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Langmuir 1994,10,1650-1653
Effect of Counterion Radius on Surfactant Properties in Winsor I1 Microemulsion Systems Julian Eastoe* and Stewart Chatfield School of Chemistry, Cantock's Close, University of Bristol, Bristol BS8 1 TS,U.K.
Richard Heenan ISIS, Rutherford Appleton Laboratory, Chilton, Oxon OX11 OQX,U.K. Received January 7, 1994. I n Final Form: March 24,1994" The effects of varying the surfactant counterion radius r on the oil-water interfacial tensions yowand water-in-oil (w/o) microemulsion droplet radii R are reported. The s u r f a h t a are derivativesof AerosolOT (AOT-is bis(2-ethylhexyl)sulfosuccinate)with quatern ammonium counterions [NH4++1, [(CH&N+I, and [(C2H&N+I;in this series r increases from 1.5 to 2.8 We have used spinning drop tensiometry to measure yowas a function of electrolyte concentration [NaClI with n-heptane as oil and small-angle neutron scattering (SANS) to determine the w/o droplet spontaneous radii R, in cyclohexane systems. Apparently the counterion radius only has a small effect on the yowvs [NaCll behavior. For example, for NH4+the electrolyteconcentration at the tension minimum [NaCll- is -0.0425 and for [(CzH&N+] 0.0525 mol dma. Similarly, the effect of the ion radius r on R, in the Winsor I1 system is also small; for NH4+R, 21 %,andwith [(C2H5)4N+lR0 30 A. The weak dependence of these parameters on ion radius r is discussed in the light of recent work by Binks et al. demonstrating a much stronger dependence on alkyl chain length.
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Introduction Microemulsions are fascinating complex fluid mixtures that have aroused intense interest over recent years.' These systems often contain highly curved surfactant-stabilized droplets and their thermodynamic stability arises from very low values of oil-water interfacial tension yow 10-I 10-4 mN m-l. Electrolyte (NaC1) has an important effect on the surfactant partitioning and interfacial properties; a deep minimum in the yowvs [NaCll profile is usually This behavior corresponds to an optimal packing of the surfactant molecules obtained by changing the area per head group ah relative to that of the tails at via screening of head group interactions. I t is also linked to the effect of salt on the relative solubility of surfactant in water and oil. A good model system is Na(AOT)/n-heptane/NaCl/ water where at 25 O C the minimum in tension occurs at -0.05 mol dm-3NaC1.213 At low salt concentrations ( 1,and a Winsor I system (o/w + excess oil) forms. At the tension minimum adat 1and a Winsor I11 phase is formed (water + middle phase + oil). After the minimum ([NaCll > 0.05 mol dm3) AOT partitions preferentially into the oil, adat < 1,and a Winsor I1 system is favored (w/o+ aqueous phase). In this high salt regime,no micelles are detected in the aqueous phase (by dynamic light scattering) and the concentration of nonaggregated surfactant is low, typically
