1. n. 
With reference to formation evaluation, the layer between a clay particle and the formation water that has a particular distribution of ions. Clays have an excess negative charge on their surface. When in contact with formation water, this charge attracts an excess of positive cations, normally Na+ together with their molecules of hydration water, into a region near the interface. The layer next to the clay surface, the Stern layer, has no anions, and is always present. Outside the Stern layer is the Gouy layer, through which the ion concentration gradually approaches that of free brine. The thickness of this layer increases as brine salinity decreases.