Head group interactions, surface

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). 

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. 

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. 

The main stabilizing feature of biological membranes is hydrophobic interactions among the molecules in the lipid bilayer. The phospholipids in the lipid bilayer orient themselves so that their polar head groups interact with water. Proteins in the lipid bilayer interact favorably in their hydrophobic milieu because they typically have hydrophobic amino acid residues on their outer surfaces. 

An amphiphilic molecule (a surfactant) can arrange itself at the surface of the water such that the polar part (head group) interacts with the water, and the hydrocarbon part (tail group) is held above the surface, as shown in Figure 5.3. This kind of organization of sur-... 

When placed in aqueous solution, phospholipids spontaneously form a lipid bilayer (Figure 26.13) in which polar head groups lie on the surface, giving the bilayer an ionic coating. Nonpolar hydrocarbon chains of fatty acids lie buried within the bilayer. This self-assembly of phospholipids into a bilayer is a spontaneous process driven by two types of noncovalent forces (1) hydrophobic effects, which result when nonpolar hydrocarbon chains cluster together and exclude water molecules and (2) electrostatic interactions, which result when polar head groups interact with water and other polar molecules in the aqueous environment. 

Figure 20.15. The interaction between two hydrophobized mica surfaces with adsorbed nonionic surfactant C12E5. As the temperature is increased from room temperature ( ), the profile changes from purely repulsive (steric plus a small residual double-layer interaction) to strongly attractive ( ) as the cloud point is surpassed and the head-group interaction becomes favourable (water becomes a poorer solvent for polyoxyethylene) (148), reproduced by permission of The Royal Society of Chemistry...

Landau E.M. and Leshem Y. 1989. A monolayer model study of surface tension - associated parameters of membrane phospholipids II. Anionic head-group interactions with pH, Ca + and auxins. Jour. Exp. Bot. (in press). 

Figure 20.6 Example of one configuration obtained for the phospholipid head group interacting with the silica surface sites via water molecules. (Adapted with permission from Ref [29]. Copyright 2011, American Chemical Society.)...

The acid monolayers adsorb via physical forces [30] however, the interactions between the head group and the surface are very strong [29]. While chemisorption controls the SAMs created from alkylthiols or silanes, it is often preceded by a physical adsorption step [42]. This has been shown quantitatively by FTIR for siloxane polymers chemisorbing to alumina illustrated in Fig. XI-2. The fact that irreversible chemisorption is preceded by physical adsorption explains the utility of equilibrium adsorption models for these processes. 

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