Interactions amphiphilic

ZnPc NaYF4 Yb,Er 980 nm Non-covale nt hydro-phobic interactions Amphiphilic chitosan n/a S180 PDT 70... 

Key words Self-assembled mono-layers - binding sites - receptors -host-guest interactions - amphiphilic structures - sensors -voltammetry - modified electrodes... 

LS. In the LS phase the molecules are oriented normal to the surface in a hexagonal unit cell. It is identified with the hexatic smectic BH phase. Chains can rotate and have axial symmetry due to their lack of tilt. Cai and Rice developed a density functional model for the tilting transition between the L2 and LS phases [202]. Calculations with this model show that amphiphile-surface interactions play an important role in determining the tilt their conclusions support the lack of tilt found in fluorinated amphiphiles [203]. 

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

A fiirther step in coarse graining is accomplished by representing the amphiphiles not as chain molecules but as single site/bond entities on a lattice. The characteristic architecture of the amphiphile—the hydrophilic head and hydrophobic tail—is lost in this representation. Instead, the interaction between the different lattice sites, which represent the oil, the water and the amphiphile, have to be carefiilly constmcted in order to bring about the amphiphilic behaviour. 

Slightly more complex models treat the water, the amphiphile and the oil as tliree distinct variables corresponding to the spin variables. S = +1, 0, and -1. The most general Hamiltonian with nearest-neighboiir interactions has the fomi... 

This Blume-Eiiiery-GrifSths (BEG) model [74] has been studied both by mean field calculations as well as by simulations. There is no pronounced difference between the amphiphile molecules S= 0, the oil or the water. Indeed, the model was first suggested in a quite different context. An extension of the model by Schick and Shih [75] includes an additional interaction of tlie fomi... 

There has been considerable interest in the simulation of lipid bilayers due to their biological importance. Early calculations on amphiphilic assemblies were limited by the computing power available, and so relatively simple models were employed. One of the most important of these is the mean field approach of Marcelja [Marcelja 1973, 1974], in which the interaction of a single hydrocarbon chain with its neighbours is represented by two additional contributions to the energy function. The energy of a chain in the mean field is given by ... 

A typical biomembrane consists largely of amphiphilic lipids with small hydrophilic head groups and long hydrophobic fatty acid tails. These amphiphiles are insoluble in water (<10 ° mol L ) and capable of self-organization into uitrathin bilaycr lipid membranes (BLMs). Until 1977 only natural lipids, in particular phospholipids like lecithins, were believed to form spherical and related vesicular membrane structures. Intricate interactions of the head groups were supposed to be necessary for the self-organization of several ten thousands of... 

Whereas the main challenge for the first bilayer simulations has been to obtain stable bilayers with properties (e.g., densities) which compare well with experiments, more and more complex problems can be tackled nowadays. For example, lipid bilayers were set up and compared in different phases (the fluid, the gel, the ripple phase) [67,68,76,81]. The formation of large pores and the structure of water in these water channels have been studied [80,81], and the forces acting on lipids which are pulled out of a membrane have been measured [82]. The bilayer systems themselves are also becoming more complex. Bilayers made of complicated amphiphiles such as unsaturated lipids have been considered [83,84]. The effect of adding cholesterol has been investigated [85,86]. An increasing number of studies are concerned with the important complex of hpid/protein interactions [87-89] and, in particular, with the structure of ion channels [90-92]. 

The Larson model and Larson-type models have been widely used to study micelles [37,111,114-120], amphiphiles at oil/water interfaces [121,122] bilayers [117,123] and various other problems [125-128]. The models differ from each other in the range of the interactions and in the treatment of the amphiphile monomers. Other than in Larson s original model, most authors include only nearest-neighbor interactions, sometimes in combination with a... 

Simulations of monolayers have focused on internal phase transitions, e.g., between the expanded phase and the condensed phases, between different tilted phases, etc. These phenomena cannot be reproduced by models with purely repulsive interactions. Therefore, Haas et al. [148,149] represent the amphiphiles as stiff Lennard-Jones chains, with one end (the head bead) confined to move in a plane. In later versions of the model [150-152], the head bead interactions differ from those of the tail beads they are taken to be purely repulsive, and the head size is variable. 

More recently suggested models for bulk systems treat oil, water and amphiphiles on equal footing and place them all on lattice sites. They are thus basically lattice models for ternary fluids, which are generalized to capture the essential properties of the amphiphiles. Oil, water, and amphiphiles are represented by Ising spins 5 = -1,0 and +1. If one considers all possible nearest-neighbor interactions between these three types of particle, one obtains a total number of three independent interaction parameters, and... 

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