Two-dimensional molecular interaction

In the present book, we aim at the unified description of ground states and collective excitations in orientationally structured adsorbates based on the theory of two-dimensional dipole systems. Chapter 2 is concerned with the discussion of orientation ordering in the systems of adsorbed molecules. In Section 2.1, we present a concise review on basic experimental evidence to date which demonstrate a variety of structures occurring in two-dimensional molecular lattices on crystalline dielectric substrates and interactions governing this occurrence. 

In previous sections we have considered collective properties of Frenkel excitons in three-dimensional molecular crystals only. In one- and two-dimensional molecular crystals the kinematic interaction appearing in the transition from Pauli to Bose operators can be, generally speaking, quite large, so that the description of Frenkel excitons in terms of Bose excitations can be very rough. Below we consider this problem in more detail. 

Felix, O. Hosseini, M.W. DeCian, A. Fischer. J. The simultaneous use of H-bonding and Coulomb interactions for the self-assembly of fumaric acid and cyclic bisamidine into one- and two-dimensional molecular networks. Angew. Chem.. Int. Ed. Engl. 1997. 36. 102. 

The length of the alkyl chain (number of carbon atoms) is the same for C,g-SAM and CH-LB films therefore, the van der Waals interaction between the alkyl chains was probably the same for these two kinds of monolayers. The friction coefficient on the CH-LB film, however, was less than half that of the Cjg-SAM. The difference was likely caused by the difference in the two-dimensional molecular density or in the degree of crystalline perfection. The CH-LB film has a more crystalline structure than the C,g-SAM because the structme of SAM on SiOj is complex [30]. The difference in the structme would result in differences in the stiffness and contact area. Thus, the CH-LB film showed a lower friction coefficient than C,g-SAM. 

Aiming toward the functional applications of DNA as novel molecular devices, polynucleic acids have been immobilized as two-dimensional molecular assemblies by means of the specific intermolecular interaction at the air-water interface [82,83] as well as by the polyion complex technique [84]. We describe here the effect of DNA on photoinduced electron transfer in polyion complex LB monolayers deposited on an ITO electrode [85], or cast in polyion complex films [86]. 

Nonionic and ionic surfactant molecules adsorb on solid surface at water-insoluble solid-water interface such as silica gel-water and alumina-water interface through van der Waals, hydrophobic, hydrogen bonding, and polar/ionic interactions at low concentrations of surfactant. Then, at a relatively higher and specific concentration known as critical hemimicelle concentration (CHMC), the adsorption increases dramatically as hemimicelles form on the adsorbent involving forces that characterize hydrophobic, van der Waals, and polar/ionic forces. Hemimicelles are two-dimensional molecular aggregates whose structural and physicochemical behavior have not been studied as extensively as those of normal micelles and, consequently, they are not understood even at a very rudimentary level. 

SAMs provide the needed design flexibUity, both at the individual molecular and at the material levels, and offer a vehicle for investigation of specific interactions at interfaces, and of the effect of increasing molecular complexity on the stmcture and stabUity of two-dimensional assembHes. These studies may eventuaUy produce the design capabUities needed for assembHes of three-dimensional stmctures (109). 

Phase transitions in two-dimensional layers often have very interesting and surprising features. The phase diagram of the multicomponent Widom-Rowhnson model with purely repulsive interactions contains a nontrivial phase where only one of the sublattices is preferentially occupied. Fluids and molecules adsorbed on substrate surfaces often have phase transitions at low temperatures where quantum effects have to be considered. Examples are molecular layers of H2, D2, N2 and CO molecules on graphite substrates. We review the path integral Monte Carlo (PIMC) approach to such phenomena, clarify certain experimentally observed anomalies in H2 and D2 layers, and give predictions for the order of the N2 herringbone transition. Dynamical quantum phenomena in fluids are analyzed via PIMC as well. Comparisons with the results of approximate analytical theories demonstrate the importance of the PIMC approach to phase transitions where quantum effects play a role. 

We start with some elementary information about anisotropic intermolec-ular interactions in liquid crystals and molecular factors that influence the smectic behaviour. The various types of molecular models and commonly accepted concepts reproducing the smectic behaviour are evaluated. Then we discuss in more detail the breaking of head-to-tail inversion symmetry in smectic layers formed by polar and (or) sterically asymmetric molecules and formation of particular phases with one and two dimensional periodicity. We then proceed with the description of the structure and phase behaviour of terminally fluorinated and polyphilic mesogens and specific polar properties of the achiral chevron structures. Finally, different possibilities for bridging the gap between smectic and columnar phases are considered. 

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