Interactions between headgroups

The free counterions form an electrical double layer in which the counterion concentrations around each micelle decrease in a Poisson-Boltzmann distribution into the aqueous phase. Figure 6 illustrates the double layer and the radial distributions of counterions at different salt concentrations obtained by solving the Poisson-Boltzmann equation. Note that the thickness of the double layer depends on the ion salt concentration. The graph also illustrates a two-site model for ion distribution used in the pseudophase models to describe measured ion distributions in solutions of ionic association colloids, that is, counterions are either bound or free (see below). However, explanations based only on coulombic interactions between headgroups and counterions fail to account for commonly observed trends in ion-specific effects, for example, the Hofmeister... 

In the case of chemisoriDtion this is the most exothennic process and the strong molecule substrate interaction results in an anchoring of the headgroup at a certain surface site via a chemical bond. This bond can be covalent, covalent with a polar part or purely ionic. As a result of the exothennic interaction between the headgroup and the substrate, the molecules try to occupy each available surface site. Molecules that are already at the surface are pushed together during this process. Therefore, even for chemisorbed species, a certain surface mobility has to be anticipated before the molecules finally anchor. Otherwise the evolution of ordered stmctures could not be explained. 

For ammonium surfactants there is evidence for the existence of an additional specific interaction between the headgroups of the surfactant and the aromatic solubilisate . This is in line with the observation that partition coefficients for benzene in CTAB solutions are much higher than those for... 

In 1997, a Chinese research group [78] used the colloidal solution of 70-nm-sized carboxylated latex particles as a subphase and spread mixtures of cationic and other surfactants at the air-solution interface. If the pH was sufficiently low (1.5-3.0), the electrostatic interaction between the polar headgroups of the monolayer and the surface groups of the latex particles was strong enough to attract the latex to the surface. A fairly densely packed array of particles could be obtained if a 2 1 mixture of octadecylamine and stearic acid was spread at the interface. The particle films could be transferred onto solid substrates using the LB technique. The structure was studied using transmission electron microscopy. 

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 pH-sensitive liposomes consist of mixtures of several saturated egg phosphatidylcholines and several A -acylamino acids. The release of drug is suggested to be a function of acid-base equilibrium effected by the interaction between ionizable amino acids and N-acy-lamino acid headgroups of the liposomes. There appears to be a close relation between Tc and pH effect [72],... 

Although the stabilizing interactions between the amino acid side chains of PLC/j, and the choline headgroup are readily apparent in the PLC fc-phosphonate inhibitor complex, it is more difficult to identify contacts between the protein and the acyl chains of the inhibitor [45]. In part this is because thermal motion in the acyl side chains, especially the sn-1 chain, renders them somewhat disordered. Consequently, the measured distances between the side chain carbons... 

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 may be caused by two factors. First of all, in the case of pyridinium salts there may be a contribution from the hydrophobic interactions between neighbouring bound headgroups (an effect which would not contribute to the free energy of micelle formation). Secondly, a steric hindrance effect may prevent the positive chrge on the trlmethylammonium head group from approaching close to the polylon charge. 

Modification typically takes advantage of electrostatic interactions between charges on the surface of the macromolecules and the polar headgroups of surfactants. We reasoned that the host-guest interactions at the nanoparticle-solution interface investigated in this work could be used for similar purposes. (From Liu et ah, 2001)... 

The phase behavior of monolayers is determined by the molecular structure of the am-phiphile and the conditions of the subphase. Phospholipids, for example, attract each other because of van der Waals interactions between the alkyl chains. The longer the alkyl chains, the more strongly the phospholipids attract each other. Thus, the LE-LC transition pressure will decrease with increasing chain length (at constant temperature). Double bonds in the alkyl chains increase this phase transition pressure. Charges and oriented dipole moments (see Chapter 6) in the headgroups, lead to a repulsion between the phopholipids and increase the pressure at which the transition occurs. Salts in the subphase, screen this repulsion and decrease the transition pressure. 

The transition moment vector of va, S-O points between surfactant molecules, and is therefore extremely sensitive to surfactant-surfactant interactions. The enhanced perturbation of vas S-O indicates that the electrostatic interactions between the dissimilar headgroups reduces a0 sufficiently to "drive" rod micelle formation in mixtures of DTAC and SDS. 

---