Subphase interaction, chain

In view of the present results, however, one is forced to conclude that there must be another more general factor which causes ethylenic fatty acids to be expanded, and this factor must be related to a chain-subphase interaction. 

New factors for tlie establislmient of multilayer stmctures are, for example, tire replacement of tire hydrocarbon chain by a perfluorinated chain and tire use of a subphase containing multivalent ions [29]. The latter can become incoriDorated into an LB film during deposition. The amount depends on tire pH of tire subphase and tire individual ion. The replacement of tire hydrocarbon by a rodlike fluorocarbon chain is one way to increase van der Waals interaction and tlierefore enlrance order and stability in molecular assemblies [431. 

Indeed, macrocycles 111 and 112 form such monolayers [8.195, 8.203]. Comparison of measured surface areas with molecular models indicated that the macrocycles were lying flat on the water surface while the aliphatic chains were tilted upright in the compressed film, as shown in Figure 24 [8.203]. Monolayers containing binding units may interact selectively with ions or other substances present in the subphase [8.204]. 

The phase behavior of monolayers is determined by the molecular structure of the am-phiphile and the conditions of the subphase. Phospholipids, for example, attract each other because of van der Waals interactions between the alkyl chains. The longer the alkyl chains, the more strongly the phospholipids attract each other. Thus, the LE-LC transition pressure will decrease with increasing chain length (at constant temperature). Double bonds in the alkyl chains increase this phase transition pressure. Charges and oriented dipole moments (see Chapter 6) in the headgroups, lead to a repulsion between the phopholipids and increase the pressure at which the transition occurs. Salts in the subphase, screen this repulsion and decrease the transition pressure. 

They also studied the interaction of stearic acid (Cig) monolayers at the air-water interface with bivalent cations (Cd2+, Pb2 +, Ca2+, Ba2+, Cu2+, Ni2+, and Zn2+) in aqueous subphase using the IRRAS technique [45-47]. However, the information on molecular orientation was limited due to the use of unpolarized IR radiation. Recently, the headgroup interaction and chain orientation in the monolayers of stearic acid on pure water and ion (Ag+, Co +, Zn2+, and Pb2+)-containing subphases have been investigated using the IRRAS technique [48]. 

The interaction of soluble cations with the phospholipid phosphate groups has been investigated on a mixture of DPPC and 1,2-dipalmitoyl-sn-glycero-3-phosphoserine DPPS as a function of surface pressure and Ca2+ ion presence [51], The presence of Ca2+ in the subphase induce an acyl chain ordering at all surface pressures in both components of the binary mixture that was not observed in the case of pure DPPC alone. Unlike the bulk phase mixture of... 

A Langmuir film consists of a monomolecular layer of amphiphiles spread onto a liquid subphase (usually water) via deposition from a volatile solvent. The solvent is allowed to evaporate, leaving the molecules free to orient themselves in the two-dimensional environment at the interface (with their hydrophilic headgroups in the water and their hydrophobic tails in the atmosphere above the water surface). The molecules dissolve or evaporate only to a very limited extent, due to the insolubility of the hydrocarbon chains and the strong headgroup interaction with the water, respectively. 

Figure 3.24. Equilibrium configuration of a monolayer. Head groups are represented as terminally anchored discs. The 90 chains have seven repeating units, the CHg groups at the end are represented by an . Truncated Lennard-Jones interactions between the head groups, which can move in the subphase surface. (Reproduced from Kox et al. with permission.)...

Gorg(A, P) results from the van der Waals forces and conformational energy of the hydrocarbon chains and the van der Waals and electrostatic interactions of the head-groups within the organic subphase. It is related, but different, to the free energy of micellization for ionic surfactants defined by Evans et al.[79]... 

Several research groups have begun molecular dynamics simulations of Langmuir monolayers in which the chain interactions and chain conformations are accurately modeled. (A few simulations with more simplified models have already been carried out. ) Full-scale simulations in which the details of the subphase are also included are complex, and in the two papers that have already appeared the scope of the problem has been reduced by using a structureless planar substrate. 

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