Chapter 2: Techniques
II. 5. 4. Importance of hydration in W/O microemulsions
As water influences micellization in W/O microemulsions, a minimal Ww is necessary to promote this mechanism. As a maximum of ciappwas reached at Ww ~ 0.15, it could be that the hydration is complete, while the heagroup effective solvatation increases continuously with Ww. Therefore, SDS charge dissociation cannot be considered as constant within the complete experimental path. Compared to path 2, path 1 showed generally higher values of ZIB (Ww) (figure (III. 36)). This difference, that is well illustrated by ZIB (Ww) of path 3 and path 4 (figure (III. 38)), may be related to the molar ratio water/SDS that is higher for path 1 than for path 2. However, in path 2 the behaviour of ciapp(Ww) that does not reach a plateau value, suggests that surface hydration is not completed in this path. In paths 3 and 4 (figure (III. 38)), ZIB is higher at low WSDS+1-pentanol where the molar ratio water/SDS is high; additionally ZIB in path 4 is more important that in path 3 for which the water content is lower. It is clear that less dissociated charges are involved in path 2, and therefore, electrostatic interaction between reverse micelles should be modified, with a displacement of percolation threshold as consequence. These differences in SDS headgroup hydration may induce differences in the counterion self diffusion between the two experimental path 1 and 2. This is well exemplified by figure (III. 23) that compares amplitudes S1 for both experimental path 1 and 2; path 1 showed a higher maximum peak than path 2. It could be also observed that τ1 was higher for path 1 than for path 2, although additionally to water effects, the structures involved for both paths are not of the same kind, as well as the molar ratio 1-pentanol/SDS at the interface may not be the same.
0,3 0,4 0,5 0,6 0,7 0,8 0,9
0 4 8 12 16
Z
IBW
SDS + 1-pentanolFigure (III. 38) Irrotationally bound water ZIB (b) vs. (SDS + 1-pentanol) weight fraction, WSDS+1-pentanol for path 3 (closed circles and solid line) and path 4 (opened circles and dotted line) at 25°C.
II. 6. Conclusion
Our studies revealed that the different contributions to DRS spectra of SDS micelles in water, SDS/1-pentanol micelles and SDS/1-pentanol W/O microemulsions are of the same kinds. The low-frequency relaxation processes (centred at τ1 ~ 2-10 ns and τ2 ~ 400-800 ps) are related to counterion motions, whereas the high-frequency relaxation steps (centred at τ3 ~ 120 ps and τ4 ~ 8 ps for SDS micelles in water; centred at τ3 ~ 100 ps, τ4 ~ 15 ps, and τ5 ~ 2-3 ps for W/O SDS microemulsions) are linked to water with different relaxation rates.
Our work has shown that theories (model of Grosse142, Cavell equations6, 7) previously applied to cationic surfactant micelles9, 10 are extrapolable to SDS micelles in water. For this system, the model of Grosse142 with input parameters (micelle radius and aggregation number) found in the literature could describe the behaviour of the low-frequency relaxation parameters centred at τ1 ~ 5 ns and τ2 ~ 600 ps. According to Grosse’s theory, the slowest relaxation process (with τ1 ~ 5 ns) is related to the fluctuations of the counterion cloud, whereas the relaxation process centred at τ2 ~ 600 ps occours due to the motions of surface counterions. This model describes also the behaviour of the low-frequency relaxation parameters centred at τ1 ~ 6 ns and τ2 ~ 700 ps for SDS/1-pentanol micelles. By the model of Pauly and Schwan143 (dealing with interfacial polarization) the behaviour of relaxation step centred at τ2 ~ 600-700 ps for both SDS (with aggregation number and micelle core radius from literature data) and SDS/1-pentanol micellar systems could be succefully reproduced.
Unfortunately, in the case of bicontinuous structures, reverse micelles and W/O microemulsions, we failed in finding a model able to describe the behaviour of the two low-frequency relaxation processes. For instance, the model of Pauly and Schwan could explain the relaxation process 2 only in a narrow water concentration range, well below the percolation threshold13, 188, 201
(where the reverse micelles start to merge). It should be remarked that the conductivity behaviour of bicontinuous and W/O microemulsions reflects their transport properties. A correlation was found between the amplitude of the relaxation step 1 and the conductivity results below the percolation threshold, suggesting that this relaxation process is directly related to the charge exchange between reverse micelles.
In water/SDS system two different kinds of water were observed, with relaxation times τ3 ~ 120 ps (water bounded to the micellar surface) and τ4 ~ 8 ps (bulk-like water). The addition of 1-pentanol causes that the relaxation process related to bulk-like water separates into two different relaxation steps with relaxation times τ4 ~ 15 ps and τ5 ~ 2-3 ps, whereas another kind of water, namely irrotationally bound water, which does not contribute at all to the dielectric relaxation, appeares. This water is strongly bound to sodium ions. With help of Cavell´ s6,7 equations, an apparent water concentration for all kinds of water could be calculated. The interfacial changes related to the transition of direct SDS/1-pentanol micelles into SDS/1-pentanol reverse micelles and then into W/O microemulsions was observed. It appears that the amount of irrotationally bound water increases with the alcohol addition for SDS/1-pentanol micelles. This behaviour can be correlated to a SDS headgroup dissociation promoted by 1-pentanol molecules, which leads to an increase of the amount of irrotationally bound water. In W/O microemulsions, the amount of irrotationally bound water grows with the increasing water concentration. This may be inferred to a change of angle curvature ehancing the effective solvation of SDS headgoups. As for SDS micelles, interfacial water characterized by a relaxation time of τ3 ~ 100 ps could be found in W/O microemulsions and this corresponds to a second interfacial hydration layer. This kind of water, together with bulk-like water seems to be related to the percolation in W/O microemulsions.
This work should be completed by examining of other W/O microemulsions systems, first by changing 1-pentanol by other n-alkanols, and then considering other surfactants. The necessary experiments have already begun (see Annexe) but are still incomplete.