 Solution

Diffuse-interface free energy. A sharp boundary assigns one mathematical surface to two uniform fluids. Molecular mixing instead changes composition over a finite distance. In square-gradient theory of a liquid interface, let $\varphi (\mathbf{r})$ be a dimensionless local composition and writeThe bulk free-energy density $f_b$ favors the equilibrium phase compositions, while the gradient term penalizes changing composition too abruptly. Since $f_b$ has units $\mathrm{J}{\mathrm{m}}^{-3}$ and $\varphi$ is dimensionless, $K$ has units $\mathrm{J}{\mathrm{m}}^{-1}$. The theory balances bulk mixing cost against this smoothness cost, rather than imposing a zero-width surface from the start.

At coexistence the two compositions ${\varphi }_{-},{\varphi }_{+}$ have a common-tangent construction for phase coexistence. Subtract this tangent from $f_b$ to define an excess grand-potential density $\Delta \omega (\varphi )$, zero at both bulk compositions. For a planar profile varying along $z$, minimizing the excess free energy givesMultiply by ${\varphi }^{\prime }$ and integrate using ${\varphi }^{\prime }\to 0$ in the bulk:It follows that the interfacial surface tension isThis connects a continuous composition profile to a positive interfacial energy per area.

Broadening near criticality. Close to a symmetric mean-field thermodynamic critical point, an order parameter $m$ measuring departure from the critical composition has bulk densityThe coexistence values are $m=\pm m_0$, $m_0=\sqrt{|r|/u}$. The scalar quartic domain wall profile isThe curvature at either bulk minimum is $f_b^{\prime \prime }(m_0)=2|r|$, so the bulk correlation length is $\xi =\sqrt{K/(2|r|)}$ and this convention gives $w=2\xi$. As $T\to T_c^{-}$, the bulk distinction between phases weakens, its free-energy curvature and barrier decrease, and composition can vary gradually over an increasing distance at little bulk cost. The gradient penalty then favors a wider profile:The same model gives $\gamma =2\sqrt{2K}|r{|}^{3/2}/(3u)$, which vanishes at criticality. The exponents here are mean-field results, not universal exact values for every real mixture.

The figure uses two illustrative parameter values in this quartic model, not experimental parameters supplied by the paper. In addition to the intrinsic gradient-theory width, thermal capillary waves move and roughen an interface. Their increasing amplitude as surface tension decreases can broaden the spatially averaged measured profile further. Thus a measured broad interface can include both intrinsic composition smoothing and long-wavelength interface-position fluctuations.