Surfactants Young equation

Ruch and Bartell [84], studying the aqueous decylamine-platinum system, combined direct estimates of the adsorption at the platinum-solution interface with contact angle data and the Young equation to determine a solid-vapor interfacial energy change of up to 40 ergs/cm due to decylamine adsorption. Healy (85) discusses an adsorption model for the contact angle in surfactant solutions and these aspects are discussed further in Ref. 86. 

For the solid-liquid system changes of the state of interface on formation of surfactant adsorption layers are of special importance with respect to application aspects. When a liquid is in contact with a solid and surfactant is added, the solid-liquid interface tension will be reduced by the formation of a new solid-liquid interface created by adsorption of surfactant. This influences the wetting as demonstrated by the change of the contact angle between the liquid and the solid surface. The equilibrium at the three-phase contact solid-liquid-air or oil is described by the Young equation ... 

If Xjj is reduced by adsorption of surfactants and xs is constant, the Young equation predicts that the contact angle will be smaller, i.e., the wetting is... 

WDD requires a non-aqueous solution of an enabling surfactant. The surfactant is retained in the non-polar fluid and the dissolved aqueous salts and colloids are removed with the water. The basis of the phenomenon is illustrated by the cartoon shown in Figure 3. A water-covered hydrophilic solid is immersed in a non-polar solution containing dissolved surfactant molecules (designated as V). The water initially is a film, but increasing adsorption of surfactant at the oil/water and oil/solid interfaces rapidly causes the water to bead up. Then the water rolls off as the contact angle approaches 180. This phenomenon is controlled by the venerable Young Equation ... 

Contact angle measurements can also be used to obtain information about adsorption of surfactants on solid surfaces (in our case on fiber materials). In such a case contact angles have to be measured as a function of the surfactant concentration c. Under certain circumstances, but generally if the liquid-vapor surface tension is larger than the solid surface tension no adsorption of the liquid vapor on the solid surface takes place, and therefore the solid surface tension will stay constant with changing surfactant concentration. The Young equation [Eq. (3)] becomes therefore ... 

Such analysis is based on the theories presented in this chapter, the concept of the contact angle and the associated Young equation discussed in Chapter 4. The analysis of solid interfaces and its application in understanding wetting and adhesion will be illustrated in Chapter 6, after the concept of contact angle is presented in Chapter 4 and surfactants in Chapter 5. Theories for interfacial tension wiU be discussed in more detail in Chapter 15. 

The work of adhesion was determined from the a versus P measurements (see Eq. 11). The work of adhesion between two rubber spheres was found to be 71 4 mJ/m. The work of adhesion reduced to 6.8 0.4 mJ/m in the presence of 0.01 M solution of dodecyl sulfate. Using these measurements of adhesion between rubber in air and a surfactant solution, Johnson et al. [6] provided the first direct experimental verification of the Young s equation (Eq. 40). They also measured... 

Detergency, or the power of a detergent product to remove soil, depends on the ability of surfactants to lower the interfacial tension between different phases. This can be explained for a typical case where removal of liquid soil is aided by surfactant adsorption onto the soil and substrate surfaces from the cleaning bath (Figure 2) using Young s equation,... 

The wetting of a liquid drop placed on a solid surface has already been described (Section 3.4) by the critical surface tension of the surface and by Young s equation. Temperature is a factor in wetting by aqueous solutions since it influences surfactant solubility. For example, the fastest wetting for polyoxyethylenated non-ionic surfactants is produced by those whose cloud points are just above the use temperature [193]. 

How surface tension translates to commercial applications will now be examined. Surfactants are often added to reduce surface tension of a liquid enabling it to wet a surface and the equation governing this phenomenon is attributed to Young [3] ... 

A variant is the micro-pipette method, which is also similar to the maximum bubble pressure technique. A drop of the liquid to be studied is drawn by suction into the tip of a micropipette. The inner diameter of the pipette must be smaller than the radius of the drop the minimum suction pressure needed to force the droplet into the capillary can be related to the surface tension of the liquid, using the Young-Laplace equation [1.1.212). This technique can also be used to obtain interfacial tensions, say of individual emulsion droplets. Experimental problems include accounting for the extent of wetting of the inner lumen of the capillary, rate problems because of the time-dependence of surfactant (if any) adsorption on the capillary and, for narrow capillaries accounting for the work needed to bend the interface. Indeed, this method has also been used to measure bending moduli (sec. 1.15). 

A general relationship between contact angle, surfactant concentration, and specific adsorption can be obtained by differentiating Young s equation with respect to In c... 

A variation of Equation (17.44) useful for predicting whether the addition of a surfactant will improve wetting is to differentiate Young s equation and combine with the Gibbs equation to yield... 

The adsorption of surfactant at the solid/liquid interface also lowers ysL- From Young s equation,... 

Equilibrium will occur when the presence of a surface-active agent does not lower Y any further. This concentration is typically at the critical micelle concentration [47]. Modification of fhe confacf angle (and fherefore emulsion sfability) can be achieved by a modification of the aqueous, oil, or solid phase so as to alter Yow, Yos, or Ysw- As described later, this can be achieved with the use of surfactants. The interfacial tension and surface tension are measurements of droplet deformation. Neither Ysw nor Yso can be directly measured, because the solid cannot be deformed. To solve Young s equation and to determine the solid/water and solid/oil interfacial surface tensions, the equation-of-state approach for interfacial surface tensions is required [48] ... 

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