CHAPTER 3: ANALYTICAL EVALUATION OF THE EFFECTS OF
B. Experimental Methods
1. Effect of Monovalent Counterions
Figure 4-1 confirms that the average reverse micelle size is proportional to the electrical double layer thickness when monovalent counterions were introduced, which contains data for electrolyte concentrations up to 0.5 M (the point at which the solutions became turbid). While the transition from transparent to turbid represents the point of instability for the NaBH4 system, the transition does not capture instability for the HAuCl4 system. In addition to the reverse micellar phase, the presence of an additional size distribution at larger sizes in the DLS results indicate the presence of a coalesced phase and is demonstrated in Figure 4-1b.42 Thus, all solutions were unstable when HAuCl4 was added although the solutions were transparent when less than 0.5 M HAuCl4
was added.
The points of instability are at concentrations much lower than expected from the relationship developed in Chapter 2 (1.2 M) and might be explained by the longer electrical double layer thickness of monovalent ions compared to higher valence ions such that they cannot be fully accommodated into the confined space of the reverse micelle structures. Furthermore, monovalent electrolytes are considered special cases in
the model because the Na+ counterion is identical to the counterion of the surfactant molecules and therefore do not displace the surfactant counterions as is expected for higher valence counterions.37,43 The results demonstrate that the average reverse micelle size decreases linearly as (z2c)1/2 is decreased (i.e. as electrolyte concentration is increased) when NaBH4 is added to the reverse micelle system, which can be expected from the work in Chapter 2. However, the average reverse micelle size increases linearly as (z2c)1/2 is decreased when HAuCl4 is added to the reverse micelle system. This result is unique from all other data collected in this study and may be used to explain the lack of stability for this system.37,42
Figure 4 - 1. (a) Average reverse micelle size with respect to counterion valence and concentration of NaBH4 and HAuCl4 in water/AOT/isooctane reverse micelle solutions with ωo =10. (b) Size distributions of water/AOT/isooctane reverse micelle solutions with ωo =10 and varying amount of HAuCl4 added.
Reverse micelle growth with HAuCl4 additions cannot be directly accounted for in the qualitative model developed in Chapter 2. The proposed mechanism for reverse
micelle size changes, compression of the electrical double layers with increasing counteiron valence or electrolyte concentration, only predicts size decreases with increasing compression of the EDLs. Three possible explanations for the reverse micelle growth discrepancy are described here. The first explanation is that the size of the (AuCl4)- co-ion complex is too large to be effectively accommodated into the core of the reverse micelle. Hydrated ion sizes are considered to investigate this argument. An ion in water will form a hydration complex because of ion-dipole interactions. An example of a hydration complex is demonstrated by the schematic given in Figure 4-2, where the negative dipole of coordinating water molecules (i.e. the oxygen poles) orient towards the sodium cation. The average radial distance of the water coordination shell is given as rNa- O and can be written more generally as rM-O for any metal ion, M, in solution. A summary of rM-O values has been given by Ohtaki and Radnai.44 In three dimensions, tetrahedral, octahedral, or no specific symmetry can be assumed for the water coordination around Na+ ions and rNa-O is in the range of 230-250 pm for a dilute electrolyte solution. The coordination complex is expected to be perturbed by the local electric field effects of other ions and the surfactant headgroups in the confined space of a reverse micelle. Nevertheless, the local field effects should be similar for varying types of ions and the structure of the dilute coordination complex is assumed to be a representative measure of the ion interaction with water in a reverse micelle since the actual structure is unknown. Thus, rM-O values will be used to describe the size of the ion hydration complexes with respect to behavior in reverse micelles. The validity of using this assumption to establish effects within reverse micelles will be highlighted in a
subsequent section by demonstrating a relationship between reverse micelle sizes and ion hydration sizes.
Figure 4 - 2. Schematic demonstrating a hydrated ion complex in dilute solution.
Chapter 2 demonstrated that NH4OH, Al(NO3)3, or ZrOCl2 cause a decrease in the average reverse micelle size. The sizes of the ion hydration complexes for these electrolytes are estimated to range from 187-365 pm, with hydrated Al3+ and Cl- ions being at small and large extremes, respectively. Thus, ion sizes within this range cannot be used to justify average reverse micelle growth. The size of the (BH4)- anion complex in NaBH4 is estimated to be between 192-395 pm, assumed by considering the range of distances with coordinated metals because rM-O is unknown for this complex.45,46 The structure of the (AuCl4)- anion complex is also unknown, but expected to be more than 357-400 pm based on considering distances with coordinated metals and larger than that for all other ions studied in this work.44,47
An alternate explanation of the anomalous reverse micelle size trend with HAuCl4
additions is considered by slight modifications to the model from Chapter 2 based on the results of Biswas et al.39 If the negative surface charge and net negatively charged core
developed in the model are replaced by a frustrated hydrogen bond network that forms a symmetric well containing counterions, then it is possible that H+ ions react with water molecules to form hydronium ions and become part of the hydrogen bond network when HAuCl4 is added to the reverse micelle system. The high reactivity of H+ with water molecules would make it the potential-determining ion in the well. Thus, the well in the hydrogen bond network could be positively charged and contain the (AuCl4) complex counterion that decays from the resulting positive surface potentials. The interaction between the negatively charged complex in the well and the AOT surfactant at the reverse micelle interface results in Coulombic repulsion and can be used to explain the expansion in reverse micelle size. A revised model describing this is demonstrated in Figure 4-3. Also incorporated into the new model is a Stern layer, which is a strongly bound layer immediately in contact with the charged surface. The Stern layer is considered to be a capacitor and reduces the surface potential in a linear manner to the ζ- potential at the shear plane (edge of the Stern layer). In the Stern model, the Gouy- Chapman diffuse layer extends from the shear plane and the potential decays according to the ζ-potential instead of the surface potential.
Alternatively, a combination of large ion complex size and effects of the hydrogen bond network may be responsible for the reverse micelle growth when HAuCl4
is added since neither of these effects can be ruled out. The large (AuCl4) ion size could also be the driving force for it to selectively reside at the interface because of the already disrupted hydrogen bond network near the AOT headgroups, accommodating large ions more readily, and the need to maximize water-water and ion-water interactions in the system.47 An analysis of pH effects on electrolyte accommodation in reverse micelles
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.