Air-water interfacial tension

Explain why the air-water interfacial tension decreases with adsorption of a surfactant at the interface. 

Surface active substances (surfactants) are chemicals which accumulate at the water surface and reduce the air-water interfacial tension. The influence of such films on air-water exchange is twofold (1) they create an additional transport barrier, and (2) they change the hydrodynamics at the water surface such that the transport of solutes by eddies approaching the water surface is reduced (hydrodynamic damping). 

The solution behavior of surfactants can be illustrated with a curve of the air-water interfacial tension (the so-called surface tension yAW) vs. surfactant concentration Cw (Figure 17.2). An increase of Cw results in a decrease of yAW at low Cw until an inflection in the curve occurs. Beyond the inflection region, increasing Cw does not result in a change of yAW. The inflection indicates saturation of the water-air interface with surfactant molecules. Additional surfactant molecules cannot adsorb to the interface and are forced to remain in the water phase. 

Extremely hygroscopic powder. Dec 166. Usually furnished as a colorless aqueous soln. Also sol in benzene. A surface-active agent a 5%aq soln (pH 6.75-7.50) has an air-water interfacial tension of 42 dynes/cm at 25 also reported as 47 dynes/cm Swan, White, Am. J. Ophthalmol. 27, 933 (1944). Atnpuled aq solns may be stored at room temp for at least One year without precipitation, darkening, change of pH. or loss of potency. Heating of aq solns at 100 causes decompn. Sterilization by filtration is recommended. therap Cat Anticholinergic. 

We measured the initial diameters of the deposited drop as a function of the CTAB concentration and observed the same trend as for the air/water interfacial tension (Fig. 4). Below the critical micellar concentration (cmc = 0.9 mM), 7w/a increases as does the initial diameter. After the cmc, both 7 /a nd the initial... 

When an oil droplet within a liquid film arrives at one of the surfaces of the film, it may or may not enter the surface (see Fig. 10) [14,15,37]. It is possible that a metastable thin aqueous film is formed between the oil drop and air. Assuming that this film, if present, can be ruptured in some way, then in an equilibrated system it is thermodynamically feasible for the drop to enter the surface if the sum of the equilibrium oil/water and air/water interfacial tensions (yow and yaw respectively) exceeds that of the oil/air interface (yoa). For this reason it is usual to define an equilibrium spreading coefficient, 5w,oa for water spreading on the oiFair interface as... 

Thus, adding surfactants to minimize the oil-water and solid-water interfacial tensions causes removal to become spontaneous. On the other hand, a mere decrease in the surface tension of the water-air interface, as evidenced, say, by foam formation, is not a direct indication that the surfactant will function well as a detergent. The decrease in yow or ysw implies, through the Gibb s equation (see Section III-5) adsorption of detergent. 

Suppose that water is not originally in contact with the solid surface and adheres to it (i.e., adhesional wetting). When the drop of water is laid on a flat, smooth, solid surface, three forces are at work surface tension between the solid and air, ySA interfacial tension between the solid and water, ysw and surface tension between water and air, yWA. Incorporating only the horizontal components of these forces leads to ... 

In conclusion we will note that the main difference between aqueous emulsion films and foam films involves the dependences of the various parameters of these films (potential of the diffuse double electric layer, surfactant adsorption, surface viscosity, etc.) on the polarity of the organic phase, the distribution of the emulsifier between water and organic phase and the relatively low, as compared to the water/air interface, interfacial tension. 

Fig. 19. Dynamic surface tension of a cloud-water sample, at constant air/water interfacial area, as observed a few seconds after formation of a bubble inside the liquid phase. Temperature T = 20 °C. ...

Kellay et al. [42, 67] attribute the correlation between the maximum film thickness and the minimum oil-water interfacial tension in this decane-aqueous AOT system to the presence of thermally induced fluctuations at the oil-water surface. The maximum film thickness of 7 nm suggests that alkane molecules form a film that separates a probably weakly interdigitated monolayer from the air. Kellay et al. [42, 67] assume this film has a tension equal to that of the bulk oil-air surface. We then have a thin film with one surface having a high surface tension (the oil-air... 

The film formation process is extremely complex, and there are a number of theories — or more accurately, schools of theories — to describe it. A major point of difference among them is the driving force for particle deformation surface tension of the polymer particles. Van der Waals attraction, polymer-water interfacial tension, capillary pressure at the air-water interface, or combinations of the above. These models of the mechanism of latex film formation are necessary in order to improve existing waterborne paints and to design the next generation. To improve the rate of film fonnation, for example, it is important to know if the main driving force for coalescence is located at the interface between polymer and water, between water and air, or between polymer particles. This location determines which surface tension or surface energies should be optimized. 

Gauthier and colleagues have pointed out that polymer-water interfacial tension and capillary pressure at the air-water interface are expressions of the same physical phenomenon and can be described by the Young and Laplace laws for surface energy [5]. The fact that there are two minimum film formation temperatures, one wet and one "dry," may be an indication that the receding polymer-water interface and evaporating interstitial water are both driving the film formation (see Section 3.4). 

In order to obtain foams and emulsions, a high amount of energy is required so as to create free air-water interfacial area and oil-water interfacial area, respectively. This energy equals the free energy of the system (yA, where y is the interfacial tension and A is the interfacial area). Since the area... 

The Krafft point, the area occupied by a surfactant molecule at the air-water and octane-water interfaces, and the aggregation number of micelles increase with an increase in fluorocarbon and/or hydrocarbon chain length of these hybrid surfactants [209]. The cmc, surface tension, and octane-water interfacial tension at the cmc decrease with an increase in fluorocarbon and/or hydrocarbon chain length. The surfactants lower the surface tension, as well as the hydrocarbon oil-water and the water-fluorocarbon oil interfacial tension. Their solutions in water can float on hydrocarbon liquids, such as benzene, cyclohexane, and decane. The 10% solution of the surfactant with m = 6 and n = 4 chains exhibit rubberlike viscoelasticity [210]. 

Pure water does not penetrate spontaneously into hydrophobized quartz capillaries however, surfactant solutions penetrate spontaneously, and the penetration rate depends on the concentration of surfactant. Both the air-Uquid interfacial tension, y, and the contact angle of the moving meniscus, are concentration dependant, where a subscript a indicates the advancing contact angle. 

TABLE 19 Efficiencies and Effectiveness of Water-Air Interfacial Tension Reduction of Alcohol and Alcohol Ether Sulfates... 

The presence of calcium and magnesium ions increases the adsorption of the surfactants at the water-air interface and leads to a corresponding lowering of the surface tension at the CMC as shown by the data in Table 4. A C16 branched AOS gives a lower surface tension than a linear C16 AOS this too is in agreement with other model studies and theoretical predictions [42, and Sec. 2 on interfacial tension). 

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