It is generally well known that not only the sodium itself, but also the non-bridging oxygen (NBO) sites associated with sodium ions are largely responsible for the surface reactivity of soda-lime-silica (SLS) glass. Thermal poling can modify the distribution of sodium in the subsurface region. In this work, a commercial SLS float glass was thermally poled using nonblocking electrodes in air. The Na+-depleted anode surface and the Na+-gradient cathode surface were characterized using a variety of methods to find the compositional, structural and morphological effects of thermal poling. Of particular significance is the use of nondestructive vibrational spectroscopy methods, which can lead to new and improved understanding of water interactions with sodium and its sites in the glass. It was found that during thermal poling, the Na+-depleted glass network on the anode side undergoes condensation reactions of NBO sites accompanied by the increase in concentrations of silanol (SiOH) groups and molecular water species. In contrast, silanol and water species do not increase and the silicate network change is negligible in the Na+-gradient cathode side. Vibrational sum frequency generation (SFG) spectroscopy analysis revealed the difference in distributions of hydrous species in the Na+-depleted and Na+-gradient surfaces. The structural information of the thermally-poled surfaces provides critical insights needed to understand the mechanical and mechanochemical properties of the Na+-concentration modified SLS glass surfaces reported in the Part 2 companion paper.