Solubilization of Hydrophobic Dyes in Surfactant Solutions

Abstract

In this paper, the use of surfactants for solubilization of hydrophobic organic dyes (mainly solvent and disperse dyes) has been reviewed. The effect of parameters such as the chemical structures of the surfactant and the dye, addition of salt and of polyelectrolytes, pH, and temperature on dye solubilization has been discussed. Surfactant self-assemble into micelles in aqueous solution and below the concentration where this occurs-the critical micelle concentration (CMC)-there is no solubilization. Above the CMC, the amount of solubilized dye increases linearly with the increase in surfactant concentration. It is demonstrated that different surfactants work best for different dyes. In general, nonionic surfactants have higher solubilization power than anionic and cationic surfactants. It is likely that the reason for the good performance of nonionic surfactants is that they allow dyes to be accommodated not only in the inner, hydrocarbon part of the micelle but also in the headgroup shell. It is demonstrated that the location of a dye in a surfactant micelle can be assessed from the absorption spectrum of the dye-containing micellar solution.

Keywords: dye; hydrophobic; micelle; solubilization; surfactant; water insoluble.

Figure 1

(a) An anionic azo dye with a sulfonate substituent; (b) and a cationic dye with a diphenylmethane structure and a quaternary ammonium group.

Figure 2

The intensely colored and hydrophobic indigo dye can be reduced to water soluble leuco form. The leuco form is easily oxidized back to indigo.

Figure 3

(a) an example of a sulfur dye [37]; (b) The intensely colored and hydrophobic sulfur dye can be reduced to the water soluble leuco form. The leuco form is oxidized back to the colored form.

Figure 4

A surfactant micelle in water. Three different regions can be identified: the outer region, the palisade region and the core.

Figure 5

Solubilization of two hydrophobic dyes C.I. Solvent Yellow 14 (Sudan I) and C.I. Solvent Orange 86 (Quinizarin)in the presence of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) at 21 °C. Data from [44,45].

Figure 6

Solubilization of C.I. Solvent Red 25 (Sudan IV) in aqueous solutions of a series of cationic surfactants (C_nTA⁺X⁻; n = 12,14,16,18 and X = Cl, Br) at 25 °C. Data from [54].

Figure 7

Solubilization of C.I. Solvent Yellow 6 (Yellow OB ) in aqueous solutions of a series of nonionic surfactants (C_nE_m; n = 8,10,12, m = 6,15,29,49) at 30 °C. Data from [59].

Figure 8

Amount of solubilized C.I. Solvent Yellow 6 (Yellow OB) as a function of the concentration of different surfactants. C₁₂E₆ is the nonionic surfactant and SDS stands for sodium dodecyl sulfate. SDS can also be written C₁₂H₂₅(OCH₂CH₂)_nOSO₃⁻Na⁺ with n = 0. The measurements were made at 25 °C. Data from [47,59].

Figure 9

Effect of polar head group on molar solubilization power at 25 °C of the dye C.I. Solvent Orange 2 (Orange OT) by different cationic surfactants, all based on a decyl chain as hydrophobic tail and all having bromide as counterion. The aggregation numbers (N_agg) are also given for each surfactant. Data from [69,70,71].

Figure 10

Amount of solubilized dye C.I. Solvent Yellow 14 (Sudan I) as a function of temperature for three different surfactants, the anionic sodium dodecyl sulfate (SDS), the cationic dodecyltrimethylammonium bromide (DTAB) and the nonionic penta(ethylene glycol)monoundecyl ether (C₁₁E₅) at a fixed concentration of 10 g/L [44].

Figure 11

Aggregation number, solubilization power, intrinsic viscosity and critical micelle concentration (CMC) of dodecyltrimethylammonium chloride as a function of NaCl concentration. The dye used is C.I. Solvent Red 25 (Sudan IV) and the temperature was 25 °C. Data from [76,79,80].

Figure 12

Solubilization of a water insoluble dye by surfactant-polyelectrolyte mixtures can take different paths. See text for explanation of the curves.

Figure 13

Effect of pH on solubilization of C.I. Solvent Yellow 14 (Sudan I) at 21 °C for dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulfate (SDS), and penta(ethylene glycol)monoundecyl ether (C₁₁E₅), all at a concentration of 10 g/L [45].

Figure 14

Assumed location of the dye C.I. Solvent Orange 86 (Quinizarin) in a micelle of a cationic surfactant.

Figure 15

Absorption spectra of the dye C.I. Solvent Yellow 14 (Sudan I) in micellar solutions of DTAB and SDS and in different solvents. “Ethanol 50 v/v %” stands for equal volumes of water and ethanol.