Interaction of head groups

In reverse micelles (formed in non-polar solvent systems containing surfactant), polar additives may be solubilized in the core where a polar interaction of head groups occurs. 

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 ... 

The order parameter is directly available from the calculations and the SCF results are given in Figure 17. The absolute values of the order parameter are a strong function of head-group area. Unlike in most SCF models, we do not use this as an input value it comes out as a result of the calculations. As such, it is somewhat of a function of the parameter choice. The qualitative trends of how the order distributes along the contour of the tails are rather more generic, i.e. independent of the exact values of the interaction parameters. The result in Figure 17 is consistent with the simulation results, as well as with the available experimental data. The order drops off to a low value at the very end of the tails. There is a semi-plateau in the order parameter for positions t = 6 — 14,... 

If the two substituents are not identical, one may still focus on the interaction of the group MO s of the substituents and those of the — CH2— CH2 - fragment. As an example, we will consider 1-fluoropropane. This molecule constitutes system where a hydrogen bond determines conformational preference and merits special attention. This hydrogen bond represents a bonding situation which can be classified under the heading of nonbonded attractive interactions. 

Surfactant aggregation in an anhydrous, nonpolar medium differs in several important respects from aggregation in water. The most apparent of these differences is that the hydrophobic effect plays no role in the formation of reverse micelles. The amphipathic species are relatively passive in aqueous micellization, being squeezed out of solution by the water. In contrast, surfactant molecules play an active role in the formation of reverse micelles, which are held together by specific interactions between head groups in the micellar core. 

The nature of the surfactant and its concentration is expected to play a role. To achieve a mechanically strong interfacial film, which can ensure the stability of the emulsion, the interfacial film of adsorbed surfactant molecules should be condensed in order to have strong lateral intermolecular interactions. A blend of two surfactants with different areas of head groups rather than an individual surfactant can more easily generate a close-packed and mechanically strong interfacial film. 

Having established that bilayer flexibility and bilayer interaction are the mesoscopic determinants, the next question is whether these determinants can be coupled to molecular parameters. In fact, this has been done to quite some extent. In general, bilayer flexibility can be shown (both experimentally as well as theoretically by simulation methods) to be directly related to bilayer thickness, lateral interaction between heads and tails of the surfactants, type of head group (ethoxylate, sugar, etc.), type of tail (saturated, unsaturated) and specific molecular mixes (e.g. SDS with or without pen-tanol). The bilayer interaction is known to be related to characteristics such as classical electrostatics. Van der Waals, Helfrich undulation forces (stemming from shape fluctuations), steric hindrance, number, density of bilayers, ionic strength, and type of salt. Two examples will be dicussed. 

The first reason lies in the fact that the interaction between solvent molecules (usually water) is stronger than the interaction between the solvent and the solute. This effect alone would lead to a precipitation of the solute. In the case of amphiphiles which form micelles, however, the head groups are strongly hydrated and repulse each other. The hydration forces and steric forces which are made responsible for this repulsion effect prevent crystallization above the Krafft point and also above the cmc. Where the formation of 3D crystals is impeded, the smallest possible droplet is formed, removing the alkyl chains from the solvent. The interactions between solvent molecules are therefore disturbed to a minimal extent, allowing the head groups to be solvated with a minimal entropy loss. It is irrelevant whether the solvent contains clusters or not. Micelle formation only occurs as a result of a solvation of head groups and non-solvation of a solvophobic core. ... 

LDLDs are mixtures of surfactants. Surfactant mixtures often perform better than the sum of the individual surfactant contributions, or perform synergisti-cally. The origin of this synergistic interaction is head group interactions and is dipolar in nature. The surfactant pairs having the greatest dipolar forces have the... 

Lipids do not form an ideal fluid, but exist as a mixture of diverse species that show preferences in associating with each other as a result of head group attractions or repulsions, and packing effects in the hydrocarbon core. Small transient microdomains are formed by specific and nonspecific protein—protein, protein-lipid, and lipid-lipid interactions. A variety of techniques have shown that membrane proteins are surrounded by a dynamic boundary layer of lipids with an average composition distinct from the bulk phase. Membrane proteins, through their preferential association with specific lipids, can induce microdomains consisting of these boundary lipids and the lipids that interact preferentially with the boundary lipids. In turn, these microdomains enhance the formation of protein clusters. 

D19.6 The formation of micelles is favored by the interaction between hydrocarbon tails and is opposed by charge repulsion of the polar groups which are placed close together at the micelle surface. As salt concentration is increased, the repulsion of head groups is reduced because their charges are partly shielded by the ions of the salt. This favors micelle formation causing the micelles to be larger and the critical micelle concentration to be smaller. 

In reverse micelles, the water molecules in the internal pool can be divided into two subensembles. One of the ensemble consists of those water molecules which are near the charged head groups and involved in stronger polar interaction with head groups. This subensemble is termed the shell as these water molecules form the outer shell of the nano-pool water. The other suhensemble contains water molecules in the core of the reverse micelle and has dynamical character similar to those found in bulk water. This subensemble is termed the core (see Figure 17.6). 

With mixed surfactants, the CMC of the mixed micelle varies according to the CMCs of the individual surfactants, and their proportions. Clearly, micelle composition varies with concentration since the micelles that form at lowest concentration are rich in the lowest-CMC surfactant, while the higher-CMC materials become more abundant in micelles as the overall concentration is increased. The detailed dependence of CMC values on mixed surfactant composition varies according to whether there are specific interactions between head-groups which lead to non-ideal mixing in the micelle. This applies particularly with mixtures of nonionic and ionic surfactants, and ionic surfactants of opposite charge. Various treatments are available to describe the behaviour (see Chapter 19), for example, as outlined in the text by Clint (3) (Chapters 5 and 6). 

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