Langmuir 1999, 15, 1981-1992
1981
Atomic-Level Molecular Modeling of AOT Reverse Micelles. 1. The AOT Molecule in Water and Carbon Tetrachloride Bela Derecskei,† Agnes Derecskei-Kovacs,‡ and Z. A. Schelly*,† Center for Colloidal and Interfacial Dynamics, Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019-0065, and the Laboratory of Molecular Simulation, Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255 Received April 10, 1998. In Final Form: August 21, 1998 The anionic surfactant bis(2-ethylhexyl) sodium sulfosuccinate (Aerosol OT or AOT) is studied by atomiclevel molecular modeling, using the second-generation ESFF (extensible systematic force field). The geometries of seven representative conformers are analyzed. The energies of these conformers correspond to those of the most probable ones based on random-sampling statistics and differ by, at most, 10 kcal/mol. Thus, these conformers should be available for the system under typical ambient conditions. Interactions with water and carbon tetrachloride modify the geometries only to a modest extent. The solvation by water is found to be exoergic, and the analysis of individual AOT-water interactions identified four strongly bound water molecules (with >10 kcal/mol of interaction energy), in accordance with experimental results. A CCl4 box was generated for the investigation of the effects of carbon tetrachloride as a solvent. A truncatedcone-geometry model of the AOT molecule yields 14.5 as the estimated aggregation number N of AOT reverse micelles in CCl4, in good agreement with the experimental value of the mean aggregation, 〈n〉 ) 15-17, of the solution. The predicted diameter of the dry reverse micelles d ) 2.8 nm is comparable with the experimental apparent hydrodynamic diameter, 〈Dh〉 ) 3.2 nm (at wo ) 0.8).
Introduction Organized assemblies have generated considerable interest not only within the boundaries of traditional colloid chemistry but in analytical, synthetic, and medicinal chemistry as well. Compartmentalized and guesthost systems such as aqueous and reverse micelles, microemulsions, cyclodextrins, vesicles, and liposomes have improved the performance of numerous analytical and separation methods and have opened entirely novel strategies for synthetic methods and drug-delivery systems. Major questions related to such applications concern the structure, size (mainly of the interior compartment), and surfactant-aggregation number of the particular vehicle. At the present stage of computational methods, molecular modeling may complement and augment the experimental efforts aimed at selecting the suitable amphiphile and estimating the relevant parameters of its aggregates. To examine this capacity, we chose the commonly used anionic surfactant, bis(2-ethylhexyl) sodium sulfosuccinate (Aerosol OT or AOT) as a test system. The advantages of AOT are that it forms normal as well as reverse micelles and that its typical aggregation number is conveniently low from a computational point of view (10-60, depending on the solvent and possible other components present). The choice of carbon tetrachloride as the solvent was prompted by its symmetry, by our previous experience of its favorable optical properties in thermal lens1 and laser E-jump studies of reverse micelles,2 and by the small number of atoms in the molecule. The relatively moderate size of the aggregates and solvent molecules allows atomic-level modeling instead of the use of lower level approximations for the description of the surfactant molecule.3-6 The results of a series of such theoretical calculations are presented, and estimates for † ‡
University of Texas at Arlington. Texas A&M University.
(1) Chen, H. M.; Schelly, Z. A. Chem. Phys. Lett. 1994, 224, 61. (2) Chen, H. M.; Schelly, Z. A. Langmuir 1995, 11, 758.
the aggregation number and diameter of AOT/CCl4 reverse micelles are compared with experimental findings. Commercially available program packages permit the generation and analysis of molecular information such as geometry and energetics, as well as electronic, spectroscopic, and bulk properties. They also provide 3-dimensional representation and visualization of the chemical system studied. A wide variety of methods have been developed and implemented. Ab initio methods have been successfully utilized in the study of small- and middlesize systems (most often 20-30 atoms), yielding results at the level of experimental accuracy. In the study of larger systems (100-200 atoms), approximate methods (e.g., semiempirical techniques) have been proven to be useful. However, chemical systems in the range of several hundreds or thousands of atomsswhich is the case when dealing with drugs, biomolecules, polymers, aggregates, etc.srequire treatments involving further approximations in the computation. One such treatment is molecular mechanics, which relies on the laws of classical physics and on parameters derived from experimental data or results of more accurate theoretical methods. These methods utilize quantum mechanics only implicitly (for the calculation of geometrical parameters, partial charges, etc.) in the development of methodology and not at all in the direct applications. The calculations are computationally very efficient. Even systems containing several thousand atoms can be examined theoretically, but the results for new systems must be validated since the calculations rely on approximations. Presently, we apply molecular modeling to the description of AOT reverse micelles in carbon tetrachloride. The (3) Smit, B.; Hilbers, P. A. J.; Esselink, K. Int. J. Mod. Phys. C. 1993, 4, 393. (4) Esselink, K.; Hilbers, P. A. J.; van Os, N. M.; Smit, B.; Karaborni, S. Colloids Surf., A 1994, 91, 155. (5) Karaborni, S.; Smit, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 411. (6) Palmer, B. J.; Liu, J. Langmuir 1996, 12, 746.
10.1021/la980419r CCC: $18.00 © 1999 American Chemical Society Published on Web 02/25/1999
1982 Langmuir, Vol. 15, No. 6, 1999
Derecskei et al.
Table 1. Energy and Geometrical Parameters for the Seven Representatives of the Most Probable Conformers of the AOT Molecule Depicted in Figure 2 (Energy in kcal/mol, Distances in nm, and Volume in nm3) parameter
conf. 1
conf. 2
conf. 3
conf. 4
conf. 5
conf. 6
conf. 7
energy max xa max y max z volume l1b l2 l3 l4 l5 l6
-122.11 0.69 1.38 1.17 1.11 0.248 0.412 1.230 0.637 1.077 0.925
-123.96 0.61 1.31 1.02 0.815 0.248 0.392 1.059 0.630 1.088 0.993
-124.60 0.56 1.32 0.98 0.724 0.248 0.419 1.020 0.671 1.023 0.939
-122.95 0.67 1.35 0.88 0.796 0.248 0.464 0.970 0.588 1.038 0.901
-125.68 0.60 1.17 1.83 1.28 0.240 0.379 1.376 1.904 1.074 0.869
-120.72 0.82 0.92 1.70 1.28 0.249 0.433 1.364 1.574 0.978 0.893
-131.04 0.68 0.98 1.26 0.839 0.250 0.428 0.652 1.257 1.084 0.844
a The max x, -y, and -z values are the maximum extents |q max - qmin| of the AOT molecule along the corresponding coordinate axes. Their product for a particular conformer is the volume () max x × max y × max z) of the enclosing right-rectangular prism. b The linear distances li between selected atoms are defined in Figure 1.
dynamic process of micelle formation occurs on a time scale that is nowadays still prohibitive for theoretical approaches, unless one uses less than atomic-level description of surfactant molecules.3-6 Therefore, we restrict ourselves to molecular mechanics and conformation search. Although this approach does not yield certain thermodynamic data about the system (e.g., free energy or enthalpy), it does provide useful qualitative and quantitative information. At every possible step of the modeling process, results are compared to experimental data available. The present paper deals with one AOT molecule and its interactions separately with water and with carbon tetrachloride. Future studies will be directed toward the description of the interaction of AOT molecules with each other in a vacuum, in water, in carbon tetrachloride, and in reverse micelles with variable water content. Experimental Section The surfactant bis(2-ethylhexyl) sodium sulfosuccinate (AOT) was obtained in purum grade (>98%) from Fluka and further purified by precipitation, titration, and drying as described elsewhere.7-9 Reverse micellar solutions were prepared by dissolving the appropriate amount of AOT in CCl4 and solubilizing the desired amount of water. The CCl4 was HPLC grade (water content
