Head groups interactions between

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. 

The surface excess T is a positive quantity even when the interactions between surfactant and water become as favorable as those between surfactant and oil. In the latter case Ah0 = 0 and the concentrations in the two phases become equal. T does not vanish in that case, because the interactions between water and head group and between oil and hydrocarbon chain are stronger than those between water and hydrocarbon chain and oil and head group, respectively. It is, however, small because the interactions at the interface are not much stronger than those in the bulk. 

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... 

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... 

Emulsions are mixtures of immiscible liquid phases in the presence of an emulsifier. In the case of classic oil in water (o/w-) emulsions the oil phase is dispersed into an aqueous phase by use of surfactants. Surfactants consist of a hydrophilic head group interacting with the polar phase and a hydrophobic tail which interacts with the non-polar phase. Thus, the usually used low molecular emulsifier adsorbs on fhe inferface between the... 

The first molecular interaction model was developed by Nagarajan [3, 4, 5]. For the description of the microstructure of the complex the necklace model was adopted. The free-energy expression developed earlier for micelles was modified in order to incorporate the interaction with the polymer. This interaction was described by two parameters. One of them was the micelle-core area shielded by the polymer and the other one was an interaction parameter due to the hydrophobic contribution of the polymer segments interacting with the core. The shielding of the micelle core has two opposite effects on the micelle formation. On the one hand, it reduces the contact area between the hydrophobic micelle core and water on the other hand, it increases the polar head group interactions. Since the shielding parameter is some kind of mean value, at present no a priori method for the estimation of this area is available ... 

The hydrocarbon chain interacts weakly with water molecules, whereas the polar or ionic head group interacts strongly with water molecules (ion-dipole or dipole-dipole interaction). The strong interaction of the head group with water molecules renders the surfactant molecule soluble in water. Cooperative action of dispersion and hydrogen bonding between the water molecules tends to squeeze the hydro-phobic group out of the water (hydrophobic chains). 

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...

FIGURE 2.4 Organization of a lipid monolayer at the air-water interface. All fatty acid molecules are oriented with respect to water so that polar head groups interact with water and apolar chains are rejected in the air. The limit between air and water in such systems is referred to as an air-water interface. 

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-... 

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