Difference in Screening Effect of Alkali Metal Counterions on H-AOT-Based W/O Microemulsion Formation

• Published in: Langmuir Vol. 26; no. 4; pp. 2274 - 2278
• Main Authors: Oshitani, Jun, Takashina, Shiho, Yoshida, Mikio, Gotoh, Kuniaki
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 16.02.2010
• Subjects: Chemistry; Colloidal state and disperse state; Colloids: Surfactants and Self-Assembly, Dispersions, Emulsions, Foams; Emulsions. Microemulsions. Foams; Exact sciences and technology; General and physical chemistry; Micelles. Thin films; Surface physical chemistry; Water; Binding; Hydrocarbon; Hydration; Potassium hydroxide; Emulsification; Micelle; Alkali metal; Isooctane; Condensed benzenic compound; Sulfonate; Infrared spectrometry; Pyrene; Phase separation; Water content; Counter ion; Electrostatic repulsion; Aqueous solution; Microemulsion
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la902700j

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.