Structure sheetlike

A number of substances such as graphite, talc, and molybdenum disulfide have sheetlike crystal structures, and it might be supposed that the shear strength along such layers would be small and hence the coefficient of friction. It is true... 

Biological membranes provide the essential barrier between cells and the organelles of which cells are composed. Cellular membranes are complicated extensive biomolecular sheetlike structures, mostly fonned by lipid molecules held together by cooperative nonco-valent interactions. A membrane is not a static structure, but rather a complex dynamical two-dimensional liquid crystalline fluid mosaic of oriented proteins and lipids. A number of experimental approaches can be used to investigate and characterize biological membranes. However, the complexity of membranes is such that experimental data remain very difficult to interpret at the microscopic level. In recent years, computational studies of membranes based on detailed atomic models, as summarized in Chapter 21, have greatly increased the ability to interpret experimental data, yielding a much-improved picture of the structure and dynamics of lipid bilayers and the relationship of those properties to membrane function [21]. 

Surface evaporation can be a limiting factor in the manufacture of many types of products. In the drying of paper, chrome leather, certain types of synthetic rubbers and similar materials, the sheets possess a finely fibrous structure which distributes the moisture through them by capillary action, thus securing very rapid diffusion of moisture from one point of the sheet to another. This means that it is almost impossible to remove moisture from the surface of the sheet without having it immediately replaced by capillary diffusion from the interior. The drying of sheetlike materials is essentially a process of surface evaporation. Note that with porous materials, evaporation may occur within the solid. In a porous material that is characterized by pores of diverse sizes, the movement of water may be controlled by capillarity, and not by concentration gradients. 

Plant cell walls are made of bundles of cellulose chains laid down in a cross-hatched pattern that gives cellulose strength in all directions. Hydrogen bonding between the chains gives cellulose a sheetlike structure. 

When three of the oxygens in the tetrahedra are shared (Si O ratio = 2 5), the complex ions that form take on a sheetlike configuration. The sheets can be stacked, and the associated cations are found between the sheets. Micas and clays are sheet-structure minerals with distinctive habits and physical properties, that reflect the planar silicate sheet structure (Fig. 2.1G). These normally platey minerals may also occur with fibrous-growth habits. The special crystal chemistry that produces such a distinctive habit is discussed later. 

It was suggested for the serpentines that cbmpositionally induced distortions favored the production of fibers, but how can this be accomplished in materials with a basic sheetlike structure Several specific examples of sheet stmcture minerals that occur as fibers (in addition to the micas) will be described before we examine some of these suggestions. 

Lignin is a noncellulosic resinous component of wood. It is the second most abundant renewable natural resource. It has alcohol and ether units with many aromatic units. Much of lignin is sheetlike in structure. 

The plasma membrane is a delicate, semipermeable, sheetlike covering for the entire cell. Forming an enclosure prevents gross loss of the intracellular contents the semipermeable character of the membrane permits the selective absorption of nutrients and the selective removal of metabolic waste products. In many plant and bacterial (but not animal) cells, a cell wall encompasses the plasma membrane. The cell wall is a more porous structure than the plasma membrane, but it is mechanically stronger because it is constructed of a covalently cross-linked, three-dimensional network. The cell wall maintains a cell s three-dimensional form when it is under stress. 

The Structure of the a-Keratins Was Determined with the Help of Molecular Models The fi-Keratins Form Sheetlike Structures with Extended Polypeptide Chains Collagen Forms a Unique Triple-Stranded Structure Globular Protein Structures Are Extremely Varied and Require a More Sophisticated Form of Analysis Folding of Globular Proteins Reveals a Hierarchy of Structural Organization... 

The fi-Keratins Form Sheetlike Structures with Extended Polypeptide Chains... 

P sheet. A sheetlike structure formed by the interaction between two or more extended polypeptide chains. 

The special property of surfactants in solution is that they associate into a monolayer or sheetlike structure with the water-soluble moieties (hydrophilic heads) on one side of the sheet and water-insoluble moieties on the other side [8], These sheetlike structures provide the building blocks for a rich variety of fluid microstructures, which, depending on thermodynamic... 

At an air-water interface, a monolayer forms with heads lying down and tails up (toward air), whereas at an air-hydrocarbon interface the monolayer lies with tails down. By closing on the tail side, the sheetlike structure can be dispersed in aqueous solutions as spherical, rodlike, or disklike micelles (Fig. 3). Closure on the head side forms the corresponding inverted micelles in oil. Oil added to a micellar solution is incorporated into the interior of the micelle to form a swollen micellar solution. Thus, surfactant acts to solubilize substantial amounts of oil into aqueous solution. Similarly, a swollen inverted micellar solution enables significant solubilization of water in oil. 

Micellar solutions are isotropic microstructured fluids which form under certain conditions. At other conditions, liquid crystals periodic in at least one dimension can form. The lamellar liquid crystal phase consists of periodically stacked bilayers (a pair of opposed monolayers). The sheetlike surfactant structures can curl into long rods (closing on either the head or tail side) with parallel axes arrayed in a periodic hexagonal or rectangular spacing to form a hexagonal or a rectangular liquid crystal. Spherical micelles or inverted micelles whose centers are periodically distributed on a lattice of cubic symmetry form a cubic liquid crystal. 

It appears that the role of increasing salinity is to change the mean curvature of the surfactant sheetlike structure from a value favoring closure on the oil-rich regions (swollen inverted micelles). In between, in bicontinous microemulsion having comparable amounts of oil and water, the preferred mean curvature must be near zero. 

The ideas underlying elemental structures models are to establish microstructures experimentally, to compute free energies and chemical potentials from models based on these structures, and to use the chemical potentials to construct phase diagrams. Jonsson and Wennerstrom have used this approach to predict the phase diagrams of water, hydrocarbon, and ionic surfactant mixtures [18]. In their model, they assume the surfactant resides in sheetlike structures with heads on one side and tails on the other side of the sheet. They consider five structures spheres, inverted (reversed) spheres, cylinders, inverted cylinders, and layers (lamellar). These structures are indicated in Fig. 12. Nonpolar regions (tails and oil) are cross-hatched. For these elemental structures, Jonsson and Wennerstrom include in the free energy contributions from the electrical double layer on the water... 

Widom has recently formulated a lattice model that takes into account the amphiphilic nature of surfactant and introduces molecular interactions that explicitly affect curvature of surfactant sheetlike structures [25]. 

The isomers of [Co(L-cys)(en)2](N03) AgN03 have been structurally characterized. Significantly, the stmcture derived from AL-[Co(L-cys)(en)2]+ leads to a two-dimensional sheetlike structure, while AL-[Co(L-cys)(eu)2]+ affords a oue-dimensional left-handed helix, where the left-handed heUcity is predetermined by the chiral configuration in the starting Co complex. ... 

Membranes are sheetlike structures, only two molecules thick, that form closed boundaries between different compartments. The thickness of most membranes is between 60 A (6 nm) and 100 A (10 nm). 

Biological membranes are sheetlike structures, typically from 60 to 100 A thick, that are composed of protein and lipid molecules held together by noncovalent interachons. Membranes are highly selechve permeability barriers. They create closed compartments, which may be enhre cells or organelles within a cell. Proteins in membranes regulate the molecular and ionic compositions of these compartments. Membranes also control the flow of informahon between cells. 

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