Cloud point: defined

The method consists of preparation of a series of mixtures near the phase separation condition then causing the separation to occur. The onset of turbidity is observed either visually, using a photoelectric cell, or UV-visible spectrophotometer. The ensemble of the cloud points defines the cloud-point curves (CPC) that closely follows the binodal. The method can be extended to rigorous studies of the phase separation by measuring the light scattering intensity. Many refinements of the original turbidity method have been made [Utracki, 1989]. 

Turbidity Polymer can be precipitated fi om dilute solution by adding a non-solvent. The cloud point defines the onset of two phases. The enthalpic component of the interaction parameter, xh, can be related to the solubility parameter difference between the solvent and the polymer (Eq. B 17). Two different non-solvents are used one having a solubility parameter above that of the solvent, and another having a solubility parameter below that of the solvent. The enthalpic components of the interaction parameter for each phase are equal at the cloud point, which can be used to measure the solubility parameter of the polymer (116). The two expressions for xh arc... 

The nature of these paraffins and their concentration in diesel fuel affect the three temperatures that characterize the cold behavior. The cloud point is the temperature at which crystals of paraffins appear when the temperature is lowered. The cold filter pluming point is defined as the temperature under which a suspension no ionger flows through a standard filter. Finally, the pour point is the temperature below which the diesel fuel no longer flows by simple gravity in a standard tube. These three temperatures are defined by regulations and the refiner has three types of additives to improve the quality of the diesel fuel of winter. 

Poly(methyl vinyl ether) [34465-52-6] because of its water solubility, continues to generate commercial interest. It is soluble in all proportions and exhibits a well-defined cloud point of 33°C. Like other polybases, ie, polymers capable of accepting acidic protons, such as poly(ethylene oxide) and poly(vinyl pyrroHdone), each monomer unit can accept a proton in the presence of large anions, such as anionic surfactants, Hl, or polyacids, to form a wide variety of complexes. 

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. 

When two atoms approach each other so closely that their electron clouds interpenetrate, strong repulsion occurs. Such repulsive van der Waals forces follow an inverse 12th-power dependence on r (1/r ), as shown in Figure 1.13. Between the repulsive and attractive domains lies a low point in the potential curve. This low point defines the distance known as the van der Waals contact distance, which is the interatomic distance that results if only van der Waals forces hold two atoms together. The limit of approach of two atoms is determined by the sum of their van der Waals radii (Table 1.4). 

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. 

Cloud Point Measurements Cloud points were recorded by the visual observation of aqueous solutions containing 1% W/V surfactant. The measurement defines the temperature at which the system under test shows a characteristic transitional change from a clear solution to an opalescent or cloudy state. All cloud points were recorded in both ascending and descending temperature cycles to ensure data confidence. The influence of salt and/or oils on the cloud point were systematically evaluated. 

Turbidimetry is ideally suited to detect the temperature at which a transparent polymer solution turns opaque. The temperature corresponding to the onset of the increase of the scattered light intensity is usually taken as the cloud-point temperature, TcP, although some authors define the cloud point as the temperature for which the transmittance is 80% (or 90%) of the initial value. This technique is commonly known as the cloud-point method [199]. Turbidimetry was employed, for instance, to show that the cloud-point temperature of aqueous PNIPAM solutions does not depend significantly on the molar mass of the polymer [150]. 

Frequently the cloud boundary defined by a fixed concentration is required. The line connecting points of equal concentration around the cloud boundary is called an isopleth. For a specified concentration (C) the isopleths at ground level are determined by dividing the equation for the centerline concentration (Equation 5-40) by the equation for the general ground-level concentration (Equation 5-39). This equation is solved directly for y ... 

Lower Critical Solution Temperatures LCSTs were determined from plots of optical density at 600 nm versus temperature for 0.03% solutions of each polymer in PBS and were defined as the temperature at which Asoo = 0.1. Temperatures were raised at less than 0.3 C per minute and were measured with a thermometer that had been calibrated against an NBS primary standard thermometer. LCSTs for Figure 6 were determined from the cloud points of 0.01% solutions. 

Curve of temperature vs. composition defined by the cloud points over range of compositions of two substances. 

The properties of solutions of macromolecular substances depend on the solvent, the temperature, and the molecular weight of the chain molecules. Hence, the (average) molecular weight of polymers can be determined by measuring the solution properties such as the viscosity of dilute solutions. However, prior to this, some details have to be known about the solubility of the polymer to be analyzed. When the solubility of a polymer has to be determined, it is important to realize that macromolecules often show behavioral extremes they may be either infinitely soluble in a solvent, completely insoluble, or only swellable to a well-defined extent. Saturated solutions in contact with a nonswollen solid phase, as is normally observed with low-molecular-weight compounds, do not occur in the case of polymeric materials. The suitability of a solvent for a specific polymer, therefore, cannot be quantified in terms of a classic saturated solution. It is much better expressed in terms of the amount of a precipitant that must be added to the polymer solution to initiate precipitation (cloud point). A more exact measure for the quality of a solvent is the second virial coefficient of the osmotic pressure determined for the corresponding solution, or the viscosity numbers in different solvents. 

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. 

Figure 6. Conventional two-component phase behavior in poly disperse Flory-Huggins theory, shown in the (p, p0) plane for three values of % As in Fig. 5, the parent has Ln = 100 and Ly/ = 150 (hence a = 2). Along the y-axis, we plot L//p0 rather than p0 so that the dilution line p = LNp0, shown as the thick solid line in (a-c), is simply along the diagonal. With x considered as an additional variable, the dilution line constraint defines a plane (p, = L/vPq, x)- The last plot, (d), shows the cut by this plane through the phase behavior in (a-c) the solid line is the cloud point curve, and the dashed line is the spinodal stability condition.

Phase separation begins when the cloud-point curve reaches the point a at (4>mo>T ). This is defined as the cloud-point conversion, xCp, which is usually lower than the gel conversion, xgei. 

A variety of well-defined poly(m-benzamide)s were synthesized from the corresponding monomers (Scheme 96). All these polymers have higher solubility than the para-substituted counterparts [322], Especially, the polyamides having an oligo(ethylene glycol) are soluble in water and show reversible cloud points by heating [323]. 

The cloud point is defined as the temperature at which a molten fat sample becomes cloudy, owing to the appearance of fat crystals, while being cooled. Versions of the test, which is a simple subjective one, are described in AOCS Official Method Cc 6-25 (Firestone, 1998) and in British Standard BS 684 Section 1.5 1987 (BSI, 1987). As supercooling is necessary to induce fat crystallization, the cloud point is always lower than the clear point (Kayle-gian and Lindsay, 1994). 

Phase separation in ternary systems may be studied using the cloud point isotherm method. Solvent, here defined as component 1, is added to a turbid mixture containing a mixture of components 2 and 3 until the agitated mixture becomes clear. A series of such experiments at various component 2-component 3 weight ratios establishes the cloud point... 

Haze point is another quality assurance tool that can be used in conjunction with cloud point. Haze point is defined as the point at which the sample starts to become hazy as it is cooled to the cloud point. In a study of partially reacted versus fully reacted methyl esters of tallow and lard, it was found that fully reacted methyl esters exhibit haze points much higher on esters in which approximately 95% of the original fatty acids as acylglycerols had been reacted to methyl ester, the haze points ranged from 5-8°C higher than the cloud points. As measured in this manner, haze points are not approved methods by AOCS standards however, the manufacturer or user of methyl esters may find these data useful. 

To investigate the effect of adding monomeric and dimeric CD on the change of phase transition temperature of the polymers P18 and P19, we performed turbidity measurements at the same polymer concentration as above in the presence of a defined amount of Me-P-CD and CD dimer, respectively (Fig. 25 a,b) [61,62], As reported previously [56-59], we found that addition of Me-p-CD led to cloud points of polymers P18 and P19 of 32°C, which correlates to the LCST of pure poly(NIPAAM). This increase of the cloud points relative to the cloud points of pure P18 and P19 results from the inclusion of the hydrophobic adamantyl units by Me-P-CD. 

---