Retention mechanisms hydrophobic interaction

The second mechanism assumes that the stationary phase is dynamically coated with oxine. As a result, the coordination compounds are dynamically equilibrated between the mobile and stationary phases. The third mechanism proposed, and the one most favored by the authors, is based upon retention by hydrophobic interactions. The authors offer linear and positively correlated graphs of In k versus the number of carbon atoms in each organotin species in support of this mechanism. 

Hydrophobic interactions and trapping of molecules in a molecular sieve formed by humic materials have been hypothesized as retention mechanisms for prometryn. It has been shown that fluridone, fluazifop, and bipyridyhum herbicides penetrate into interlamellar spaces of smectites and can become trapped. 

The mechanism of reversed phase chromatography can be understood by contrast with normal phase chromatography. Normal phase liquid chromatography (NPLC) is usually performed on a polar silica stationary phase with a nonpolar mobile phase, while reversed phase chromatography is performed on a nonpolar stationary phase with a polar mobile phase. In RPLC, solute retention is mainly due to hydrophobic interactions between the solutes and the nonpolar hydrocarbon stationary surface. The nonpolar... 

One of the component mechanisms is, of course, hydrophobic interactions. Retention is proportional to column hydrophobicity, and elution order is expected to generally follow solute hydrophobicity. However, it is important to keep in mind that proteins bind preferentially to columns by their dominantly hydrophobic surface. Two proteins with very similar average surface hydrophobicity may exhibit very different retention characteristics due to differences in their respective distribution of hydrophobic residues.1,2... 

The PO mode is a specific elution condition in HPLC enantiomer separation, which has received remarkable popularity especially for macrocyclic antibiotics CSPs and cyclodextrin-based CSPs. It is also applicable and often preferred over RP and NP modes for the separation of chiral acids on the cinchonan carbamate-type CSPs. The beneficial characteristics of the PO mode may arise from (i) the offset of nonspecific hydrophobic interactions, (ii) the faster elution speed, (iii) sometimes enhanced enan-tioselectivities, (iv) favorable peak shapes due to improved diffusive mass transfer in the intraparticulate pores, and last but not least, (v) less stress to the column, which may extend the column lifetime. Hence, it is rational to start separation attempts with such elution conditions. Typical eluents are composed of methanol, acetonitrile (ACN), or methanol-acetonitrile mixtures and to account for the ion-exchange retention mechanism the addition of a competitor acid that acts also as counterion (e.g., 0.5-2% glacial acetic acid or 0.1% formic acid) is required. A good choice for initial tests turned out to be a mobile phase being composed of methanol-glacial acetic acid-ammonium acetate (98 2 0.5 v/v/w). 

CSPs has, overall, a hydrophobic character (very similar to RP phases with C4-C8 ligands) which stems from contributions of the chiral selectors itself and (capped) linker groups (only a portion of the linkers are utilized for selector attachment) which constitutes a kind of hydrophobic basic layer on the support surface. Hence under typical RP-conditions, hydrophobic interactions between lipophilic residues of the solute and hydrophobic patches of the sorbent may be active and thus a reversed-phase like partition mechanism may be superimposed upon the primary ion-exchange process k = A rp -I- A ix). This A Rp-retention contribution may be especially important for eluents with high aqueous content. 

Factors that influence the retentive powers and selectivity of such bonded phases include the surface concentrations of hydrodartenaceous ligates and free silanol groups. The thermodynamic aspectitm solute interactions with the hydrocarbonaceous ligates at the surface, which are hydrophobic interactions in the case of aqueous eluents, are discussed later in this chapter within the framework of the solvophobic theory. In practice, however, solute interactions with surface silanol which may be termed silanophilic interactions can also contribute ]to retention (71, 75, 93), particularly in the case of amino compounds. Consequently the retention mechanism may be different from that which would be ol served with an ideal nonpolar phase. Therefore, increasing attention is paid to the estimation of the concentration of accessible sianols and to their elimination from the surface of bonded phases. 

It should be emphasized that here we are considering competing mechanisms for stereodiscriminative complex formation and retention. For example, the adsorption strength of the two diastereomeric [SO-SA] complexes will be influenced by the hydrophobic sorbent (e g., RP18 or phenyl etc.) as additional parameter. If a situation is considered where the chelation complex [SOs-SAs] is more stable than [SOs-SAx], this would lead to an elution order of (/ )-SA before (S)-SA (case B). But, with an additional competing mechanism as in the case of a strong, nonstereoselective, hydrophobic interaction of the (S)-SA molecule (case A), the overall retention characteristics of the total [SO-SA] complex is reversed. In other words, these different retention mechanisms can work in opposition in terms of enantioselectivity and thus can lead to peak coalescence and a = 1. 

Each of the types of SPE sorbents discussed retains analytes through a primary mechanism, such as by van der Waals interactions, polar dipole-dipole forces, hydrogen bonding, or electrostatic forces. However, sorbents often exhibit retention by a secondary mechanism as well. Bonded silica ion-exchange sorbents primarily exhibit electrostatic interactions, but the analyte also experiences nonpolar interaction with the bonded ligand. Nonpolar bonded silicas primarily retain analytes by hydrophobic interactions but exhibit a dual-retention mechanism, due to the silica backbone and the presence of unreacted surface silanol groups [72], Recognition that a dual-... 

Two main theories, the so-called solvophobic and partitioning theories, have been developed to explain the separation mechanism on chemically bonded, non-polar phases, as illustrated in Figure 2.4. In the solvophobic theory the stationary phase is thought to behave more like a solid than a liquid, and retention is considered to be related primarily to hydrophobic interactions between the solutes and the mobile phase14-16 (solvophobic effects). Because of the solvophobic effects, the solute binds to the surface of the stationary phase, thereby reducing the surface area of analyte exposed to the mobile phase. Adsorption increases as the surface tension of the mobile phase increases.17 Hence, solutes are retained more as a result... 

In common with other application areas of chromatographic separation, a considerable amount of effort has been expended recently on the development of different elution conditions and types of stationary phases for peptide separations in attempts to maximize column selectivities without adversely affecting column efficiences. Peptide retention will invariably be mediated by the participation of electrostatic, hydrogen bonding, and hydrophobic interactions in the distribution phenomenon. The nature of the predominant distribution mechanism will be dependent on the physical and chemical characteristics of the stationary phase as well as the nature of the molecular forces which hold the solute molecules within the mobile and stationary zones. The retention of the solute in all HPLC modes can be described by the equation... 

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