In addition, an outer-sphere complex (OSC) was observed. In the present context, an M complex is formed when calcium interacts with the carboxylate groups using one oxygen as a donor site. Similarly, in a B complex, calcium ions interact with two oxygen atoms, which act as two donor sites. In an OSC, the calcium ions are surrounded by a monolayer of water, resulting in a greater distance between the ions and the carboxylate groups.