LINEAR-CHAIN SURFACTANTS AT A PLANAR INTERFACE - A COMPARATIVE MONTE-CARLO STUDY OF SEVERAL LATTICE MODELS

Linear chain surfactants in a densely packed arrangement (such as alka ne chains in lipid monolayers in the ''uniform tilt'' or ''no tilt'' s tructures) are described by a crude coarse-grained model where the end groups grafted on the interface form a regular lattice and the chains are described by the bond fluctuation model with chains containing N = 4 effective monomers only. Square-well interactions between the monom ers are studied for both the attractive and repulsive case for three c hoices of the interaction range. None of these models exhibits a struc ture with uniform tilt. For attractive interactions the last bond has a strong tendency to fold back thus leading to a very high density clo se to the interface. Only when an intrachain-potential favoring stiff chain configurations also is included one can obtain configurations wi th uniform tilt order. Although related models (with much longer chain lengths and lower grafting densities) are very useful for the study o f polymer brushes, the present case of very short chains in a high-den sity state clearly is plagued by various lattice artefacts and it is c oncluded that for modelling linear chain surfactants one should use an off-lattice model even on a coarse-grained level.