Solvents, adsorption energy

Several workers [e.g.. Refs. 5,45,46)] have pointed out that solvent adsorption energy per unit of surface (and therefore solvent strength, from its definition) is related to the interfacial tension between solvent and... 

An eluotropic series ranks solvents by their abilities to displace solutes from a given adsorbent. Eluent strength in Table 22-2 is a measure of solvent adsorption energy, with the value for pentane defined as 0. The more polar the solvent, the greater its eluent strength. The greater the eluent strength, the more rapidly solutes are eluted from the column. 

Such displacement effects, although often very pronounced, have not yet been studied systematically. They will be the subject of the present paper. We will discuss the adsorption of polymer from a mixture of two solvents and we will see that in some cases drastic effects occur as a function of the mixture composition. Also, we explore some consequences and practical applications of displacement. It turns out that displacement studies not only increase our insight on the role of the solvent in polymer adsorption but can also be used to determine the segmental adsorption energy. So far, experimental data for this quantity were very scarce. Some illustrative experiments will be discussed briefly. 

NEP and PVP. In the polymer where monomers are linked in a chain, hydrophobic parts are largely screened from interactions with the solvent. For free monomers such screening is not possible so that they experience more unfavourable interactions with the solvent. The adsorption energy parameter xs is not affected by the different chemical surrounding of free monomer and polymer segments, since the mechanism for interaction with the surface is hydrogen bonding in both cases (8).)... 

Determination of the segmental adsorption energy. The determina-tion of x ° is also possible. Since x d can be found from Equation 5 if Xsc an the solvency terms are known, we can add xf° and find x ° by Equation 1. The determination of xf° calls for a separate experiment, e.g., an adsorption isotherm of the displacer from solvent, in the absence of polymer. Following such a scheme we used the values of cr obtained from the displacement isotherms of Figure 3 and 4 to determine segmental adsorption energy parameters Xg° for PVP on silica. The required additional information on xdo was obtained from the initial slopes of dis-... 

Porous silica is the most widely used sorbent in HPLC and has many advantages 23). Its surface silanols are capable of adsorbing solutes according to their structure and polarity. In solution, the molecules of the solute and solvent compete for the active sites on the silica surface. The adsorption energy of solvents can be characterized by their e° values24>. The figures are usually tabulated for alumina adsorbents. For silica as an adsorbent, a plain proportionality is assumed25 ... 

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