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 aggregation behavior of sodium taurocholate (TC) in deuterium oxide without salt was investigated by one-and two-dimensional NMR spectroscopy. Analysis of the concentration dependence of the chemical shift suggests that TC forms a dimer and a pentamer. The equilibrium constants of dimerization and pentamerization are close to those already determined by chromatography in the presence of 154 mM sodium chloride. The structure of the dimer is estimated from the NOESY and ROESY spectra of a 8 mM TC solution and molecular mechanics calculations. The inter-proton distances calculated from the molecular mechanics structure are consistent with the NOE and ROE intensities, whereas those calculated from the X-ray crystal structure (hydrogen-bonded structure) are inconsistent. The molecular mechanics structure is stabilized by hydrophobic interactions between the steroid nuclei and by reduced electrostatic repulsion between the sulfonate ions. The local structures of the pentamer are estimated on the basis of the ROESY spectrum of a 30 mM TC solution. The pentamer of TC is formed mainly by hydrophobic interactions. Thus, a novel NMR method in surfactant chemistry has provided the first step to resolve the 20-year debate about the structures of dimers and micelles of TC. This novel approach in surfactant chemistry will serve to estimate the structures of micelles of other natural and synthetic surfactants.

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.

Proton NMR spectroscopy was applied to determine the binding constants and the solution structures of 1:1 R-cyclodextrin (R-CD) complexes with hexyltrimethylammonium (HTAB) and octyltrimethylammonium (OTAB) bromides. Chemical shift data of all protons, referred to an internal standard, were used to determine reliable binding constants and chemical shift variations ∆δ complex induced by complex formation. The ROESY spectra of aqueous solutions containing R-CD and HTAB or OTAB provide information about rough structures of the complexes. The alkyl chains of HTAB and OTAB are incorporated from the wide rim of the R-CD cavity. The ROE intensities of intermolecular cross-peaks are plotted against the effective interproton distances for many structures different in the penetration depth of the alkyl chain. Detailed structures of the R-CD complexes with HTAB and OTAB are determined from the best correlations between the ROE intensities and the interproton distances. The ∆δ complex values for the protons of propanol, HTAB, and OTAB depend on the position located in the R-CD cavity. For instance, the proton near the proton H3 of R-CD exhibits the largest variation (ca. 0.2 ppm). The present data on single-and short-chain surfactants will provide the basis for determining the stoichiometry, the binding constants, and the structures of CD complexes with longchain surfactants and double-chain surfactants.