Adsorption at Interfaces - American Chemical Society

11 Interaction of Calcium Ions with the Mixed Mono-

Downloaded by NORTH CAROLINA STATE UNIV on November 8, 2013 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0008.ch011

layers of Stearic Acid and Stearyl Alcohol at pH 8.8 D. O. SHAH Departments of Anesthesiology and Chemical Engineering, University of Florida, Gainesville, Fla. 32611

Introduction Molecular interactions in mixed monolayers are of impor tance in understanding the phenomena such as stability of foams and emulsions, retardation of evaporation by films, and reactions occuring at the cell-surface (1-12). Earlier studies on mixed monolayers of acids, amines, alcohols, ethers, and triglycerides were reported by Schulman and his coworkers (13-15). Similar studies were also reported by Harkins and his group (16-18). Various investigators (19-29) have studied the effect of di- and trivalent cations as well as the effect of pH of the subsolution on the ionic structure of fatty acid mono layers. The present paper reports our studies on the ion-dipole interaction between stearic acid and stearyl alcohol at pH 8.8, as well as the interaction of calcium ions with these mixed monolayers.

Materials and Methods Highly purifies (>99%) stearic acid and stearyl alcohol were purchased from Applied Science Laboratories, Inc., (State College, Penn. 16801). Lipid solutions of 0.8 to 1.0mg/ml concentration were prepared in methanol-chloroform-hexane (l/l/3v/v/v) mixture, where all solvents were of spectroscopic grade. Inorganic chemicals of reagent grade, and distilled— deionized water of electrical resistance 1.2 x 10 ohms/cm were used in all experiments. 6

The surface pressure was measured by a m o d i f i e d Wilhelmy p l a t e method, and the surface p o t e n t i a l was determined by using a r a d i o a c t i v e e l e c t r o d e , as described p r e v i o u s l y (30). The s t a t e of the monolayer was i n f e r r e d from the m o b i l i t y o f t a l c p a r t i c l e s s p r i n k l e d on the monolayers when a gentle stream of a i r was blown a t the p a r t i c l e s by means o f a dropper (31). Various s t a t e s of monolayers were q u a l i t a t i v e l y d i s t i n g u i s h e d on the b a s i s o f movement of t a l c p a r t i c l e s and designated as 170

In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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the s o l i d , g e l , v i s c o u s l i q u i d , o r the l i q u i d s t a t e . I n the s o l i d s t a t e , the t a l c p a r t i c l e s do not move a t a l l ; i n the g e l s t a t e they move very l i t t l e and stop; i n the v i s c o u s l i q u i d s t a t e they move but not f r e e l y ; whereas i n the l i q u i d s t a t e of monolayers the p a r t i c l e s move f r e e l y when the a i r c u r r e n t i s blown a t them. The s u r f a c e measurements were taken on the f o l l o w i n g subs o l u t i o n s : 0.05M t r i s b u f f e r + 0.02M NaCl, and 0.05M t r i s b u f f e r + 0.01M C a C l a t pH 8.8. The b u f f e r s o l u t i o n s were prepared according t o Biochemists' Handbook (32).


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Theory The average area per molecule i n a mixed monolayer i s c a l c u a l t e d by d i v i d i n g the t o t a l area by the t o t a l number of molecules o f both components i n the mixed monolayer. I f the molecules o f both components occupy the same molecular areas as i n t h e i r i n d i v i d u a l monolayers, the p o i n t s f o r the average area per molecule o f the mixed monolayers would l i e on a s t r a i g h t l i n e j o i n i n g the two end p o i n t s f o r the pure components a t the same s t a t e of compression. A d e v i a t i o n from t h i s ' a d d i t i v i t y r u l e ' i n d i c a t e s condensation of the mixed monolayers e i t h e r due to an i n t e r a c t i o n o r i n t e r m o l e c u l a r c a v i t y e f f e c t i n the mixed monolayers (12,33). Surface p o t e n t i a l (AV) of a monolayer can be expressed as AV = Κημ, where Κ i s a constant, J ^ i s the number o f molecules per cm o f the f i l m ( i . e . , η = 10 /area per molecule i n 8 ) , and μ i s the r e s u l t a n t v e r t i c a l component of the d i p o l e moment of the molecule. Thus, AV/n = Κμ, where the term on the l e f t hand s i d e of the equation, r e p r e s e n t i n g the average p o t e n t i a l per molecule (mv/molecule), i s p r o p o r t i o n a l to the surfaçg d i p o l e moment μ o f the^molecule. Hence, AV/n = AV χ 10 χ area per molecule i n X , where the term on the r i g h t - h a n d s i d e can be used f o r c a l c u l a t i o n s . The advantages o f u s i n g AV/n i n s t e a d o f AV alone a r e the f o l l o w i n g : f i r s t l y , the average p o t e n t i a l per molecule (AV/n) i s a parameter which i s a c h a r a c t e r i s t i c o f the molecule, and i s analogous to average area per molecule; and secondly, i t e l i m i n a t e s changes i n s u r f a c e p o t e n t i a l s caused by expansion or condensation o f the mixed monolayers. C o n c l u s i v e evidence f o r i o n i c i n t e r a c t i o n can o n l y be obtained from s u r f a c e p o t e n t i a l measurements when AV/n i s p l o t t e d a g a i n s t mole f r a c t i o n o f the components i n mixed monolayers a t the same s t a t e o f compression. In t h i s case a d e v i a t i o n from the a d d i t i v i t y l i n e i n d i c a t e s i o n - i o n , o r i o n - d i p o l e i n t e r a c t i o n between the two components i n mixed monolayers (12,33). Results Average molecular areas and p o t e n t i a l s o f the mixed



ADSORPTION A T INTERFACES

Figure 1. Average area per molecule of stearic acid-stearyl alcohol monohyers at different surface pressures on subsolutions of 0.05M tris buffer + 0.02M NaCl, pH 8.8, at 22°C. The broken line indicates the additivity rule of molecular areas. The optimum condensation of mixed monohyers occurs at 9:1 and 1:3 molar ratios.

Figure 2. Average potential per molecule of stearic acidr-stearyl alcohol monolayers at different surface pressures on subsolutions of 0.05M tris buffer + 0.02M NaCl, pH 8.8 22°C. The broken lines indicate the additivity rule of average potentials. 9

MOLE FRACTION

(I ALCOHOL)



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monolayers of stearic acid and stearyl alcohol on subsolutions of 0.05M t r i s buffer + 0.02M NaCl at pH 8.8 are shown i n Figures 1 and 2. Average molecular areas i n the mixed monolayers show optimum condensation at molar ratios 9:1 and 1:3 between stearic acid and stearyl alcohol (Figure 1). The average potentials show deviations from the additivity rule at a l l surface pressures (Figure 2). The deviation i s less prominent when stearyl alcohol i s the major fraction i n the mixed monolayers. Figures 3 and 4 show average molecular areas and potentials on subsolutions of t r i s buffer + 0.01M CaCl at pH 8.8. The average areas follow the additivity rule (Figure 3), whereas average potentials show deviation from i t (Figure 4). However, in contrast to the nonlinear decrease i n the absence of CaCl^ (Figure 2), there i s a linear increase i n average potential i n the presence of CaCl i n subsolutions. Figure 5 shows the maximum values of surface potential of mixed monolayers near their limiting areas on both subsolutions. It should be noted that surface potentials of mixed monolayers on t r i s buffer + NaCl subsolutions show distinct kinks at 9:1, 3:1, and 1:3 molar ratios of stearic acid to stearyl alcohol. Table I summarizes the state of the monolayers near their collapse pressures (~40dynes/cm) on various subsolutions. The mixed monolayers did not show significant differences i n their collapse pressure values. The stearic acid monolayer was i n the solid state i n the presence of CaCl^. However, the presence of 10 mole % of stearyl alcohol converted the monolayer to the gel state, suggesting a disording of solid l a t t i c e structure of calcium stéarate by stearyl alcohol. 2

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Discussion At pH 8.8, the average molecular areas show two minimia at molar ratios 9:1 and 1:3 between stearic acid and stearyl alcohol i n mixed monolayers (Figure 1). We have shown (11) previously that the rate of evaporation through these mixed monolayers also show a minimum at the 1:3 molar ratio between stearic acid and stearyl alcohol. However, there was no such minimum i n the evaporation through mixed monolayers at the 9:1 molar ratio i s presumably due to structural alterations (e.g., aggregation or micelle formation) i n the mixed monolayer. Using various mixed surfactant systems such as mixed monolayers, foams and emulsions, we have proposed (11) that the striking change i n the properties of these systems at the 1:3 molar ratio i s due to a hexagonal arrangement of molecules of the two components such that one component occupies the corners and the other component occupies the center of a hexagon. The mechanism of structural alteration at the 9:1 molar ratio i n the mixed monolayers may be as follows. At pH 9, i t i s expected that there w i l l be ionic repulsion between stearic acid molecules i n the monolayer. Perhaps a small number of stearyl


ADSORPTION A T INTERFACES

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Adsorption at Interfaces - American Chemical Society

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