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Why does the boiling point of a cluster rise as the cluster becomes larger?

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cluster Figure 1: The four stable geometric structures of the seven-atom cluster of argon, in order of increasing energy: (A) A pentagonal bipyramid. (B) A regular octahedron with one face capped by the seventh atom. (C) A regular tetrahedron with three of its faces capped by other atoms. (D) A trigonal bipyramid with two of its faces capped by other atoms; although this has the highest energy of the four structures, it is very close in energy to the tricapped tetrahedron.

The boiling point of a cluster rises as it becomes larger because of the relationship between surface area and particle escape. In a cluster, a significant fraction of the particles are located on the surface. Due to the cluster's large surface area and curved shape, it is easier for particles to escape from a cluster than from the flat surface of bulk matter.1

This increased ease of escape results in a higher vapour pressure for a cluster compared to the corresponding bulk substance. Because the boiling point is defined as the temperature at which a liquid's vapour pressure equals the surrounding atmospheric pressure, the higher vapour pressure of a small cluster means it reaches this state at a lower temperature than the bulk liquid. As the cluster size increases, its vapour pressure decreases, which consequently causes the boiling point to rise toward that of the bulk material.1