Determination of Association Constants for Cyclodextrin–Surfactant Inclusion Complexes: A Numerical Method Based on Surface Tension Measurements
✍ Scribed by Runhua Lu; Jingcheng Hao; Hanqing Wang; Linhui Tong
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 129 KB
- Volume
- 192
- Category
- Article
- ISSN
- 0021-9797
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✦ Synopsis
factant molecules into their cavities in aqueous solutions. Inclusion complexes of b-cyclodextrin (b-CD) with sodium oc-The stoichiometries and formation constants of such inclutyl sulfonate (C 8 As), sodium dodecyl sulfonate (C 12 As), and sosion complexes have been investigated mainly by conductivdium hexadecyl sulfonate (C 16 As) in aqueous solutions are studied ity measurements (7-16). UV-visible spectroscopy has by surface tension measurement at the air/water interface at difbeen applied to determine the association constants of cycloferent temperatures. At fixed concentrations of the surfactants, dextrin inclusion complexes by using the changes in the the surface tension increases with an increase in b-CD concentraabsorbance of species in the UV region (17)(18)(19). Therefore, tion to a maximum value, at which it holds. The surface tension it is impossible to determine association constants if the curves of the surfactants in the presence of b-CD are higher than those in the absence of b-CD. The curves rise higher with the species have no chromophoric groups using this method. In increase in b-CD concentration for each surfactant. The apparent recent years, calorimetry, vapor pressure, and NMR methods critical micelle concentrations (CMC*) of the surfactants vary have also been developed to infer association constants of linearly with b-CD concentration. The CMC* and surface tension cyclodextrins with various guest molecules (20-27).
values (including those after the CMC*) for the same system
Surface tension measurements have proved suitable for decrease with increase in temperature. A numerical method based studying complexation of surfactants with cyclodextrins on surface tension measurements is developed to determine the (28)(29)(30)(31)(32)(33)(34). To date, relatively few surface tension studies association constants for 1:1 inclusion complexes. This method is have been performed in this field. Okubo et al. initiated such very reliable and easy to perform. The results demonstrate that studies for the b-cyclodextrin-cetyltrimethylammonium the longer the hydrophobic tail of the surfactant, the greater the bromide (CTAB) system (7). They assumed that the incluassociation constant with b-CD, and that for the same surfactant the association constant is higher at lower temperatures. ᭧ 1997 sion complexes have no surface activity, the surface tension Academic Press (g) of this system is attributed to the monomeric state elec-Key Words: b-cyclodextrin; surfactant; surface tension; associatrolyte (CTAB), and the association constant, therefore, can tion constant. be determined by using the g-[CTAB] profile in the presence and absence of cyclodextrin. The determination, however, was not able to be carried out because of the compara-1 This work was supported by the Director Foundation of Lanzhou Instisurfactants with cyclodextrins when ternary and quaternary tute of Chemical Physics.
2 To whom correspondence should be addressed.
complexes as well as their binary complexes are taken into 37