On the connection between the complexation and aggregation thermodynamics of oxyethylene nonionic surfactants

Density and sound velocity data for aqueous solutions containing nonionic surfactants of the homologue series of polyoxyethylene(n) nonyl phenyl ethers (NPEn, n = 5 and 40) were analyzed in the absence and presence of beta-cyclodextrin (beta-CD) at 298 K. Thus, the critical micelle concentration of the surfactants and their apparent and partial molar volumes and compressibilities were measured. From a pseudophase separation model, the partial molar volumes and compressibilities of both pure surfactants in the micelle state and those of NPE40 in the monomer phase have been determined directly. For the ternary systems, increases of the molar volumes and compressibilities of NPE5 and NPE40 at infinite dilution and shifts of the cmc were observed compared to the binary systems. Luminescent measurements of the complexation process between NPE40 and beta-CD showed 1:1 + 1:2 (NPEn/2 beta-CD) stoichiometries for the complexes, with thermodynamic equilibrium constants that were in good agreement with previous results for NPE5 in the presence of beta-CD. This resemblance allowed us to use these results to indirectly determine the molar partial properties of NPE5 in the monomer state and understand the changes in the thermodynamic properties of NPE5 due to aggregation. From the aggregation and complexation data, a folding of the surfactants at the monomer state, in which the hydrophobic moieties of NPEn are surrounded by the EO chain, has been found. The oxyethylene group contributions at the monomer and micelle state of the NPEn homologue series have been estimated. The values of the transfer thermodynamic properties of both surfactants and beta-CD at infinite dilution conditions have been discussed in terms of a new extended model, in which the balance between the released water from the cavities of two beta-CDs and the different hydrophobic moieties of the surfactant that enter the macrocycle was considered.