- April 29, 2013
-- In one embodiment, a lipid bilayer is the effective encapsulating mechanisim, multiple layers may be used to extend the release profile. A lipid bilayer is a thin polar membrane made of two layers of lipid molecules. This structure is called a "lipid bilayer" because it is composed of two layers of fatty acids organized in two sheets. The lipid bilayer is typically about five to ten nanometers thick and surrounds all cells providing the cell membrane structure. It forms a continuous barrier around cells and thus provides a semipermeable interface between the interior and exterior of a cell and between compartments within the cell. The cell membrane of almost every living organism is made of a lipid bilayer, as are the membranes surrounding the cell nucleus and other sub-cellular structures. The lipid bilayer is the barrier that sustains ions, proteins and other molecules and prevents them from diffusing into areas where they should not be. Lipid bilayers are ideally suited to this role because, even though they are only a few nanometers in width, they are impermeable to most water-soluble (hydrophilic)
molecules. With the hydrophobic tails of each individual sheet interacting with one another, a hydrophobic interior is formed and this acts as a permeability barrier. The hydrophilic head groups interact with the aqueous medium on both sides of the bilayer. The two opposing sheets are also known as leaflets. Bilayer-
forming lipids are amphipathic molecules (containing both hydrophilic and hydrophobic components). The hydrophilic fragment, typically termed the lipid head-group, is charged, or polar, whereas the hydrophobic section consists of a pair of alkyl chains (typically between 14 and 20 carbon atoms in length). The structure of the lipid bilayer explains its function as a barrier. Lipids are fats, like oil, that are insoluble in water. There are two important regions of a lipid that provide the structure of the lipid bilayer. Each lipid molecule contains a hydrophilic region, also called a polar head region, and a hydrophobic, or nonpolar tail region (Figure 4). The phospholipid molecule's polar head group contains a phosphate group. It also sports two nonpolar fatty acid chain groups as its tail. The phospholipids organize themselves in a bilayer to hide their hydrophobic tail regions and expose the hydrophilic regions to water. This organization is spontaneous, meaning it is a natural process and does not require energy. This structure forms the layer that is the wall between the inside and outside of the cell. Natural bilayers are usually composed of phospholipids. The phospholipid bilayer is the two-layer membrane that surrounds many types of plant and animal cells. It's made up of molecules called phospholipids, which arrange themselves in two parallel layers, forming a membrane that can only be penetrated by certain types of substances. This gives the cell a clear boundary, and keeps unwanted substances out. Though the phospholipid bilayer works well most of the time, it can be damaged, and some types of unwanted substances can bypass it. In an aqueous environment the lipids self-assemble into structures that minimize contact between water molecules and the hydrophobic components of the lipids by forming two leaflets (monolayers);
this arrangement brings the hydrophobic tails of each leaflet in direct contact with each other, and leaves the head groups in contact with water. Importance: Controlled release provides for lower hydrogen levels resulting in a more favorable condition for reductive dechlorinators, allowing them to effectively out compete methanogens for growth substrates. The result is less substrate is lost to methane formation, therefore requiring less overall material to be introducing into the subsurface and increasing the efficacy of the remedial program!!!!!
Two different dosages (0.5 g/L and 1 g/L) of both the regular calcium propionate (RCP) and the encapsulated 80% calcium propionate (ECP) were tested in order to compare the calcium release rates of both materials. The materials were placed in capped 250-ml flasks and were mixed with the use of magnetic stirring plates. All the experiments were performed in duplicates. As the results on Table 1 show, ECP showed much slower release rates upon the completion of the 14-day experimental procedure. In fact the 0.5 g/L ECP did not show any release of calcium during the first 2 days of the mixing procedure, while the release was increased to 5.8% of total calcium content 14 days upon the start of the experiment. Similarly the 1 g/L ECP showed a 2-day calcium release of 11.7%, which increased to 17.5% during the 14-day sampling period. Conversely RCP showed much higher calcium release rates in the solution. For the 0.5 g/L RCP the amount of calcium released was at 37.4% after 2 days of mixing and 56.1% after 14 days. For the 1 g/L RCP calcium release was at 56.1% after 2 days and at 65.4% after 14 days. IET-INC.NET