Chaotropic effects

With the increase of the counteranion concentration, the solvation of the protonated basic analyte decreases. The primary sheath of water molecules around the basic analytes is disrupted, and this decreases the solvation of the basic analyte. The decrease in the analyte solvation increases the analyte hydrophobicity and leads to increased interaction with the hydrophobic stationary phase and increased retention for the basic analytes. 

Disruption of the basic analyte solvation shell should be possible with practically any counteranion employed, and the degree of this disruption will be dependent on the chaotropic nature of the anion. Chaotropic activity of counteranions has been established according to their ability to destabilize or bring disorder (bring chaos) to the structure of water [148,149]. 

Even a very low counteranion concentration in the mobile phase will cause significant initial disruption of the solvation shell, thus leading to the significant increase of the analyte retention, while in the high concentration region 

As was shown above, the chaotropic effect is related to the influence of the counteranion of the acidic modifier on the analyte solvation and is independent on the mobile-phase pH, provided that complete protonation of the basic analyte is achieved. Analyte interaction with a counteranion causes a disruption of the analyte solvation shell, thus affecting its hydrophobicity. Increase of the analyte hydrophobicity results in a corresponding increase of retention. This process shows a saturation limit, when counteranion concentration is high enough to effectively disrupt the solvation of all analyte molecules. A further increase of counteranion concentration does not produce any noticeable effect on the analyte retention. 

1 Chaotropic Model. If the counteranion concentration is low, some analyte molecules have a disrupted solvation shell, and some do not due to the limited amount of counteranions present at any instant within the mobile phase. If we assume an existence of the equilibrium between solvated and desolvated analyte molecules and counteranions, this mechanism could be described mathematically [151]. 

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Pilorz, K. and Choma, I. Isocratic reversed-phase high-performance liquid chromatographic separation of tetracyclines and flmnequine controlled by a chaotropic effect. J. Chromatogr. A. 2004, 1031, 303-305. 

The chaotropic effect is dependent on the concentration of the free counteranion and not the concentration of the protons in solution at pH < basic analyte Ka. This suggests that change in retention of the protonated basic analyte may be observed with the increase in concentration of the counteranion by the addition of a salt at a constant pH as shown in Figure 4-47 for a pharmaceutical compound containing an aromatic amine with a pKa of 5. 

We typically do not use protease inhibitors. The combination of the lysis buffer with its reducing ability, the chaotropic effects of the urea and the surfactant, and the cold temperature seems to inactivate proteolytic activity. We also do not perform any steps requiring room temperature or protein activity (such as the DNAse-RNAse treatment found in some protocols). Furthermore, the presence of the inhibitors may sometimes interfere with the fluorescent labeling. The sample cups on the lEF gel have about 100-pl maximum capacity. However, if necessary, more volume can be handled by ordering more sample cup holder bars separately from the dry strip kit and used to spread one sample between several cups. Because IFF is a focusing technique, the sample does not necessarily have to be applied in exactly the same spot. The dye synthesis is detailed elsewhere. The dyes are not commercially available as of the time of this writing. 

The electrolyte can also have another effect, not commonly recognized by many researchers, the chaotropic effect.40 This effect, more commonly discussed in biochemical circles, is related to variations in the water-structuring ability of different salts. This ability is used, for example, to dehydrate and salt out macromolecular proteins. The same effect can be expected with conducting polymers. As well as ion-size effects, this may be used to explain the large shifts in switching potentials observed in different electrolytes41 when the cation was varied and the difference in overoxidation potentials observed in different electrolytes.42... 

Figure 5-15 shows how the retention of 4-ethylpyridine varies with pH. The greatest increase in retention occurred when trifluoroacetic acid and perchloric acid were employed. The phosphoric modifier did not increase the retention of the 4-ethylpyridine significantly at decreasing pH values. Therefore the trifluoroacetate and perchlorate had a greater influence than phosphate as chaotropic counteranions at any particular pH. It was shown that this chaotropic effect began to occur at pHs less than 3. At all pHs greater than 4 the retention factors of the basic compound were similar and were independent of what type of modifier was used in the mobile phase. 

Figure 5-21 illustrates the chaotropic effect for several basic analytes. The effect of the chaotropic agent on the disruption of the basic analytes solvation shell is dependent on the type and position of substituents. At the various concentrations, the effect on the retention was different for the analytes of different stereochemistry and led to increased resolution of certain components of similar structure. 

This chaotropic effect is predominant under highly aqueous conditions, hut is it significant where the organic content is higher ... 

The overall chaotropic effect within a given concentration range is not similar at different temperatures. A plot of k vs counteranion concentration is given in Fig. 5-26. 

As can be seen from Fig. 5-30, the retention factor decreases together with the pH of the mobile phase, until pH 2.6 is reached. Then the retention factor starts to increase with the increase of the concentration of the perchlorate anion (decrease in pH). This is due to the chaotropic effect. At pHs close to the p/f of aniline (4.6), the peak shape is broad and severe fronting is observed. The increase of the perchlorate concentration at low pHs gave retention factors comparable to those at higher pH values. 

The state of water at surfaces of unmodified and modified (e.g., partially hydrophobized) silicas differs because any complete modification displaces water from the surface in the region with lower electrostatic field, or partial modification results in the formation of a clustered adsorption layer. The latter is due to surface hydrophobic functionalities playing a role of barriers for separated water clusters located between them. Additionally, kosmotropic or chaotropic effects, the type of which depends on the structure of surface groups (Chaplin 2011, Gun ko et al. 2005d), lead to changes... 

To study the chaotropic effects of cations and anions of dissolved salts, KCl was selected because both ions are chaotropic but not maximum strong. It is known that interfacial water at a surface of nanosilica is in the strongly associated state at Ce o>200 mg/g (Gun ko et al. 2005d, 2009d). Therefore, nanosilica A-380 at Ce o=275 mg/g was selected as an initial sample. The H NMR spectra of the sample in deuterochloroform are given in Figure 1.113 at210[Pg.128]

Flieger, J. and Markowski, W. Application of gradient elution optimized by Chromsword software in chromatography of phenothiazines in reversed phase systems controlled by chaotropic effect. Chem. Anal. 54 187-202, 2009. 

As earlier mentioned, the resin does not hind thiocyanate hy an ion-exchange mechanism hut hy adsorption forces, prohahly related to the strong chaotropic effect of thiocyanate. This interpretation is supported hy the fact that thiocyanate is displaced from the resin hy other chaotropic ions such as perchlorate, trichloro-acetate and nitrate, hut not hy non-chaotropic ions such as chloride or sulfate (Table l). 

Flieger J (2009) Improvement of chiral discrimination of acidic enantiomers on teicoplanin stationary phase by the use of chaotropic effect. J liq Chromatogr Relat Technol 32 948-963... 

Sebastiani and coworkers used ab initio molecular dynamics and experimental and simulated CS to study to the hydrogen bond network dynamics in low and high salt water solutions of Lil. The combined approach allows reaching a considerable insight on the competitive chaotropic effects of the anion and the kosmotropic effect of the cation, indicating that the former has longer range effects. 

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