Water surface tension, surfactant

Stabilization in water (surface tension 72 mN/m) can be achieved through the use of surfactants [77] such as SDBS [78] or sodium cholate [79], or long chain polymers such as polyvinyl alcohol. While this allows a non-toxic and easily removed solvent to be used, the presence of the surfactant or polymer molecules can be detrimental to the subsequent applications [80]. 

In the wetting process, if we decrease yl of water by adding small amount of surfactant or alcohol (50 vol%), the value of A(y)sL can be reduced significantly. The values of surface tension of the liquids used in the study are listed in Table 25.1. The influence of the reduction of water surface tension on the surface configuration change of Teflon PFA was given in Figure 23.14. 

Surface chemistry and thermodynamics, Surface Tension Surfactants. Defoamers, antifoams, anti flood and float, wetting aids, emulsifiers, levelling agents, pigment dispersants. Colorant acceptor Water repellents. Numerous aniottic, non-ioruc, cationic and an hoteric agents. Silicones an hipafhic polymers. Polysiloxanes... 

The air-water surface tension decreases with increasing surfactant concentration below the CMC and... 

Acetone, IPA, and methanol were purchased on lab or from local chemical vendors and were used, as received. All chemicals were of high grade purity (>99.5% pure). Standard distilled water was used for the pure water and diluted water/methanol tests. To reduce uncertainty in the data due to a reduction in water surface tension due to the presence of impurities, the water bottles were subjected to the shake test to ensure minimum surfactant concentration. The shake test uses the property that foam fraction is an effective separation process for surfactants in aqueous solutions. Simply shake a sample of the water in a clean volumetric flask. Any bubbles will immediately break if the surfactant concentration due to contamination is ignorable. 

Even on micellar solutions of this compound, the PDMS oil forms lenses with zero spreading pressure. Similar behavior is seen with submicellar solutions even up to air-water surface tensions of 45 mN m. Since the oil spreads on water, this suggests a possible first-order wetting transition between partial wetting and pseudo-partial wetting at even higher air-water surface tensions. Rather surprisingly then, this means that no penetration of the monolayer of this silicone surfactant by the PDMS must occur. 

The rate of duplex film spreading of PDMS oils on either water or surfactant solution is given by a simple power law expression provided that, in the case of surfactant solutions, the air-water and oil-water surface tensions are maintained constant by rapid transport of surfactant to the relevant surfaces. Thus, Fay [86], and later Hoult [87], derived by dimensional analysis that the time dependence of the distance y from the origin to the spreading front, in the case of linear spreading as a duplex film, is given by... 

The polydimethylsiloxane oils used for antifoams usually spread on the air-water surfaces of surfactant solutions (see Section 3.6.2). At equilibrium, this process produces either complete wetting and duplex films for which 5 = 0 or pseudo-partial wetting and oil films in contact with lenses of bulk oil for which 5 0 (see Section 3.6.2.1). It has been shown by Racz et al. [3], and later confirmed by Denkov et al. [6, 7, 21], that deactivation of hydrophobed silica-polydimethylsiloxane antifoams correlates with the disappearance of this spread oil film. These studies used solutions of both anionic (SDS [3] and AOT [6,7]) and non-ionic surfactants (alkyl glu-copyranoside) [21]. Loss of the spread layer during deactivation is accompanied by an increase in surface tension to that of the pure surfactant solution [6]. It has also been directly observed using ellipsometry [21]. This finding is key to the understanding of deactivation because the presence of a spread layer of polydimethylsiloxane at the air-water surface is a clear indicator that oil has emerged into that surface. 

For simple case of the water-air interface (usually referred to as surface ), increasing the amphiphile concentration in the liquid phase results in an increasing number of surfactant molecules adsorbed at interface, and the water surface tension decreases accordingly. The surface tension y is an intensive thermodynamic function of state defined as the rate of increase with the area (A) of the interface of the Helmholtz (at constant volume) free energy (F) at constant temperature and composition ... 

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