Difference in Screening Effect of Alkali Metal Counterions on H-AOT-Based W/O Microemulsion Formation
The purpose of this study was to estimate the screening of electrostatic repulsions between the polar headgroups of AOT− by alkali metal counterions and to explore the relationships between the screening effect and the phase behavior of H-AOT-based W/O microemulsions. The screening effect was evalua...
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Published in | Langmuir Vol. 26; no. 4; pp. 2274 - 2278 |
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Main Authors | , , , |
Format | Journal Article |
Language | English |
Published |
Washington, DC
American Chemical Society
16.02.2010
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Subjects | |
Online Access | Get full text |
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Summary: | The purpose of this study was to estimate the screening of electrostatic repulsions between the polar headgroups of AOT− by alkali metal counterions and to explore the relationships between the screening effect and the phase behavior of H-AOT-based W/O microemulsions. The screening effect was evaluated by means of critical micelle concentration (CMC) data using the pyrene 1:3 ratio method with aqueous solutions containing M-AOT (where M+ = Li+, Na+, K+, Rb+ and Cs+) to form normal micelles, and by counterion binding constants, determined from plots of CMC versus counterion concentration. The order of the screening effect was found to be K+ ≈ Rb+ > Cs+ > Na+ > Li+. Interestingly, the order does not follow the hydration size dependence of the alkali metal counterions. An aqueous MOH solution containing a given concentration/H-AOT/isooctane was emulsified at a water content (w 0 = [water]/[H-AOT]) of 10 to produce H-AOT-based W/O microemulsions. The phase behavior and size variation were investigated by FT-IR and DLS measurements. The emulsified mixture separates into two phases at lower MOH concentration due to an insufficient screening effect. When the concentration is increased to a level sufficient to intensify the screening effect, W/O microemulsions are formed without phase separation at lower KOH and RbOH concentrations compared to CsOH. A period of standing after the emulsification and a higher concentration of NaOH compared to KOH, RbOH, and CsOH are required to form W/O microemulsions. W/O microemulsions are not formed in the case of LiOH. These results indicate that the formation of a W/O microemulsion with H-AOT is strongly correlated with the order of the screening effect. A possible cause for the difference in the screening effect is proposed based on hydration of the polar headgroups and counterions, as evidenced by FT-IR spectral data, i.e., symmetrical sulfonate stretching and O−H stretching. |
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Bibliography: | ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 0743-7463 1520-5827 |
DOI: | 10.1021/la902700j |