**2. Supramolecular chemistry**

Supramolecular chemistry is the discipline of chemistry which involves all intermolecular interactions where covalent bonds are not established between the interacting species: i.e.,

Application of Cyclodextrins in Textile Dyeing 375

groups are situated on the larger side of the two edges of the ring, whereas all the primary ones are placed on the smaller side of the ring. These hydroxyl groups ensure good water solubility. The cavity is lined by the hydrogen atoms of C3, by the glycosidic oxygen bridges and hydrogen atoms of C5. The nonbonding electron pairs of the glycosidic oxygen bridges are directed toward the inside of the cavity producing a high electron density there and because of this the inner side of the cavity has some Lewis base characteristics. The C-2-OH group of one glucopyranose unit can form a hydrogen bond with the C-3-OH group of the adjacent glucopyranose unit. In the cyclodextrin molecule, a complete secondary belt is formed by these H bonds, therefore the -cyclodextrin has a rather rigid structure. Because of this arrangement, the interior of the toroids is not hydrophobic but considerably less hydrophilic than the aqueous environment and thus able to host hydrophobic molecules. Cyclodextrins behave more or less like rigid compounds with two degrees of freedom, rotation at the glucosidic links C4-O4 and C1-O4 and rotations at the O6 primary hydroxyl

groups at the C5-C6 band.

(Connors, 1997).

**2.2 Toxicological considerations** 

Fig. 1. Structure and dimensions of -, - and -cyclodextrin

The intramolecular hydrogen bond formation is probably the explanation for the observation that -cyclodextrin has the lowest water solubility of alls. The hydrogen-bond belt is incomplete in the -cyclodextrin molecule, because one glucopyranose unit is in a distorted position. Consequently, instead of the six possible hydrogen-bonds, only four can be established fully. -Cyclodextrin is a noncoplanar with more flexible structure; therefore, it is the most soluble of the three cyclodextrins. Fig. 2 shows a sketch of the characteristic structural features of cyclodextrins. On the side where the secondary hydroxyl groups are situated, the diameter of the cavity is larger than on the side with the primary hydroxyls, since free rotation of the primary hydroxyls reduces the effective diameter of the cavity

Since fabrics are in direct contact with human skin, toxic specification of cyclodextrins have been studied (Martin Del Valle, 2004; Dajstjerdi & Montazer, 2010). Since year 2000, -

molecules, ions, or radicals. The majority of these interactions are of the host-guest type. Among all potential hosts, the cyclodextrins seem to be the most important ones, for the following reasons (Szejtli, 1998; Vögtle, 1991).


#### **2.1 Cyclodextrins**

Cyclodextrins (CDs) comprise a family of three well-known industrially produced substances. The practically important, industrially produced CDs are the -, -, and cyclodextrins (Fig. 1). There are some rare, minor cyclic oligosaccharides as well which are due to their costs not applicable to textiles (Vögtle, 1991).

The three major cyclodextrins are crystalline, homogeneous, nonhygroscopic substances, which are torus-like macro-rings built up from glucopyranose units. The -cyclodextrin (Schardinger's -dextrin, cyclomaltohexaose, cyclohexaglucan, cyclohexaamylose, -CD, ACD, C6A) comprises six glucopyranose units, -cyclodextrin (Schardinger's -dextrin, cyclomaltoheptaose, cycloheptaglucan, cycloheptaamylose, -CD, BCD, C7A) comprises seven such units, and -cyclodextrin (Schardinger's -dextrin, cyclomaltooctaose, cyclooctaglucan, cyclooctaamylose, -CD, GCD, C8A) comprises eight such units (Fig. 1). Cyclodextrins can be obtained by enzymatic degradation of starch. In this process compounds with six to twelve glucopyranose units per ring are produced. Depending on the enzyme and the way the reaction is controlled, the main product is , or -cyclodextrin (6, 7 and 8 glucopyranose units, respectively). They are of circular and conical conformation, where the height is about 800 pm. The inner diameter of the cavity varies from 500 to 800 pm.

Crystal structure analysis has demonstrated that all glucopyranose units in the torus-like ring possess the thermodynamically favoured chair conformation because all substituents are in the equatorial position. As a consequence of the 4C1 conformation of the glucopyranose units, all secondary hydroxyl groups are situated on one of the two edges of the ring, whereas all the primary ones are placed on the other edge. All secondary hydroxyl

molecules, ions, or radicals. The majority of these interactions are of the host-guest type. Among all potential hosts, the cyclodextrins seem to be the most important ones, for the

1. Cyclodextrins are seminatural products; they are produced from a renewable natural

2. They are produced in thousands of tons per year by environmentally friendly

3. Because of their huge production, the initially high prices of cyclodextrins have dropped to levels where they become acceptable for most industrial purposes. The total output of -cyclodextrin is in excess of 1000 tons/year, and the price is only several dollars per kilogram, depending on quality and delivered quantity. Also - and cyclodextrins, as well as several derivatives, (hydroxypropyl--cyclodextrin and - cyclodextrin, randomly methylated - and -cyclodextrin, maltosyl--cyclodextrin, acetylated cyclodextrins, etc.) are produced industrially. A large number of other derivatives are available as fine chemicals, and used in various chromatographic

methods, or are studied as potential drug carriers, stabilizers, catalysts, etc.

solubility, possible increase in bioavailability, slow release and others.

4. Cyclodextrin molecules are of a great interest for scientists because of their capacity to include guest molecules in their cavities. Such inclusion is considered as a molecular encapsulation and it results in better stability of guests to air, heat or light, higher water

5. In general, cyclodextrins are not toxic, but any of their toxic effects are of secondary character and can be eliminated by selecting the appropriate cyclodextrin type or

Cyclodextrins (CDs) comprise a family of three well-known industrially produced substances. The practically important, industrially produced CDs are the -, -, and cyclodextrins (Fig. 1). There are some rare, minor cyclic oligosaccharides as well which are

The three major cyclodextrins are crystalline, homogeneous, nonhygroscopic substances, which are torus-like macro-rings built up from glucopyranose units. The -cyclodextrin (Schardinger's -dextrin, cyclomaltohexaose, cyclohexaglucan, cyclohexaamylose, -CD, ACD, C6A) comprises six glucopyranose units, -cyclodextrin (Schardinger's -dextrin, cyclomaltoheptaose, cycloheptaglucan, cycloheptaamylose, -CD, BCD, C7A) comprises seven such units, and -cyclodextrin (Schardinger's -dextrin, cyclomaltooctaose, cyclooctaglucan, cyclooctaamylose, -CD, GCD, C8A) comprises eight such units (Fig. 1). Cyclodextrins can be obtained by enzymatic degradation of starch. In this process compounds with six to twelve glucopyranose units per ring are produced. Depending on the enzyme and the way the reaction is controlled, the main product is , or -cyclodextrin (6, 7 and 8 glucopyranose units, respectively). They are of circular and conical conformation, where the height is about

Crystal structure analysis has demonstrated that all glucopyranose units in the torus-like ring possess the thermodynamically favoured chair conformation because all substituents are in the equatorial position. As a consequence of the 4C1 conformation of the glucopyranose units, all secondary hydroxyl groups are situated on one of the two edges of the ring, whereas all the primary ones are placed on the other edge. All secondary hydroxyl

material, starch, by a relatively simple enzymatic conversion.

following reasons (Szejtli, 1998; Vögtle, 1991).

derivative or mode of application.

due to their costs not applicable to textiles (Vögtle, 1991).

800 pm. The inner diameter of the cavity varies from 500 to 800 pm.

technologies.

**2.1 Cyclodextrins** 

groups are situated on the larger side of the two edges of the ring, whereas all the primary ones are placed on the smaller side of the ring. These hydroxyl groups ensure good water solubility. The cavity is lined by the hydrogen atoms of C3, by the glycosidic oxygen bridges and hydrogen atoms of C5. The nonbonding electron pairs of the glycosidic oxygen bridges are directed toward the inside of the cavity producing a high electron density there and because of this the inner side of the cavity has some Lewis base characteristics. The C-2-OH group of one glucopyranose unit can form a hydrogen bond with the C-3-OH group of the adjacent glucopyranose unit. In the cyclodextrin molecule, a complete secondary belt is formed by these H bonds, therefore the -cyclodextrin has a rather rigid structure. Because of this arrangement, the interior of the toroids is not hydrophobic but considerably less hydrophilic than the aqueous environment and thus able to host hydrophobic molecules. Cyclodextrins behave more or less like rigid compounds with two degrees of freedom, rotation at the glucosidic links C4-O4 and C1-O4 and rotations at the O6 primary hydroxyl groups at the C5-C6 band.

Fig. 1. Structure and dimensions of -, - and -cyclodextrin

The intramolecular hydrogen bond formation is probably the explanation for the observation that -cyclodextrin has the lowest water solubility of alls. The hydrogen-bond belt is incomplete in the -cyclodextrin molecule, because one glucopyranose unit is in a distorted position. Consequently, instead of the six possible hydrogen-bonds, only four can be established fully. -Cyclodextrin is a noncoplanar with more flexible structure; therefore, it is the most soluble of the three cyclodextrins. Fig. 2 shows a sketch of the characteristic structural features of cyclodextrins. On the side where the secondary hydroxyl groups are situated, the diameter of the cavity is larger than on the side with the primary hydroxyls, since free rotation of the primary hydroxyls reduces the effective diameter of the cavity (Connors, 1997).

#### **2.2 Toxicological considerations**

Since fabrics are in direct contact with human skin, toxic specification of cyclodextrins have been studied (Martin Del Valle, 2004; Dajstjerdi & Montazer, 2010). Since year 2000, -

Application of Cyclodextrins in Textile Dyeing 377

complex formation is the substitution of the high-enthalpy water molecules by an

The binding of guest molecules within the host cyclodextrin is not fixed or permanent but rather is a dynamic equilibrium. Binding strength depends on how well the 'host–guest' complex fits together and on specific local interactions between surface atoms. Complexes can be formed either in solution or in the crystalline state and water is typically the solvent of choice. Inclusion complexation can be accomplished in a co-solvent system and in the presence of any non-aqueous solvent (Martin Del Valle, 2004). Generally, one guest molecule is included in one cyclodextrin molecule, although in the case of some low molecular weight molecules, more than one guest molecule may fit into the cavity, and in the case of some high molecular weight molecules, more than one cyclodextrin molecule may bind to the guest. In principle, only a portion of the molecule must fit into the cavity to form a complex. Cyclodextrin inclusion is a stoichiometric molecular phenomenon in which usual only one guest molecule interacts with the cavity of the cyclodextrin molecules to become entrapped. 1:1 complex is the simplest and most frequent case. However, 2:1, 1:2, 2:2, or even more complicated associations, and higher order equilibrium exist almost

Inclusion in cyclodextrins has a profound effect on the physical and chemical properties of guest molecules as they are temporarily locked or caged within the host cavity (Martin Del

stabilisation of labile guests against the degradative effects of oxidation, visible or UV

Therefore, cyclodextrins are used in food, pharmaceuticals, cosmetics, environment

The ring structure of cyclodextrins allows them to act as hosts and form inclusion compounds with various small molecules. Such complexes can be formed in solution, in the solid state, as well as when cyclodextrins are linked to a solid surface where they can act as permanent or temporary hosts to those small molecules that provide certain desirable attributes such as adsorption of dyestuff molecules, fragrances or antimicrobial agents. This "molecular encapsulation" is already widely utilized in many industrial products,

Cyclodextrins can be considered as a new class of auxiliary substances for the textile industry. Cyclodextrins can be used for textile application because of their natural origin

appropriate "guest" molecule.

always simultaneously.

These properties are:

light and heat,

solubility enhancement of highly insoluble guests,

physical isolation of incompatible compounds,

retarding effect in dyeing and finishing baths

taste modification by masking off flavours, unpleasant odours

protection of dyes from undesired aggregation and adsorption.

removal of dyes and auxiliaries from dyeing effluents

protection, bioconversion, packing and the textile industry.

control of volatility and sublimation,

controlled release of drugs and flavours

chromatographic separations,

technologies, and analytical methods.

**3. Cyclodextrins in textile applications 3.1 Cyclodextrins in textile dyeing processes** 

Valle, 2004).

cyclodextrin has been introduced as a food additive in Germany. With respect to OECD experiments, this compounds has shown no allergic impact.

Fig. 2. Structural features of -cyclodextrin

In general, the natural cyclodextrins and their hydrophilic derivatives are only able to permeate lypophilic biological membranes, such as the eye cornea, with considerable difficulty. All toxicity studies have demonstrated that orally administered cyclodextrins are practically non-toxic, due to lack of absorption from the gastrointestinal tract. The main properties of -cyclodextrin (-CD), the most important cyclodextrin in textile application are: less irritating than -cyclodextrin after i.m. injection, binds cholesterol, small amount (1- 2%) is adsorbed in the upper intestinal tract, no metabolism in the upper intestinal tract, metabolised by bacteria in caecum and colon, LD50 oral rat > 5000 mg/kg, LD50 i.v., rat: between 450 – 790 mg/kg, however, application of high doses may be harmful and is not recommended.

#### **2.3 Inclusion complex formation**

The most notable feature of cyclodextrins is their ability to form solid inclusion complexes ("host–guest" complexes) with a very wide range of solid, liquid and gaseous compounds by a molecular complexation.

In these complexes a guest molecule is held within the cavity of the cyclodextrin host molecule. Complex formation is a dimensional fit between host cavity and guest molecule. The lipophilic cavity of cyclodextrin molecules provides a microenvironment into which appropriately sized non-polar moieties can enter to form inclusion complexes. No covalent bonds are broken or formed during formation of the inclusion complex. The main driving force of complex formation is the release of enthalpy-rich water molecules from the cavity. The water molecules located inside the cavity cannot satisfy their hydrogen bonding potentials and therefore are of higher enthalpy. The energy of the system is lowered when these enthalpy–rich water molecules are replaced by suitable guest molecules which are less polar than water. In an aqueous solution, the slightly apolar cyclodextrin cavity is occupied by water molecules which are energetically unfavoured, and therefore can be readily substituted by appropriate "guest molecules" which are less polar than water. This apolar– apolar association decreases the cyclodextrin ring strain resulting in a more stable lower energy state. The dissolved cyclodextrin is the "host" molecule, and the "driving force" of the

cyclodextrin has been introduced as a food additive in Germany. With respect to OECD

In general, the natural cyclodextrins and their hydrophilic derivatives are only able to permeate lypophilic biological membranes, such as the eye cornea, with considerable difficulty. All toxicity studies have demonstrated that orally administered cyclodextrins are practically non-toxic, due to lack of absorption from the gastrointestinal tract. The main properties of -cyclodextrin (-CD), the most important cyclodextrin in textile application are: less irritating than -cyclodextrin after i.m. injection, binds cholesterol, small amount (1- 2%) is adsorbed in the upper intestinal tract, no metabolism in the upper intestinal tract, metabolised by bacteria in caecum and colon, LD50 oral rat > 5000 mg/kg, LD50 i.v., rat: between 450 – 790 mg/kg, however, application of high doses may be harmful and is not

The most notable feature of cyclodextrins is their ability to form solid inclusion complexes ("host–guest" complexes) with a very wide range of solid, liquid and gaseous compounds

In these complexes a guest molecule is held within the cavity of the cyclodextrin host molecule. Complex formation is a dimensional fit between host cavity and guest molecule. The lipophilic cavity of cyclodextrin molecules provides a microenvironment into which appropriately sized non-polar moieties can enter to form inclusion complexes. No covalent bonds are broken or formed during formation of the inclusion complex. The main driving force of complex formation is the release of enthalpy-rich water molecules from the cavity. The water molecules located inside the cavity cannot satisfy their hydrogen bonding potentials and therefore are of higher enthalpy. The energy of the system is lowered when these enthalpy–rich water molecules are replaced by suitable guest molecules which are less polar than water. In an aqueous solution, the slightly apolar cyclodextrin cavity is occupied by water molecules which are energetically unfavoured, and therefore can be readily substituted by appropriate "guest molecules" which are less polar than water. This apolar– apolar association decreases the cyclodextrin ring strain resulting in a more stable lower energy state. The dissolved cyclodextrin is the "host" molecule, and the "driving force" of the

experiments, this compounds has shown no allergic impact.

Fig. 2. Structural features of -cyclodextrin

recommended.

**2.3 Inclusion complex formation** 

by a molecular complexation.

complex formation is the substitution of the high-enthalpy water molecules by an appropriate "guest" molecule.

The binding of guest molecules within the host cyclodextrin is not fixed or permanent but rather is a dynamic equilibrium. Binding strength depends on how well the 'host–guest' complex fits together and on specific local interactions between surface atoms. Complexes can be formed either in solution or in the crystalline state and water is typically the solvent of choice. Inclusion complexation can be accomplished in a co-solvent system and in the presence of any non-aqueous solvent (Martin Del Valle, 2004). Generally, one guest molecule is included in one cyclodextrin molecule, although in the case of some low molecular weight molecules, more than one guest molecule may fit into the cavity, and in the case of some high molecular weight molecules, more than one cyclodextrin molecule may bind to the guest. In principle, only a portion of the molecule must fit into the cavity to form a complex. Cyclodextrin inclusion is a stoichiometric molecular phenomenon in which usual only one guest molecule interacts with the cavity of the cyclodextrin molecules to become entrapped. 1:1 complex is the simplest and most frequent case. However, 2:1, 1:2, 2:2, or even more complicated associations, and higher order equilibrium exist almost always simultaneously.

Inclusion in cyclodextrins has a profound effect on the physical and chemical properties of guest molecules as they are temporarily locked or caged within the host cavity (Martin Del Valle, 2004).

These properties are:


Therefore, cyclodextrins are used in food, pharmaceuticals, cosmetics, environment protection, bioconversion, packing and the textile industry.

The ring structure of cyclodextrins allows them to act as hosts and form inclusion compounds with various small molecules. Such complexes can be formed in solution, in the solid state, as well as when cyclodextrins are linked to a solid surface where they can act as permanent or temporary hosts to those small molecules that provide certain desirable attributes such as adsorption of dyestuff molecules, fragrances or antimicrobial agents. This "molecular encapsulation" is already widely utilized in many industrial products, technologies, and analytical methods.
