Interlayer Structure and Dynamics of HDTMA+-Intercalated Rectorite with and without Water: A Molecular Dynamics Study

Rectorite is a special kind of clay mineral, consisting of illite layer and smectite layer in a regular order. In this study, we employ classical molecular dynamics simulations to study the microscopic interlayer properties of HDTMA(+) (hexadecyl trimethyl ammonium)-intercalated rectorites with and without water at different HDTMA(+) loading levels. The simulation results show that as the loading level increases, monolayer, bilayer, transition, and trilayer configurations of HDTMA(+) occur in succession. The different layer charge characteristics between illite sheet and smectite sheet lead to special interlayer distributions of HDTMA(+). In the systems without water, the headgroups of HDTMA(+) and Na+ are controlled by six-membered rings of clay surface. With water addition, water molecules show the highest mobility among the interlayer species. However, HDTMA headgroups are still controlled by clay surfaces mainly with low mobility. At the same time, most of Na+ cations escape from surfaces because of the attractions from water. But water has little influence on the mobility of Na+. Water can decrease the mobility of alkyl chains because water fills up the empty region in the organic phases. In the sample with loading level exceeding 1 CEC, anions (Cl- in this study) present headgroups. very poor mobility due to the electrostatic attractions with