Molecular dynamics study of a hexadecyltrimethylammonium chloride monolayer at the interface between two immiscible liquids

We have simulated a hexadecyltrimethylammonium (CTA) chloride monolayer adsorbed at the water/ air and water/1,2-dichloroethane (DCE) interfaces. A diffusive electrical double layer was found formed by the chloride and CTA ions. It was found that the water molecules change their dipole orientation to help on the neutralization of this electrical charged layer. The DCE molecules penetrate the CTA tails and an increased concentration exists inside the tails. The tilt angle maximum probability for the CTA tails is 20degrees at the water/DCE interface, while at the water/air interface the maximum probability is located at 40degrees. The average tilt angle values are 27degrees and 40degrees, respectively. The average number of trans conformations per chain is similar at both interfaces with a maximum probability corresponding to 12 trans conformations. Nevertheless, the percentage of trans conformations in the chains is greater for the water/DCE interface and consequently the CTA tails at the water/air interface are more folded, present more defects, and are less stretched. The distribution of the chains is broader when the monolayer is adsorbed at the water/DCE interface. The motion in the monolayer plane of the terminal methyl group is less at the water/DCE interface.