o f t h e c h a i n s , and can produce s y n e r g i s t i c e f f e c t s . Another s t u d y has c o n f i r m e d (22) t h a t m i x t u r e s o f d o d e c y l d i m e t h y l a m i n e o x i d e and sodium dodecane s u l f o n a t e show v e r y pronounced s u r f a c e t e n s i o n l o w e r i n g upon m i x i n g , and t h a t p r e c i p i t a t i o n o c c u r s o n l y i n d i l u t e solutions. Hence the e x p e c t e d but n o t d e t e c t e d i n t e r a c t i o n between C 2 2 amine o x i d e and n o n a d e c y l benzene s u l f o n a t e i s p r o b a b l y a s t e r i c e f f e c t due t o the p r e s e n c e o f a b u l k y group i n the mixed m o l e c u l a r assembly. Acknowledgments The a u t h o r s w i s h discussions.

t o thank Dr. E. D. Goddard f o r many h e l p f u l

Literature Cited 1.

Chang, D. L., and Rosano, H. L., In "Structure/Performance Relationships in Surfactants"; ACS SYMPOSIUM SERIES No. 253, American Chemical Society: Washington, D.C., 1984; p. 129-140.



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February 3, 1986


Phenomena in Mixed Surfactant Systems - ACS Publications

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