Contact angle, surface tension and

Detergents may be produced by the chemical reaction of fats and fatty acids with polar materials such as sulfuric or phosphoric acid or ethylene oxide. Detergents emulsify oil and grease because of their abiUty to reduce the surface tension and contact angle of water as well as the interfacial tension between water and oil. Recent trends in detergents have been to lower phosphate content to prevent eutrification of lakes when detergents are disposed of in municipal waste. 

One may also be able to determine the work of adhesion for cases in which the contact angle is zero by using probe liquids, as described later in this chapter. There are also other ways of determining the work of adhesion, such as inverse gas chromatography, which do not depend solely on capillary measurements (surface tension and contact angle). This too will be discussed later. 

As can be seen in Table 6.5, ONB in APG solution of concentration C = 100 ppm took place at significantly higher surface temperatures. It should be noted that the ONB in surfactant solutions may not be solely associated with static surface tension Sher and Hetsroni (2002). Other parameters such as heat flux, mass flux, kind of surfactant, surface materials, surface treatments, surface roughness, dynamic surface tension and contact angle need to be considered as well. 

This shows that, to remove a liquid from a solid surface, surface tension and contact angle 0 are required. If the liquid wets the solid surfaces (water on glass 0 = 0),... 

Hiemenz, P. C., Rajagopalan, R. (1997). Surface tension and contact angle. In P. C. Hiemenz (Ed.), Principles of colloid and surface chemistry (pp. 248-255). Marcel Dekker, New York. 

Cipriano, B.H., Raghavan, S.R., and McGuiggan, RM., Surface tension and contact angle measurements of a hexadecyl imidazolium surfactant adsorbed on a clay surface. Colloids and Surfaces A Physicochem. Eng. Aspects, 262, 8, 2005. 

As we see in Chapter 6, surface tension and contact angle measurements provide information on liquid-liquid and solid-liquid adhesion energies (Fig. 1.26c). Contact angles measured under different atmospheric environments or as a function of time provide valuable insights into the states of surfaces and adsorbed films and of molecular reorientation times at interfaces. 

Surface tension and contact angle, wetting phenomena, effects of the curvature of the surface on capillarity and phase equilibria, and porosimetry (Chapter 6)... 

Surface tension and contact angle, however, are two different things, although they are... 

Surface tension and contact angle phenomena play a major role in many practical things in life. Whether a liquid will spread on a surface or will break up into small droplets depends on the above properties of interfaces and determines well-known operations such as detergency and coating processes and others that are, perhaps, not so well known, for example, preparation of thin films for resist lithography in microelectronic applications. The challenge for the colloid scientist is to relate the macroscopic effects to the interfacial properties of the materials involved and to learn how to manipulate the latter to achieve the desired effects. Vignette VI provides an example. 

Our major objectives in this chapter are to define surface tension and contact angle and describe how they are measured and what effects they have on the equilibrium behavior of materials. 

As mentioned in Section 6.1a, surface tension and contact angle determine wetting phenomena we examine this in Section 6.6. We take a closer look at the definition of contact angle and some complications associated with it in Section 6.7. 

Finally, we close the chapter with a discussion of the relation between molecular interactions and surface tension and contact angle. 

SURFACE TENSION AND CONTACT ANGLE A FIRST LOOK 6.2a Surface Tension as a Force... 

The situation shown in Figure 6.2b is one in which surface tension and contact angle considerations pull a liquid upward in opposition to gravity. A mass of liquid is drawn up as if it were suspended by the surface from the supporting walls. At equilibrium the upward pull of the surface and the downward pull of gravity on the elevated mass must balance. This elementary statement of force balance applies to two techniques by which 7 can be measured if 6 is known the Wilhelmy plate and capillary rise. 

Figure 6.3a represents a thin vertical plate suspended at a liquid surface from the arm of a tared balance. For simplicity, the plate is positioned so that the lower edge is in the same plane as the horizontal surface of the liquid away from the plate as shown in the figure. The manifestation of surface tension and contact angle in this situation is the entrainment of a... 

It is impossible to complete a discussion of the measurement of surface tension without saying something about the need for extreme cleanliness in any determination of 7. Any precision chemical measurement requires attention to this consideration, but surfaces are exceptionally sensitive to impurities. It is often noted that touching the surface of 100 cm2 of water with a fingertip deposits enough contamination on the water to introduce a 10% error in the value of 7. Not only must all pieces of equipment be clean, but also the experiments must be performed within enclosures or in very clean environments to prevent outside contamination. In addition, both surface tension and contact angle should be measured under constant temperature conditions. 

EXAMPLE 6.3 Determination of Heat of Immersion from Surface Tension and Contact Angle. Estimate the heat of immersion for the system for which y and 6 are 22 mJ m 2 and 30°,... 

In addition to the methods discussed here and in Section 6.2, there are a few other methods for measuring surface tension that are classified as dynamic methods as they involve the flow of the liquids involved (e.g., methods based on the dimensions of an oscillating liquid jet or of the ripples on a liquid film). As one might expect, the dynamic methods have their advantages as well as disadvantages. For example, the oscillating jet technique is ill-suited for air-liquid interfaces, but has been found quite useful in the case of surfactant solutions. A discussion of these methods, however, will require advanced fluid dynamics concepts that are beyond our scope here. As our primary objective in this chapter is simply to provide a basic introduction to surface tension and contact angle phenomena, we shall not consider dynamic methods here. Brief discussions of these methods and a comparison of the data obtained from different techniques are available elsewhere (e.g., see Adamson 1990 and references therein). 

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