Water attraction

Cost is always a factor in selecting a component for a pyrotechnic composition, as is the water-attracting abiHty of a substance (hygroscopicity). In general, substances that attract water from the atmosphere are avoided in pyrotechnic formulations for a variety of reasons, thereby supporting the centuries-old adage, "Keep your powder dry."... 

If three oxygens of each tetrahedron are shared, sheet struetures form (Fig. 16.4d). This is the basis of clays and micas. The additional M attaches itself preferentially to one side of the sheet - the side with the spare oxygens on it. Then the sheet is polarised it has a net positive charge on one surface and a negative charge on the other. This interacts strongly with water, attracting a layer of water between the sheets. This is what makes clays plastic the sheets of silicate slide over each other readily, lubricated... 

Having disguised each particle of oil or grease, it can readily enter solution while sheathed in its water-attracting overcoat of surfactant. And if the oil particles enter the solution, then the oil is removed from the plate, and is cleaned. 

Molecules possessing distinct hydrophobic (water-repelling) and hydrophilic (water-attracting) regions. The terms surface-active agents , detergents , and amphiphiles are often used synonymously with the term surfactants . 

Figure 11.10 A positively charged oxide particle in water attracts anionic species including organic ones (e.g., i ) to the near-surface water. Some of these anionic species may also react with the surface, displacing other ligands (e.g., H20 or OH ), to form surface-bound sorbate. M in the solid refers to atoms like Si, Al, or Fe.

Ion-dipole attractions are much weaker than ionic bonds. However, a large number of ion-dipole attractions can act collectively to disrupt an ionic bond. This is what happens to sodium chloride in water. Attractions exerted by the water molecules break the ionic bonds and pull the ions away from one another. 

Cotton, by contrast, is made of the polysaccharide cellulose, which has many water-attracting polar hydroxyl groups. Cotton, therefore, will soak up water much more readily than will silk. 

Chapter 2). However, AH is distinctly negative for association of heterocyclic bases. This has also been attributed to a decrease in the ordering of solvent around the bases as a result of exclusion of water. Attraction or repulsion of partial charges on the polar groups comprising the purine and pyrimidine bases may also be an important factor.43 46 8 For addition of a base pair to an RNA helix, the change in aithalpy, AH, varies from about -24 to -60 kj/mol.44,45... 

FIGURE 8.17 The hydrocarbon tails of a soap or surfactant molecule dissolve in grease, leaving the water-attracting head groups on the surface where they can interact favorably with water. As a result, the whole blob can dissolve in water and be washed away. 

The intermolecular attractions between the hydrocarbon chains in the interior of the micelle represent an energetically favourable situation but it is not one which is significantly more favourable than that which results from the alternative hydrocarbon-water attraction in the case of single dissolved surfactant molecules. Comparison of the surface tension of a typical hydrocarbon oil with the dispersion component of the surface tension of water (as discussed on page 67) illustrates this point. 

The method used to prepare the solid surface may affect the contact angle for example, substances which have crystallised in contact with water often have a lower contact angle than if they crystallise in air, owing to the orientation of the water-attracting groups outwards in the former case. Penetration and entrapment of traces of water in the surface layers, which decreases 0, is also probable in these circumstances. 

Formation of 2,7-octadien-l-ol (136) by the reaction of water attracts attention as a new commercial process for -octanol (155). The Pd-catalysed reaction of water under... 

Hydrophilic Water-soluble or water-attracting molecules or systems that strongly interact with water. 

Detergents have water-attracting (hydrophilic) groups on one end of the molecule and water-repelling (hydrophobic) groups on the other. 

Looking at the barrier in more detail, we find that it can be described as composed of two main components. Interspersed between the corneocytes we find the hydrophobic (water-repellent) substance, the barrier lipids. The keratinized corneocytes containing fibrous and amorphous proteins represent a hydrophilic (water-attracting) component. Neutral lipids (fatty acids, cholesterol) and ceramides dominate the lipid phase, and it is mainly these lipids that are responsible for the control and limitation of water transport through the skin.14 Visualization of the penetration pathway through the skin by tracer methods has demonstrated that the extracellular pathway is likely to be the only route through the barrier for substances other than water.15 Water diffusion through the keratinocytes... 

Bile salts are substances derived from sterols, which make up a substantial part of the solid matter in bile and which play a central role in lipid absorption, by virtue of their surface-active properties. The structure and properties of these salts have been reviewed by Haslewood (305) and Heaton (316). Bile salts essentially have molecules of detergent type hydrocarbon, with a fat-dissolving part and a polar, water-attracting part. The fat-dissolving part consists of the bulk of the steroid nucleus. The hydroxyl groups are so distributed that hydration can readily take place the remainder of the molecule will dissolve the fatty phase. Emulsification of fat/water complexes can thus occur easily. The terms bile acid and bile salt are used somewhat interchangeably in the literature. 

Amphoteric surfactants have both a positive and a negative component at the water-attracting end. Amphoteric surfactants are used in washing-up liquids, but rarely in laundry detergents because they cost too much. The best known amphoteric surfactants are the cocamides. 

Pentaerythritol tetrapalmitate, an octopus-like molecule with four water-attracting groups, each esterified by a long chain and arranged symmetrically round a central carbon atom,... 

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