Effect of cyclodextrin modification on complexation thermodynamics with hexadecyltrimethylammonium cation with emphasis on subsequent surfactant micellization.

Surfactant-cyclodextrin based complexes, are ideal guest-hosts for fundamental studies since both surfactant hydrophobic region with head group, and cyclodextrin cavity can be systematically varied. This allows, better understanding of the contributions that different chemical entities do to the stabilization of the complex and subsequent surfactant micellization. We studied systematically the interaction of hexadecyltrimethylammonium bromide (▪) with native α -, β - and γ -CDs, 2-hydroxypropylated and methylated α - and β -CDs, using isothermal titration calorimetry (ITC), measuring below critical micellization concentration (CMC) at least at three temperatures in the range (288.15 to 318.15) K. Data were treated simultaneously allowing the estimation of thermodynamically consistent temperature dependences of the equilibrium constant, the enthalpy and heat capacity for the inclusion complex formation. The data for cation/CD interaction were analysed mostly by the sequential binding model with 1:1 and 1:2 (cation:CD) stoichiometries, while some cation-CD combinations were examined using only 1:1, or more complicated 1:1, 2:1 and 2:2 stoichiometries. Thermodynamic quantities for complexation are discussed in terms of structural features, their temperature dependence with previous literature being examined. The shift of the CMC in presence of CDs was calculated founding large discrepancies with data in the literature. An extended mass-action chemical equilibrium model is proposed including free surfactant, free CD, complexes of the above mention stoichiometries, also including micelle, surfactant-cyclodextrin complexes of higher order stoichiometry, similar to that found for sodium dodecylsulfate-CDs systems. The new model provides a qualitative match with our own calorimetric titration curves measured above CMC for all studied ▪/CD combinations, and matches quantitatively with literature CMC values. • Binding of α -, β -, γ -, HP- α -, HP- β -, randomly Me- α -, and 2,6-DiMe- β -CDs with ▪ cation examined using ITC. • Formation of C+:CD inclusion complexes of 1:1; 1:1 and 1:2; or more complicated stoichiometries considered. • T -dependence of binding K and mostly also of binding Δ H established for formed inclusion complexes, but 2,6-DiMe- β -CD. • Effect of cyclodextrin structure and temperature variation on thermodynamic properties disclosed below surfactant CMC. • Extended mass-action model which includes micelle, [C+.CD], [C+.CD 2 ] and [C a +.CD b ] used to predict CMC shift. [ABSTRACT FROM AUTHOR]