Hydrophobic or Micellar Behavior of 2-butyl and 2-pentyl-2-(hydroxymethyl)-1,3-propaneDiol in Water. Volumes and Osmotic Coefficients

Abstract

n-butyl-2 and n-pentyl-2-(hydroxymethyl)-1,3-propanediol, named OT and NT, respectively, have been examined in aqueous solutions at 30, 40, and 50°C for NT and at 40°C for OT, using density and vapor pressure osmometry techniques. OT does not form true micelles but rather hydrophobic aggregates at the critical hydrophobic interaction concentration (CHIC). CHIC = 0.68 mol kg -1 for this compound. NT gives micellar association and the CMC has been determined at these temperatures. The values obtained are 0.247 (30°C) and 0.239 (40 and 50°C) mol kg -1 using density measurements, and 0.260 (30°C), 0.273 (40°C), and 0.276 (50°C) mol kg -1 using vapor pressure osmometry data. The volume changes during micellization are 3.81 (30°C), 3.74 (40°C), and 3.75 (50°C) cm 3 mol -1. The concentration dependence of properties such as the osmotic coefficient, the molal activity coefficient, the nonideal Gibbs energy of the solute, and the relative chemical potential are also studied in the temperature range explored. The aggregation numbers of NT micelles n = 11.7 (30°C), 12.0 (40°C), and 15.8 (50°C), with an accuracy ± 0.2, have been calculated using the phase-separation model. Results are discussed and compared to those obtained from the 1,2,3-alkane triol series.