Nanotechnology & MEMS ★★☆ Moderate

🧲 Molecular Self-Assembly — Micelles & Bilayers

Watch amphiphilic molecules spontaneously organise into micelles and bilayers. Control concentration to cross the critical micelle concentration (CMC) and see vesicle formation driven by the hydrophobic effect.

0.50×
p = 0.25
kT = 1.0
Monomers: 0 Aggregates: 0 Avg Nagg: — Phase: — ΔG: — kT

Monomers — below CMC

At low concentration molecules remain as individual amphiphiles. The hydrophobic tails are exposed to water, raising free energy. No stable aggregates form.

The Physics

Self-assembly is driven by minimising free energy: ΔG_transfer ≈ −kT·ln(CMC⁻¹). Below CMC, monomers dominate. Above CMC, aggregation number N_agg determines shape via packing parameter p = v/(a₀·l_c): p < 1/3 → spherical micelle; 1/3 < p < 1/2 → cylindrical; p ≈ 1 → bilayer/vesicle.

About this simulation

Amphiphilic molecules — a hydrophilic head joined to a hydrophobic tail — stay as free monomers in dilute solution, but past the critical micelle concentration (CMC) they spontaneously assemble to shield their tails from water. Which shape they form is set almost entirely by the packing parameter p = v/(a₀·l_c): small p gives curvy spherical micelles, intermediate p gives cylinders, and p ≈ 1 gives flat bilayers or closed vesicles — the same geometry that builds every cell membrane.

🔬 What it shows

Each molecule is drawn as a cyan hydrophilic head with an orange hydrophobic tail, drifting under Brownian motion until concentration crosses the CMC. Above it, molecules snap into spherical micelles (heads outward), cylindrical micelles, or a two-leaflet bilayer/vesicle — with aggregation number N and phase shown live in the stats bar.

🎮 How to use

Drag Concentration (×CMC) past 1.0 to trigger aggregation, and set the Packing parameter p to switch shape: below 1/3 for spherical micelles, 0.33-0.5 for cylindrical, near 1 for bilayers. The Temperature (kT) slider adds thermal noise, or jump straight to a scenario with the Dilute, At CMC, Micelles or Bilayer preset buttons.

💡 Did you know?

The stats bar's ΔG value is computed as −kT·ln(concentration) — a direct simulation of the free-energy argument that explains why surfactants aggregate at all: below the CMC, forming a micelle costs more energy than it saves, so molecules stay as monomers.

Frequently asked questions

What is the critical micelle concentration (CMC)?

It is the concentration threshold above which amphiphilic molecules stop existing mostly as free monomers and start spontaneously forming aggregates called micelles. In this simulation, the Concentration slider is expressed directly in multiples of the CMC, so a value of 1.0× is exactly the threshold.

What is the packing parameter and why does it decide the shape?

The packing parameter p = v/(a₀·l_c) compares a molecule's tail volume v and length l_c to its head-group area a₀. Geometrically, cone-shaped molecules (small p) pack most efficiently into curved spheres, wedge shapes (p around 0.4) favour cylinders, and cylinder-shaped molecules (p near 1) pack flat into bilayers, exactly the thresholds the simulation's aggType() function uses.

What happens if I drop the concentration below the CMC while aggregates exist?

The simulation immediately dissolves every existing aggregate, scattering its member molecules back out as free monomers with fresh random velocities, mirroring how real micelles are dynamic structures that disassemble once concentration falls below the CMC rather than persisting indefinitely.

Why does the Temperature (kT) slider matter?

Higher kT increases the Brownian noise applied to free molecules each step and speeds up aggregate diffusion, representing greater thermal energy fighting against the ordering tendency of self-assembly; very high kT can make it harder for stable aggregates to persist, similar to heating a real surfactant solution.

Is this the same self-assembly that forms real cell membranes?

Yes — the bilayer/vesicle regime (p ≈ 1, reached with two-tailed lipids) is the same packing geometry that phospholipids use to form the lipid bilayer of every biological cell membrane, closing into a vesicle that encloses an aqueous interior exactly as the simulation's bilayer preset shows.