Difference in screening effect of alkali metal counterions on H-AOT-based W/O microemulsion formation

Jun Oshitani, Shiho Takashina, Mikio Yoshida, Kuniaki Gotoh

Research output: Contribution to journalArticle

5 Citations (Scopus)

Abstract

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-basedW/Omicroemulsions. The screening effectwas evaluated bymeans of criticalmicelle concentration (CMC) data using the pyrene 1:3 ratio method with aqueous solutions containing M-AOT (whereM+ =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 (w0 = [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.Aperiod of standing after the emulsification and a higher concentration ofNaOHcompared toKOH, RbOH, and CsOHare required to formW/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.

Original languageEnglish
Pages (from-to)2274-2278
Number of pages5
JournalLangmuir
Volume26
Issue number4
DOIs
Publication statusPublished - Feb 16 2010

Fingerprint

Alkali Metals
Microemulsions
Alkali metals
alkali metals
Screening
screening
Phase behavior
Hydration
Stretching
hydration
aqueous solutions
Emulsification
Pyrene
Micelles
pyrenes
sulfonates
Phase separation
Water content
moisture content
low concentrations

ASJC Scopus subject areas

  • Electrochemistry
  • Condensed Matter Physics
  • Surfaces and Interfaces
  • Materials Science(all)
  • Spectroscopy

Cite this

Difference in screening effect of alkali metal counterions on H-AOT-based W/O microemulsion formation. / Oshitani, Jun; Takashina, Shiho; Yoshida, Mikio; Gotoh, Kuniaki.

In: Langmuir, Vol. 26, No. 4, 16.02.2010, p. 2274-2278.

Research output: Contribution to journalArticle

Oshitani, Jun ; Takashina, Shiho ; Yoshida, Mikio ; Gotoh, Kuniaki. / Difference in screening effect of alkali metal counterions on H-AOT-based W/O microemulsion formation. In: Langmuir. 2010 ; Vol. 26, No. 4. pp. 2274-2278.
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