Headgroup concentration

The ernes of ionic surfactants are usually depressed by tire addition of inert salts. Electrostatic repulsion between headgroups is screened by tire added electrolyte. This screening effectively makes tire surfactants more hydrophobic and tliis increased hydrophobicity induces micellization at lower concentrations. A linear free energy relationship expressing such a salt effect is given by ... 

Below some critical surfactant concentration, the system is two-phase with excess oil or water depending on the oil/water concentration. On adding more surfactant, the system moves into a one-phase region with normal micelles forming in water-rich systems. The water constitutes the continuous phase, solvating the headgroups of the surfactant whose hydro-phobic tails solubilise oil in the core of the micelle. In oil rich systems, reverse-micelles form. With further increases in surfactant composition. 

The interactions between the exposed headgroups are responsible for the association of lamellae into three-dimensional structures. The high Ca concentration ( 20mM), low temperature ( i 2°C and the low pH ( 6.0) required for crystallization presumably promote these interactions. 

The results of TRFSS experiments have shown us that the reverse micellar interior has the effect of limiting solvent mobility [30,31,38 0,42,43] The origin of this immobilization is still unclear. While specific interactions with the surfactant headgroups seem to play a role, the reverse micellar milieu seems more important than a high concentration of ions. 

FIG. 9 Simulated electrical potential and space charge density profiles at the water-1,2-DCE interface polarized at/= 5 in the absence (a) and in the presence (b) of zwitterionic phospholipids. The supporting electrolyte concentrations are c° = 20 mM and c = 1000 mM. The molecular area of the phospholipids is 150 A, and the corresponding surface charge density is a = 10.7 xC/cm. The distance between the planes of charge associated with the headgroups is d = 3 A. 

A theoretical approach based on the electrical double layer correction has been proposed to explain the observed enhancement of the rate of ion transfer across zwitter-ionic phospholipid monolayers at ITIES [17]. If the orientation of the headgroups is such that the phosphonic group remains closer to the ITIES than the ammonium groups, the local concentration of cations is increased at the ITIES and hence the current observed due to cation transfer is larger than in the absence of phospholipids at the interface. This enhancement is evaluated from the solution of the PB equation, and calculations have been carried out for the conditions of the experiments presented in the literature. The theoretical results turn out to be in good agreement with those experimental studies, thus showing the importance of the electrostatic correction on the rate of ion transfer across an ITIES with adsorbed phospholipids. 

Cationic surfactants contain a positively charged headgroup and are typically used as conditioners to improve hair manageability and reduce static. Cationic surfactants are especially irritating to eyes when used in high concentrations but are safe and useful in low amounts. Quatemium-15 (chloroallyl methanamine chloride, Fig. 7.9.4) is a cationic surfactant included in shampoo formulations... 

Surfactant molecules (also called amphiphiles or detergents) combine a polar or ionic head and a non-polar tail within the same molecule. The non-polar part, which is typically made up of one or more alkyl chains, causes these compounds to be sparingly soluble in water, whereas the polar or ionic part interacts strongly with water. Upon increasing the concentration of the amphiphilic compound in water, the solubility limit will be reached at a certain point and phase separation will set in. Due to the efficient interactions between the polar headgroups and the surrounding water molecules, a complete phase separation is usually unfavourable. Instead, the process halts in an intermediate stage... 

This calculation is for spherical micelles, but a similar calculation could be used to obtain estimates of salt concentrations for ionic wormlike micelles. Such salt concentrations for wormlike micelles are expected to be increased in comparison to spherical micelles. In fact, the addition of counterions or a sufficient increase in surfactant concentration often leads to a transition from spherical micelles to wormlike micelles. As the free counterion concentration in solution increases, so does the counterion binding. As a result, electrostatic repulsion between the charged head-groups is increasingly shielded and the mean cross-sectional (effective) headgroup... 

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. 

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