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Why do clusters have higher vapour pressures than the same substances in bulk form?

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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.

Clusters exhibit higher vapour pressure than the same substances in bulk form primarily due to their high surface-to-volume ratio and the curved geometry of their surfaces.1

In a cluster, a large fraction of the constituent particles are located on the surface rather than in the interior. Because surface atoms are bonded to fewer neighbors than those in the interior, they are less tightly held to the main body of the substance. This increased surface area, combined with the curvature of the cluster, makes it energetically easier for particles to escape the cluster than to leave the flat surface of a bulk solid or liquid.12

Consequently, this enhanced particle mobility leads to a higher vapour pressure for clusters compared to their bulk counterparts. As the size of a cluster increases, the proportion of surface atoms decreases, causing the vapour pressure to decline and eventually merge into the properties of the bulk material.12