SANS study of a fluorinated water - in - oil microemulsion

168 P. Bagliori et al.IJournal ofMolecular Strutture 383 (1996) 165-169

( x 1 0

À -

) C

Fig. 6. The microemulsion interfacial area per surfactant molecule deduced by the Porod analysis. Open circles correspond to values calculated for the samples of Fig. 1 (T = 20°C); filled circles to values calculated for the samples of Fig. 2 (T = 35°C).

interpret the incoherent contribution of the hydrogen atoms of the sample and of any other incoherent part. The fitting of the experimental data to this last relationship allows the calculation of A = 2irK2CJl

(the intercept of the straight line with the ordinate axis at Q4 = 0) and B (the slope of the straight line).

Once the contrast K and the Cs value are known, E can

be deduced from A. In Fig. 4 the family of curves at T = 20°C and in Fig. 5 the family of curves at T = 35°C demonstrate that the Porod law is followed by ali the samples studied, thus a sharp interface exists in the

Fig. 7. The background as a function of 0 deduced by the Porod analysis. The meaning of the symbols is as in Fig. 6.

system. The straight lines drawn in Fig. 4 and Fig. 5 represent the best fit of the experimental points to the equation reported above. The error bars (standard deviation) are drawn except when they are smaller than the symbol used. In Fig. 6 and Fig. 7, E and B as a function of concentration are reported for ali the samples studied. We should point out that the area per polar head of the surfactant molecule, obtained by measuring the air-water interfacial tension after surfactant addition, is 50 A2 [10]. The values of Fig.

6 at surfactant concentrations up to 0 = 0.327 agree with the value of ref. [10]. A n increase in </> produces a decrease in the interfacial area per surfactant molecule. Similar results are shown at T = 35°C. The increase in the background as a function of the increase in <j> (see Fig. 7) is compatible with the increased contribution of the hydrogen atoms due to the increase in water content. The temperature does not affect this result.

In conclusion, under the hypothesis that the system is composed of interacting droplets, the droplets have a radius of about 23 A, which is presumably Constant in the range 20-35°C. The interface is sharp and the interfacial area per surfactant molecule is = 50 A2

for <j> values in the range 0.0205-0.327, and decreases to « 35 A2 for higher <j> values.

Acknowledgements

Ausimont s.p.A. is acknowledged for having pro-vided the products. We thank A. Chittofrati for helpful discussions and J. Teixeira for very useful criticisms and suggestions. This research is supported by MPI 60% and 40% (INFM) funds.

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