Precipitation phase boundary

Fujiwara et al. studied the precipitation phase boundary diagrams of the sodium salts of a-sulfonated myristic and palmitic acid methyl esters in the presence of calcium ions [61]. The time dependency of the precipitation showed that the calcium salts have an extremely slow crystallization rate at room temperatures. This is the reason for the good hardness tolerance of the a-sulfonated fatty acid methyl esters. 

Studies of surfactant precipitation have concentrated on predicting the precipitation phase boundaries (i.e., the amount of added electrolyte necessary to cause... 

Glassification of Phase Boundaries for Binary Systems. Six classes of binary diagrams have been identified. These are shown schematically in Figure 6. Classifications are typically based on pressure—temperature (P T) projections of mixture critical curves and three-phase equiHbria lines (1,5,22,23). Experimental data are usually obtained by a simple synthetic method in which the pressure and temperature of a homogeneous solution of known concentration are manipulated to precipitate a visually observed phase. 

Contaminant precipitation involves accumulation of a substance to form a new bulk solid phase. Sposito (1984) noted that both adsorption and precipitation imply a loss of material from the aqueous phase, but adsorption is inherently two-dimensional (occurring on the solid phase surface) while precipitation is inherently three-dimensional (occurring within pores and along solid phase boundaries). The chemical bonds that develop due to formation of the solid phase in both cases can be very similar. Moreover, mixtures of precipitates can result in heterogeneous solids with one component restricted to a thin outer layer, because of poor diffusion. Precipitate formation takes place when solubility limits are reached and occurs on a microscale between and within aggregates that constitute the subsurface solid phase. In the presence of lamellar charged particles with impurities, precipitation of cationic pollutants, for example, might occur even at concentrations below saturation (with respect to the theoretical solubility coefficient of the solvent). 

Figure 23.1 is a phase diagram of a system that exhibits precipitation. If cooled along the path indicated, the a phase will become supersaturated with respect to the (3 phase when it crosses the phase boundary, and if there is no intervening spinodal, the /3 phase will then precipitate discontinuously in the a phase (matrix phase) as the system attempts to reach equilibrium. 

In general, the nuclei are not spherical. There are several reasons for this. One is that the a- 3 surface energy term, y p, depends on the orientations of the a and 3 phases. Another reason is that when nucleation occurs on oc-oc grain boundaries, the angle of contact, 0, between the a and 3 phases depends on the ratio of y p/yagb- A third reason is that the elastic strain energy is minimized if the precipitating phase is lenticular. 

The present review deals mainly with two examples of polyelectrolyte phase behavior as discussed above. As an example for an H-type precipitation, the solution properties of polyvinylpyridinium chains are monitored as function of added inert salt. Here, we focus on the determination of the effective charge density and of the solvent quality parameter which are supposed to play a central role for the understanding of polyelectrolyte solution without specific counterion interactions. The second system under investigation comprises the interaction of polyacrylic acid with alkaline earth cations which exhibit very specific interactions, thus representing an example for type L-precipitation. Here the coil dimensions close to the phase boundary are compared to those close to type H-precipitation with inert added salt. 

From the experiments just outlined [74-76], a few points are worth being emphasized A powerful procedure was developed to gradually approach phase boundaries of polyelectrolyte precipitation. The approaches can be performed in a highly systematic manner and lead to states which are located extremely close to the precipitation threshold. Approaches could successfully be accompanied by LS experiments. The experiments demonstrated, that the polyelectrolyte chains shrank dramatically in size immediately before the phase boundaries were reached. A sudden increase of the scattering intensity indicated the phase boundaries. These developments give rise to the hope that intermediates may be revealed which have not become accessible in preceding investigations [78-81]. 

As described in Sect. 3.2, the size of the NaPA coils shrink drastically when the precipitation line is approached. This is revealed by a decrease of the radius of gyration Rg and the hydrodynamically effective radius Rh- In exploiting this effect, data from various approaches to phase boundaries belonging to different inert salt levels and NaPA samples had been collected. Facing such an extensive set of data, a meaningful tool for its interpretation seems highly desirable. 

Clearly, the phase boundaries lie on top of each other. A closer look reveals a slight decrease in the slope r0 of Eq. (26) according to Ca2+>Sr2+>-Ba2+, which indicates that the larger the bivalent earth alkaline cation is, the smaller is the stoichiometric amount of M2+ necessary to precipitate NaPA. Although based on a different method, the present results suggest a comparison with data from Pochard et al. [73]. In doing so, we have to keep in mind that our own r0 values are of fair accuracy at best because they were evaluated from slopes based on a few data points only. Still, this trend in rQ is the opposite to the observation of Pochard et al. [73]. They found a decrease of the amount of M2+ per COO function at the precipitation threshold, if Ca2+... 

Interactions at the liquid-solid phase boundary 1.1.4.1 Dissolution and precipitation... 

Polymers Interact with surfactants and mlcroemulslons In diverse. Interesting and technologically Important wavs(1.21. The mechanisms that are responsible for the Interactions Include the usual panoply of forces Involved In the interaction of any two different molecules lon-lon, lon-dlpole, dlpole-dlpole, and van der Waals forces all modulated by the presence of solvent and/ or other species such as dissolved salts. All may play a role. The special factors Involved in surfactant/polymer and polymer-/mlcroemulslon Interactions that form the basis for their particular interest lies in their tendencies to form a variety of supermolecular clusters and conformations, which In tuim may lead to the existence of separate phases of coexisting species. Micelles may form In association with the polymer, polymer may precipitate or be solubilized, mlcroemulslon phase boundaries may change, and so on. 

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