Solvent excluding surface, solution

Figure 2.1.7. The Solvent Excluding Surface of a particular solute is the envelope of the volume excluded to the solvent considered as a whole sphere that represents its charge density.

Crystals can be grown from the molten state just as water is frozen into ice, but it is not easy to remove impurities from crystals made in this way. Thus most purifications in the laboratory involve dissolving the material to be purified in the appropriate hot solvent. As the solvent cools, the solution becomes saturated with respect to the substance, which then crystallizes. As the perfectly regular array of a crystal is formed, foreign molecules are excluded and thus the crystal is one pure substance. Soluble impurities stay in solution because they are not concentrated enough to saturate the solution. The crystals are collected by filtration, the surface of the crystals is washed with cold solvent to remove the adhering impurities, and then the crystals are dried. This process is carried out on an enormous scale in the commercial purification of sugar. 

Steric descriptors and/or -> size descriptors representing the volume of a molecule. The volume of a molecule can be derived from experimental observation such as the volume of the unit cell in crystals or the molar volume of a solution or from theoretical calculations. In fact, analytical and numerical approaches have been proposed for the calculation of molecular volume where the measure depends directly on the definition of - molecular surface-, -> van der Waals volume and -> solvent-excluded volume are two volume descriptors based on van der Waals surface and solvent-accessible surface, respectively. 

B and A symbolize the dissolved and adsorbed polymer states, respectively, while S represents a free site on the surface, l.e. an interfacial microdomain (including solvent and other solutes) having the lateral dimensions of an adsorbed and isolated macromolecule. If we postulate that the only interactions airising between adsorbed macromolecules etre surface excluded interactions, due to short-range (hard core) forces, the thermodynamic equilibrium condition leads to the Langmuir equation. 

The non-polar contribution to the free energy of solvation includes van der Waals solute-solvent interactions and the cost of cavity formation. The cost of cavity formation can be modeled effectively with a term that is simply proportional to the solvent accessible surface area or the solvent excluded volume [28-30]. Often, van der Waals interactions are not considered separately, but new formalisms have been suggested to include such contributions effectively [31]. Recently, it was also recognized that the length-scale dependence of non-polar interactions may further complicate the development of accurate implicit non-polar models [32]. It is clear that further studies will be needed to identify the optimal implicit non-polar formalism. 

Each of these types of molecular surfaces is adequate for some applications. The van der Waals surface is widely used in graphic displays. However, for the representation of the solute cavity in a continuum model the Accessible and the Excluding molecular surfaces are the adequate models as long as they take into account solvent. The main relative difference between both molecular surface models appears when one considers the separation of two cavities in a continuum model and more precisely the cavitation contribution to the potential of mean force associated with this process (Figure 2.1.6). In fact, we have shown that only using the Excluding surface the correct shape of the potential of mean force is obtained. The cavitation term cannot be correctly represented by interactions among only one center by solvent molecule, such as the construction of the Accessible... 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

Properly folded native proteins tend to aggregate less than when unfolded. Solution additives that are known to stabilize the native proteins in solution may inhibit aggregation and enhance solubility. A diverse range of chemical additives are known to stabilize proteins in solution. These include salts, polyols, amino acids, and various polymers. Timasheff and colleagues have provided an extensive examination of the effects of solvent additives on protein stability [105]. The unifying mechanism for protein stabilization by these cosolvents is related to their preferential exclusion from the protein surface. With the cosolvent preferentially excluded, the protein surface is... 

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