Head group interactions

Kim K S, M A Moller, D J Tildesley and N Quirke 1994a. Molecular Dynamics Simulations Langmuir-Blodgett Monolayers with Explicit Head-group Interactions. Molecular Simidati 13 77-99. 

A significant change of the lipid head group interactions is observed [123]... 

We can also describe the differences between these reaction types in terms of Pearson s hard-soft description (Pearson, 1966 Pearson and Songstad, 1967). Cationic micellar head groups interact best with soft bases, e.g. relatively large anions of low charge density such as bromide or arenesulfo-nate, or anionic transition states such as those for nucleophilic aromatic substitution. They interact less readily with hard bases, e.g. high charge density anions such as OH ", or anionic transition states for deacylation. 

Surfactants can aggregate in nonpolar solvents in the presence of small amounts of water with the tails oriented towards the bulk nonpolar solution and head groups interacting with water in the center (Fig. 2). The water pool formed in reverse micelles has been used as a medium to study chemical and biological reactions [22]. 

Beardmore etal. [Ill] have presented a realistic empirical potential function to model the head-group interaction for SAMs of alkanethiols on Au(lll). The main result of these calculations is that the barriers within the surface corrugation potential are too small to pin S atoms at any particular site. 

The second factor, namely the head group interaction, can also influence the surface properties of mixed surfactant markedly. In particular, anionic/catlonic surfactant mixtures exhibit the largest effect (17,18). In nonionic/anionic surfactant mixtures, synergistic effects can still take place to a significant extent, as revealed in Figure 3 (pH 10.9, nonionic amine oxide with anionic long chain sulfate), since insertion of nonionic surfactant molecules into an ionic surfactant molecular assembly minimises electrostatic repulsion (19). 

Dispersants Keep contaminants of the lubricant in suspension and avoid their aggregation. Dispersants are amphiphilic molecules. Their long hydrocarbon tail helps to solubilize polar molecules in the base oil. The polar head group interacts with contaminants and facilitates the formation of (inverse) micelles around them. 

REPULSIVE HEAD-GROUP INTERACTIONS AND MONOLAYER CURVATURE... 

In non-polar solvents, hydrophilic head groups interact due to dipole-dipole attractions and produce aggregates called reverse micelles. With this structure, head groups of surfactant molecules orientate towards the interior and the hydrophobic tails orientate towards the nonpolar solvents. In the absence of additives such as water, the aggregation numbers of reverse micelles are small (mostly less than 10). On the other hand, in polar solvents such as glycol,... 

The second contribution to the size-dependent standard free-energy term arises from the repulsive head group interactions. The magnitude of this interaction depends on the separation between the head groups the available area per head group is used as a measure of this separation. Tanford has shownsb that an expression of the form... 

A cell membrane is a fluid mosaic of lipids and proteins. Phosphoglycerides are the major membrane lipids that form a bilayer with their hydrophilic head groups interacting with water on both the extracellular and intracellular surfaces, and their hydrophobic fatty acyl chains in the central and hydrophobic regions of the membrane. Peripheral proteins are embedded at the periphery, while integral proteins span from one side to the other. Biomembranes separate the contents of the cell from the external environment. 

Alternatively, the strongly opposed preferences of the hydrophilic and hydrophobic moieties of membrane lipids can be satisfied by forming a lipid bilayer, composed of two lipid sheets (Figure 12,10). A lipid bilayer is also called a bimolecular sheet. The hydrophobic tails of each individual sheet interact with one another, forming a hydrophobic interior that acts as a permeability barrier. The hydrophilic head groups interact with the aqueous medium on each side of the bilayer. The two opposing sheets are called leaflets. 

Membrane lipids spontaneously form extensive bimolecular sheets in aqueous solutions. The driving force for membrane formation is the hydrophobic interactions among the fatty acid tails of membrane lipids. The hydrophilic head groups interact with the aqueous medium. Lipid bilayers are cooperative structures, held together by many weak bonds. These lipid bilayers are highly impermeable to ions and most polar molecules, yet they are quite fluid, which enables them to act as a solvent for membrane proteins. 

Most single-chain surfactants do not lower the oil-water interfacial tension sufficiently to form microemulsions nor are they of the correct molecular structure, and short- to medium-chain length alcohols are necessary as cosurfactants. The cosurfactant also ensures that the interfacial film is flexible enough to deform readily around each droplet as their intercalation between the primary surfactant molecules decreases both the polar head group interactions and the hydrocarbon chain interactions. Medium-chain alcohols such as pentanol and hexanol have been used by many investigators as they are particularly effective... 

The polar, ionic and even non-ionic head-group interactions of lipid membranes and other surfactants, (as indeed for many polymers and polyelectrolyte intra-molecular interactions) and the associated curvature at interfaces formed by such assemblies will be dependent on dissolved gas in quite complicated ways. Fluctuating nanometric sized cavities at such surfaces will be at extremely high pressure, (P = 2y/r, y= surface tension, and r the radius) resulting in formation of H and OH radicals. The immediate formation of Cl radicals and consequent damage to phospholipids offers em explanation of exercise-induced immunosuppression (through excess lactic acid CO2 production), perhaps a clue to the aging process. 

Phospholipids form a bilayer, with their hydrophilic head groups interacting with water on both the extracellular and intracellular surfaces, and g their hydrophobic fatty acyl chains in the central portion of the membrane. 

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