Effect of solvent size

Effect of the Solvent Size. The effect of solvent size and geometry is reflected in the apparent activation energy for diffusion, Ed, for diffusion in the limit of zero solvent concentration defined by... 

In the paper entitled A remarkable effect of solvent size on the stability of a molecular complex , a strong correlation was shown between binding energy and surface area of the solvent molecule [61]. This study compared closely related families of solvents to determine the effect of solvent surface area on complex formation. Correlation was also shown between binding energy and solvation energies for the complexation of two families of solvents. It was proposed that the increase in solvation energy (positive values) that occurs as solvent size increases makes it thermodynamically favourable... 

The authors also investigated the effect of solvent composition on the retention of a series of solutes including a dispersion of silica smoke (mean particle diameter 0.002 pm). The silica smoke was used to simulate a solute of very large molecular size... 

Paine et al. [85] extensively studied the effect of solvent in the dispersion polymerization of styrene in the polar media. In their study, the dispersion polymerization of styrene was carried out by changing the dispersion medium. They used hydroxypropyl cellulose (HPC) as the stabilizer and its concentration was fixed to 1.5% within a series of -alcohols tried as the dispersion media. The particle size increased from only 2.0 /itm in methanol to about 8.3 /itm in pentanol, and then decreased back to 1 ixm in octadecanol. The particle size values plotted against the Hansen solubility parameters... 

The hydrodynamic radius reflects the effect of coil size on polymer transport properties and can be determined from the sedimentation or diffusion coefficients at infinite dilution from the relation Rh = kBT/6itri5D (D = translational diffusion coefficient extrapolated to zero concentration, kB = Boltzmann constant, T = absolute temperature and r s = solvent viscosity). 

We can obtain a crude estimate the time required for a precise quantum mechanical calculation to analyse possible syntheses of bryosta-tin. First, the calculation of the energy of a molecule of this size will take hours. Many such calculations will be required to minimise the energy of a structure. A reasonable estimate may be that a thousand energy calculations would be required. Conformation searching will require many such minimisations, perhaps ten thousand. The reactivity of each intermediate will require a harder calculation, perhaps a hundred times harder. Each step will have many possible combinations of reagents, temperatures, times, and so on. This may introduce another factor of a thousand. The number of possible strategies was estimated before as about a million, million, million. In order to reduce the analysis of the synthesis to something which could be done in a coffee break then computers would be required which are 10 times as powerful as those available now. This is before the effects of solvents are introduced into the calculation. 

A simple estimate of the diffusion coefficients can be approximated from examining the effects of molecular size on transport through a continuum for which there is an energy cost of displacing solvent. Since the molecular weight dependence of the diffusion coefficients for polymers obeys a power law equation [206], a similar form was chosen for the corneal barriers. That is, the molecular weight (M) dependence of the diffusion coefficients was written as ... 

TP Gall, RC Lasky, EJ Kramer. Case II diffusion Effect of solvent molecule size. Polymer 31 1491-1499, 1990. 

Figure 3. Effect of solvent on the effective linear sizes of molecules in solution (Fuel 1982) (14) ...

As an attempt to quantify the effect of solvent on individual faces of sucrose crystal we have ertqrloyed a solvent accessibility of Van der Waals sized surface atoms for a spheric probe representing the solvent molecule flSI. The smooth surface generated by rolling such a probe along the crystal surface consists of... 

In mobile phases that are good sample solvents, the retention mechanism of high molecular compounds can be explained on the basis of conventional theory of RPC or NPC gradient elution applying for small molecnles, considering the effect of increasing size of molecules on the values of the constants a, b, ko, and m of the retention equations— Equation 5.7, 5.10, or 5.11. 

The effect of solvent composition on the retention of a series of solutes, commonly used to measure column dead volumes, was also investigated by these authors. They employed mixtures of methanol and water as the mobile phase and measured the retention volume of the same salts together with a silica gel dispersion (containing particles 0.002 micron in diameter). They also measured the retention volume of the components of the mobile phase methanol, and water. The silica dispersion was chosen to simulate a solute of very large molecular size. The results they obtained are shown in figure (2). 

Table 4.1 Effect of solvent used for dehydrochlorination of the polymer precursor on pore size distribution in activated carbon. Experimental conditions chlorinated PVC was treated with KOH at 20°C for 5 h thermal treatment conditions carbonization at 400°C for 30 min followed by activation with CO at 900°C for 5 min...

Table III describes the effect of solvent change on the lignin model compounds. None of the model compounds exhibited evidence of association all had unimodal elution in the different solvents. The Kp of the fully derivatized model compounds tended to increase as solvent polarity was increased, as had that of the polymer standards. As these are fully derivatized, relatively small molecules, the possibility for size change through interaction with the solvents is small. Increasing affinity for the column gel as the solvent polarity increased is the most probable explanation for their greater retention.

Some redox couples of organometallic complexes are used as potential references. In particular, the ferrocenium ion/ferrocene (Fc+/Fc) and bis(biphenyl)chromium(I)/ (0) (BCr+/BCr) couples have been recommended by IUPAC as the potential reference in each individual solvent (Section 6.1.3) [11]. Furthermore, these couples are often used as solvent-independent potential references for comparing the potentials in different solvents [21]. The oxidized and reduced forms of each couple have similar structures and large sizes. Moreover, the positive charge in the oxidized form is surrounded by bulky ligands. Thus, the potentials of these redox couples are expected to be fairly free of the effects of solvents and reactive impurities. However, these couples do have some problems. One problem is that in aqueous solutions Fc+ in water behaves somewhat differently to in other solvents [29] the solubility of BCr+BPhF is insufficient in aqueous solutions, although it increases somewhat at higher temperatures (>45°C) [22]. The other problem is that the potentials of these couples are influenced to some extent by solvent permittivity this was discussed in 8 of Chapter 2. The influence of solvent permittivity can be removed by... 

---