Relaxation processes, microemulsions

The first type of relaxation processes reflects characteristics inherent to the dynamics of single droplet components. The collective motions of the surfactant molecule head groups at the interface with the water phase can also contribute to relaxations of this type. This type can also be related to various components of the system containing active dipole groups, such as cosurfactant, bound, and free water. The bound water is located near the interface, while free water, located more than a few molecule diameters away from the interface, is hardly influenced by the polar or ion groups. For ionic microemulsions, the relaxation contributions of this type are expected to be related to the various processes associated with the movement of ions and/ or surfactant counterions relative to the droplets and their organized clusters and interfaces [113,146]. 

For percolating microemulsions, the second and the third types of relaxation processes characterize the collective dynamics in the system and are of a cooperative nature. The dynamics of the second type may be associated with the transfer of an excitation caused by the transport of electrical charges within the clusters in the percolation region. The relaxation processes of the third type are caused by rearrangements of the clusters and are associated with various types of droplet and cluster motions, such as translations, rotations, collisions, fusion, and fission [113,143]. 

The third relaxation process is located in the low-frequency region and the temperature interval 50°C to 100°C. The amplitude of this process essentially decreases when the frequency increases, and the maximum of the dielectric permittivity versus temperature has almost no temperature dependence (Fig 15). Finally, the low-frequency ac-conductivity ct demonstrates an S-shape dependency with increasing temperature (Fig. 16), which is typical of percolation [2,143,154]. Note in this regard that at the lowest-frequency limit of the covered frequency band the ac-conductivity can be associated with dc-conductivity cio usually measured at a fixed frequency by traditional conductometry. The dielectric relaxation process here is due to percolation of the apparent dipole moment excitation within the developed fractal structure of the connected pores [153,154,156]. This excitation is associated with the selfdiffusion of the charge carriers in the porous net. Note that as distinct from dynamic percolation in ionic microemulsions, the percolation in porous glasses appears via the transport of the excitation through the geometrical static fractal structure of the porous medium. 

The two relaxation processes were evaluated both for the alcohol/dodecane mixtures (Fig. 29) and for the microemulsions (Fig. 30). In order to understand the way to calculate the amount of alcohol at the interface and in the oil phase, let us first analyze the alcohol/ dodecane binary mixture. 

The short dielectric relaxation process (presented here by 12) is associated with the anisotropic motion of the monomer alcohol species in a chain cluster (149). In microemulsions, the short process is the superposition of several dielectric relaxation processes, which have similar relaxation times such as movement or rotation of the alcohol monomers, hydrate water, and surfactant polar head groups. The short relaxation time is barely affected by the alcohol concentration in the mixture since it is less sensitive to the aggregation process. 

Information about the bound water fraction in some colloid systems, silica gels, and biological systems is usually inferred on die basis of the frequency- and time-domain DS measurements from the analysis of the dielectric decrements or die relaxation times (64, 150-152). However, the nonionic microemulsions are characterized by a broad relaxation specfrum as can be seen from the Cole-Cole plot (Fig. 33). Thus, these dielectric methods fail because of the difficulties of deconvoluting die relaxation processes associated widi the relaxations of bound water and surfactant occurring in the same frequency window. 

In microwave dielectric measurements (> 30 GHz) the dieleclric permittivity and dielectric losses for bound and free water show significantly different magnitudes. Thus, in measurements at high microwave frequencies the contribution from bound water in the dieleclric losses will be negligibly small, and the contribution from the free water fraction can be found. In contrast to the above-mentioned procedures used for calculation of bound water from the relaxation spectrum analysis, this approach will not involve analyses of overlapping relaxation processes and can thus easily be applied to microemulsions having a complex relaxation spectrum. 

Microemulsions consist of either three or four components two solvents, a surfactant, and sometimes an alcohol/cosurfactant. This complexity of composition means that there are potentially many relaxation processes. Despite this, microemulsion kinetics has been relatively well researched due to sustained interest in their structure and optimization. There have been several important reviews of the area, including summaries of work on the dynamic processes in such systems [100,101]. 

The effect of alcohol on the dynamic properties of micellar systems has been considered as a first approach toward the understanding of microemulsion systems. In mixed alcohol + surfactant micelles, the theory predicts the existence of three relaxation processes, which have been experimentally observed using chemical relaxation techniques a slow process associated with the formation/breakdown of mixed micelles and two fast processes associated with the exchange of the surfactant and alcohol, respectively, between the mixed micelles and the bulk aqueous phase. With g representing a mixed micelle with a alcohol (A) molecules and s surfactant (S) molecules, these two exchange reactions can be written in the form... 

In a first approach, Lang et al. have studied the ultrasonic relaxation of microemulsions whose compositions were chosen so as to follow specific pathways in the ternary or pseudoternary phase diagrams. Many different systems were studied, varying the nature of the components. The ultrasonic relaxation spectra were measured in the range 1 to 156 MHz, showing the existence of one or two relaxation processes ... 

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-... 

The majority of the different chemical and physical properties, as well as the morphology of microemulsions, is determined mostly by the micro-Brownian motions of its components. Such motions cover a very wide spectrum of relaxation times ranging from tens of seconds to a few picoseconds. Given the complexity of the chemical makeup of microemulsions, there are many various kinetic units in the system. Depending on their nature, the dynamic processes in the microemulsions can be classified into three types ... 

We have also shown in a recent study (41) that the addition of a short chain alcohol (propanol) increases the rate of micelle formation-dissolution significantly. The amplitude of Xgi, process in the water-rich region is very small and hence the resolution of relaxation time xsiow is poor. Within the range of experimental accuracy, Xgioj, was found to be independent of the microemulsion composition. 

Another approach to determining the viscoelastic properties of dense microemulsions at high frequencies is to conduct ultrasonic absorption experiments. In such experiments it has been found that the percolation process is correlated to a shift of the ultrasonic dynamics from a single relaxation time to a distribution of relaxation times [121]. Other experiments showed an increase in the hypersonic velocity for samples at and beyond the percolation threshold. The complex longitudinal modulus deduced from such experiments is also correlated with the occurrence of the percolation phenomenon, which suggests that the velocity dispersion is clearly correlated with structural transformations [122]. 

Unlike the interphase peak, the other main depolarization peaks observed, respectively, at + 35°C and + 0.4°C, do occur within a well defined concentration interval and are no more detectable outside a given liquid-crystalline region we interpreted these peaks as structure-peaks due to the presence of either liquid-crystalline structure. It should be noted that, since liquid-crystalline meso-phases of w/o microemulsions are both lyotropic and thermotropic, the structure peaks depend on the polarizing temperature, whereas the interphase peak does not. The above result indicates that each liquid—crystalline phase has a threshold temperature above which that given structure is destroyed. Considering the behavior of both the activation energy and the relaxation time of the interface--polarization process (Fig. 6), we may conclude that, as far as the interphase peak is concerned, a transition occurs at c = 0.580. 

We have seen earlier that the microemulsion formation is a spontaneous process which is controlled by the nature of amphiphile, oil, and temperature. The mechanical agitation, heating, or even the order of component addition may affect microemulsification. The complex structured fluid may contain various aggregation patterns and morphologies known as microstmctures. Methods like NMR, DLS, dielectric relaxation, SANS, TEM, time-resolved fluorescence quenching (TRFQ), viscosity, ultrasound, conductance, etc. have been used to elucidate the microstructure of microemulsions [25,26]. 

In three-component microemulsion systems there are two order parameters that describe the fluctuation of the system i.e. oil volume fraction against water P = (pj ((/>o + 0s) and surfactant volume fraction 0s- The two decaying processes observed in NSE spectra might be correlated to the relaxation of these two order parameters. 

Many modem technologies depend on the optimum use of surfactants. The applied concentrations are often above the critical micelle concentration (CMC) and special effects are direcdy related to the presence of micelles. This is tme for example in cleaning and detergency [1], encapsulation of drugs in micelles (2, 3] or microemulsions [4], and many others [5]. The important parameters of micellar solutions are the CMC and the aggregation number n. The formation and dissolution of aggregates or the release or incorporation of single molecules are controlled by the relaxation times of slow and fast processes. Their values, however, depend on the models applied. 

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