In order to study the growth/shrinking of a hygroscopic nanoparticle during hydration/dehydration in an atmosphere of water vapour we have employed a thermodynamic approach proposed by Shchekin, Shabaev, and Rusanov (2008). This approach uses the mechanic and thermodynamic concept of disjoining pressure of thin films and allows, among others, the prediction of the humidity growth factor of both (i) a homogeneous solution droplet with completely dissolved residual core, and (ii) a heterogeneous solution droplet with partially dissolved residual core as a function of the ambient relative humidity. For application to a nanometric sodium chloride particle we have extended the original approach by (i) consideration of the nonideality of the solution through the dependence of molecular volumes of the solvent and solute molecules and the solute and solvent activities on the solution concentration, by (ii) derivation of an equation for the estimation of the efflorescence properties of a homogeneous solution droplet,