Intermediate-salinity diffusion

Figure 9. Diagram of intermediate-salinity diffusion phenomena in the TRS system.

High-Salinity Diffusion Phenomena. Just as at intermediate salinities, the TRS system exhibited extensive convection at high salinities. The rate of phase equilibration was extremely rapid. Typically, the interfaces in this system moved further in a few hours than those in the PDM system moved in two weeks. 

FIGURE 9.12 High-salinity diffusion path for contact of composition D with oil (O) indicating an intermediate brine phase (b) and spontaneous emulsification in the oil phase (s.e.). Ic and w/o denote the lamellar liquid crystalline phase and a water in oil microemulsion, respectively. S/A denotes the surfactant/alcohol mixture in this pseudoternary diagram. (From Raney, K.H. and Miller, C.A., AIChE J., 33, 1791, 1987. With permission.)... 

Below the main pycnocline, one finds the layer that is named sometimes generally as the deep layer. The present-day concepts about the vertical structure of its upper part have already been considered (see Fig. 2b). Below the intermediate isothermal layer, in the depth range from 700 to 1700 m, one observes a layer with a slow increase in the temperature and salinity with depth sometimes broken by T,S inversions with vertical scales about 10 m, which is typical of the fine T,S structure of the waters [11] (Fig. 3b). Theoretical estimates [13] show that they may result from the thermal type of double diffusion (layered convection), which is the principal mechanism of the vertical heat and salt exchange in this layer. 

The identity of the intermediate phase formed at these conditions can be deduced from the relative movement of the interfaces. Because the phase grew quickly in the direction of the aqueous surfactant solution, it contained predominantly brine. Although small in quantity, some oil did diffuse into it. From this information and from its isotropic appearance, one can conclude that the intermediate phase was an oil-in-water microemulsion. Additional support for this conclusion is that this type of microemulsion is an equilibrium phase at low salinities. 

A comparison between experimental and theoretical results shows that diffusion path analysis can qualitatively predict what is observed when an anionic surfactant solution contacts oil. Experimentally, one or two intermediate phases formed at all salinities. The growth of these phases was easily observed through the use of a vertical-orientation microscope. Except when convection occurred due to an intermediate phase being denser than the phase below it, interface positions varied as the square root of time. As a result, diffusion path theory could generally he used to correctly predict the direction of movement and relative speeds of the interfaces. 

Complete information on phase behavior including tie-lines and on diffusion coefficients is rarely available for oil-water-surfactant systems. Nevertheless, Raney and Miller used plausible phase diagrams for an anionic surfactant-NaCl brine-hydrocarbon system as a function of salinity to calculate diffusion paths that exhibited intermediate phase formation and spontaneous emulsification in agreement with experimental observations made using the vertical cell technique. For example. Figure 9.12 shows a diffusion path for a surfactant-alcohol-brine mixture of composition D in contact with oil for a case when initial salinity is high. An intermediate brine phase is predicted as well as spontaneous emulsification in the oil phase, both of which were, in fact, observed. 

Mixtures containing 1 wt% of the pure nonionic surfactant C,2E5 in water were contacted with pure n-hexadecane and n-tetradecane at various temperatures between 25 and 60°C using the vertical cell technique. Similar experiments were performed with C,2E4 and n-hexadecane between about 15 and 40°C. In both cases the temperature ranged from well below to well above the phase inversion temperature (PIT) of the system, i.e., the temperature where hydrophilic and lipophilic properties of the surfactant are balanced and a middle phase microemulsion forms (analogous to the optimal salinity for ionic surfactants mentioned above). The different intermediate phases that were seen at different temperatures and the occurrence of spontaneous emulsification in some but not all of the experiments could be understood in terms of known aspects of the phase behavior, e.g., published phase diagrams for the C12E 5-water-n-tetradecane system, and diffusion path theory. That is, plausible diffusion paths could be found that showed the observed intermediate phases and/or spontaneous emulsification for each temperature. 

Figure 6.24a (Raney and Miller, 1987) shows the calculated diffusion path for a situation where an anionic snrfactant-alcohol-brine mixture of composition D is contacted with a comparable volume of a hydrocarbon near the optimal salinity. The main features of the phase diagram, including the three-phase region and the existence of the initial mixture as a dispersion of the lamellar liquid crystal and brine, are consistent with known behavior. HowevCT, the precise locations of the phase boundaries were assumed because of the lack of experimental information. The diffusion path (dotted lines) indieates that both brine and a microemulsion should form as intermediate phases, as was in fact observed expraimentally in such systrans. Figure 6.24b is an expanded view of the oil...

---