CHAPTER 3: ANALYTICAL EVALUATION OF THE EFFECTS OF
B. Experimental Methods
2. Effect of Divalent and Trivalent Counterions
may provide insight for determining if changing the hydrogen bond network or ion size is causal because potential determining ions H+ and OH- could be used to switch the effect of the hydrogen bond network to cause either size decrease or increase for the same electrolyte solution, depending on the pH, if the hydrogen bond network is causal. Some pH studies on reverse micelle behavior or reverse micelle synthesis exist, but not in a systematic way that establishes the effect on reverse micelle size changes with added salt.6,48-51 At least one study establishes improved solubility in reverse micelles by addition of an acid, which can be a result of changes to the hydrogen bond network.4
Figure 4 - 3. Qualitative model for opposite reverse micelle size changes as a result of structural changes to the hydrogen bond network from negative or positive potential determining ions.
containing divalent counterions as a function of electrical double layer thickness are given in Figure 4-4a and the linear fits demonstrate the dependence on the electrical double layer thickness. The linear dependence on the electrical double layer thickness and similar slope can be expected from the compression of the electrical double layer thickness with increasing electrolyte concentration and because there are no changing variables in the electrical double layer thickness (equation 1). However, the size intercept of the linear fits cannot be accounted for using the electrical double layer thickness and can be considered a specific ion effect. This is demonstrated in Figure 4- 4b, which shows a linear dependence of the size intercept from the fits given in Figure 4- 4a on the hydrated ion complexes (cation and anion) for the four electrolytes, assuming that rM-O is a representative measure.44 Thus, the size of the reverse micelles can be fully described using the electrolyte concentration and size of the ion hydration complexes.
The analysis allows the average reverse micelle size to be predicted for other electrolytes using the sizes of the associated ion hydration complexes.
An interesting feature of Figure 4-4 is that the slopes of the linear fits in Figure 4- 4b and Figure 4-3d have opposite sign, which could indicate that the effect of ion hydration sizes are different for the electrolytes with divalent counterions when compared to the effect when electrolytes with trivalent counterions are added. The electrolytes having divalent counterions follow expectations that larger average reverse micelle sizes are obtained with larger hydrated ion sizes. However, the opposite is true of electrolytes with trivalent counterions; smaller reverse micelles are obtained with larger ion hydration sizes. Thus ion size cannot be the only important consideration. This could also be a unique consequence of specific ions, because trivalent counterions provide a greater
extent of electrostatic screening and result in a greater difference between the surface potential and the ζ-potential. Furthermore, smaller counterions also provide a greater
Figure 4 - 4. (a) Effect of electrolytes with divalent counterions on average reverse micelle size. (b) Effect of average ion hydration radii on reverse micelle size intercept of linear fits when electrolytes with divalent counterions are added. (c) Effect of electrolytes with trivalent counterions on average reverse micelle size. (d) Effect of average ion hydration radii on reverse micelle size intercept when electrolytes with trivalent counterions are added.
extent of electrostatic screening. In the model presented in Chapter 2, the inner potential is generated by the co-ion core. Thus, when the ζ-potential is reduced, then the surface charge density of the core must also be reduced. Reduced surface potential of the core could be accommodated by additional ordering within the core or by swelling of the core size. The increased size with smaller counterions might be a consequence of core swelling, but the exact mechanism has not been resolved.
The point of instability with Fe(SO4), Mg(NO3)2, MgCl2, and CuCl2 additions occurred at 0.2 M, 0.5 M, 0.4 M and 0.3 M, respectively. The point of instability of Mg(NO3)2 is in reasonable agreement with the results from Chapter 2. However, the points of instability when Fe(SO4), MgCl2, or CuCl2 are added are lower than would be expected. Figure 4-5 depicts a possible explanation for the discrepancy. The edges of the two shaded regions connect the points of instability. The point of instabilities for the Mg(NO3)2, MgCl2, and CuCl2 systems occur at concentrations for which the average reverse micelle size is similar. Thus, the lower point of instability of MgCl2 and CuCl2
when compared to Mg(NO3)2 is likely to be a reverse micelle size effect. The destabilization at similar sizes suggests a salting out mechanism, where the surface tension is increased to a point where a single phase reverse micelle solution is no longer supported because of increased interfacial area and reduced surfactant coverage with decreasing reverse micelle size. A similar analysis can demonstrate why the points of instability for Fe(NO3)3 and Y(NO3)3 were 0.3 M, less than the point of instability of 0.4 M for Al(NO3)3. The reverse micelle sizes with dissolved Fe(SO4) are not small enough to justify a salting out mechanism. Thus, this is a specific ion effect. The hydrated iron and sulfate ion complexes are both greater in size than the respective ions for the other
electrolytes. This specific ion size effect leading to destabilization is also in agreement with the instability of HAuCl4 containing reverse micelles, because the size of the hydrated (AuCl4) ion complex is too large and manifests as larger reverse micelles (average size of 21 nm was measured for (z2c)-1/2 = 1.5). The shaded region in Figure 4-5 corresponding to instability as a result of ion size effect was generated using only two data points. It would be beneficial to evaluate a greater number of electrolytes having large ion hydration complexes to establish a more accurate boundary. The effect of counterion valence on these boundaries also needs to be established.
Figure 4 - 5. The shaded regions demonstrate unstable reverse micellar solutions as a result of reverse micelles that are too small and hydrated ion sizes that are too large.