Hydrophilic headgroup

When a monolayer of phospholipids is adsorbed at the ITIES, there must be a modification of the electrical structure of the interface [60]. Since we aim at describing the effect of this monolayer on the rate of ion transfer in a simple way, we assume a sharp interface also in the presence of phospholipids. The hydrophobic tails are located in the organic phase (negative x region), and the hydrophilic headgroups are located in the aqueous phase (positive X region). 

Biomembranes mainly consists of phospholipid matrices, and the major component is phosphorylcholines (PC). PC is an amphiphile consisting of hydrophilic headgroup and hydrophobic long chains. In view of the amphiphilic feature of PC, we can divide hydrated lipid bilayers into the three zones, I, II, and III. The zone model, which has been used in a recent NMR study of DD [46-48], is illustrated in Fig. 2. 

Fig. 3. Bilayer of a liposomal membrane composed of hydrophilic headgroups - represented as circles - with two long alkyl chains...

The effect of additives betrays the intricacy of the balance of rate effects even more. The addition of cholesterol to catalytic bilayers has been found to be beneficial for the Kemp eleminiation but to inhibit the decarboxylation of 6-NBIC. In general, the effects of additives on the decarboxylation of 6-NBIC appear to subtly depend on the structure of the hydrophobic tail and hydrophilic headgroup of additives. Similarly subtle effects were found for the Kemp elimination and nucleophilic attack by Br and water on aromatic alkylsulfonates depending on the choice of additive, hydrogen bonding effects, reactivity of partially dehydrated OH , and local water concentrations all played a role and vesicular catalysis could be increased or decreased. 

Fig. 1. Lipid substitution patterns for the polymerization of lipid bilayers, featuring polymerization of the lipid tails at (A) the chain terminus, (B) near the lipid backbone or polymerization of reactive groups (C) covalently or (D) electrostatically associated with the hydrophilic headgroup.

Molecules consisting of a long hydrophobic part and one or two hydrophilic headgroups. Able to form micelles and/or liposomes depending on the hydrophilic-lipophilic balance (HLB). 

Spontaneous aggregation of lipids — double chain amphiphiles — in aqueous medium (smectic phase). Hydrophobic core of long alkyl chains (lamellar orientation) is covered by hydrophilic headgroups (see Figs. 2, 15). 

Due to their structure (Figure 17.1), all surfactants have the tendency to accumulate at interfaces because there the hydrophobic tail can be shielded from interacting with water molecules while the hydrophilic headgroup remains solvated by water molecules. As a result of this orientation, surfactant molecules displace water molecules at the interface. Consequently, the number of hydrogen bonds decreases per unit interface area. This can be... 

The energetically unfavorable interactions of the hydrophobic tails with the water molecules are then minimized by the surfactants forming aggregates with other surfactant molecules. In those aggregates, the hydrophilic headgroups remain solvated by water molecules while the hydrocarbon moieties are shielded from water and create a hydrophobic microenvironment. Examples of these spontaneously formed aggregates are micelles and lamellae. The intersection of the extrapolations of the linear parts of the surface tension curve (Figure 17.2) is the critical micelle concentration (CMC). 

The non-specific adsorption of surfactants is based on the interaction of the hydrophilic headgroup and the hydrophobic alkyl chain with the pigment and substrate surfaces as well as the solvent. For the adsorption of surfactants, different models have been developed which take into account different types of interactions. A simple model which excludes lateral interactions of the adsorbed molecules is the Langmuir equation ... 

The HLB concept assumes that the emulsion type is mainly governed by the curvature of the interface. Large headgroups may need considerable space on the outside of oil droplets in a continuous water phase and cause a positive curvature of the interface. On the other hand, small hydrophilic headgroups can be forced together inside a water droplet whereas large hydrophobic moieties extend into the continuous oil phase. The interface now has a negative curvature. 

Aqueous micelles are 40-80 A diameter spherical aggregates which are dynamically formed from surfactants in water above a characteristic concentration, the CMC (9). Depending on the chemical structure of their hydrophilic headgroups, surfactants can be neutral or charged (positively or negatively). The alkyl chain of the surfactants typically contains between 5-20 carbon atoms. Micelles rapidly break up and reform by two known processes. The first process occurs on the microsecond time scale and is due to the release and subsequent reincorporation of a single surfactant from and back to the micelle. The second process occurs on the millisecond time scale and is ascribed to the dissolution of the... 

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