The Solubility of Macromolecules

Because of the size of macromolecules, rates of diffusion and conformation changes accompanying dissolution (or precipitation) may be very slow, and significant amounts of time may be required for the system to reach equilibrium. The fine line between miscibility and inuniscibility and the relatively long equilibrium times involved are very important to the comportment of macromolecules in solution and at various surfaces and interfaces. Understanding the effects produced by the special behavior of polymers is of great theoretical and practical importance. 

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For m -> oo, the critical value is identical with that in a d- solvent, i.e., A2 = 0 and X = 0-5- Since the solubility of macromolecules decreases with increasing molecular weight, it is possible to separate these materials with respect to their molecular weights by changing the composition of the solvent and/or the temperature. In general, one roughly distinguishes between two methods, namely fractional precipitation and fractional extraction. 

The solubility of macromolecules like proteins in water generally increases in the presence of a suitable concentration of salt, so-called salting in. Furthermore, increasing the concentration of salt further leads to a decrease in the solubility of the proteins and their precipitation (salting out). 

The discussion of real (i.e., non-ideal) polymer solutions will be deferred till section 6 d. We shall first consider the solubility" of macromolecules from the point of view of ideal solutions. This aspect has played an important part in the physical chemistry of high polymers especially in the theory of fractional precipitation. 

The solubility of macromolecules as a rule improves with the rising temperature. Solvent - polymer mixtures usually exhibit the upper consolute temperature or upper critical solution temperature, UCST, with a maximum on the plot of system concentration versus temperature. Above the critical solution temperature, polymer is fully soluble at any concentration. For practical work, the systems with UCST below ambient temperature are welcome. There are, however numerous polymer - solvent systems, in which the solvent quality decreases with increasing temperature. The plot of system concentration versus temperature exhibits a minimum. The phenomenon is called lower consolute temperature or lower critical solution temperature, LCST Polymer is only partially soluble or even insoluble above lower critical solution temperature. This unexpected behavior can be explained by the dominating effect of entropy in case of the stiff polymer chains or by the strong solvent - solvent interactions. The possible adverse effect of rising temperature on polymer solubility must be kept in mind when woiking with low solubility polymers and with multicomponent mobile phases. It may lead to the unforeseen results especially in the polymer HPLC techniques that combine exclusion and interaction retention mechanisms, in coupled methods of polymer HPLC (see section 11.8, Coupled Methods of Polymer HPLC). 

Liquid chromatography at the critical adsorption point (LC LCS) was investigated. The solubility of macromolecules, the sensitivity of LC LCS to such variables as the injection volume, concentration injected, mobile phase flow rate and mixing between the injection zone, were investigated, using polymethyl methacrylate (PMMA) and polyacrylamide standards. It is proposed that using tetrahydrofuran and n-hexane as eluents for PMMA, at the LC LCS, limited polymer insolubility or even local precipitation is combined with the size exclusion of macromolecules. 12 refs. 

A carboxylic acid can be represented as R — CO2 H. Many different carboxylic acids participate in organic chemistry and biochemishy. Although carboxylic acids react in many different ways, breaking the C—OH bond is the only reaction that is important in polymer formation. A carboxylic acid is highly polar and can give up H to form a carboxylate anion, R — CO2. The carboxyl group (— CO2 H) also forms hydrogen bonds readily. These properties enhance the solubility of carboxylic acids in water, a particularly important property for biochemical macromolecules. 

Laurent, T. C., The interaction between polysaccharides and other macromolecules. 5. The solubility of proteins in the presence of dextran, Biochem.., 89, 253, 1963. 

Several block and graft copolymers have been shown to form stable aggregates under thermodynamically poor solvent conditions, as a result of differences in the solubility of different parts of a macromolecule. Whereas in a good solvent the experimentally measured value of A2 for a copolymer represents the balance of all the multiple interactions, under thermodynamically poor conditions A2 is mainly determined by the interaction of the groups situated on the polymer-solvent interface. Groups which form the hydrophobic core and are not in a contact with the solvent do not contribute significantly to the solution properties of the copolymer. 

The presence of water-soluble macromolecules in solution at submicel-lar concentrations has been reported to enhance the water solubility of hydro-phobic organic chemicals in several instances [19, 106, 113]. The presence of macromolecules in solution can enhance the apparent solubility of solutes by sorptive interactions in the solution phase. The processes by which macromolecules enhance the solubility of pollutants are probably variable as a function of the particular physical and chemical properties of the system. A macromolecule possessing a substantial nonpolar region can sorb a hydrophobic molecule, thereby minimizing the interfacial tension between the solute and the water. 

Although the soaking method for heavy-atom derivative preparation is by far the simplest and most common, it is not the only method used. One can first derivatize the macromolecule, and then crystallize. This procedure is less frequently used because of drawbacks such as the inabihty to produce isomorphous crystals due to the disruption of intermolecular contacts by the heavy atoms. Other frequent problems are the introduction of additional heavy-atom sites (a potential complicating factor in phasing) by exposing sites hidden by crystal contacts, and changing the solubility of the derivatized macromolecule. 

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. 

The solubility of polymers is determined by the interactions between macromolecules and the molecules of the solvent. But the prediction of the solubility of a macromolecule and hence the correlation to its chemical (and morphological) structure is much more complicated than for a low-molecular-weight compound. Nevertheless, some general rules do exist ... 

The dissolution of macromolecules is a prerequisite for the application of liquid chromatography for their separation and characterization. Compared to HPLC of small molecules, concentration of the polymer solutions injected into the analytical HPLC columns is higher and usually assumes lmg mL and more. This is mainly due to detection problems the detectors used in polymer HPLC are much less sensitive (Section 16.9.1) than detectors for small molecules, which often carry the UV-absorbing chromophores. This means that samples subject to polymer HPLC must exhibit rather high solubility. 

Basically all factors are important which display an effect on the solubility of the macromolecule and its flexibility, i.e., the capacity for deformation of the polymer backbone. According to Brostow s theory, there are two effects of central importance for practical application degradation and relaxation. Every type of energy transferred to a polymer chain in solution does so via one of these two processes. Thus, all parameters which directly affect these processes are of corresponding significance. 

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