Agreed that in nano composite materials or nanoparticles research, the water absorption depends on powder's grain size. Herewith this, I am attaching 2 papers for your kind review (2006 & 2009 publications). Hope these help.
Most of the particles or nanoparticles dispersed in water have a surface charge, caused by ionization phenomena or absorption of charged species. The charged particles are surrounded by several layers ionic in solution, the composition of which is different from that of the bulk. When they move in solution (eg Brownian motion) the particles move together to a double layer ion.