Surfactant-cycladextrin binding, mixed surfactant micelles, and new surface tension methods.

Abstract

A stainless steel rod with precisely known radius is used to measure surface tensions. One of the most important aspects in two new methods is that a top-loading analytical laboratory balance of 0.1 to 1 mg precision can be used. Another important feature is that the probe can be held rigidly above the solution so it is possible to make measurements of small samples where the probe may be near the container wall. Although the methods can be used with traditional platinum rings or plates, a stainless steel rod provides excellent results.

The natural tendency of surfactants to form aggregates (micelles) in solution can be substantially enhanced when present as mixtures, compared to either of the pure surfactants. The synergistic lowering of the critical micelle concentration (CMC) of a series of binary cationic/nonionic mixed surfactant systems is the subject of the present work. The cationic surfactant used was cetylpyridinium chloride (CPC). The series of nonionic surfactants used were polyethylene oxides (PEO's) with 7 and 8 EO linkages, and hydrocarbon tails of C12, C14, and C16. The CMCs for the mixtures were used in a model for which a single parameter □ , is a measure of the magnitude of the enhancement. Results show that synergistic effect is stronger as the hydrocarbon tail of the PEO increases and increases slightly as the number of ethylene oxide subunits decreases.

Cyclodextrins are macrocyclic compounds with the unusual and useful structural feature permitting formation inclusion complexes in which a nonpolar structural feature of a guest molecule is bound inside the cavity. In the work presented here, the binding constants for cyclodextrin (CD) with a series of n-alkyl trimethylammonium bromide (TAB) cationic surfactants have been determined. The activity of the TAB compounds was measured in solution by using a surfactant sensitive electrode. The electrode response was used in a least-squares regression fit to model the equilibrium constants. Results show that the TAB surfactants form complexes of varying strength and different stoichiometries with a , b , and g -cyclodextrins depending on the relative length of the surfactants hydrocarbon tail and on the size of the cyclodextrins cavity.