Mixtures cloud points

Figuer4.31 shows polystyrene-polybutadiene mixture cloud point data taken from Fig.4.3 in Roe and Zin (1980) PS2-PBD2 (solid circles) PS3-PBD2 (open squares), and PS5-PBD26 (open triangles) m psi = 2220, m pss = 3500, m pss = 5200, tnpBDi = 2350, mpBD26 = 25000. Note that 4>i refers to PS. The solid lines are... 

The tendency to separate is expressed most often by the cloud point, the temperature at which the fuei-alcohol mixture loses its clarity, the first symptom of insolubility. Figure 5.17 gives an example of how the cloud-point temperature changes with the water content for different mixtures of gasoline and methanol. It appears that for a total water content of 500 ppm, that which can be easily observed considering the hydroscopic character of methanol, instability arrives when the temperature approaches 0°C. This situation is unacceptable and is the reason that incorporating methanol in a fuel implies that it be accompanied by a cosolvent. One of the most effective in this domain is tertiary butyl alcohol, TBA. Thus a mixture of 3% methanol and 2% TBA has been used for several years in Germany without noticeable incident. 

A melamine laminating resin used to saturate the print and overlay papers of a typical decorative laminate might contain two moles of formaldehyde for each mole of melamine. In order to inhibit crystallization of methylo1 melamines, the reaction is continued until about one-fourth of the reaction product has been converted to low molecular weight polymer. A simple deterrnination of free formaldehyde may be used to foUow the first stage of the reaction, and the build-up of polymer in the reaction mixture may be followed by cloud-point dilution or viscosity tests. 

Solubility of resins can be predicted in a similar way as for the solubility of polychloroprene rubbers in a solvent mixture (see Section 5.5) by means of solubility diagrams (plots of the hydrogen bonding index (y) against the solubility parameter (5). Another more simple way to determine the solubility of resins is the determination of the cloud point, the aniline and the mixed aniline points. 

Cloud point. Measures the solubility/compatibility of a resin with solvents. The value reported is the temperature at which a specific mixture of a resin and a solvent or solvents blend gives a cloudy appearance, having been cooled from a temperature at which the mixture was clear. Commonly, a test tube of a given diameter is used and the temperature is noted when the lower end of the thermometer, placed at the bottom of the tube, disappears. Resins with cloud points below 0°C are commonly regarded as soluble and cloud points greater than 10°C indicate poor solubility/compatibility. White spirit with various aromatic contents is a widely used solvent in the determination of cloud point, but other solvents or solvents mixtures are also used. 

Aniline and mixed aniline point (DIN 51 775 modified). It is similar to the cloud point test except that the solvent is aniline, a very polar liquid. The aniline point is defined as the temperature at which a mixture of equal parts of aniline and the resin show the beginning of phase separation (i.e. the onset of clouding). Phase separation for aromatic resins occurs between I5°C and below zero for resins with intermediate aromaticity, it lies between 30 and 50°C and for non-aromatic resins, it is 50 to 100°C. Sometimes the mixed aniline point is used. It is similar to the aniline point except that the solvent is a mixture of one part of aniline and one part of w-heptane. The problem of both procedures is that precipitation of resins can be produced before the cloud is generated. 

The critical point (Ij of the two-phase region encountered at reduced temperatures is called an upper critical solution temperature (UCST), and that of the two-phase region found at elevated temperatures is called, perversely, a lower critical solution temperature (LCST). Figure 2 is drawn assuming that the polymer in solution is monodisperse. However, if the polymer in solution is polydisperse, generally similar, but more vaguely defined, regions of phase separation occur. These are known as "cloud-point" curves. The term "cloud point" results from the visual observation of phase separation - a cloudiness in the mixture. 

The theta (0) conditions for the homopolymers and the random copolymers were determined in binary mixtures of CCl and CyHw at 25°. The cloud-point titration technique of Elias (5) as moaified by Cornet and van Ballegooijen (6) was employed. The volume fraction of non-solvent at the cloud-point was plotted against the polymer concentration on a semilogarithmic basis and extrapolation to C2 = 1 made by least squares analysis of the straight line plot. Use of concentration rather than polymer volume fraction, as is required theoretically (6, 7 ), produces little error of the extrapolated value since the polymers have densities close to unity. 

The surfactant selected for CPE technique should not have too high a cloud point temperature. In practice, it is possible to obtain almost any desired temperature by choosing an appropriate mixture of surfactants, as cloud point temperatures of mixtures of surfactants are intermediate between those of the two pure surfactants, or by the choice of an appropriate additive (i.e., salts, alcohols, organic compounds) [105]. 

O.OOl C) the BE-H2O systems were titrated (2.5 ml Gilmont syringe) with DEC until a slight cloudiness appeared, corresponding to the formation of two or more phases. For temperature studies, the temperature of a known mixture of BE-DEC-H2O was varied until a cloud point was observed. The temperature was read to 0.01 C with a pre-calibrated thermistor. In general this technique is fast and quite reproducible but difficulties are often encountered. 

Chen [8] studied mixtures of the pure surfactants Ci2(EO)4 and sodium dodecyl sulfate (SDS) at 30 °C. At this temperature the former is a liquid which does not dissolve in water (see Fig. 3), and the latter is a solid. The SDS was doubly recrystallized from ethanol to remove n-dodecanol and other impurities. The solubility of SDS in pure Ci2(EO)4 at 30 °C was found to be approximately 9 wt. %. When small drops of an 8 wt. % mixture were injected into water at 30 °C, complete dissolution was observed, the time required being a linear function of the square root of initial drop radius. For instance, a drop having an initial radius of 70 (xm required approximately 100 s to dissolve, significantly more than the 16 s cited above for a slightly larger drop of pure Ci2(EO)6. Behavior was similar to that of nonionic mixtures below their cloud points discussed previously in that most of the drop dissolved rapidly, but the final small volume dissolved rather slowly with some observable emulsification. 

Bai [2] performed similar drop dissolution experiments with sodium oleate (NaOl) and Ci2(EO)4. For drops initially containing 7 and lOwt. % NaOl (particle size < 38 jim) the behavior was similar to that described above for drops having 8 wt. % SDS. However for drops with 15 and 17 wt. % NaOl dissolution was faster—comparable to that of the pure nonionics—and neither a surfactant-rich liquid immiscible with water nor emulsification was seen. Instead a concentrated liquid crystalline phase transformed directly into a micellar solution, as seen for the pure nonionics and nonionic mixtures well below their cloud points. 

The explanation for this behavior is similar to that given in the preceding section for nonionic surfactant mixtures. Adding a hydrophihc anionic surfactant raises the temperature at the cloud point and other phase transitions above those for pure Ci2(EO)4. If the amount of anionic added exceeds only slightly that needed for complete solubility, the final stages of the dissolution process are slow because preferential dissolution of the anionic causes the remaining drop to rise above its cloud point and nucleate small droplets of surfactant-rich liquid. But if the amount added is sufficiently large, drop composition remains below the cloud point in spite of preferential dissolution, with the result that dissolution is fast as with pure nonionic surfactants below their cloud points. 

The effect of using mixtures of surfactants on micelle formation, monolayer formation, solubilization, adsorption, precipitation, and cloud point phenomena is discussed. Mechanisms of surfactant interaction and some models useful in describing these phenomena are outlined. The use of surfactant mixtures to solve technological problems is also considered. 

This overview will outline surfactant mixture properties and behavior in selected phenomena. Because of space limitations, not all of the many physical processes involving surfactant mixtures can be considered here, but some which are important and illustrative will be discussed these are micelle formation, monolayer formation, solubilization, surfactant precipitation, surfactant adsorption on solids, and cloud point Mechanisms of surfactant interaction will be as well as mathematical models which have been be useful in describing these systems,... 

In order to define a ionic/nonionic surfactant solution with high salinity/hardness tolerance, the following criterion should be followed. The mixed micelle should have as large of a negative deviation from ideality as possible. Surfactant mixture characteristics which result in this have already been discussed. The nonionic surfactant should have a high cloud point. Otherwise the amount of nonionic surfactant which can be added to the system is limited to low levels before phase separation occurs. If possible, a mixed ionic surfactant should be used for reasons Just discussed. There is no such benefit to using mixed nonionic surfactants, although this is not necessarily detrimental either. 

As the temperature of dilute aqueous solutions containing ethoxylated nonionic surfactants is increased, the solutions may turn cloudy at a certain temperature, called the cloud point. At or above the cloud point, the cloudy solution may separate into two isotropic phases, one concentrated in surfactant (coacervate phase) and the other containing a low concentration of surfactant (dilute phase). As an example of the importance of this phenomena, detergency is sometimes optimum just below the cloud point, but a reduction in the washing effect can occur above the cloud point (95). However, the phase separation can improve acidizing operations in oil reservoirs (96) For surfactant mixtures, of particular interest is the effect of mixture composition on the cloud point and the distribution of components between the two phases above the cloud point. 

The cloud point is close to, but not necessarily equal to the lower consolute solution temperature for polydisperse nonionic surfactants (97). These are equal if the surfactant is monodisperse. Since the lower consolute solution temperature is like a critical point for liquid—liquid mixtures, the dilute and coacervate phases have the same composition, and the volume fraction of solution which the coacervate comprises is a maximum at this temperature (98). If a coacervate phase containing a high concentration of surfactant is desired, the solution should be at a temperature well above the cloud point. 

The cloud point of a mixture of nonionic surfactants is intermediate between the pure nonionic surfactants involved (95.99) The cloud point of a dilute nonionic surfactant solution increases upon addition of ionic surfactant (95.98—104). The coacervate phase forms because of attractive forces between the micelles in solution. The incorporation of ionic surfactant into the nonionic micelles introduces electrostatic repulsion between micelles, causing coacervate phase formation to be hindered, raising the cloud point. 

B. Diphenaldehydic acid. A mixture of 10 g. (0.0368 mole) of 3,8-dimethoxy-4,5,6,7-dibenzo-l,2-dioxacyclooctane, 50 ml. of 10% sodium hydroxide solution, and 200 ml. of 95% ethyl alcohol is heated under reflux for 15 minutes, during which time the solid dissolves (Note 3). The solution is cooled, acidified with concentrated hydrochloric acid, and diluted to the cloud point with water. Crystallization is induced by rubbing the side of the vessel with a stirring rod (Note 4). More water is then added slowly until crystallization is complete. Filtration yields 6.7-7.3... 

The observation of CST with practical precision is usually very simple. The two liquids are placed in a test tube and are stirred with a thermometer while heating or cooling until the liquids just mix (while heating) or just cloud (on cooling). Determinations of the cloud point are usually more precise than determinations of the temperature of disappearance of two phases. There is very little risk of subcooling a liquid mixture below the CST, and having it remain homogeneous. When the upper layer becomes small before it disappears, more of the major component of the upper layer is added, and the observation is repeated until the interface disappears near the middle of the system. This is necessary in order to... 

Figure 5.1-2. Phase behaviour of ethylene LDPE mixtures [10]. Thick line, cloud-point curve dotted line, shadow curve thin lines, co-existence curves total polymer concentration a, 6.1 wt.% b, 11.4 wt.% c, 18.6 wt.% d, 28.0 wt.% e, 36.5 wt.%.

---