Critical radius

The critical radius of a system can be determined from its Gibbs free energy.^[1]

${\displaystyle \Delta G_{T}=\Delta G_{V}+\Delta G_{S}}$

It has two components, the volume energy ${\displaystyle \Delta G_{V}}$ and the surface energy ${\displaystyle \Delta G_{S}}$. The first one describes how probable it is to have a phase change and the second one is the amount of energy needed to create an interface.

The mathematical expression of ${\displaystyle \Delta G_{V}}$ , considering spherical particles, is given by:

${\displaystyle \Delta G_{V}={\frac {4}{3}}\pi r^{3}\Delta g_{v}}$

where ${\displaystyle \Delta g_{v}}$ is the Gibbs free energy per volume and obeys ${\displaystyle -\infty <\Delta g_{v}<\infty }$. It is defined as the energy difference between one system at a certain temperature and the same system at the fusion temperature and it depends on pressure, the number of particles and temperature: ${\displaystyle \Delta g_{v}(T,p,N)}$. For a low temperature, far from the fusion point, this energy is big (it is more difficult to change the phase) and for a temperature close to the fusion point it is small (the system will tend to change its phase).

Regarding ${\displaystyle \Delta G_{S}}$ and considering spherical particles, its mathematical expression is given by:

${\displaystyle \Delta G_{S}=4\pi r^{2}\gamma >0}$

where ${\displaystyle \gamma }$ is the surface tension we need to break to create a nucleus. The value of the ${\displaystyle \Delta G_{S}}$ is never negative as it always takes energy to create an interface.

The total Gibbs free energy is therefore:

${\displaystyle \Delta G_{T}=-{\frac {4\pi }{3}}r^{3}\Delta g_{v}+4\pi r^{2}\gamma }$

The critical radius ${\displaystyle r_{c}}$ is found by optimization, setting the derivative of ${\displaystyle \Delta G_{T}}$ equal to zero.

${\displaystyle {\frac {d\Delta G_{T}}{dr}}=-4\pi r_{c}^{2}\Delta g_{v}+8\pi r_{c}\gamma =0}$

yielding

${\displaystyle r_{c}={\frac {2\gamma }{|\Delta g_{v}|}}}$,

where ${\displaystyle \gamma }$ is the surface tension and ${\displaystyle |\Delta g_{v}|}$ is the absolute value of the Gibbs free energy per volume.

The Gibbs free energy of nuclear formation is found replacing the critical radius expression in the general formula.

${\displaystyle \Delta G_{c}={\frac {16\pi \gamma ^{3}}{3(\Delta g_{v})^{2}}}}$

When the Gibbs free energy change is positive, the nucleation process will not be prosperous. The nanoparticle radius is small, the surface term prevails the volume term ${\displaystyle \Delta G_{S}>\Delta G_{V}}$. Contrary, if the variation rate is negative, it will be thermodynamically stable. The size of the cluster surpasses the critical radius. In this occasion, the volume term overcomes the superficial term ${\displaystyle \Delta G_{S}<\Delta G_{V}}$.

From the expression of the critical radius, as the Gibbs volume energy increases, the critical radius will decrease and hence, it will be easier achieving the formation of nuclei and begin the crystallization process.

In order to decrease the value of the critical radius ${\displaystyle r_{c}}$ and promote nucleation, a supercooling or superheating process may be used.

Supercooling is a phenomenon in which the system's temperature is lowered under the phase transition temperature without the creation of the new phase. Let ${\displaystyle \Delta T=T_{f}-T}$ be the temperature difference, where ${\displaystyle T_{f}}$ is the phase transition temperature. Let ${\displaystyle \Delta g_{v}=\Delta h_{v}-T\Delta s_{v}}$ be the volume Gibbs free energy, enthalpy and entropy respectively.

When ${\displaystyle T=T_{f}}$, the system has null Gibbs free energy, so:

${\displaystyle \Delta g_{f,v}=0\Leftrightarrow \Delta h_{f,v}=T_{f}\Delta s_{f,v}}$

In general, the following approximations can be done:

${\displaystyle \Delta h_{v}\rightarrow \Delta h_{f,v}}$ and ${\displaystyle \Delta s_{v}\rightarrow \Delta s_{f,v}}$

Consequently:

${\displaystyle \Delta g_{v}\simeq \Delta h_{f,v}-T\Delta s_{f,v}=\Delta h_{f,v}-{\frac {T\Delta h_{f,v}}{T_{f}}}=\Delta h_{f,v}{\frac {T_{f}-T}{T_{f}}}}$

So:

${\displaystyle \Delta g_{v}=\Delta h_{f,v}{\frac {\Delta T}{T_{f}}}}$

Substituting this result on the expressions for ${\displaystyle r_{c}}$ and ${\displaystyle \Delta G_{c}}$, the following equations are obtained:

${\displaystyle r_{c}={\frac {2\gamma T_{f}}{\Delta h_{f,v}}}{\frac {1}{\Delta T}}}$

${\displaystyle \Delta G_{c}={\frac {16\pi \gamma ^{3}T_{f}^{2}}{3(\Delta h_{f,v})^{2}}}{\frac {1}{(\Delta T)^{2}}}}$

Notice that ${\displaystyle r_{c}}$ and ${\displaystyle \Delta G_{c}}$ diminish with an increasing supercooling. Analogously, a mathematical derivation for the superheating can be done.

Supersaturation is a phenomenon where the concentration of a solute exceeds the value of the equilibrium concentration.

From the definition of chemical potential ${\displaystyle \Delta \mu =-k_{B}Tln\left({\frac {c_{0}}{c_{eq}}}\right)}$, where ${\displaystyle k_{B}}$ is the Boltzmann constant, ${\displaystyle c_{0}}$ is the solute concentration and ${\displaystyle c_{eq}}$ is the equilibrium concentration. For a stoichiometric compound and considering ${\displaystyle \mu ={\frac {\partial G}{\partial N}}}$ and ${\displaystyle N={\frac {V}{v_{a}}}}$, where ${\displaystyle v_{a}}$ is the atomic volume:

${\displaystyle \Delta g_{v}={\frac {\Delta \mu }{v_{a}}}=-{\frac {k_{B}T}{v_{a}}}ln\left({\frac {c_{0}}{c_{eq}}}\right).}$

Defining the supersaturation as ${\displaystyle S={\frac {c_{0}-c_{eq}}{c_{eq}}},}$ this can be rewritten as

${\displaystyle \Delta g_{v}=-{\frac {k_{B}T}{v_{a}}}ln\left(1+S\right).}$

Finally, the critical radius ${\displaystyle r_{c}}$ and the Gibbs free energy of nuclear formation ${\displaystyle \Delta G_{c}}$ can be obtained as

${\displaystyle r_{c}={\frac {2\gamma v_{a}}{k_{B}Tln\left(1+S\right)}}}$,

${\displaystyle \Delta G_{c}={\frac {16\pi \gamma ^{3}V_{M}^{2}}{3(RTln\left(1+S\right))^{2}}},}$

where ${\displaystyle V_{M}}$ is the molar volume and ${\displaystyle R}$ is the molar gas constant.

• Nucleation
• Ostwald ripening
• Supercooling
• Superheating