Liquids cohesive forces

The size of the contact angle depends on the magnitude of the liquid-solid adhesive force compared with that of the liquid-liquid cohesive force. Specifically, Young s equation (also called the Young and Dupre equation) indicates that... 

The rising of a liquid against the pull of gravity through a narrow space, such as a thin tube, is called capillary action, or capillarity. Capillarity results from a competition between the intermolecular forces within the liquid (cohesive forces) and those between the liquid and the tube walls (adhesive forces). Let s look at the difference between the capillarities of water and mercury in glass ... 

Solid Dispersion If the process involves the dispersion of sohds in a liquid, then we may either be involved with breaking up agglomerates or possibly physically breaking or shattering particles that have a low cohesive force between their components. Normally, we do not think of breaking up ionic bonds with the shear rates available in mixing machineiy. 

In the limit of high viscosity, immobile liquid bridges formed from materials such as asphalt or pitch fail by tearing apart the weakest bond. Then adhesion and/or cohesion forces are Lilly exploited, and binding ability is much larger. 

The solubility parameter is calculated at 20 MPa and therefore the polymer is swollen by liquids of similar cohesive forces. Since crystallisation is thermodynamically favoured even in the presence of liquids of similar solubility parameter and since there is little scope of specific interaction between polymer and liquid there are no effective solvents at room temperature for the homopolymer. 

The greater the viscosity of a liquid, the more slowly it flows. Viscosity usually decreases with increasing temperature. Surface tension arises from the imbalance of intermolecular forces at the surface of a liquid. Capillary action arises from the imbalance of adhesive and cohesive forces. 

If the principal cohesive forces between solute molecules are London forces, then the best solvent is likely to be one that can mimic those forces. For example, a good solvent for nonpolar substances is the nonpolar liquid carbon disulfide, CS2-It is a far better solvent than water for sulfur because solid sulfur is a molecular solid of S8 molecules held together by London forces (Fig. 8.19). The sulfur molecules cannot penetrate into the strongly hydrogen-bonded structure of water, because they cannot replace those bonds with interactions of similar strength. 

Molecules in contact with the surface of their container experience two sets of intermolecular forces. Cohesive forces attract molecules in the liquid to one another. In addition, adhesive forces attract molecules in the liquid to the molecules of the container walls. 

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. 

For wetting to occur, the adhesive forces of the liquid for the solid must exceed the cohesive forces of the liquid for itself. 

For cavitation to occur in a liquid, it has to overcome the natural cohesive forces present in the liquid. Any increase in these forces will tend to increase the threshold pressure and hence the energy required to generate cavitation. In highly viscous liquids, severe attenuation of the sound intensity occurs and the active cavitating zone gets reduced substantially. Moholkar et al. [56] have confirmed this fact with experiments with different liquids and reported that for highly viscous liquids, cavitational effects are not observed. 

The fact that the pressure is negative implies that a negative pressure must be applied to overcome the cohesive forces of a liquid. Those readers interested in the derivation of this equation should turn to Appendix 2. 

Since it is necessary for the negative pressure in the rarefaction cycle to overcome the natural cohesive forces acting in the liquid, any increase in these forces will increase the threshold of cavitation. One method of increasing these forces is to increase the viscosity of the liquid. Tab. 2.1 shows the influence of viscosity on the pressure amplitude (Pft) at which cavitation begins in several liquids at 25 °C, at a hydrostatic pressure of 1 atm. 

The fact that the pressure is negative implies that a negative pressure must be applied to overcome the cohesive forces of a liquid and produce a bubble of radius R. Writing P] = Pjj — Pg allows the estimation of Pg, (known as Blake threshold pressure), which is the negative (or rarefaction) pressure which must be applied in excess of the hydrostatic pressure (Pj ) to create a bubble of radius R. e.g. for large bubbles (i.e. 2a/R, Ph)... 

In Chapter 2 we explained why there existed a cavitation threshold i. e. a limit of sound intensity below which cavitation could not be produced in a liquid. We suggested that only when the applied acoustic amplitude (P ) of the ultrasonic wave was sufficiently large to overcome the cohesive forces within the liquid could the liquid be tom apart and produce cavitation bubbles. If degradation is due to cavitation then it is expected that degradation will only occur when the cavitation threshold is exceeded. This is confirmed by Weissler who investigated the degradation of hydroxycellulose and observed that the start of degradation coincided with the onset of cavitation (Fig. 5.21). 

Hansen [28,29] expanded this theory by dividing the cohesive forces of liquids into three components—dispersive (d), polar (p) and hydrogen bonding (h) forces—and defined the three component solubihty parameter 3o as ... 

The three kinds of forces described above, collectively known as the cohesive forces that keep the molecules of liquids together, are responsible for various properties of the liquids. In particular, they are responsible for the work that has to be invested to remove molecules from the liquid, that is, to vaporize it. The energy of vaporization of a mole of liquid equals its molar heat of vaporization, Ay//, minus the pressure-volume work involved, which can be approximated well by Rr, where R is the gas constant [8.3143 J K" mol" ] and T is the absolute temperamre. The ratio of this quantity to the molar volume of the liquid is its cohesive energy density. The square root of the cohesive energy density is called the (Hildebrand) solubility parameter of the liquid, 8 ... 

The first stage is the creation of the cavity in the liquid to accommodate the solute. Obviously, work must be done against the cohesive forces of the liquid that hold its molecules together. This work should be proportional to the required size of the cavity, and increase as the volume of the solute increases. 

If the solute A does not undergo any reaction in the two solvents, except for the solubility caused by the solvation due to the nonspecific cohesive forces in the liquids, the distribution of the solute follows the Nernst distribution law, and the equilibrium reaction can be described either by a distribution constant or an (equilibrium) extraction constant... 

Table 6.2 presents data showing the effect of various CMOS on the activity coefficient or mole fraction solubility of naphthalene, for two different solvent/water ratios. To examine the cosolvent effect, Schwarzenbach et al. (2003) compare the Hildebrand solubility parameter (defined as the square root of the ratio of the enthalpy of vaporization and the molar volume of the liquid), which is a measure of the cohesive forces of the molecule in pure solvent. 

It is the cohesive forces that maintain water, for example, in the liquid state at room temperature and pressure. This becomes obvious when one compares two different molecules, such as H20 and H2S. At room temperature and pressure, H20 is... 

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