This proposal is to explore adsorption of tetraalkylammonium (R4N+) counterions into monolayers of surfactant anions (see Figure 1) at the air-water interface, as a function of chain substitution on both cation and anion. This will provide important insight into cation-anion interactions with a new class of surfactant ionic liquids (SAILs). These SAILs are beginning to find applications as structured reaction media, and to understand their function it is important to study specific cation-anion effects. Interrogating 2-D monolayers by neutron reflectivity (NR) is the best way to gain unambiguous information on interfacial association, since SANS suffers from strong repulsive S(Q) effects being difficult to model with certainty (see below) and surface tension relies on thermodynamic assumptions.