Surfactant-modified stationary

In addition to the factors listed in Table VIII, the nature of the surfactant-modified stationary phase affects P (partition coefficient for distribution of solute between bulk solvent and modified stationary phases) and thus will influence the retention observed. It should be realized that most of the normal and reversed-phase packing materials will adsorb/absorb surfactant molecules from the mobile phase solution and become coated to different degrees when surfactant mobile phases are passed through them. Numerous adsorption isotherms have been reported for various surfactant - stationary phase combinations illustrating this point (82,85,106,115-128,206). The presence of additives can mediate the amount of surfactant surface coverage obtained (110-129,175,206). It has been postulated that the architecture which adsorbed surfactant molecules can assume on conventional stationary phases can range from micellar, hemi-micellar, or admicellar to mono-,bi-, or multilayered, and/or other liquid crystalline-type structures (93,106,124,128,129,... 

The major contributions which result in the reduced chromatographic efficiency have been ascribed to slow mass transfer principally due to poor wetting of the surfactant modified stationary phase (109), poor mass transfer between the micelle and stationary phase (113), and poor mass transfer in the stationary phase (100,106). In some cases, the use of small amounts of alcohol additives (MeOH, n-PrOH) and operation at elevated temperature (MO0 C) result in chromatographic efficiencies comparable to that seen in traditional LC using hydro-organic mobile phases (109,113,154,206). In our own work, we have found n-pentanol to be superior to n-propanol in this regard (refer to Table IX) (112). Further work is clearly needed in this efficiency area in order to clarify the exact reason(s) for the reduction in efficiency. It appears that a combination of factors can contribute to this effect with the dominant efficiency reduction mode dependent upon the nature of the solute, micellar mobile phase, and stationary phase packing material employed (100,112,135). 

The problem with using surfactant-modified stationary phases in LC is that the surfactant will usually slowly elute (bleed) from the support thus resulting in different retention behavior of solutes with time. This is why most applications are in the area of GC or GLC. An exciting recent advance has been reported by Okahata, et al (181). Namely, a procedure has been developed for immobilizing a stable surfactant vesicle bilayer as the stationary phase in GC. A bilayer polyion complex composed of DODAB vesicles and sodium poly(styrene sulfonate) was deposited on Uniport HP and its properties as a GC stationary phase evaluated. Unlike previous lipid bilayers which exhibited poor physical stability, the DODAB polyion phase was stable. Additionally, the temperature-retention behavior of test solutes exhibited a phase transition inflection point. The work demonstrates that immobilized surfactant vesicle bilayer stationary phases can be employed in GC separations (181). Further work in this direction will likely lead to many such unique gas chromatographic supports and novel separations. 

Inside the column, solutes are affected by the presence of micelles in the mobile phase and by the nature of the alkyl-bonded stationary phase, which is coated with monomers of surfactant (Fig. 1). As a consequence, at least two partition equilibria can affect the retention behavior. In the mobile phase, solutes can remain in the bulk water, be associated to the free surfactant monomers or micelle surface, be inserted into the micelle palisade layer, or penetrate into the micelle core. The surface of the surfactant-modified stationary phase is micelle-like and can give rise to similar interactions with the solutes, which are mainly hydrophobic in nature. With ionic surfactants, the charged heads of the surfactant in micelles and monomers adsorbed on the stationary phase are in contact with the polar solution, producing additional electrostatic interactions with charged solutes. Finally, the association of solutes with the nonmodified bonded stationary phase and free silanol groups still exists. 

The retention behavior of solutes will depend on the type of interactions with the micelles and with the surfactant-modified stationary phase. Nonpolar solutes should only be affectedby hydrophobic interactions (Fig. 5.1a). For these solutes, different proportions of nonpolar, dipole-dipole and proton donor-acceptor interactions between solutes and... 

The first situation is encoimtered when an anionic solute is eluted with an anionic surfactant, or a cationic solute is eluted with a cationic surfactant [e.g., dissociated phenol with the anionic surfactant sodium dodecyl sulfate (SDS), and protonated benzylamine with the cationic surfactant dodecyl trimethylammonium bromide (DTAB), on a C18 column] [2]. Electrostatic repulsion from the micelle should not affect the retention as the solute will still reside in the bulk solvent phase, and therefore, will move down the column. In contrast, repulsion from the surfactant-modified stationary phase should cause a decreased retention, and the solute may elute in the dead volume. However, it may be retained by hydrophobic interaction with the stationary phase, although this effect will be reduced by the electrostatic repulsion. Because due to different hydrophobic interactions, dissociated phenol and 2-naphthol are well separated with SDS. 

In contrast, for MLC, it has been postulated that the free energy of cavity formation would not contribute to the overall free energy of retention, because the cavity created when the solute partitions from the micellar pseudo-phase to bulk water would be lost when the solute partitions from bulk water to the surfactant-modified stationary phase, producing no net free energy change [9]. 

On the other hand, the environment of the surfactant-modified stationary phase is independent of micelle concentration in the mobile phase (for most surfactants and stationary phases), and similar to that of pure aqueous eluent systems. As a result, the alkyl-bonded stationary phase will have both invariable amphiphilic and anisotropic properties. In contrast, in aqueous-organic RPLC, the composition and structure of the alkyl-bonded phase change with the concentration of organic modifier in mobile phase. 

Usually, diminishes as the organic solvent concentration increases. For ionic surfactants, solute-surfactant electrostatic interactions are responsible for the shift in sign in at increasing surfactant concentration. These interactions also explain the sign in the trend of the fe-pH dependence, which is sigmoidal and resembles conventional acid-base titration curves. The observed behaviors indicate that the interaction of solutes with the surface of the surfactant-modified stationary phase is stronger than that with micelles. 

The retention of polar amino acids can be enhanced by the inclusion of decyl sulfate (203) or other anionic surfactants (234) in the eluent. The use of such agents in the mobile phase modifies stationary phase interactions... 

Equations 5.8 and 5.9 describe the retention of solutes that can form associates, or inclusion complexes, both with micelles and surfactant adsorbed on the stationary phase. This is the case for neutral compounds and compounds with an opposite charge to the surfactant. However, as indicated above, compounds having the same charge as the surfactant will be excluded from the micelles and repelled by the modified stationary phase, unless other interactions exist that neutralize the electrostatic repulsion. For solutes repelled from the micelles, the retention increases with the concentration of surfactant, which supposes an apparent negative value for Kam Since this constant is the ratio of the equilibrium concentrations of solute between bulk water and micelles, it should be positive. 

Although mentioned in theories of pseudo-phase chromatography, the possibility of a direct transfer of an insoluble or a sparingly water-soluble solute, from the micelle in the mobile phase to die surfactant-coated stationary phase, was largely ignored. For this situation, a modified form of the equation of Armstrong and Nome (eq. 5.1) was derived [29], which successfully accounts for the dependence between k and [M], observed in the elution of such hydrophobic solutes ... 

Figure 5.8 Representation ofthedirecttransferprocessfordistributionofasolute between the micellar pseudo-phase and the surfactant-modified C18 stationary phase. Reprinted from...

The effect of pH on the retention factors of solutes, eluted with an anionic surfactant, is very similar on Cl8 columns to that obtained using cyano columns, when hydrophobic interactions dominate. However, less hydro-phobic and negatively charged solutes will elute very quickly on C18 columns, because of repulsion from both micelles and negatively charged modified stationary phase. Fig. 5.11 shows plots of k vs. pH with the typical... 

In MLC, solutes are separated on the basis of their differential partitioning between bulk solvent and micelles in the mobile phase or surfactant-coated stationary phase. For water-insoluble species or for species strongly bound to micelles, partitioning can also occur via direct transfer between the micellar pseudophase and the modified stationary phase. Partition equilibria are affected by a variety of factors, such as the nature and concentration of surfactant and organic modifiers, temperature, ionic strength, and pH. 

For charged solutes eluted with ionic surfactants, two situations are possible repulsion or attraction, depending on the mutual charges of solutes and surfactant. In the case of electrostatic repulsion with the stationary phase, charged solutes cannot be retained and elute at the dead volume, unless significant hydrophobic interactions with the modified bonded layer exists. In contrast, combined electrostatic attraction and hydrophobic interactions with the modified stationary phase may give rise to strong retention. 

Mass-action model of surfactant micelle formation was used for development of the conceptual retention model in micellar liquid chromatography. The retention model is based upon the analysis of changing of the sorbat microenvironment in going from mobile phase (micellar surfactant solution, containing organic solvent-modifier) to stationary phase (the surfactant covered surface of the alkyl bonded silica gel) according to equation ... 

Solutions to the above problea are required if efficient open tubular colunns are to be prepared. The energy of the saooth glass surface can Sse Increased by roughening or chemical Modification, or the surface tension of the stationary phase can be lowered by the addition of a surfactant. Roughening and/or cheMical modification etre the most widely used techniques for column preparation the addition of a surfactant, although effective, modifies the separation properties of the stationary phase and may also limit the thermal sted>ility of columns prepared with high temperature stable phases. 

In the most recent method described by Hu et al. [239] for the direct determination of ultraviolet-absorbing inorganic anions in saline matrixes, an octadecylsilica column modified with a zwitterionic surfactant [3-(N,N-di-methylmyristylammoniojpropanesulfate] is used as the stationary phase, and an electrolytic solution is used as the eluent. Under these conditions, the matrix species (such as chloride and sulfate) are only retained weakly and show little or no interference. It is proposed that a binary electrical double layer is established by retention of the eluent cations on the negatively charged (sulfonate) functional groups of the zwitterionic surfactant, forming a cation-binary electrical double layer. 

This problem was resolved by Nakae et al. [7] using non-polar octadecylsilica as the stationary phase and a solution of 0.1 M of sodium perchlorate in methanol/water (80 20) as the mobile phase. The ternary system (water-alcohol-salt), previously used by Fudano and Konishi [8] as an eluent for the separation of ionic surfactants at higher concentrations, induced the so-called salting out effect . The addition of the organic solvent to the water modified the polarity of the eluent and produced a good separation within a short period of time [9]. It also has the function of dissociating the surfactant micelles in individual molecules that are dissolved in the eluent [8], The presence of the salt (NaC104) in the mobile phase has a considerable influence on... 

In MLC, the mobile phase consists of surfactants at concentrations above their critical micelle concentration (CMC) in an aqueous solvent with an alkyl-bonded phase (52). Retention behavior in MLC is controlled by solute partitioning from the bulk solvent into micelles and into stationary phase as well as on direct transfer from the micelles in the mobile phase into the stationary phase. Eluent strength in MLC is inversely related to micelle concentration. A linear relationship exists between the inverse of retention factor and micelle concentration. Similar to what is observed in RPLC, a linear relationship exists between retention in MLC and , the volume fraction of the organic modifier. Modeling retention in MLC is much more complicated than in RPLC. The number of parameters is important. Micelles are obviously a new domain in both liquid chromatography and electrophoresis. Readers interested in the topic will appreciate Ref. 53, a special volume on it. 

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