Microemulsions droplet clustering

Lagues et al. [17] found that the percolation theory for hard spheres could be used to describe dramatic increases in electrical conductivity in reverse microemulsions as the volume fraction of water was increased. They also showed how certain scaling theoretical tools were applicable to the analysis of such percolation phenomena. Cazabat et al. [18] also examined percolation in reverse microemulsions with increasing disperse phase volume fraction. They reasoned the percolation came about as a result of formation of clusters of reverse microemulsion droplets. They envisioned increased transport as arising from a transformation of linear droplet clusters to tubular microstructures, to form wormlike reverse microemulsion tubules. 

The most reasonable explanation for the increase in apparent hydrodynamic diameter measured by DLS is the enhanced micelle-micelle interactions as the boundary of a two-phase system is approached (i.e., the pressure is lowered). Figure 4 illustrates this concept of micelle-micelle interactions, which is manifested as aggregation (or clustering) of the reverse micelle or microemulsion droplets. Since the solvent environment is essentially unchanged by this "macromolecular aggregation" (Ui) we exclude the possibility of (other than transitory) micelle-micelle coalescence to form stable, larger micelles. The micelles may coalesce briefly to form transitional species (which might be a "dumbbell" or more cylindrical structures), in which the water cores collide and intermix. 

Interesting effects are observed when a dispersion contains both larger and smaller particles the latter are usually polymer coils, spherical or cylindrical surfactant micelles, or microemulsion droplets. The presence of the smaller particles may induce clustering of the larger particles due to the depletion attraction (see Section 5.4.S.3.3, above) such effects are described in the works on surfactant-flocculated and polymer-flocculated emulsions. Other effects can be observed in dispersions representing mixtures of liquid and solid particles. Yuhua et al. ° have established that if the size of the solid particles is larger than three times the size of the emulsion drops, the emulsion can be treated as a continuous medium (of its own average viscosity), in which the solid particles are dispersed such treattnent is not possible when the solid particles are smaller. 

The transition from non-adsorbing to adsorbing polymers can be achieved simply by changing the surfactant and thus increasing the attraction between polymer and surfactant. Hydrophilic polymers in w/o-droplet microemulsions lead to polymers incorporated in the droplets (Scheme 4.5). Attractive interactions lead to adsorption at the inside of the surfactant film. With increasing chain length confinement effects eventually occur (Scheme 4.6). In this case, the polymer is incorporated in more droplets and the droplets form clusters. Polymers adsorbing on the outside can also lead to droplet clusters. 

Scheme 4.6 Increasing the polymer size of an adsorbed confined polymer inside a droplet microemulsion. Large polymers lead to droplet clusters. Polymers adsorbed on the outside can also lead to droplet clusters.

The applied electric field interacts with the instantaneous dipole moments that may arise from uneven charge distribution due to the fluctuating shape of the individual microemulsion droplets. The ensuing reorientation of an instantaneous dipole by the field may involve the rotation of a droplet as a whole or a peristaltic type of rotation in which the instantaneous dipole moment vector rotates in (and the associated structural distortion propagates as a wave on the surface of) the droplet, which itself is either stationary or may also rotate but not necessarily in a correlated fashion. A peristaltic rotation of the instantaneous dipole may be the only mode of rotation on the observed time scale for droplets locked in clusters. If any of these descriptions applies, the time constant of the associated rise of the birefringence (t/) may be referred to as rotational relaxation time. 

Figure 8 Schematic representation of the processes leading to birefringence (and turbidity) in a W/O microemulsion, in relation to an applied electric square pulse E. Below a (second) threshold value of the field strength and far from critical conditions, or under any conditions if the pulse is terminated at a time indicated by the dashed line, only birefringence is observed due to the formation of AJ, and Above the threshold of the field strength, close to critical conditions, and with a sufficiently long square pulse, turbidity contributes to the signal due to phase separation or/and percolation. The double wall of the particles symbolizes the water/oil interface. Symbols A, surfactant monomer An, microemulsion droplet (An), cluster LCmp, liquid-crystalline microphase or/and percolation structure. Primed symbols stand for polarized structures oriented parallel to E (- ) reversible step with respect to turning the field on or off (->) irreversible step. (Reprinted with permission from Refs. 6 and 41. Copyright 1989 and 1994 American Chemical Society.)...

In typical microemulsion systems, a prominent composition region shows bicontinuous structures. With probes and quenchers confined to either oil or water, the domains in the bicontinuous region may be so large in all dimensions that normal exponential decays are observed. Only in the region with discrete droplets, and in a transition region where droplets cluster and merge, can the micellar type of quenching be expected. However, if the amphiphilic probes and quenchers are bound to the interfacial surfactant film in the bicontinuous microemulsions, one would expect 2-D behavior. 

Figure 8 The three different quenching pathways within droplet clusters in microemulsions. L denotes an excited lumophore and Q a quencher molecule. Although in reality the different reaction pathways compete, for illustration purposes they are pictured here as subsequent processes. After a quenching reaction has finished, the considered droplet is shaded.

For those systems near a phase transition, the apparent hydrodynamic diameter of the droplets (or the correlation length), as calculated using the Stokes-Einstein equation, appears to decrease as pressure increases [2,4,39]. For example, the apparent hydrodynamic diameter of a microemulsion droplet (for [surfactant] = 150 mM and 5) in supercritical xenon [2] decreases from 6.5 to 4.5 nm as pressure is increased from 350 to 550 bar (10 bar = 1 MPa). This effect is due to the change in the extent of micelle clustering rather than an actual change in the micelle size. 

The above energetic rationale for clustering of microemulsion droplets has been amply exploited by workers in this discipline [48-54]. To estimate A7/°i, microemulsion at a fixed o) was diluted with several portions of oil, and the for all the diluted preparations were determined from conductance measurements. The AG°i values were calculated from the system compositions. The rest was graphical and... 

It may be mentioned here that the conductance of w/o microemulsions also becomes a function of the pressure. Boned et al. [70,71] have studied the effect of pressure up to 1000 bar on the conductance and viscosity of two microemulsion systems (a) water-AOT-undecane, and (b) glycerol-AOT-isooctane. The first system was an aqueous microemulsion, and the second was a nonaqueous one. The systems were also studied as a function of volume fraction, in the dispersion ( = water plus AOT or glycerol plus AOT). In both systems, the percolation thresholds, were obtained. In aqueous microemulsion, as the pressure increased, the f continuously decreased indicating higher droplet-droplet attractive interaction whereas in nonaqueous microemulsion there was almost no change in the , values. It seems from the above that there was droplet cluster formation in aqueous... 

B.K. Paul, R.K. Mitra, and S.P. Moulik 2006 Microemulsions Percolation of condnc-tion and thermodynamics of droplet clustering, in Encyclopedia of Surface and Colloid Science ed. P. Somasundaran, 2nd Edition, Taylor Francis, Boca Raton, EL, pp. 3927-3956. 

S.P. Moulik, G.C. De, B.B. Bhowmik, and A.K. Panda 1999 Physicochemical studies on microemulsions. 6. Phase behavior, dynamics of percolation, and energetics of droplet clustering in water/AOT/ -heptane system influenced by additives (sodium cholate and sodium salicylate), J. Phys. Chem. B 103, 7122-7129. 

P. Alexandradis, J.P. Holzwarth, and T.A. Hatton 1995 Thermodynamics of droplet clustering in percolating AOT water-in-oU microemulsions, J. Phys. Chem. 99, 8222-8232. 

S.K. Hait, A. Sanyal, and S.P. Moulik 2002 Physicochemical studies on microemulsions. 8. The effects of aromatic methoxy hydrotropes on droplet clustering and understanding of the dynamics of conductance percolation in water/oil microemulsion systems, J. Phys. Chem. B 106, 12642-12650. 

Physiochemical studies on microemulsions. 7. Dynamics of percolation and energetics of clustering in water/AOT/isooctane and water/AOT/decane w/o microemulsions in the presence of hydrotopes (sodium salicylate, a-napthol, 3-napthol, resorcinol, catechol, hydroquinone, pyrogallol and urea) and bile salt (sodium cholate). J Phys Chem 105, 7145-7154 (b) Hait, S. K., Sanyal, A., and Moulik, S. P. (2002). Physiochemical studies on microemulsions. 8. The effects of aromatic methoxy hydrotropes on droplet clustering and understanding of dynamics of conductance percolation in water/oU microemulsions. J Phys Chem B 106,12642-12650. 

Malo de Molina P, Appavou M-S, Gradzielski M (2014) Oil-in-water microemulsion droplets of TDMAO/decane interconnected by the telechelic C18-EO150-C18 clustering and network formation. Soft Matter 10(28) 5072-5084... 

Tekle and Schelly have studied the electric birefringence of AOT/isooctane/water W/0 microemulsions, which revealed two distinct relaxation processes on timescales of the order of 10 and 100 ps, respectively. The fast relaxation was attributed to the polarization/alignment of the individual reverse microemulsion droplets the slow relaxation, of smaller amplitude, was assigned to the linearization/reorien-tation of the micellar clusters. The rates of both processes became slower when w, or the AOT concentration, or the temperature was increased. Transient phase separation could occur beyond some threshold values of the preceding param-... 

---