Surfactant-micellar mobile phases

Figure 4. Artistic representation of the species and equilibria present when employing surfactant micellar mobile phases in LC.

M.F. Borgerding and W.L. Hinze, Characterization and Evaluation of the Use of Nonionic Surfactant Micellar Mobile Phases in RP-HPLC, Anal Chem., 57 2183 (1985). 

The model was tested by the micellar liquid chromatography separ ation of the five rarbornicin derivatives and four ethers of hydroxybenzoic acid. Micellar mobile phases were made with the sodium dodecylsulfate and 1-pentanol or isopentanol as modifier. In all cases the negative signs of the coefficients x and y indicate that at transition of the sorbat from the mobile on the stationar y phase the number of surfactant monomers as well as the number of modifier molecules increases in its microenvironment. 

The pur pose of work is to develop the technique of separ ation of purine bases (caffeine, theophylline, theobromine) and the technique of detection of purine bases in biological fluid by TLC using micellar mobile phases containing of different surfactants. 

The basis for separation employing micellar mobile phases stems from their ability to differentially solubilize and bind structurally similar solutes. Skeptics view MLC as a fascinating example of the incorporation of secondary equilibria for control or adjustment of retention (101). However, it is the ultimate of secondary equilibria since the types of interactions possible with micellar aggregates cannot be duplicated by any single other equilibrium system, or for that matter, any one or mixture of traditional normal or reversed phase mobile phase systems. This is due to the fact that solutes can interact with the surfactant aggregates via hydrophobic, electrostatic, hydrogen bonding, and/or a combination of these factors. 

A micellar mobile phase can be viewed as being composed of both the surfactant micellar aggregates (pseudophase) and the rest of the... 

Examination of equations 5, 6, and 7 reveals that retention can be controlled by variation of the surfactant micelle concentration, variation of pH (for ionizable species), and by manipulation of the solute-micelle binding constant (K. ) which, in turn can be influenced by additives (salt, alcohol referto data on DDT, Table VI) or the type (charge and hydrophobicity) of micelle-forming surfactant employed (refer to data in Table VII for 1-pentanol). Table VIII summarizes some of the factors that influence retention for surfactant-containing mobile phases and compares the effect of changes in these factors upon the retention behavior observed in both micellar liquid and ion-pair chromatography (81). 

Lastly, the use of micellar mobile phases allows a convenient means of studying micelle - solute interactions (i.e. determination of binding constants) (1,10 4,105) as well as determination of surfactant CMC values (from breaks in the log k gQ vs. log C, plots)... 

The main disadvantages of micellar chromatography are the observed diminished chromatographic efficiency, higher column back pressure, and in preparative work, the need to separate the final resolved analyte from the surfactant (95) (a later section of this review will discuss this latter problem and its resolution in further detail). The higher column back pressure and part of the decreased efficiency stem from the fact that surfactant-containing mobile phases are more viscous compared to the usual hydro-organic mobile phases employed in conventional RP-HPLC (refer to viscosity data in Table X)... 

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). 

A cursory review of the literature reveals that the ELC technique with micellar mobile phases has proven to be very beneficial in the characterization of micellar systems (184-186,190-192,227,228). For example, microcolumn exclusion LC has been applied to the determination of the CMC value of surfactants (or micellar-forming proteins), determination of the kinetic rate and equilibrium association constants for surfactant (or protein) micellization (184,192), determination of the size or size distribution of micelles (especially those formed from block copolymers or milk casein) (185,186,191,192,225) as well as for estimation of the time required for formation of micelles (or micelle-forming macromolecules) (186) among others. The size and stability of reversed micelles has also been evaluated using ELC (195). 

We have found that the use of 3% n-propanol in the micellar mobile phase and column temperatures of 40° C appear to offer a broadly applicable solution to the low efficiency previously reported for micellar mobile phases. These conditions have resulted in reduced plate heights of 3-4 for SDS, cetyltrimethylammonium bromide (CTAB), and Brij-35 (15). This efficiency optimization scheme then appears to be a broadly-based solution for micellar mobile phases of any surfactant. This means that the surfactant type can be varied to affect separational selectivity with no loss in column efficiency. 

The stationary phases play an important part in Liquid Chromatography using micellar mobile phases. They interact with both the surfactant and with solutes. To study the interactions with surfactants, adsorption isotherms were determined with two ionic surfactants on five stationary phases an unbonded silica and four monomeric bonded ones. It seems that the surfactant adsorption closely approaches the bonded monolayer (4.5 pmol/m2) whatever the bonded stationary phase-polarity or that of the surfactant. The interaction of the stationary phase and solutes of various polarity has been studied by using the K values of the Armstrong model. The KgW value is the partition coefficient of a solute between the... 

The concentration of organic solvent should be low enough to make the existence of micelles possible. Such maximal amount depends on the type of surfactant and organic solvent, and is usually unknown. For SDS, the maximal volume fractions of acetonitrile, propanol, butanol, and pentanol that seem to guarantee the presence of micelles are 20%, 15%, 10%, and 7% (v/v), respectively. However, analytical reports where authors claim the use of hybrid micellar mobile phases and these maximal values are exceeded—micelles do not exist— are not unusual. In such conditions, the system bears closer resemblance to an aqueous-organic system, although the surfactant monomers still affect the retention and efficiencies. 

Fig. 2 Correlation between octanol-water partition coefficients of the organic solvents (log Po/w). and the retention factors of (a) benzene and (b) 2-ethylanthraquinone in hybrid SDS micellar mobile phases. The concentration of surfactant and organic solvent was 0.285 M and 5% (v/v), respectively. (From Ref [11].)...

Chromatographic efficiency seems to be linked to the additive-to-surfactant concentration ratio in the micellar mobile phase. The plate numbers increase with this ratio but reach a maximum level (e.g., at pentanol/SDS = 6 and acetonitrile/CTAC= 12). - The organic solvent/surfactant ratio affects the exchange rates of the solute between micelle/stationary and aqueous phases. It also controls the extent of the surfactant coverage and the fluidity of the organic layer on the stationary phase. 

Lopez-Grio, S. Baeza-Baeza, J.J. Garcfa-Alvarez-Coque, M.C. Evaluation of the elution strength of organic modifier, and surfactant in micellar mobile phases. J. Liq. Chromatogr. Relat. Technol. 2001, 24, 2765-2783. 

Halko R, Hutta M. Study of high-performance liquid chromatographic separation of selected herbicides by hydro-methanolic and micellar liquid chromatography using Genapol X-080 nonionic surfactant as mobile phase constituent. Anal Chim Acta 2002 466 325-333. 

To overcome this problem, our laboratories have initiated a program of study in the area of micellar liquid chromatography (MLC). The mobile phase in a MLC experiment consists of a surfactant that is at a concentration above the critical micellization concentration (cmc). We have learned that the addition of a co-surfactant to a micellar mobile phase will result in the formation of lamellar liquid crystals at the surface of the reversed phase (i.e., Cjg) material... 

Biological Separations. As early as 1985, Cline-Love showed that the surfactant molecules contaiaed in the micellar mobile phase were able to bind to proteins and to reduce greatly their adsorption on the silica based stationary phases. The direct injection of biological samples such as serum or urine samples can be done m MLC without protein precipitation inside the column [51], This property initiated a great interest for MLC and a high number of works in this area [32, 52-55]. 

The organic additives of the micellar mobile phase affect the surfactant-adsorbed layer. This changes the chromatographic selectivity and efficiency obtained for a set of analytes with the same column. Selectivity and efficiency are studied in other parts of this book. 

To answer to the introduction question, a study of the retention behavior of a set of solutes of various polarities was done with different stationary phases and identical micellar mobile phases [26]. SDS and CTAB micellar phases were used with five different HypersiKD phases. The retention of the polar, apolar as well as ionic solutes studied obeyed Armstrong s three phase model (eq. 3. 4, Chapter 3). Table 4.3 lists the P m solute-micelle partition coefficient and the Pws solute-stationary phase affinity coefficient for different solutes on four bonded phases. The Pwm values are similar within experimental errors (see Chapter 5). The Pws values are similar for the C8 and C18 bonded phases. These two phases do behave similarly in classical RPLC. The more polar Cl and CN bonded phases do show significantly lower affinity coefficients. The surfactant layer is there cationic solutes are not retained by the four bonded phases and hydro-organic mobile phases. Their high affinity for the stationary phases with SDS mobile phases is due... 

It is always recommended to use the same column with the same type of surfactant. A column should be dedicated to the anionic surfactants, a second one to the cationic surfactants, etc. The reproducibility of the results in MLC depends on the column equilibration. The adsorbed layer of surfactant should be done correctly. It was shown that the time to reach the equilibrium between the stationary phase and the mobile phase could be very long in ion-pair chromatography with sub-micellar mobile phases. Two days at 1 mL/min were necessary to equilibrate a 15 cm x 4.6 mm i.d. column of Hypersil ODS with a mobile phase containing 0.0003 M CTAB [19]. These low surfactant concentration solutions do not contain micelles. So, they are not used in MLC. With a micellar phase, the equilibration time is reduced. It is possible to use the rapid gradient capability just mentioned above. Typically, a mobile phase containing a high surfactant concentration (10 to 100 cmc) can be used to quickly saturate the column with surfactant. Then 5 to 10 column volumes are used to rinse the column with the mobile phase containing the desired amount of surfactant. 

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