Contribution of Na+ counterions to H-AOT&Na-AOT-based W/O microemulsion formation using aqueous NaOH solutions as estimated by pyranine absorbance

Jun Oshitani, Shiho Takashina, Mikio Yoshida, Kuniaki Gotoh

Research output: Contribution to journalArticlepeer-review


The objective of this study is to estimate the contribution of Na+ as a counterion in the formation of H-AOT&Na-AOT-based W/O microemulsions using aqueous NaOH solution by pyranine absorbance measurements. A mixture of an aqueous NaOH solution containing pyranine/H-AOT&Na-AOT/isooctane was emulsified by changing the mixing ratio of Na-AOT (XNa-AOT = 0-1) and the mole fraction of NaOH (XNaOH = [NaOH]/the AOT- concentration in the water pool = 0-1). The phase behavior of the emulsified mixture was evaluated from the absorbance of pyranine at the isosbestic point and by visual observations. W/O microelumsions are formed at the mid-range of XNa-AOT, whereas the emulsified mixture separates into two phases at lower XNa-AOT and higher XNa-AOT. The two phase boundaries shift toward lower XNa-AOT as with increasing XNaOH. The phase behavior depends on the degree of screening of electrostatic repulsions between the polar headgroups of AOT- by the Na+ counterion. Interestingly, nano-sized W/O microemulsions are formed without phase separation using a highly concentrated NaOH aqueous solution when the Na-AOT mixing ratio is appropriately adjusted. The phase behavior was plotted as XNaOH versus XNa-AOT, and the correlation equations for the two phase boundaries were obtained by fitting the points. The contribution of the Na+ counterion from NaOH to W/O microemulsion formation was estimated by the correlation equations. The absorbance of pyranine and the size of W/O microemulsions, as measured by DLS, were plotted as a function of XN a+ = (x [ N a+ from NaOH ] + [ N a+ from Na-AOT ]) / [ AO T- ], in which x is the ratio contributed by NaOH. The absorbance and size correlates well with XN a+, indicating that XN a+ is a meaningful parameter for quantitatively estimating phase behavior and size variation.

Original languageEnglish
Pages (from-to)136-140
Number of pages5
JournalColloids and Surfaces A: Physicochemical and Engineering Aspects
Issue number1-3
Publication statusPublished - Oct 20 2009


  • Phase separation
  • Polar headgroup
  • Screening effect

ASJC Scopus subject areas

  • Surfaces and Interfaces
  • Physical and Theoretical Chemistry
  • Colloid and Surface Chemistry


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