Effects of temperature and chain length on the properties of long chain polyoxyethylene dodecyl ethers in aqueous solution

Abstract

The micelle formation and adsorption phenomena at the airwater interface of four long-chain polyoxyethylene dodecyl ethers, C₁₂H₂₅O(C₂H₅O)_n H abbreviated as C₁₂E_n (n=6, 7, 10, 20, 30, and 40), which have newly been synthesized to have more narrow polyoxyethylene chain-length distribution than previously used, have been investigated in aqueous solution by surface tension measurements over a concentration range of the surfactants (10⁻⁵−10⁻³ m) at different temperatures (25, 35, 40, and 45 °C). The critical micelle concentration (CMC), the area (A) per surfactant molecule, and the standard Gibbs free-energy change of micellization (ΔG _m) were determined from the surface-tension data of the solutions and the Gibbs adsorption equation at the various temperatures. It has been found that the CMC values of C₁₂E_n increase with increasing chain length of polyoxyethylene but decrease with increasing temperature. The area per molecule A increases with elevating temperature, their increment from 25 °C to 45 °C being 3.3, 12.2, 20.5, and 36.9 Ų for the C¹²E_n (n=10, 20, 30, and 40). respectively. Application of a scaling rule in polymer science to parameters A and ΔG _m has suggested that a coil-globule transition takes place between 35 and 45 °C both at the micellar surface and at the air-water interface for the C₁₂E₃₀ and C₁₂E₄₀ around 40 °C