Complementary Use of Simulations and Molecular-Thermodynamic Theory to Model Micellization

Abstract

Molecular-thermodynamic descriptions of micellization in aqueous media can be utilized to model the self-assembly of surfactants possessing relatively simple chemical structures, where it is possible to identify a priori what equilibrium position they will adopt in the resulting micellar aggregate. For such chemical structures, the portion of the surfactant molecule that is expected to be exposed to water upon aggregate self-assembly can be identified and used as an input to the molecular-thermodynamic description. Unfortunately, for many surfactants possessing more complex chemical structures, it is not clear a priori how they will orient themselves within a micellar aggregate. In this paper, we present a computational approach to identify what portions of a surfactant molecule are hydrated in a micellar environment through the use of molecular dynamics simulations of such molecules at an oil/water interface (modeling the micelle core/water interface). The local environment of each surfactant segment is determined by counting the number of contacts of each segment with the water and oil molecules. After identifying the hydrated and the unhydrated segments of the surfactant molecule, molecular-thermodynamic modeling can be performed to predict: (i) the free-energy change associated with forming a micellar aggregate, (ii) the critical micelle concentration (CMC), and (iii) the optimal shape and size of the micellar aggregate. The computer simulation results were found to be sensitive to the atomic charge parameters utilized during the simulation runs. Two different methods of assigning atomic charges were tested, and the computer simulation and molecular-thermodynamic modeling results obtained using both sets of atomic charges are presented and compared. The combined computer simulation/molecular-thermodynamic modeling approach presented here is validated first by implementing it in the case of anionic (sodium dodecyl sulfate, SDS), cationic (cetyltrimethylammonium bromide, CTAB), zwitterionic (dodecylphosphocholine, DPC), and nonionic (dodecyl poly-(ethylene oxide), C 12 E 8 ) surfactants possessing relatively simple chemical structures and verifying that good predictions of CMCs and micelle aggregation numbers are obtained. In the case of C 12 E 8 , the challenges and limitations associated with simulating a single, polymeric E 8 moiety at the oil/water interface to model its behavior at the micelle/water interface are discussed. Subsequently, the combined modeling approach is implemented in the case of the anionic surfactant 3-hydroxy sulfonate (AOS) and of the nonionic surfactant decanoyl-n-methylglucamide (MEGA-10), which possess significantly more complex chemical structures. The good predictions obtained for these two surfactants indicate that the combined computer simulation/molecular-thermodynamic modeling approach presented here extends the range of applicability of molecular-thermodynamic theory to allow modeling of the micellization behavior of surfactants possessing more complex chemical structures.

Aggregation and adsorption of reactive dyes in the presence of an anionic surfactant on mesoporous aminopropyl silica

“…The lower affinity suggested for the red dye/DBS aggregates may also be due to negative charge-charge repulsions of the sulfonate groups of the red dye chemical structure. On the other hand, the differences of the cac values may also be due to a combination of interactions in the hydrophobic region of the mixed micelles and the polar micelle-water interface [27].…”

Ionic Surfactant Aggregates in Saline Solutions: Sodium Dodecyl Sulfate (SDS) in the Presence of Excess Sodium Chloride (NaCl) or Calcium Chloride (CaCl2)

“…37,38 Lazaridis et al 39 simulated 960 DPC molecules in an implicit environment providing to date the temporally most extensive micellization data but without explicit solvent. Very recently, models that combine atomistic simulations and a thermodynamic approach for micelle formation 41 have emerged.…”

Molecular dynamics simulation of a polysorbate 80 micelle in water

“…Just above the limiting monomer solubility (the critical micelle concentration, CMC), surfactant molecules begin to aggregate into micellar structures that are characterized by an inner core comprising the non-polar moieties and an outer interface of polar moieties in contact with the aqueous environment. The driving force for micelle formation is in general terms understood [1][2][3][4][5] , the key components being the hydrophobic effect, which excludes the non-polar moieties from the water to the interior of the micelle structure, and an interfacial free energy penalty for the water-micelle interface. These define an energy barrier akin to nucleation that must be surmounted before a micellar cluster can form a stable micelle.…”

“…It is not obvious, unlike linear surfactants, as to what conformation this molecule adopts either as a monomer or within a micelle structure in an aqueous environment, how the molecules pack into a micelle, nor the nature of the surfactant-water interface. Knowledge of the micelle structure of this class of surfactant structures is of fundamental interest being required, for instance, for successful applications of molecular-thermodynamic theories, [21][22] and is important for technological applications to enable development and control of formulations on a rational basis.…”