COUNTERION SPECIFIC INTERACTIONS IN DIOCTADECYLDIMETHYLAMMONIUM BROMIDE MONOLAYERS AT THE MONOLAYER SUBPHASE INTERFACE

We have investigated the influence of halogen counterions on the molec ular organization characteristics of dioctadecyldimethylammonium bromi de (DOMA) monolayers at the air/water interface. Surface pressure- and surface potential-area isotherms and Brewster angle microscopy were u sed to characterize the head group-counterion interactions. The counte rions used were F-, Cl-, Br- and I-. The presence of halogen ions in t he subphase leads to compression of the pi-A and a decrease in the sur face potential. The effects of counterions on the microscopic features of molecular organization are, however, the most dramatic. DOMA at th e air/water interface shows a homogeneous and featureless morphology d uring the whole compression process. The presence of F-and Cl- ions in the subphase does not lead to any significant changes. However, the i ntroduction of Br- ions results in the appearance of dendritic structu res in the liquid/solid coexistence region. The I- ions have an even g reater effect on the morphology, in that solid domains are formed imme diately after spreading. The influence of counterions on DOMA isotherm s and morphology may be rationalized in terms of the degree of hydrati on of the various ions.