Micelle formation by a Short Chain Cationic Surfactant

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Micelle formation by a Short Chain Cationic Surfactant

The formation of micelles by surfactants was fol-lowed by molecular dynamics calculations per-formed with MedeA® [1] LAMMPS and using thePCFF+ forcefield. An initial model with a ran-dom distribution of C9TAC surfactant moleculeswas built using the MedeA building capabilities,such as the MedeA Molecular Builder and theMedeA Amorphous Materials Builder. Results arein agreement with previous simulation studies [2]and available experimental data [3].

Keywords: surfactants, self-assembly, micelle for-mation, colloids, n-nonyltrimethylammonium chlo-ride, PCFF+

1 Surfactants

Surfactants are used as performance additives fora wide variety of industrial as well as personal andhome care applications: household detergents,cosmetics, industrial cleaning, enhanced oil re-covery, hydraulic fracturing, crop protection, paintsand coatings, textile softeners, etc. Surfactants re-duce the surface tension of a liquid, the interfacialtension between two liquids, or that between a liq-uid and a solid.

They are usually organic amphiphilic compoundsconsisting of a lipophilic (hydrophobic) tail and ahydrophilic head :

• Tail : they can have one or two tails usuallyconsisting of hydrocarbon chains which arefairly similar in most surfactants

• Head : depending on the nature of the hy-drophilic part they can be classified as (seeFigure 1):

– Anionic

– Cationic

[1] MedeA and Materials Design are registered trademarksof Materials Design, Inc.

[2] J. B. Maillet et al., “Large scale molecular dynamics sim-ulation of self-assembly processes in short and long chaincation surfactants”, PCCP 1, 5227 (1999) (DOI)

[3] T. Imae et al., “Sphere-rod transition of micelles oftetradecyltrimethylammonium halides in aqueous sodiumhalide solutions and flexibility and entanglement of longrodlike micelles”, J. Phys. Chem. 90, 5216 (1986) (DOI)

Figure 1: Classification of surfactants according tothe nature of the head (a) anionic (b) cationic (c)amphoteric (d) non-ionic.

– Amphoteric

– Non-ionic

Due to the amphiphilic nature of these systems,aggregates such as micelles (Figure 2) are formedin bulk aqueous phase at intermediate surfactantconcentrations. The lipophilic tails form the core ofthis self-assembled structure and the hydrophilicheads form an outer layer in contact with water.

Figure 2: Surfactant micelle in water. Micelles areparticles of colloidal dimensions that exists in equi-librium with the molecules or ions in solution fromwhich it is formed.

The mechanism of micelle formation can be fol-

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https://doi.org/10.1039/A905216J

https://doi.org/10.1021/j100412a065

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lowed at the molecular level using molecular dy-namics simulations starting from a random distri-bution of surfactant molecules in aqueous solution.

2 Molecular Modeling

Figure 3 shows a molecule of C9TAC (n-nonyltrimethylammonium chloride), a cationic sur-factant with a relatively short chain (9 carbonatoms).

Figure 3: C9TAC surfactant molecule (only thecationic part of the molecule is displayed).

A molecule of C9TAC can easily be sketchedusing the MedeA Molecular Builder. Using theMedeA Amorphous Materials Builder, we gener-ate an amorphous model containing 48 moleculesof C9TAC and 3,000 molecules of H2O at a tem-perature of 300 K and at an initial density of 0.8g/ml.

This constructed cell is first subjected to an en-ergy minimization. After minimization, moleculardynamics at a constant temperature of 300 K andpressure of 1 atm (NPT-MD) takes place for a pe-riod of at least 3.5 ns. The time step is 0.5 fs forthe initial 100 ps and 1 fs for the rest of the dy-namics. The LAMMPS simulation program (Large-scale Atomistic-Molecular Massively Parallel Sim-ulator ) was used for this computation. The force-field used is PCFF+ forcefield [4].

[4] PCFF was developed by the Biosym Potential EnergyFunctions and Polymer Consortia between 1989 and2004. Since 2009, PCFF+ has been developed at Mate-rials Design in order to improve the density and cohesiveproperties of a large number of organic liquids and poly-mers under ambient conditions.

3 Results

Figure 4a shows the initial random distribution ofthe surfactant cations at a concentration above theexpected critical micelle concentration. After only100 ps of NVT molecular dynamics run (see Fig-ure 4b), different areas of high and low density canbe seen in the model. The snapshot taken at 500ps NVT-MD (see Figure 4c) shows the formationof three aggregates that already suggest a spher-ical shape. The chains, which are not part of theaggregates, are clustered in smaller groups of 2-4 molecules. Figure 1d shows the arrangementof the molecules at the end of the simulation, af-ter 3.6 ns. Three spherical micelles can be distin-guished. They contain 16, 15 and 10 moleculesrespectively. The rest of the surfactant moleculesform a cluster with a rather indistinct shape.

Figure 4: Distribution of C9TAC cations (the wa-ter molecules and the chloride counterions arenot displayed for clarity) (a) Initial random distri-bution generated by the MedeA Amorphous Mate-rials Builder (b) after 100 ps NPT-MD (c) after500ps NPT-MD (d) after 3600 ps NPT-MD.

These micelles are better visualized in Figure 5.

The radial distribution function between the nitro-gen atoms of the surfactant molecules averagedover the last 400 ps of simulation is shown inFigure 6. Note the main peak at around 9 Ain agreement with previous calculations . Watermolecules and chloride ions are organized to a



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Figure 5: Distribution of C9TAC cations (the wa-ter molecules and the chloride counterions are notdisplayed for clarity) after 3600 ns NPT-MD.

high extent around the nitrogen atoms of the sur-factant molecules. This can be observed in Fig-ure 6 showing the radial distribution function cal-culated between the nitrogen atoms of the surfac-tant molecules and the oxygen atoms of the wa-ter molecules (red line) and between then nitro-gen atoms and the chloride ions (blue line). Wa-ter molecules seem to be somewhat closer to thecationic head of the surfactant than chloride ions.

4 Conclusions

Atomistic, explicit-solvent simulations can providea detailed understanding of the static and dynamicbehavior of surfactant micelles. As PCFF+ is validin a large range of conditions for numerous chem-ical families, MedeA is an excellent tool for explor-ing the influence of operating conditions (temper-

Figure 6: Radial distribution function for the ni-trogen atoms of the C9TAC surfactant molecules(black line), between the nitrogen of the C9TACmolecules and the oxygen atoms of the watermolecules (red line) and between the nitrogen ofthe C9TAC surfactant molecules and the chlorideions (blue line), averaged over the last 400 ps ofNPT-MD run.

ature, salinity, pressure, co-surfactants, etc.) foroptimal formulation of additives in a given con-text. The MedeA flowchart interface allows theconstruction of automated complex workflows thatcan be used with a collection of different initial sys-tems facilitating these types of studies. For ex-ample, the effect of salinity on micellization canbe assessed using a single flowchart that encom-passes the minimization, long molecular dynam-ics calculation and calculation of radial distributionfunctions on a set of initial models with differentsalt concentration.

The results shown here were obtained in less than3 days using a standard workstation with 12 cores.C9TAC is a short chain surfactant, but the mod-erate computational requirements needed for thiscalculation indicate that this type of study can beperformed for larger surfactants with very reason-able effort.

5 Perspectives

Molecular simulations contribute to the under-standing of surfactant dynamics in simple systemsas well as in mixtures with co-surfactants. Micelleformation mechanisms may be described quanti-



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tatively by following the time evolution of surfactantself-assembly.

The atomic-level understanding of surfactant dy-namics and kinetics in complex environments is akey ingredient in designing high-quality additives

for specialized industrial applications encounteredin the oil and gas industry: from oil well drilling, hy-draulic fracturing, reservoir injection, oil well pro-duction and surface plant processes to pipelineand seagoing transportation of petroleum emul-sions.



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Micelle formation by a Short Chain Cationic Surfactant