Type Of Head Group Research Articles

Effect of Ammonium Group on Inhibition of the Acid Corrosion of Steel and Iron by D-Alkylquaternary Ammonium Compounds

Previous studies of the inhibition of the acid corrosion of mild steel and pure iron by n-alkyltrimethylammonium (TMA) and n-alkyltriethylammonium (TEA) compounds have been extended to include the corresponding n-alkyltripropylammonium (TPA) and n-alkyltributylammonium (TBA) compounds and the performance ofthefour headgroup types have been compared at different n-alkyl chain lengths. For a given chain length, the percentage inhibition generally increases in the order TMA < TEA < TPA, in line with the expected decrease in mutual repulsion between the adsorbed cations with expanding headgroup size. However, the transition to a TBA headgroup sometimes has an adverse effect, especially at shorter chain lengths. This is possibly due to reduced Van der Waals attraction between the alkyl chains as they are forced apart by the spatial requirements of the large head group.Although the corrosion inhibition increases in the above headgroup sequence for both mild steel and pure iron, the incremental changes are appreciably greater for iron and this could indicate that the rest potential lies closer to the potential of zero charge than it does in the case of steel.Polarisation measurements show that the quaternary ammonium compounds studied in this work are predominantly anodic inhibitors and, with steel in 0·5 M H2SO4, they actually cause appreciable stimulation of the cathodic reaction at the lower levels of inhibition. The possible significance of this in relation to the choice of picking inhibitors is discussed.

Aggregation of amphiphiles as micelles or vesicles in aqueous media

The physical factors responsible for the aggregation of amphiphiles in aqueous media are examined and expressions for their contribution to the attractive or repulsive components of the free energy change of aggregation are established. Whereas in previous treatments, an arbitrary repulsive force was necessary to explain the behavior of nonionic systems, no such ad hoc assumption is made here. Rather the free energy changes due to interfacial tension at the hydrocarbon core (of the aggregates)-water interface and to the loss of a part of translational and rotational degrees of freedom of the amphiphiles when they aggregate are the two main repulsive contributions. On the other hand, the factors favoring aggregation are: (i) the van der Waals interactions between the hydrocarbon tails of the amphiphiles, and (ii) the structural changes in water and the changes in the interactions between amphiphiles and water resulting from aggregation. For ionic and zwitterionic amphiphilar systems additional free energy contributions are included to account for the repulsive electrostatic interactions between the head groups. For vesicles, the repulsion caused by the overlapping electrical double layers inside the vesicles is also considered. The expressions established for the various free energy changes associated with aggregation are used to examine the formation of micelles and vesicles, from single and double chain amphiphiles with nonionic, ionic, or zwitterionic head groups. In general, single chain amphiphiles aggregate as micelles, rather than as vesicles, for all types of polar head groups. Depending upon the nature of the head groups small and/or large micelles can form. Nonionic amphiphiles which have head groups of small cross-sectional areas form large micelles, whereas those with large cross-section aggregate as small micelles. This happens because the repulsion caused by the loss of translational degrees of freedom is larger in the latter of the two cases. Ionic or zwitterionic amphiphiles form small micelles even though they have small head groups because of the electrostatic repulsion between the head groups. At large ionic strengths, large micelles can form because the repulsive interactions between the head groups are small. Nonionic double chain amphiphiles aggregate predominantly as vesicles. Ionic or zwitterionic double chain amphiphiles aggregate as micelles when the electrostatic repulsion between the head groups is large and as vesicles when this repulsion is small. However for intermediate values of these interactions both micelles and vesicles form depending upon the length of the hydrocarbon tail. Most biologically significant double chain amphiphiles have long, complex polar head groups and they aggregate as micelles when the hydrocarbon tail length is short, even if the electrostatic repulsion between the head groups is weak; but they aggregate as vesicles when the hydrocarbon tail length is long. The size distributions calculated for different types of amphiphiles can be unimodal, representing a single population of aggregates, bimodal, or trimodal representing the coexistence of two or three distinct populations of aggregates. The three possible populations are small micelles, large micelles, and vesicles.

The molecular organization of asymmetric lipid bilayers and lipid-peptide complexes

Oriented fatty acid bilayers with asymmetric distributions of lipid head group types, hydrocarbon chain lengths, and associated polypeptides have been analyzed by a combined use of high resolution electron microscopy and X-ray diffraction techniques. The exclusion of fixatives, stains, and embedding materials has made it possible to relate unequivocally microscopic images to molecular composition. The ultrastructure of asymmetric bilayers has been determined by a novel analysis in which one half of the bilayer serves as a structural reference for the entire bilayer. Absolute electron density profiles at 7 Å resolution have been computed for bilayers formed from long and short chain length lipids either segregated to opposite sides or mixed together in both sides of the bilayer. The data indicate that the two lipids self organize in a specific paired configuration. Detailed analysis of bilayers associated with poly- l-lysine shows that although this hydrophilic peptide resides near the lipid head group region, its presence alters the arrangement of the bilayer hydrocarbon chains.

Investigation of lipid + water systems: Part 6 — Proton magnetic resonance in ordered lyotropic mesophases

P.m.r. measurements have been made on macroscopically ordered samples of the smectic mesophase in two 1-monoglyceride + water systems and the 1-amino-octane + water system. Three doublets have been observed, one from hydroxyl protons, and the remaining two from methylene protons in two sections of the alkyl chain undergoing different types of molecular motion. The protons of the glyceryl residue also contribute to one of the alkyl chain doublets. The magnitudes of the doublet splittings have been measured at a number of different temperatures and compositions in each system. Some correlations have been made between the splittings obtained and the type of head group and the length and conformation of the alkyl chain. The effects of temperature change have been interpreted in terms of the increasing occurrence of gauche configurations in segments of the alkyl chain. Correlations have been made between degrees of order obtained from p.m.r. and d.m.r. spectra.

Rhodopsin-lipid interactions.

AbstractPurified, lipid‐free rhodopsin has been incorporated into bilayers of natural and synthetic phosphatidyl cholines and a natural digalactosyl diglyceride. Successful incorporation and high regenerability of rhodopsin appears to depend on the nature of the hydrocarbon chains and not specifically on the type of polar head group of the lipids.Electron paramagnetic resonance spectra of recombinant membranes containing small amounts of spin labeled phosphatidyl cholines reveals that rhodopsin interacts directly with the hydrocarbon chains of the phospholipids in the bilayer, resulting in a dramatically decreased motional freedom of the chains.In studies of the recombinant membranes using freeze–fracture electron microscopy, the presence of rhodopsin is detected as discrete particles in the plane of fracture. The particle distribution is dependent on the nature of the lipid hydrocarbon chains and whether or not the rhodopsin has been photolyzed.From these studies, it is concluded that the rhodopsin‐bilayer structure is stabilized primarily by direct interaction of the hydrocarbon chains with the protein molecule, and that structural parameters in this model system are subject to modulation by visible light.