Classification of dyeing and printing auxiliaries by function
10.3 MACROMOLECULAR COMPLEXING AGENTS
10.3.3 Crown ethers
Crown ethers and related structures are macrocyclic organic compounds generally composed of repeating ethylene (CH2CH2) units separated by hetero atoms such as oxygen, nitrogen, sulphur or phosphorus [54]. Other alkylene sub-units such as methylene (CH2) or propylene (CH2CH2CH2) may also be included but are less common. In contrast to the cyclodextrins and cucurbituril, these macrocyclic complexing agents possess an electron-rich and highly polar cavity and a hydrophobic exterior. Usually they are readily soluble in organic solvents.
They have been known since the 1930s. Two typical structures are 10.46 and 10.47. Oxygen is the hetero atom most commonly incorporated into the ring, but nitrogen (azacrowns), sulphur (thiacrowns) or phosphorus (phosphacrowns) are also known. Organic moieties sterically equivalent to the ethylene unit (such as 1,2-benzo) can be incorporated, as can most carbohydrates with vicinal dihydroxy groupings. Crown ethers are usually named as x-crown-y, from the number (x) of atoms composing the macrocyclic ring and the number (y) of hetero atoms contained within it. If one of the oxygen atoms in structure 10.46 is substituted by nitrogen, it becomes monoaza-12-crown-4.
Not all crown ethers have been tested for ecological or toxicological properties. Some are irritants and some are known to be toxic, although those tested do not show high toxicity.
Nevertheless, amongst those that have not been tested, some may be hazardous to health.
O
The functional characteristic of these compounds that is of interest from the viewpoint of textile processing is their capability to accommodate alkaline-earth and alkali metal cations, as well as a variety of other species, within their cavities. Stability constants (Equation 10.3) are again used, both as a measure of ligand strength and as a hierarchical indicator of complexing capability.
kc = rate constant for complex formation
kd = rate constant for dissociation of the complex Ks = stability constant
The stability constant is dependent, amongst other things, on the solvating medium. For example, for a simple crown ether kc is usually very large and kd also large, but in nonpolar solvents kd is much smaller than kc, so that KS increases with decreasing polarity of the solvating medium.
It is generally accepted that for complexing to occur the cavity of the crown ether must contain convergent binding sites (as, for example, the inwardly directed oxygen atoms in 10.46 and 10.47), whilst the entity to be complexed must have divergent binding sites. An example is shown in 10.48, the formation of which is facilitated by the hydrogen atoms diverging from the central nitrogen of the methylammonium cation. It is the three N–H–O hydrogen bonds that stabilise the complex.
The potential of crown ethers for use as auxiliaries in textile coloration processes does not appear to have been evaluated recently, although their potential to complex with
alkaline-CH2
earth and alkali metal ions has been demonstrated with styryl dyes containing an aza-15-crown-5 macroheterocyclic moiety (10.49) [55].
10.3.4 Liposomes
Liposomes, also known as lipid vesicles, are aqueous compartments enclosed by lipid bilayer membranes [56,57]. Figure 10.11 shows how lipid bilayers are arranged in the liposome and the lipid structures in large unilamellar vesicles and multilamellar vesicles. Lipids consist of two components:
– an elongated hydrophobic moiety – a hydrophilic end group.
10.49 D = Styryl dye chromogen
D N
H2C H2C O
CH2 CH2 O
CH2
H2C H2C O
CH2 CH2 O
CH2
polar non polar polar cavity
Liposome
MLV LUV
Figure 10.11 Liposome structures, including multilamellar vesicles (MLV) and large unilamellar vesicles (LUV) [57]
When these lipids are dispersed in water, they spontaneously form bilayer membranes (also called lamellae) which are composed of two monolayer sheets of lipid molecules with their hydrophobic surfaces facing one another and their hydrophilic surfaces contacting the aqueous medium. In the case of phospholipids such as phosphatidylcholine (10.50), the structure consists of:
– hydrophobic component: two hydrocarbon chains (R1 and R2) – hydrophilic component: glyceryl ester, phosphate and choline groups.
CH2 O
C
R1 O
CH CH2 O C R2
O
O P O
O
O
CH2 CH2 N CH3 CH3 CH3
10.50
+ _
Phosphatidylcholine
These structures are effective encapsulating systems for either hydrophilic or hydrophobic compounds. They can be obtained not only in uni- or multilamellar forms but also in different particle sizes with varying degrees of aggregation. They are particularly useful in biological and pharmacological applications. Recent research in various areas of textile wet processing has revealed further potential in these sectors. However, the methods of preparing liposomes so far reported are not readily adaptable to commercial processing conditions, as can be seen from the following typical procedures used by de la Maza et al.
[57–60]. Clearly, substantial development work is needed before these techniques become compatible with bulk-scale textile wet processing.
Large unilamellar vesicle liposomes
Reverse-phase evaporation in a nitrogen atmosphere was used to prepare lipids. A lipid film previously formed was redissolved in diethyl ether and an aqueous phase containing the dyebath components added to the phospholipid solution. The resulting two-phase system was sonicated at 70 W and 5 °C for 3 minutes to obtain an emulsion. The solvent was removed at 20 °C by rotary evaporation under vacuum, the material forming a viscous gel and then an aqueous solution. The vesicle suspension was extruded through a polycarbonate membrane to obtain a uniform size distribution (400 nm).
Multilamellar vesicle liposomes
A lipid film was formed from a chloroform solution of egg phosphatidylcholine by rotary evaporation in a nitrogen atmosphere and under vacuum. An aqueous phase containing the dyebath components was then added to the lipid film. The solution was swirled to transfer the lipid from the flask and to disperse lipid aggregates; glass beads being added to facilitate dispersion. The resulting milky suspension was centrifuged for 5 minutes and then extruded through a polycarbonate membrane to obtain a uniform size distribution (400 or 800 nm).
Liposomes made from pure phosphatidylcholine or containing lipids that are found in the cell membrane complex of wool (e.g. cholesterol) have been used to encapsulate aqueous chlorine solutions in chlorination processes [61,62]. The results showed improvements in
the uniformity and homogeneity of oxidative treatment, minimising degradation of the wool and facilitating subsequent treatments.
The application of acid dyes to wool using liposomes has also been researched. The dyes used were the milling acid dye CI Acid Blue 90 [57] and the neutral-dyeing 1:2 metal-complex dye CI Acid Yellow 129 [60]. Dyeing conditions were 90 °C and pH 5.5, using various ratios of phosphatidylcholine:dye. In the work with CI Acid Blue 90, both uni- and multilamellar vesicles were used. Dye exhaustion decreased with increasing concentration of phospholipid (Figure 10.12) but the amount of bonded dye increased with increasing lipid concentration (Table 10.4). The percentage of dye bonded to wool (Cb) was expressed by Equation 10.4:
= a- e
b
a
100(C C )
C C (10.4)
where Ca= mg/g dye absorbed by wool
and Ce= mg/g dye extracted from wool by ethanol and ammonia.
Dye exhaustion/%
Time/min
20 60 100
100 80
60
40
20 A
Dye exhaustion/%
Time/min
20 60 100
100 80
60
40
20 B
0.0 0.5 1.0 2.0 4.0 Concentration (mmol/l)
Figure 10.12 Exhaustion of CI Acid Blue 90 by untreated wool in dyeing with LUV (A) and MLV (B) liposomes [57]
Table 10.4 Amounts of dye bonded to wool using LUV and MLV liposomes at different lipid concen-trations with CI Acid Blue 90 [57]
Bonded dye (%) Lipid concentration
(mmol/1) LUV MLV
4.0 84 77
2.0 78 74
1.0 77 73
0.5 68 68
0 62 62
The work with CI Acid Yellow 129 used only unilamellar vesicles. The liposomes again suppressed exhaustion but increased dye–fibre bonding, leading to better fastness properties.
It is claimed that liposomes can be used to control the rate of exhaustion.
The application of CI Disperse Violet 1 to wool with phosphatidylcholine [58] and phosphatidylcholine/cholesterol [59] liposomes has been investigated. Figures 10.13 and 10.14 show that exhaustion decreases with increasing concentration of liposome, an effect which may be used to control exhaustion rate. It is claimed that liposomes enhance the dye dispersion efficiency, being superior to conventional dispersing agents. Dye–fibre bonding forces and levelling of the dye are also said to be improved.
Exploration of the use of liposomes in wool processing stems from the similarity that exists between the bilayer structure of the cell membrane complex of wool and that of the liposomes. Merino wool contains about 1% by weight of lipids, these forming the hydrophobic barrier of the cell membrane complex. Cholesterol is one of the main lipid
20 60 80 100 120
Time/min 40
20 60 80 100
Dye exhaustion/%
40
Phosphatidylcholine lipid concn (mmol/l)
0.5 1.0
1.5 2.5
Figure 10.13 Exhaustion rates of CI Disperse Violet 1 on untreated wool in dyeing using liposomes at different lipid concentrations and constant dye concentration [58]
20 40 60 80 100 120
Time/min 20
40 60 100
80
Dye exhaustion/% Phosphatidylcholine/cholesterol
lipid concn (mmol/l) 1.25
2.5
3.0
Figure 10.14 Exhaustion rates of CI Disperse Violet 1 on untreated wool during dyeing in the presence of MLV liposomes at different lipid concentrations and constant dye concentration [59]
components in wool; hence its use in combination with phosphatidylcholine (Figure 10.14).
One of the ideas behind this research, which remains valid despite limited commercial prospects as yet, is to focus attention away from electrostatic forces of attraction towards hydrophobic interactions, which are now accorded greater importance. In dyeing, for example, the idea is that the hydrophobic liposome will encapsulate dye molecules by means of hydrophobic interaction, the liposome–dye complex then being absorbed into the hydrophobic centre of the cell membrane complex via further hydrophobic interaction. This is why claims are made for increased hydrophobic bonding of the dyes to the fibre.
Along similar lines, synthetic cationic (10.51) and anionic (10.52) double-chain surfactant vesicles have been investigated for the dyeing of polyester with a monoazo disperse dye [63]. The results were moderately encouraging from technological, economical and environmental viewpoints, although problems and inconsistencies were observed. It is often difficult to explain results with disperse dyes on the basis of structural chemistry alone, since they can be influenced by variations in dispersion characteristics and instability during dyeing.