Water molecules cohesion

Surface tension is responsible for several interesting phenomena. For example, consider how water beads up on a freshly waxed car. The water is a highly polar substance, and the wax is highly nonpolar. Since the polar water is not attracted to the nonpolar wax, there are few adhesive forces between the water and the waxed surface. Adhesive forces cause different substances to stick to each other. However, the surface tension in water results in significant cohesive forces between the water molecules. Cohesive forces, or cohesion, cause a substance to be more attracted to itself. The height of the bead depends on the wax surface and the surface tension of water. 

One result of adhesive forces is the curved surface of a liquid, called a meniscus. As Figure 11-18 shows, water in a glass tube forms a concave meniscus that increases the number of water molecules in contact with the walls of the tube. This is because adhesive forces of water to glass are stronger than the cohesive forces among water molecules. 

In its solid state, however, the basic structural features of ordinary hexagonal ice (ice I) are well established. In this structure (Figure 1.2), each water molecule is hydrogen bonded to four others in nearly perfect tetrahedral coordination. This arrangement leads to an open lattice in which intermolecular cohesion is large. 

Normal lungs, however, produce a chemical substance referred to as pulmonary surfactant. Made by alveolar type II cells within the alveoli, surfactant is a complex mixture of proteins (10 to 15%) and phospholipids (85 to 90%), including dipalmitoyl phosphatidyl choline, the predominant constituent. By interspersing throughout the fluid lining the alveoli, surfactant disrupts the cohesive forces between the water molecules. As a result, pulmonary surfactant has three major functions ... 

A nonpolar neutral species in a polar medium such as water experiences interfacial tension. Solvophobic theory is a general statement of hydrophobic theory, which has been developed to explain the tendency of neutral organic species to flee the water phase. It has been reported that the solution of nonelectrolytes in water is attended by a net decrease in entropy [65,158]. This has been attributed to an increased structuring of water molecules in the vicinity of the solute. The process may be conceptually rationalized by considering that a solute must occupy space in a cohesive medium. The solute must create a cavity in the water milieu and then occupy that cavity [19,65,158]. The very high cohesive density of water creates considerable interfacial tension in the... 

Certain chemicals have the ability to lower the surface tension of water. This allows water to spread out over a surface rather than bead up. Wetting agents decrease the cohesive forces between water molecules, and this helps water to spread over the surface of an object by adhesive force. 

The electronic configuration of the linear water molecule is (2o +)2( 1 ou+ )2( 1 nj4 [the ls2(0) pair of electrons occupy the lc + molecular orbital]. The 2ag+ pair of electrons is only weakly bonding, and the 1gm+ pair supplies practically the only cohesion for the three atoms, the other four electrons being non-bonding. 

Hydrogen bonds between water molecules provide the cohesive forces that make water a liquid at room temperature and that favor the extreme ordering of molecules that is typical of crystalline water (ice). Polar biomolecules dissolve readily in water because they can replace water-water interactions with more energetically favorable water-solute interactions. In contrast, nonpolar biomolecules interfere with water-water interactions but are unable to form water-solute interactions— consequently, nonpolar molecules are poorly soluble in water. In aqueous solutions, nonpolar molecules tend to cluster together. 

In the presence of each other, antigens and antibodies agglutinate (bind). The forces that bind them are principally due to hydrogen bonding and hydrophobic interactions. The hydrostatic pressure exerted by surrounding water molecules also ensures cohesion. 

In this section, we explore how water molecules in the liquid phase interact with one another via cohesive forces, which are forces of attraction between molecules of a single substance. For water, the cohesive forces are hydrogen bonds. We also explore how water molecules interact with other polar materials, such as glass, through adhesive forces, forces of attraction between molecules of two different substances. 

Cohesive and adhesive forces involving water are dynamic. It is not one set of water molecules, for example, that holds a droplet of water to the side of a glass. Rather, the billions and billions of molecules in the droplet all take turns binding with the glass surface. Keep this in mind as you read this section and examine its illustrations, which, though informative, are merely freeze-frame depictions. 

The meniscus of a liquid is the curved surface it forms in a narrow tube (Fig. 5.18). The meniscus of water in a glass capillary is curved upward at the edges (forming a concave shape) because the adhesive forces between water molecules and the oxygen atoms and —OH groups that are present on a typical glass surface are stronger than the cohesive... 

This cohesiveness produces a high surface tension, and all but the most polar or ionic solutes experience a hydrophobic force that drives them onto the stationary phase and causes retention. Most chemists are not as familiar with hydrophobic effects as with polar interactions, since the former have only recently been discussed in the chromatographic literature (1-6). The hydrophobic effect results from the strong attractive forces between water molecules. The "structure" of the water must be distorted or disrupted when a solute is dissolved. We can think of the solute as forming a cavity in the water. Highly polar or ionic solutes can themselves interact strongly with water, compensating for this distortion, and are thus easily solvated. 

Another type of force that can occur is known as adhesion, or adhesive force. These are the forces that bind different substances together. An example of adhesive forces (other than adhesive tape ) is the way that water adheres to different materials. When you first learned how to read a graduated cylinder, you had to learn to compensate for the meniscus, or the curved part on the surface of the liquid. The meniscus forms when you put water into a graduated cylinder because the adhesive force between the water molecules and the glass is stronger than the cohesive forces holding the water molecules together. 

The cohesion within water that results from these transitory hydrogen bonds allows water to move upward in plants against the force of gravity. Where water and another, less dense, substance meet, the bonds between water molecules contribute to surface tension. 

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