Anionic Surfactants

Anionic surfactant systems are preferred in MEKC because the electrophoretic migration of the micelles is in the opposite direction to the electroosmotic flow, and the micelles do not interact with the negatively charged walls of the fused silica capillaries. Anionic surfactants with alkyl chain and polar group, such as sodium decyl sulfate, sodium N-lauroyl-N-methyl-taurate, sodium tetradecyl sulfate, and especially sodium dodecyl sulfate (SDS) are the most widely used. Simultaneous separation of neutral and positively charged compounds is not possible at low pH because the EOF is too slow to carry the micelles to the cathode.

Most studies with anionic surfactants have been carried out under neutral or basic conditions. The most frequently used anionic surfactant, SDS, forms relatively spherical micelles with hydrophobic tail groups oriented towards the centre and charged head groups along the outer surface. The surfaces of SDS micelles possess a large net negative charge, giving them a large electrophoretic mobility toward the anode.

Another group of anionic surfactants, which has been widely used in separations of both neutral and ionic analytes, is bile salts. Bile salts have a hydroxyl-substituted steroidal backbone with hydrophilic and

Table 2 CMC values of SDS in selected electrolyte solutions at 25°C

Electrolyte solution

CMC (mM)

Method ofdetermination

50 mM AMPSOa (pH 9.0)

3.6

Conductometric titration

50 mM AMPSOa (pH 9.0)

3.9

CE

50 mM AMPSOa (pH 8.7)

2.7

Surface tension

20 mM PIPES', 20 mM NaOH (pH 7.0)

3.8

Conductometric titration

100 mM BESc, 100 mM NaOH (pH 7.0)

3.1

Conductometric titration

100 mM borate, 50 mM phosphate (pH 7.0)

2.9

Conductometric titration

5 M urea, 100 mM borate, 50 mM phosphate (pH 7.0)

4.4

Conductometric titration

20% DMSO (v/v), 25 mM sodium tetraborate, 50 mM sodium dihydrogen

6

Conductometric titration

phosphate (pH 7.0)

20% acetone (v/v), 25 mM sodium tetraborate, 50 mM sodium dihydrogen

6.3

Conductometric titration

phosphate (pH 7.0)

20 mM sodium tetraborate (pH 9.2)

3.1

CE

20 mM sodium tetraborate (pH 8.0)

5.5-

9.6

CE

5 mM sodium tetraborate-acetonitrile (85 : 15, v/v)

7.3

CE

5 mM sodium tetraborate (pH 9.2)

5.3

CE

100 mM sodium tetraborate, 100 mM sodium dihydrogen phosphate (pH 6.0)

2

CE

100 mM sodium tetraborate, 100 mM sodium dihydrogen phosphate (pH 6.5)

2.4

CE

100 mM sodium tetraborate, 100 mM sodium dihydrogen phosphate (pH 7.0)

3.1

CE

100 mM sodium tetraborate, 100 mM sodium dihydrogen phosphate (pH 7.7)

4

CE

50 mM CHESd (pH 10.0)

2.9-

5.2

CE

50 mM CHESd (pH 10.0)

2.7-

5.4

CE

80 mM CHESd (pH 10.0)

1.6-

2.2

CE

100 mM CHESd (pH 10.0)

1.2-

2.4

CE

50 mM ammonium acetate (pH 9.0)

1.7-

2.7

CE

aAMPSO = 3-[(1,2-dimethyl-2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid; ''PIPES = piperazine-N/,N'-bis(2-ethanesulfonic acid) monosodium salt; cBES = N,N'-bis(2-hydroxyethyl)-2-aminoethanesulfonic; dCHES = 2-(N/-cyclohexylamino)ethanesulfonic acid.

aAMPSO = 3-[(1,2-dimethyl-2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid; ''PIPES = piperazine-N/,N'-bis(2-ethanesulfonic acid) monosodium salt; cBES = N,N'-bis(2-hydroxyethyl)-2-aminoethanesulfonic; dCHES = 2-(N/-cyclohexylamino)ethanesulfonic acid.

hydrophobic faces and they form helical micelles. Bile salts have a lower solubilizing effect on hydrophobic compounds than does SDS.

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