 Solution

An amphiphile has a solvent-affine part and a solvent-averse part within one molecule. In water these are commonly called hydrophilic head and hydrophobic tail. Two nonbiological examples are sodium dodecyl sulfate, with a twelve-carbon tail and an anionic sulfate head, and cetyltrimethylammonium bromide, with a sixteen-carbon tail and a cationic quaternary-ammonium head. Their counterions balance the ionic head-group charge; the head and tail do not have interchangeable interactions with water.

Aggregation reduces unfavorable exposure of hydrocarbon tails to water but loses molecular translational entropy. Interfacial free energy, head-group solvation and electrostatic repulsion, chain packing and conformational entropy determine the preferred aggregate. The hydrophobic effect is a solvent-mediated free-energy balance, rather than a literal attractive bond between tails.

The amphiphile packing parametercompares tail volume $v$, effective head-group area $a_0$ and maximum tail length ${\ell }_c$. A spherical core of radius $R$ obeys $v/a_0=R/3$, and a long cylindrical core obeys $v/a_0=R/2$. The condition $R\le {\ell }_c$ gives the conventional spherical window $P\lesssim 1/3$ and cylindrical window $1/3\lesssim P\lesssim 1/2$. Less strongly curved bilayers become compatible for $1/2\lesssim P\lesssim 1$, with planar packing near one; values above one can favor inverse curvature in an appropriate nonpolar surrounding medium. These windows are geometric guides rather than exact phase boundaries.

A larger hydrated or electrostatically repelling head increases $a_0$ and favors positive curvature. Screening of charged heads or a smaller head can lower $a_0$ and favor rods or sheets. Multiple tails increase the tail volume relative to head area and can favor bilayers. Tail length, branching, temperature and solvent selectivity alter both geometry and interaction free energy, so one cannot predict a unique aggregate from hydrophobicity alone.^[ref]

In the supplied chemical-potential expression, $\mu$ is the equilibrium chemical potential per amphiphile, ${\mu }_{\alpha }^0$ the standard free energy per amphiphile within an aggregate of size $\alpha$, and $x_{\alpha }$ the dimensionless molecular fraction belonging to that size class. The aggregate molecular fraction versus aggregate number fraction distinction explains the logarithm: the aggregate count fraction is $x_{\alpha }/\alpha$, and dividing its translational chemical potential by $\alpha$ gives a per-molecule value. ThusHere $k_B$ is the Boltzmann constant, $T$ absolute temperature, and $\alpha$ a positive integer. The logarithm acts on a dimensionless ideal activity; a concentrated nonideal solution would require activity coefficients.

equilibrium exchange of amphiphiles gives the same per-molecule $\mu$ for every size. For monomers, $\mu ={\mu }_1^0+k_{B}T\ln x_1$. Substitution yields the finite-micelle aggregation-number distributionThis formula by itself does not prescribe a preferred micelle size: the size dependence of ${\mu }_{\alpha }^0$ must include the competing packing and interaction terms. If their balance favors a finite size ${\alpha }_{\ast }$, a monomer population and a peak around that size can coexist. Dividing by $\alpha$ would instead plot the distribution of aggregate counts.

At very low $x_{\mathrm{tot}}$, the factors $x_1^{\alpha }$ suppress large aggregates and most molecules remain monomers. As $x_1$ rises, a favorable aggregate class grows extremely rapidly because of this high power. The critical micelle concentration from standard chemical potentials has the large-size estimateMore precisely, the illustrative crossover condition $x_{{\alpha }_{\ast }}=x_1$ givesAdded material above the crossover enters aggregates predominantly. Monomer concentration then increases slowly rather than being exactly pinned: its logarithm enters $\mu$, while aggregate populations respond with a large factor ${\alpha }_{\ast }$. For finite micelles this is a sharp association crossover, not an absolute prohibition on aggregates below one exact concentration.

The drawing uses a declared illustrative standard-free-energy model favoring aggregates near $\alpha =40$. It is not an inferred distribution or critical concentration for either named surfactant. The separate monomer peak is at $\alpha =1$; a distribution of molecular fractions weights each aggregate by its number of molecules.

For the general physics of a nearly planar fluid sheet, let $h(x,y)$ be a small-slope displacement, $\kappa$ membrane bending rigidity and $\sigma$ mechanical tension. At fixed topology and symmetric reference curvature, the quadratic Helfrich energy isThe bending term is the small-slope curvature-elastic energy; additional spontaneous-curvature or boundary terms require extra information.^[ref]

For Fourier amplitudes $h_{\mathbf{q}}=A^{-1}{\int }_{A}h(\mathbf{x})e^{-i\mathbf{q}\cdot \mathbf{x}}{d}^{2}x$, equipartition gives the membrane fluctuation spectrum under tension,Higher temperature increases fluctuations, while greater membrane bending rigidity or tension suppresses them. Short-wavelength modes are predominantly bending-controlled; longer wavelengths can be tension-controlled. Finite sheet size, supports, confinement and imposed area set boundary conditions and infrared cutoffs; continuum elasticity also needs a short-distance cutoff. solvent viscosity affects relaxation rates, although it does not enter this equilibrium amplitude at fixed elastic parameters.

TODO: user-deferred biology. The requested importance of these fluctuations for bodily function is deferred. The preceding result covers only the independent continuum and thermal physics.