Chemiluminescence in Analytical Chemistry

utilized to facilitate analytical chemiluminescence (CL) measurements [8–13]. Advantages cited include elimination of solubility problems [8, 9], improved sensitivity [9–13], increased selectivity [9, 13] better precision [13], and a less strict pH requirement for observation of efficient chemiluminescence [9, 13].

In the following sections the most important features of the organized media that are most frequently used in chemiluminescent reactions (micellar media and cyclodextrins) will be summarized as well as their influence on various chemiluminescent systems, including their corresponding applications in chemical analysis.

2. ORGANIZED MEDIA

2.1 Micellar Media

2.1.1Definitions and Characteristics

Surfactants (a contraction of the term surface-active agent) are substances one of whose properties is that of being adsorbed in surfaces or interfaces of the system and altering the free energy of these surfaces (or interfaces). Here, the term interface refers to the union between two inmiscible phases, while the term surface refers to an interface in which one of the phases is a gas, generally air.

Free surface energy is the minimum work required to create an interface. The surface tension of a liquid is the free surface energy per unit of area in the bond between the liquid and the air. Thus, the minimum work required to create an interface is:

Wmin γ∆A

where γ is the surface tension and ∆A the increase in area between phases. The surfactant usually acts by reducing the free surface energy, though on

occasion it will increase it. The molecules present on the surface have greater energy than those found in the interior of a medium, as the latter interact more strongly in such a way that their energy is lowered. Therefore, a certain amount of work will be required to carry a molecule from the interior to the surface.

Surfactants have a molecular structure characteristic, called amphipathic, consisting of a group that has little affinity for the solvent, called hydrophobic, when the solvent is water, and a group with a high affinity for the solvent, called hydrophilic, in an aqueous solution. When the surfactant is dissolved in water, the presence of the hydrophobic group in the interior of the water causes a distortion, thereby bringing about an increase in the free energy of the system. This means that the work required to carry a surfactant molecule to the surface is less than the work required to transfer a water molecule. Therefore, the surfactant will concentrate on the surface. This brings about a decrease in the work required

to increase the surface unit area (surface tension). On the other hand, the presence of the hydrophilic group impedes the surfactant from being completely expelled from the solvent, which would require the dehydration of this group. The amphipathic structure of the surfactant causes not only concentration of surfactant on the surface and reduction of the surface tension of the water, but also orientation of the molecule with its hydrophilic group in the aqueous phase and its hydrophobic group toward the exterior [14].

The hydrophobic group is usually a hydrocarbon chain and is generally called the tail, while the hydrophilic group, called the head, is an ionic group and very polar. Depending on the nature of the hydrophilic group, the surfactants are classified as:

1.Anionic: the head group is negatively charged such as, for example, alkylcarboxilic or sulfonic acids salts.

2.Cationic: the head group is positively charged such as, for example, quaternary ammonium salts.

3.Nonionic: the surfactant has no groups with charge such as, for example, monoglycerides of long-chain fatty acids or polyoxyethylenated alkylphenols.

4.Zwitterionic: in the surfactant there are positively and negatively charged groups such as, for example, long-chain amino acids.

Table 1 shows examples of these four types of surfactants and their corresponding structures.

In diluted solutions (generally at concentrations less than 10 4 M), the surfactants are generally found to be monomers, though there can also be dimers, trimers, etc. If the concentration of surfactant in solution increases, it can reach a point where a process of aggregation occurs and many of the physicochemical properties undergo change. The colloidal aggregate is given the name micelle and its shape varies depending on the surfactant and the medium. The concentration at which these changes occur is known as the critical micelle concentration (cmc).

It was mentioned above that when substances that contain hydrophobic groups are dissolved in water the free energy of the system increases. To make this free energy minimal, the substance positions itself with its hydrophobic part toward the exterior of the solvent. However, there is another way to minimize this energy, which consists of the aggregation of these surface-active molecules (surfactants) in clusters (micelles), with their hydrophobic groups facing toward the interior of the cluster and the hydrophilic groups toward the solvent. Thus, the formation of micelles is an alternative mechanism to adsorption in the interfaces to avoid contact between the hydrophobic groups and the water, and so decrease the free energy of the system [14].

In spite of the fact that keeping the hydrophobic groups from being in contact with the water can produce a decrease in the free energy of the system,

the surfactant molecule can lose its freedom on finding itself in the micelle. Moreover, an electrostatic repulsion can occur in the ionic surfactants between molecules of surfactants in micelles and surfactants that are free to have the same charge. These forces increase the free energy of the system and resist the formation of micelles. As a consequence, if in a certain case micelles are formed, the concentration of surfactant of which they are produced depends on the equilibrium between the factors that favor the formation of micelles and those that resist it.

The structure of the micelles remains an object of study and controversy. A series of models has been proposed that attempt to explain the experimental evidence. In this study the chronological order of publication of the four most important models has been considered.

The classical model, as shown in Figure 1, assumes that the micelle adopts a spherical structure [2, 15–17]. In aqueous solution the hydrocarbon chains or the hydrophobic part of the surfactants from the core of the micelle, while the ionic or polar groups face toward the exterior of the same, and together with a certain amount of counterions form what is known as the Stern layer. The remainder of the counterions, which are more or less associated with the micelle, make up the Gouy-Chapman layer. For the nonionic polyoxyethylene surfactants the structure is essentially the same except that the external region does not contain counterions but rather rings of hydrated polyoxyethylene chains. A micelle of

Figure 1 Typical cross-sectional schematic representing the classical view of an aqueous micelle. Counterions are not shown. (From Ref. 2 with permission.)

this type has a radius that is approximately equal to the length of the extended hydrocarbon chain and usually fluctuates between 3 and 6 nm [3].

This model has been used for years for its simplicity, but it presents several contradictions and for this reason other researchers have proposed different alternatives. Thus, it has been noted that there is contact between the water and the tails of the surfactants that make up the micelle. This can occur in three different ways:

If there were penetration of the water in the core of the micelle.

If, even if there were no penetration of the water in the interior of the core of the micelle, a part of the hydrocarbon chains were exposed to the micelle-water interface.

If a combination of the above two situations were to occur.

The model proposed by Menger et al. (Fig. 2) shows two extreme conformations, one in which the hydrocarbon chains are fully extended and another in which they are folded [18, 19]. The surface of Menger’s micelle is less defined than in the classical model and the surfactants that form the micelle are randomly orientated. The water can penetrate and enter in contact with the hydrophobic part of the surfactants. This model, apart from being more acceptable from an esteric point of view, gives a better explanation than the classical model of a series of experimental results such as viscosity, polarity, or kinetics.

Figure 2 A possible representation of the Menger micelle in which the hydrocarbon ‘‘tails’’ are extended. Counterions are not shown. (From Ref. 2 with permission.)

Figure 4 A cross-sectional schematic of the Dill model for the micelle. Counterions are not shown. (From Ref. 2 with permission.)

changes directly from spherical to laminar as the concentration of surfactant increases.

It was mentioned previously that the narrow range of concentrations in which sudden changes are produced in the physicochemical properties in solutions of surfactants is known as critical micelle concentration. To determine the value of this parameter the change in one of these properties can be used; so normally electrical conductivity, surface tension, or refraction index can be measured. Numerous cmc values have been published, most of them for surfactants that contain hydrocarbon chains of between 10 and 16 carbon atoms [1, 3, 7]. The value of the cmc depends on several factors such as the length of the surfactant chain, the presence of electrolytes, temperature, and pressure [7, 14]. Some of these values of cmc are shown in Table 2.

It is important to point out that, in general, the micelles composed of nonionic surfactants usually have a lower cmc and higher aggregation numbers than the analogous ionic micelles. This is partly due to the absence of electrostatic repulsion between the heads of the nonionic surfactants. However, in the ionic micelles these repulsion tend to limit the aggregation number and the cmc.

Most of the applications of the micelles in aqueous medium are based on the association or solubilization of solutes. The interactions between both can be electrostatic, hydrophobic, or, more normally, a combination of both effects [23, 24]. It was thought initially that hydrophobic solutes dissolve in the core of the micelle in the same way in which they would do so in an organic solvent, but

Source: From Ref. 7 with permission.

this is probably a simplification. In accordance with the structure of the micelle of Menger, Fromherz, or Dill, the interaction between many hydrophobic solutes and the micelle can be somewhat similar to a surface adsorption phenomenon where hydrophobic and electrostatic reactions are important. This would explain not only the apparent presence of solutes that are relatively nonpolar near the surface of the micelle [23], but also the fact that some polar solutes have a higher solubility in micellar solutions than in water or organic solvents. There are solutes, such as aliphatic hydrocarbons, that are dissolved in the core of the micelle and other solutes, such as dodecanol, that have a part of their structure integrated in the core of the micelle. It is therefore believed that the micelle has at least two interaction sites: a hydrophobic one in the core and another more polar one near the surface of the micelle [15, 25].

Thus far reference has only been made to the so-called normal micelles that are formed in polar solvents such as water. However, when the surfactants are dissolved in organic nonpolar solvents, the hydrophilic groups are found in the interior of the aggregate, while the hydrophobic chains extend toward the

Figure 5 Model of a reversed micelle in a nonpolar organic solvent from which all possible water and impurities have been removed. (From Ref. 2 with permission.)

nonpolar phase (Fig. 5) [26]. The accepted term for this type of aggregate, the structure and properties of which are generally different from those of normal micelles, is reversed micelle. The interior core of the reversed micelle, i.e., the micellar interface and the inner aqueous phase, provides a unique and versatile reaction field. Depending upon its water content (which also dictates the size of the aggregate), the microscopic polarity, the local concentration (proximity), and the mobility of substrates (microviscosity) can vary markedly and, therefore, the chemical reactions can be controlled as required [26–28].

Finally, and apart from the importance of micelles in the solubilization of chemical species, mention should also be made of their intervention in the displacement of equilibria and in the modification of kinetics of reactions, as well as in the alteration of physicochemical parameters of certain ions and molecules that affect electrochemical measurements, processes of visible-ultraviolet radiation, fluorescence and phosphorescence emission, flame emission, and plasma spectroscopy, or in processes of extraction, thin-layer chromatography, or highperformance liquid chromatography [2–4, 29–33].

2.2 Cyclodextrins

2.2.1General Characteristics

Cyclodextrins are cyclic oligosaccharides formed by the connection of individual glucopyranose units through α-1,4-glycosidic oxygen bridges. These compounds form inclusion complexes with appropriate molecules, producing changes in the photophysical properties of these molecules.

The three most used cyclodextrins are known as α, β, and γ, and these contain six, seven, and eight units of glucopyranose, respectively. The structure of the cyclodextrins is in the shape of a truncated cone, the cavity of which has