softmatter:Surfactant

From NSDL Materials Digital Library Soft Matter Wiki

Surfactants are members of the general class of amphiphiles, where molecules consist of both hydrophilic and hydrophobic or lyophilic groups chemically bonded together. Often, the hydrophobic group is a long, slender hydrocarbon polymer that is referred to as the "tail". Conversely, the hydrophilic group is a short, bulky polymer, referred to as the "head". These hydrophobic/hydrophilic interactions drive solutions of surfactants to phase separate when in water or oil. For example, when placed in water, the hydrophilic groups will wish to maximize contact with the water molecules, rather than aggregate, where as the water-hating groups will try to minimize their contact with the water and will aggregate [1,2].

Common Surfactants

Lauryldimethylamine-oxide (LDAO)

• Lauryldimethylamine-oxide (LDAO):
  • Molecule Information
  • CH₃(CH₂)₁₁N(O)(CH₃)₂

• Cetyltrimethylammoniumbromide (CTAB):
  • Molecule Information
  • CH₃(CH₂)₁₅N(Br)(CH₃)₃

• Sodium dodecyl sulfate (SDS):
  • Molecule Information
  • CH₃(CH₂)₁₁OSO₃Na

Phase Separation

Surfactants are known to exhibit a wide range of structures including [2]:

• Spherical Micelles
• Cylindrical micelles
• Flexible bilayers
• Vesicles
• Planar Bilayers
• Inverted Micelles

The evolution of these structures can be rationalized and predicted by the use of the Israelachvili packing factor [2].

As one goes to higher concentration, long-range order can develop. These phases strongly resemble those of Block copolymers and can adopt liquid crystal like structures. These phases include [1]:

• Cubically Ordered Spherical Micelles
• Hexagonally Packed Cylindrical Micelles
• Double Gyroid
• Tetragonal mesh
• Lamella
• Crystalline solids
• Nematic state of oriented micelles

Simulation of Surfactants

General Model

Brownian Dynamics simulation of a surfactant showing hexagonally ordered cylindrical micelles. Full MATDL record

Typically, surfactants are modeled as bead-spring polymers, where individual beads possess different interaction types/strengths [3,4]. In these models, polymers of length 'N' are held together with springs (typically Harmonic Springs or FENE springs); more sophisticated connections may be used to help restrict bond angles and distances. Each polymer molecule is then broken up into head segment and a tail segment (for example, an 8 bead polymer may have a 2 bead head and 6 bead tail, H₂T₆). Hydrophobic (or more generally solvent-phobic) portions of the molecule will have a tendency to aggregate to minimize contact with the solvent; as such hydrophobic species interact with an attractive potential such as the Lennard-Jones Potential. Hydrophilic (or more generally solvent-philic) portions of the molecule will not aggregate, and thus a potential without substantial attraction is used, such as the Weeks-Chandler-Andersen Potential. It has been shown that even such simple models are very powerful, capable of realizing many of the complex phases demonstrated in experiment. This model has also been implemented successfully for the study of block copolymers in selective solvent.

Literature

Coarse grained simulations have been very successful at predicting and validating phenomena and phases of surfactants. A small sampling of the literature, categorized by simulation method, yields:

• Monte Carlo
  • Larson RG, Monte Carlo simulations of the phase behavior of surfactant solutions, JOURNAL DE PHYSIQUE II 6 : 1441 1996
  • Larson RG, Molecular Simulation of Ordered Amphiphilic Phases, CHEMICAL ENGINEERING SCIENCE 49 : 2833 1994
  • Lisal M, Hall CK, Gubbins KE, Panagiotopoulos AZ, Formation of Spherical Micelles in a supercritical Solvent: Lattice Monte Carlo simulation and multicomponent solution model, MOLECULAR SIMULATION 29 : 139 2003
  • Siperstein FR, Gubbins KE, Phase separation and liquid crystal self-assembly in surfactant-inorganic-solvent systems, LANGMUIR 19 : 2049 2003
  • Bedrov D, Smith GD, Freed KF, Dudowicz J, A comparison of self-assembly in lattice and off-lattice model amphiphile solutions, JOURNAL OF CHEMICAL PHYSICS 116 (12): 4765-4768 MAR 22 2002

• Brownian Dynamics
  • Bourov GK, Bhattacharya A, The role of geometric constraints in amphiphilic self-assembly: A Brownian dynamics study, JOURNAL OF CHEMICAL PHYSICS 119 : 9219 2003
  • von Gottberg FK, Smith KA, Hatton TA, Stochastic dynamics simulation of surfactant self-assembly, JOURNAL OF CHEMICAL PHYSICS 106 : 9850 1997
  • von Gottberg FK, Smith KA, Hatton TA, Dynamics of self-assembled surfactant systems, JOURNAL OF CHEMICAL PHYSICS 108 : 2232 1998
  • Iacovella CR, Horsch MA, Zhang Z, Glotzer SC, Phase diagrams of self-assembled mono-tethered nanospheres from molecular simulation and comparison to surfactants, LANGMUIR 21 (21): 9488-9494 OCT 11 2005

• Molecular Dynamics
  • Fodi B, Hentschke R, Simulated phase behavior of model surfactant solutions, LANGMUIR 16 : 1626 2000
  • Salaniwal S, Cui ST, Cochran HD, Cummings PT, Molecular simulation of a dichain surfactant water carbon dioxide system. 1. Structural properties of aggregates, LANGMUIR 17 : 1773 2001
  • Soddemann T, Dunweg B, Kremer K, A generic computer model for amphiphilic systems, EUROPEAN PHYSICAL JOURNAL E 6 : 409 2001

• Dissipative Particle Dynamics
  • Prinsen P, Warren PB, Michels MAJ, Mesoscale simulations of surfactant dissolution and mesophase formation, PHYSICAL REVIEW LETTERS 89 (14): Art. No. 148302 SEP 30 2002

Examples

• Brownian dynamics simulation of a tail aggregating surfactant: view record on MATDL Repository
• Glotzilla surfactant template

Surfactants Under Shear

• Summary of G.Arya, A.Z. Panagiotopoulos, "Molecular modeling of shear-induced alignment of cylindrical micelles" Computer Physics Communications, 169 (2005), 262–266

References

• [1] The Structure and Rheology of Complex Fluids
• [2] Intermolecular and Surface Forces
• [3] Soddemann T, Dunweg B, Kremer K, A generic computer model for amphiphilic systems, EUROPEAN PHYSICAL JOURNAL E 6 : 409 2001
• [4] Iacovella CR, Horsch MA, Zhang Z, Glotzer SC, Phase diagrams of self-assembled mono-tethered nanospheres from molecular simulation and comparison to surfactants, LANGMUIR 21 (21): 9488-9494 OCT 11 2005

Additional Sources

• Surfactant solutions are discussed at length in chapter 12 of the first edition of The Structure and Rheology of Complex Fluids
• Surfactant and amphiphile aggregration are discussed in chapters 16 and 17 in the second edition of Intermolecular and Surface Forces