General and rigorous theory for equilibrium concentrations of uncomplexed species and 1 : 1, 1 : 2, 2 : 1, and 2 : 2 complexes of cyclodextrin (D) and surfactant has been developed and applied to evaluate the binding constants of these complexations from surface tension data of aqueous solutions of dodecyl maltoside (DM) with α-, β-, and γ-D, reported by Saeger and Muller-Fahrnow. The orders of the binding constants for 1 : 1 and 2 : 1 complexations of D and DM are α-D > β-D > γ-D. Those for 1 : 2 and 2 : 2 complexations are γ-D > β-D > α-D. These orders are explained on the basis of the goodness of fit of the dodecyl chain into the cyclodextrin cavity. Furthermore, the theory for the effects of D on the molar conductivity and the critical micelle concentration (cmc) of an ionic surfactant is modified by accounting for the change of these values with the surfactant concentration and applied to evaluate the binding constants for 1 : 1, 1 : 2, and 2 : 1 complexations of sodium dodecyl sulfate (SDS) and β-D from conductance and cmc data reported by Palepu and Reinsborough. Although there are several uncertainties in experimental data and their interpretations for the β-D-SDS system, the 1 : 1 complexation is predominant and its binding constant appears to be 5000 dm3 mol−1. Based on the present analysis, it is suggested that since a surfactant is a long and fine molecule, relative to aromatic compounds, its ternary and quaternary complexes with D’s as well as its binary complex should be taken into consideration.

From frontal chromatograms and derivatives of sodium taurocholate (TC) and taurodeoxycholate (TDC) on Sephadex G-10 columns in 0.154 M sodium chloride, monomer concentrations (Cl), weight and number average aggregation numbers excluding and including the monomer contribution, critical micelle concentrations (cmc's), and minimum multimerization concentrations have been determined as a function of the total concentration (C). Since the micellization of TC occurs rather noncritically, the cmc values obtained from the same centroid volume data depend on the data treatment. The definition of cmc, that cmc is the total concentration at the inflection on the C1 vs C curve, is applicable to TC and TDC as well as surfactants. The dimerization of TC and TDC has been proven by several pieces of chromatographic evidence, and their dimerization constants are determined independently from centroid volume and peak volume data. The stepwise aggregation constants are determined and are used to calculate micelle size distributions. Micelles of TC and TDC grow above the cmc more gradually than those of surfactants. This gradual growth is the main reason for inconsistencies in the literature of bile salt micellization. Since TDC is more hydrophobic than TC, the dimerization constant and aggregation numbers of TDC are larger than those of TC. This result supports the Small model for small micelles of bile salt.

The surface tension and the bitter taste intensity of aqueous solutions of propantheline bromide alone and of complexes with R-, -, and γ-cyclodextrin have been determined, and a unique correlation between these quantities has been found to hold regardless of the kind and concentration of the cyclodextrin. The surface tension of an aqueous solution of propantheline bromide increases on addition of one of the cyclodextrins, and the effectiveness in surface tension enhancement decreases in the order -, γ-, R-cyclodextrin. Analysis of these surface tension data yields the equilibrium constants of 1:1, 1:2, and 2:1 complexations of propantheline bromide and R-, -, γ-cyclodextrin. These binding constants are in excellent agreement with those estimated by spectrophotometry. As well as the equimolar complex, -cyclodextrin forms the 1:2 complex, and γ-cyclodextrin forms the 2:1 complex. The dimer of propantheline bromide can be incorporated into γ-cyclodextrin. Although the dimer is not included into R-and -cyclodextrins, the extent of complexation is decreased by the dimerization. The bitter taste of propantheline bromide is suppressed by adding one of the cyclodextrins. The order of this suppression agrees with that of surface tension enhancement. From the relationship between the surface tension and the bitter taste intensity of aqueous solutions of propantheline bromide and the observed value of surface tension for an aqueous solution of propantheline bromide and one of the cyclodextrins, we can predict the bitter taste intensity of the mixed solution. Molecular models of some complexes of propantheline bromide and cyclodextrins have been proposed on the basis of size and shape of these molecules and the affinity of each group of propantheline bromide to the cyclodextrin cavity.

Publication costs assisted by the Kyoto College of PharmacyThe surface tension y of mixed fluorocarbon (NF) and hydrocarbon (STS) surfactants was measured in 0.01 and 0.05 M sodium chloride solutions, and two kinds of mixed micelles were found to coexist in equilibrium. The micellar composition x, was determined from the surface tension data on NF-STS solutions above the cmc. In the relation between the y at the cmc and xm, a plateau region was observed, and both ends of this region represent the mutual solubility of NF and STS in the mixed micelles. The cmc vs. monomeric composition curve breaks where two micellar phases coexist. In the relation between the y at the cmc and the composition of adsorbed monolayers on aqueous solution, there was no plateau region, indicating the complete miscibility of NF and STS in the adsorbed monolayers. From the temperature dependence of mutual solubilities in 0.01 M sodium chloride solution, the critical solution temperature was shown to exist in the NF-STS comicellar system. This fact demonstrates that micelles have a character of a liquid like phase. A quantitative analysis based on regular solution theory was made in an attempt to explain the relations of cmc to the monomeric and micellar compositions as well as the temperature dependence of the mutual solubility. The major reason for the partial miscibility of NF and STS in the mixed micelles may be the phobicity of the fluorocarbon and hydrocarbon chains in the mixed micelles. The change of electrostatic energy with comicellization, albeit a minor factor, should be taken into account.

A method for calculations of molecular surface area changes with the docking of host and guest is developed and applied to the estimation of the structures and binding constants of cyclodextrin inclusion systems. Each molecule of the host and guest is regarded to consist of hydrophilic and hydrophobic groups. The change ΔS in water-accessible surface area with the docking of these host and guest molecules is divided into four terms:  ΔSoo(HG), ΔSow(HG), ΔSwo(HG), and ΔSww(HG). For instance, ΔSoo(HG) stands for the change in host hydrophobic surface area by overlapping with guest hydrophobic surface area. When a guest molecule is moved along the symmetry axis of cyclodextrin, the structure of the complex having the maximum ΔSoo(HG) value is close to its crystal structure. Thus, we can estimate the “solution” structure of the complex from the maximum ΔSoo(HG) value. Using this method, we predict the solution structures of six cyclodextrin inclusion systems. Furthermore, we find that the logarithm of the 1:1 binding constant is linear with the maximum ΔSoo(HG) value for 11 systems including α-, β-, and γ-cyclodextrins and aliphatic and aromatic guest molecules. The present results would be applied to other cyclodextrin inclusion systems and protein−ligand systems.