Water molecules surface tension

All molecules that, when dissolved in water, reduce surface tension are called surface-active substances (e.g., soaps, surfactants, detergents). This means that such substances adsorb at the surface and reduce surface tension. The same will happen if a surface-active substance is added to a system of oil-water. The interfacial tension of the oil-water interface will be reduced accordingly. Inorganic salts, on the other hand, increase the surface tension of water. 

Soap or detergent molecules align themselves at the surface of liquid water so that their nonpolar tails can escape the polarity of the water. This arrangement disrupts the water s surface tension. 

The free surface of pure liquids. There is some evidence in favour of a tendency for the hydrocarbon ends of molecules to be oriented outwards when one end of the molecule is hydrocarbon in character and the other has a greater residual affinity.1 The most direct evidence is gained by comparing the work of cohesion, or twice the surface tension (Chap. I, 8), of compounds of related constitution with their work of adhesion to water. The surface tension is half the work that must be done in order to pull apart a bar of the liquid of 1 sq. cm. cross-section, for 2 sq. cm. of fresh surface are formed in this operation. The work of cohesion therefore measures the intensity of the attraction between two free surfaces of the same liquid about to come into contact. 

The theory that liquid water is a mixture of polymerised water molecules, or contains dissolved ice, apparently first suggested by Rowland and by Whiting,2 was independently put forward by J. J. Thomson,3 Rontgen, and Sutherland.3 The ice molecule is supposed to be more complex than the liquid but less dense. This theory has been supported by appeal to the very abnormal properties of liquid water (high surface tension and dielectric constant, great ionising power for solutes, high b.p. as compared with expansion on... 

A qualitative model for lubrication of the condensed alcohol layer is not fully developed yet. We present here the best qualitative model based on the literature. As the applied pressure is very large and the scanning speed is very low in AFM, one can rule out the hydrodynamic lubrication even in the presence of condensed alcohol layer on the substrate. The AFM tip and substrate interface must be in the boundary lubrication regime. Figs. 7-10 show that the adhesion force reduction upon alcohol adsorption is accompanied by reduction of the friction force. Part of the adhesion force reduction is because of the surface tension decrease of the water layer on the substrate. When alcohol is dissolved in water, the surface tension decreases significantly because of segregation of alcohol molecules to the liquid-air interface. ... 

Micellar dispersions, which contain micelles along with individual surfactant molecules, are the typical examples of lyophilic colloidal systems. Micelles are the associates of surfactant molecules with the degree of association, represented by aggregation number, i.e. the number of molecules in associate, of 20 to 100 and even more [1,13,14]. When such micelles are formed in a polar solvent (e.g. water), the hydrocarbon chains of surfactant molecules combine into a compact hydrocarbon core, while the hydrated polar groups facing aqueous phase make the hydrophilic shell. Due to the hydrophilic nature of the outer shell that screens hydrocarbon core from contact with water, the surface tension at the micelle - dispersion medium interface is lowered to the values othermodynamic stability of micellar systems with respect to macroscopic surfactant phases. 

In general, the stronger the attractions between particles, the greater the surface tension. Water has a high surface tension because its molecules can form multiple hydrogen bonds. Drops of water are shaped like spheres because the surface area of a sphere is smaller than the surface area of any other shape of similar volume. Waters high surface tension is what allows the spider in Figure 12.16 to walk on the surface of the pond. 

A schematic comparison of the two situations is shown in Figure 8.11. On the left, a reservoir that has a movable barrier (B) is filled with pure water. The surface tension of the water on each side of the barrier will be the same (oo). If a quantity of surfactant is added to one side of the reservoir, time is allowed for the system to reach equiUbrium, and the surface tension of each side is measured (ol and or), one finds that ol = or. Surfactant molecules... 

The defining characteristic of surfactants is their ability to lower surface tension at the air-water interface. Surface tension results from an imbalance in intermolecular forces at the surface of a liquid. There are fewer molecules on the vapour side than on the liquid side of molecules near the surface, leading to a net repulsive force and hence a gradual decrease in density (Fig. 4.5). Surface tension, y, can be defined in two equivalent ways. First, in terms of... 

They are long molecules with hydrophilic head and a hydrophobic tail. Figure 5.14(a) shows the distribution of surfactants in a liquid drop. The surfactants gather on the interface between the liquid and the surrounding gas due to its amphiphilic nature. The presence of surfactant lowers the surface tension of the liquid. When the interface gets saturated with surfactants above a critical concentration, the surfactant molecules form micelles in the bulk of the fluid. Note that pure water has surface tension equal to 72 mN/m, which reduces to 30 mN/m at the critical concentration limit. Figure 5.14(b) shows the variation of surface tension as a function of the surfactant concentration. The surface tension value remains constant after a critical surfactant concentration. 

These surface active agents have weaker intermoiecular attractive forces than the solvent, and therefore tend to concentrate in the surface at the expense of the water molecules. The accumulation of adsorbed surface active agent is related to the change in surface tension according to the Gibbs adsorption equation... 

Surface tension arises at a fluid to fluid interface as a result of the unequal attraction between molecules of the same fluid and the adjacent fluid. For example, the molecules of water in a water droplet surrounded by air have a larger attraction to each other than to the adjacent air molecules. The imbalance of forces creates an inward pull which causes the droplet to become spherical, as the droplet minimises its surface area. A surface tension exists at the interface of the water and air, and a pressure differential exists between the water phase and the air. The pressure on the water side is greater due to the net inward forces... 

Fig. III-9. Representative plots of surface tension versus composition, (a) Isooctane-n-dodecane at 30°C 1 linear, 2 ideal, with a = 48.6. Isooctane-benzene at 30°C 3 ideal, with a = 35.4, 4 ideal-like with empirical a of 112, 5 unsymmetrical, with ai = 136 and U2 = 45. Isooctane-

Make a theoretical plot of surface tension versus composition according to Eq. III-53, and compare with experiment. (Calculate the equivalent spherical diameter for water and methanol molecules and take o as the average of these.)... 

Equation 9 states that the surface excess of solute, F, is proportional to the concentration of solute, C, multipHed by the rate of change of surface tension, with respect to solute concentration, d /dC. The concentration of a surfactant ia a G—L iaterface can be calculated from the linear segment of a plot of surface tension versus concentration and similarly for the concentration ia an L—L iaterface from a plot of iaterfacial teasioa. la typical appHcatioas, the approximate form of the Gibbs equatioa was employed to calculate the area occupied by a series of sulfosucciaic ester molecules at the air—water iaterface (8) and the energies of adsorption at the air-water iaterface for a series of commercial aonionic surfactants (9). 

The monolayer resulting when amphiphilic molecules are introduced to the water—air interface was traditionally called a two-dimensional gas owing to what were the expected large distances between the molecules. However, it has become quite clear that amphiphiles self-organize at the air—water interface even at relatively low surface pressures (7—10). For example, x-ray diffraction data from a monolayer of heneicosanoic acid spread on a 0.5-mM CaCl2 solution at zero pressure (11) showed that once the barrier starts moving and compresses the molecules, the surface pressure, 7T, increases and the area per molecule, M, decreases. The surface pressure, ie, the force per unit length of the barrier (in N/m) is the difference between CJq, the surface tension of pure water, and O, that of the water covered with a monolayer. Where the total number of molecules and the total area that the monolayer occupies is known, the area per molecules can be calculated and a 7T-M isotherm constmcted. This isotherm (Fig. 2), which describes surface pressure as a function of the area per molecule (3,4), is rich in information on stabiUty of the monolayer at the water—air interface, the reorientation of molecules in the two-dimensional system, phase transitions, and conformational transformations. 

Lecithin (qv), a natural phosphoHpid possessing both hydrophilic and hydrophobic properties, is the most common emulsifier in the chocolate industry (5). The hydrophilic groups of the lecithin molecules attach themselves to the water, sugar, and cocoa soflds present in chocolate. The hydrophobic groups attach themselves to the cocoa butter and other fats such as milk fat. This reduces both the surface tension, between cocoa butter and the other materials present, and the viscosity. Less cocoa butter is then needed to adjust the final viscosity of the chocolate. 

Bilayers have received even more attention. In the early studies, water has been replaced by a continuous medium as in the monolayer simulations [64-67]. Today s bilayers are usually fully hydrated , i.e., water is included exphcitly. Simulations have been done at constant volume [68-73] and at constant pressure or fixed surface tension [74-79]. In the latter case, the size of the simulation box automatically adjusts itself so as to optimize the area per molecule of the amphiphiles in the bilayer [33]. If the pressure tensor is chosen isotropic, bilayers with zero surface tension are obtained. Constant... 

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