Head group

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. 

The interest in vesicles as models for cell biomembranes has led to much work on the interactions within and between lipid layers. The primary contributions to vesicle stability and curvature include those familiar to us already, the electrostatic interactions between charged head groups (Chapter V) and the van der Waals interaction between layers (Chapter VI). An additional force due to thermal fluctuations in membranes produces a steric repulsion between membranes known as the Helfrich or undulation interaction. This force has been quantified by Sackmann and co-workers using reflection interference contrast microscopy to monitor vesicles weakly adhering to a solid substrate [78]. Membrane fluctuation forces may influence the interactions between proteins embedded in them [79]. Finally, in balance with these forces, bending elasticity helps determine shape transitions [80], interactions between inclusions [81], aggregation of membrane junctions [82], and unbinding of pinched membranes [83]. Specific interactions between membrane embedded receptors add an additional complication to biomembrane behavior. These have been stud-... 

Figure Bl.19.8. (a) STM image (5.7 mu x 5.7 mu) of 10-alkylcyanobiphenyl on graphite (b) model showing the packing of the molecules. The shaded and unshaded segments represent the alkyl tails and the cyanobiphenyl head groups, respectively. (Taken from [38], figure 2.)...

Daniel M F, Lettington O C and Small M 1983 Investigation into the Langmuir-Blodgett film formation ability of amphiphiles with oyano head groups Thin Solid Films 99 61-9... 

Bain C D, Evall J and Whitesides G M 1989 Formation of monolayers by the ooadsorption of thiols on gold— variation in the head group, tail group, and solvent J. Am. Chem. Soc. Ill 7155-64... 

The first molecular dynamics simulations of a lipid bilayer which used an explicit representation of all the molecules was performed by van der Ploeg and Berendsen in 1982 [van dei Ploeg and Berendsen 1982]. Their simulation contained 32 decanoate molecules arranged in two layers of sixteen molecules each. Periodic boundary conditions were employed and a xmited atom force potential was used to model the interactions. The head groups were restrained using a harmonic potential of the form ... 

Fi. 7.22 Simulations of a Langniuir-Blodgett film [Kim et al. 1994a] as the area per head group increases the chains tilt away from the normal. 

Fig. 7.23 In simulations of stearic add on a hydrophobic surface hydrogen bonding between the head groups is important in controlling the orientation of the molecules [Kim et al, 1994b],...

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 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... 

Phospholipid (Section 26 4) Adiacylglycerol beanng a choline phosphate head group Also known as phosphatidyl choline... 

Hydrolysis. The first effect of either acid hydrolysis or alkaline hydrolysis (saponification) is the removal of the fatty acids. The saponification value of commercial lecithin is 196. Further decomposition into glycerol, phosphoric acid, and head groups (ie, choline, ethanolamine, etc) may foUow prolonged heating. Lecithin may also be hydrolyzed by enzymes. 

Soap is one example of a broader class of materials known as surface-active agents, or surfactants (qv). Surfactant molecules contain both a hydrophilic or water-liking portion and a separate hydrophobic or water-repelling portion. The hydrophilic portion of a soap molecule is the carboxylate head group and the hydrophobic portion is the aUphatic chain. This class of materials is simultaneously soluble in both aqueous and organic phases or preferential aggregate at air—water interfaces. It is this special chemical stmcture that leads to the abiUty of surfactants to clean dirt and oil from surfaces and produce lather. 

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