Cyclodextrins, Surfactants and Their Inclusion Complexes

Abstract

Herein, a brief overview of the cyclodextrin structure is provided, along with its most important derivatives. The difference between the water molecules in the outer hydration shell of cyclodextrin and those in its hydrophobic cavities is discussed. The structural characteristics of surfactants, along with their structural differences, are presented. An insight into the formation of surfactant micelles was given in aqueous solution. A thermodynamic model for the formation of the inclusion complex between surfactants and cyclodextrin in a solution is presented, explaining the hydrophobic effect, which drives the formation of the inclusion complex at lower and room temperatures. The influence of the size of the cyclodextrin cavity and the structure of surfactants on the stoichiometry of the inclusion complex, as well as on the affinity of the surfactant to the hydrophobic cavity of cyclodextrin, is discussed. The most important experimental methods used to study the cyclodextrin-surfactant inclusion complex are listed.

Keywords: critical micellar concentration; detailed balance; inclusion complex; micelle; stoichiometry.

Figure 1

Structure and schematic model of β-cyclodextrin (βCD).

Figure 2

Schematic representation of surfactant self-assembly and micellization. (A) At constant temperature and pressure, surfactant monomers distribute between the bulk solution, the air–water interface, and micelles once the concentration exceeds the critical micellar concentration (CMC). The hydrophilic (polar) head groups (red circles) are oriented toward the aqueous phase, while hydrophobic chains aggregate in the micelle core. (B) Surfactant molecules can locate at the micelle surface or the air–water interface, stabilized by hydrophobic and electrostatic interactions. (C) Water molecules structure around hydrophobic moieties, and their release upon micelle or inclusion complex formation contributes to the driving force of assembly. The lower panel illustrates the typical decrease in surface tension with increasing surfactant concentration, showing the breakpoint at the CMC.

Figure 3

Classification of surfactants according to their dissociation in aqueous solution. Conventional surfactants are grouped into non-ionic and ionic types (anionic, cationic, and zwitterionic), while novel classes include cleavable, polymerizable, polymeric, and catanionic surfactants, as well as those with multiple polar/nonpolar segments.

Figure 4

General structures of some surfactants and an example of DOWAX surfactant.

Figure 5

Formation of the cyclodextrin-surfactant inclusion complex: formal dehydration of the hydrophobic segment of the surfactant and the inner cavity of the cyclodextrin (A), the entry of the surfactant into the cavity of the cyclodextrin (B), the hydration of the hydrophobic segment of the surfactant that is not in the cavity of the cyclodextrin and the entry of a more or less twisted conformation of the hydrocarbon chain of the surfactant into the cavity of the cyclodextrin (D) when phase (C) does not occur.