Surfactant systems

The study of surfactants has a long history. Detailed understanding of such systems at a microscopic level has important implications for a wide range of scientific and technological research areas such as detergency, lubrication, molecular self-assembly, ion transfert, drug delivery, etc.  During the last few years, we have taken advantage of powerfull computers and sophisticated computer simulation methodologies, and used MD simulations as an alternative technique to investigate surfactant behavior at different liquid-vapor, liquid-liquid and liquid-solid interfaces. In the many system studied,   we found qualitative and quantitative agreement with experiment, which gives added confidence to the 
simulators. The success to date indicate that the use of atomistic computer simulations as a mean of probing the behavior of surfactant systems will inevitably continue.
 
 
 

• Intermolecular Interactions and the Structure of Fatty Acid-Soaps Crystals

Lynch M.L.; Wireko F.; Tarek M. and Klein M.L.     J. Phys. Chem B.  17, 511-521 (2001)

Single crystals of NaHP2 (sodium hydrogen dipalmitate) have been prepared from mixtures of NaP (sodium palmitate) and HP (palmitic acid) in ethanol. The phase compound crystallizes in the P21/a space group with a=9.906 Å, b= 7.163 Å, c=45.580Å, and ß=92.78^o with 4 molecules per unit cell. The arrangement of the head-group is unique among known soap and fatty acid structures by accomodating both hydrogen bonding and electrostatic interactions. Carboxylate and acid-like-pairs couple perpendicular to the bilayer to accomodate a 'short' hydrogen bond and the sodium is shared among laterally adjacent carboxylate anions to create a pseudo six member ring, which adds to the crystal stability. Molecular Dynamics (MD) simualtions establish a set of interaction parameters to describe the structure and energetics of acid-carboxylate bonds, which accounts for the solid state miscibility of HP and NaP. Infrared data, collected by ATR-FTIR on very pure, powdered NaH2P3, Na2H3P5 and NaHP2 acid soap standards, reveal peculiar changes in the carbonyl and hydroxyl spectral regions. These spectra where analyzed based on the crystal structure and MD simulation data. 
 

• Molecular Dynamics Study of the Poly(oxyethylene) Surfactant C₁₂E₂and Water

Bandyopadhyay S.; Tarek M.; Lynch M.L. and Klein M.L.     Langmuir  16, 942-946 (2000)

Constant pressure and temperature (NPT) molecular dynamics simulations have been carried out to investigate the properties of a binary mixture of poly(oxyethylene) surfactant C₁₂E₂ and water in its liquid crystalline lamellar phase (La). The calculated interlamellar spacing and the area per surfactant were found to be in reasonable agreement with X-ray diffraction results. The water molecules were observed to form hydrogen bonded bridged structures linking the oxygen atoms of the same surfactant chain. This interaction leads to a strong preference for the C-C bonds in the head group to attain a gauche conformation.
 
 

• Molecular Dynamics Study of the Hexagonal Mesophase of Sodium Dodecylsulfate

in Aqueous Solution"
Bandyopadhyay S.; Klein M.L.; Martyna G.L. and Tarek M..    Molec. Phys.   95, 377-384 (1998)

Molecular dynamics calculations using a recently proposed simulation methodology (Martyna, G. J., Tuckerman, M. E., Tobias, D. J., and Klein, M. L., 1996, Molec. Phys., 87, 1117) have been carried out to investigate the structural properties of the sodium dodecylsulphate (SDS)-water system in the lyotropic liquid crystalline mesophase E. The simulation system consisted of two cylindrical aggregates, each containing 128 dodecylsulphate anions, plus 256
sodium counterions and 4350 water molecules. The system had periodic boundary conditions and initially overall hexagonal symmetry. A 260 ps trajectory was then generated at constant temperature (T = 333 K) and constant pressure (P = 0) using a new molecular dynamics package (PINY-MD), which utilizes a timestep almost an order of magnitude larger than the usual value. The structural and dynamic results are compared with previous simulations reported for quasi-spherical SDS micelles and experimental data on the same system. In agreement with experiment, the simulation predicts a small but well-defined distortion from hexagonal symmetry. 
 
 

• Computer Simualtion Studies of Amphiphilic interfaces

Bandyopadhyay S.; Tarek M. and Klein M.L.   Curr. Opin. Coll. Interf. Sci.  3, 242-246  (1998) 

Computer simulation has emerged as a powerful probe for analysing the behavior of amphiphilic systems. The past year has seen several novel applications, which have given important insights into the nature of the water/amphiphile interface as well as the behavior of amphiphiles at different liquid/vapor, liquid/liquid and liquid/solid interfaces. This review focuses on surfactants and lipids where simulations have revealed for the first time an atomistic level description of not only the hydration of polar head groups but also the comportment of the hydrophobic tails. 
 
 

• Surfactant Aggregation at a hydrophobic Surface

Bandyopadhyay S.; Shelley J.C. Tarek M.; Moore P.B and Klein M.L.  J. Phys. Chem. B 
102, 6318-6322  (1998) 

Molecular dynamics (MD) simulations have been carried out to investigate the morphology of monolayers of the cationic surfactant cetyltrimethylammonium bromide (C16TAB) at the interface between a hydrophobic substrate and an aqueous solution. The simulation system consisted of two flat hydrophobic surfaces separated by a distance of 70 Angstrom along z (direction normal to the interface). Initially, one surface was covered with a uniform monolayer of 94 erect surfactant molecules, with surface area of 45 A² / molecule. The other surface was covered with two hemicylindrical aggregates, each containing 64 surfactant molecules. A 1 ns trajectory was then generated at constant volume and temperature (T = 300 K). The hemicylinder aggregates relaxed to a semielliptical rather than semicircular cross section but were observed to be otherwise stable during the simulation run. The monolayer arrangement was, however, unstable and evolved into hemicylindrical type aggregates. The present simulation provides support for the interpretation of recent atomic force microscopy (AFM) measurements, which claim C16TAB forms stable hemicylindrical aggregates at the interface between an aqueous solution and hydrophobic surfaces, such as graphite. 

tarek@jazz.ncnr.nist.gov

Last modified: Sept. 5, 1999