Experimental Methods and Prediction with COSMO-RS to Determine Partition Coefficients in Complex Surfactant Systems

Surfactant-based separation processes are a promising alternative to conventional organic solvent processes. A crucial parameter to describe the efficiency of such processes is the partition coefficient between the surfactant aggregates (micelles) and the aqueous bulk phase. In this work, several experimental methods to determine these partition coefficients (micellar liquid chromatography, micellar enhanced ultrafiltration, and cloud point extraction) are evaluated and compared. In addition, these results are compared to predictions with the thermodynamic model COSMO-RS. In particular, systems with the nonionic surfactant TritonX-100 are studied. The partition equilibria of various solutes (pyrene, naphthalene, phenanthrene, phenol, 3-methoxyphenol, and vanillin) and the influence of different additives (alcohols) are investigated. All experimental methods show very good reproducibility. Moreover, the results from different methods are in good agreement, supplementing one another concerning the temperature ranges. Notably, the COSMO-RS model is capable of predicting partition coefficients between micelles and water in the investigated temperature range and at different alcohol concentrations. The results demonstrate the potential of the model COSMO-RS to facilitate the selection of optimized process parameters for a given separation problem. By predicting partition equilibria in multicomponent systems, the selection of surfactant, temperature, and appropriate additives can be facilitated

Prediction of Micelle/Water and Liposome/Water Partition Coefficients Based on Molecular Dynamics Simulations, COSMO-RS, and COSMOmic

Liposomes and micelles find various applications as potential solubilizers in extraction processes or in drug delivery systems. Thermodynamic and transport processes governing the interactions of different kinds of solutes in liposomes or micelles can be analyzed regarding the free energy profiles of the solutes in the system. However, free energy profiles in heterogeneous systems such as micelles are experimentally almost not accessible. Therefore, the development of predictive methods is desirable. Molecular dynamics (MD) simulations reliably simulate the structure and dynamics of lipid membranes and micelles, whereas COSMO-RS accurately reproduces solvation free energies in different solvents. For the first time, free energy profiles in micellar systems, as well as mixed lipid bilayers, are investigated, taking advantage of both methods: MD simulations and COSMO-RS, referred to as COSMOmic (Klamt, A.; Huniar, U.; Spycher, S.; Keldenich, J. COSMOmic: A Mechanistic Approach to the Calculation of Membrane–Water Partition Coefficients and Internal Distributions within Membranes and Micelles. <i>J. Phys. Chem. B</i> <b>2008</b>, <i>112</i>, 12148–12157). All-atom molecular dynamics simulations of the system SDS/water and CTAB/water have been applied in order to retrieve representative micelle structures for further analysis with COSMOmic. For the system CTAB/water, different surfactant concentrations were considered, which results in different micelle sizes. Free energy profiles of more than 200 solutes were predicted and validated by means of experimental partition coefficients. To our knowledge, these are the first quantitative predictions of micelle/water partition coefficients, which are based on whole free energy profiles from molecular methods. Further, the partitioning in lipid bilayer systems containing different hydrophobic tail groups (DOPC (1,2-dioleoyl-<i>sn</i>-glycero-3-phosphocholine), SOPC (stearoyl-oleoylphosphatidylcholine), DMPC (1,2-dimyristoyl-<i>sn</i>-glycero-3-phosphocholine), and POPC (1-palmitoyl-2-oleoyl-<i>sn</i>-glycero-3-phosphocholine)) as well as mixed bilayers was calculated. Experimental partition coefficients (log <i>P</i>) were reproduced with a root-mean-square error (RMSE) of 0.62. To determine the influence of cholesterol as an important component of cellular membranes, free energy profiles in the presence of cholesterol were calculated and shown to be in good agreement with experimental data

Monoalkylcarbonate Formation in Methyldiethanolamine–H₂O–CO₂

In this work, the monoalkylcarbonate ((<i>N</i>-hydroxyethyl)­(<i>N</i>-methyl)­(2-aminoethyl) hydrogen carbonate) formation in the system methyldiethanolamine (MDEA)–water (H₂O)–carbon dioxide (CO₂) is investigated by nuclear magnetic resonance (NMR) spectroscopy. Aqueous solutions containing 0.4 g/g of MDEA were loaded with CO₂ in valved NMR tubes, and the composition of the liquid phase in equilibrium was determined <i>in situ</i> at 298 K at pressures up to 11 bar. By two-dimensional NMR, the presence of monoalkylcarbonate was verified, which has been widely overlooked in the literature so far. The experimental data of this work and reevaluated NMR data obtained in previous work of our group were used to calculate chemical equilibrium constants of the proposed monoalkylcarbonate formation. A model taken from the literature that describes the solubility of CO₂ in aqueous solution of MDEA and the corresponding species distribution is extended so that it can account for the monoalkylcarbonate in the liquid phase as well. The extended model is validated using NMR data in the temperature range 273–333 K. The study shows that more than 10 mol % of the absorbed CO₂ is bound as monoalkylcarbonate under conditions relevant for technical applications