TY - JOUR
T1 - Contribution of Na+ counterions to H-AOT&Na-AOT-based W/O microemulsion formation using aqueous NaOH solutions as estimated by pyranine absorbance
AU - Oshitani, Jun
AU - Takashina, Shiho
AU - Yoshida, Mikio
AU - Gotoh, Kuniaki
N1 - Funding Information:
This work was supported in part by the Core-to-Core Program promoted by Japan Society for the Promotion of Science (Project No.18004). A grant for promising researchers from the Hosokawa Powder Technology Foundation is gratefully acknowledged.
PY - 2009/10/20
Y1 - 2009/10/20
N2 - 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.
AB - 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.
KW - Phase separation
KW - Polar headgroup
KW - Screening effect
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U2 - 10.1016/j.colsurfa.2009.09.020
DO - 10.1016/j.colsurfa.2009.09.020
M3 - Article
AN - SCOPUS:70349783730
VL - 350
SP - 136
EP - 140
JO - Colloids and Surfaces A: Physicochemical and Engineering Aspects
JF - Colloids and Surfaces A: Physicochemical and Engineering Aspects
SN - 0927-7757
IS - 1-3
ER -