Surface tension and foams

It can be seen that surfactants which adsorb at the solid-water and dirt-water interfaces will be the best detergents. Adsorption at the air-water interface with the consequent lowering of surface tension and foaming is not necessarily an indication of detergent effectiveness for example, non-ionic detergents usually have excellent detergent... 

Tamura, T Y. Kaneko M. Ohyama. Dynamic surface tension and foaming properties of aqueous polyoxyethylene n-dodecyl ether solutions./. Coll. Interf. Sci. 1995, 173, 493-499. 

Fluorinated surfactants are effective in a variety of FOR techniques including (1) improving subterranean wetting, (2) increasing foam stability, and (3) modifying the surface properties of the reservoir formation by lowering surface tension and foaming properties to well-stimulation additives [69-72]. Both fluorotelomer [69]... 

Kitabatake N, Doi E. Surface tension and foaming of protein solutions. J Eood Sci 1982 47 1218-1225. 

Influence of Packing Shape, Irrigated Packed Beds, High Liquid Rate Performance, Liquid Holdup in Packed Beds, Pressure Drop Calculation, Effects of Surface Tension and Foaming, Concurrent Flow Operation, Cross-Flow Operation, Example Problem, Notation, References... 

Regarding the systems used in this study, we use the same proteins as in the previous section (whole casein and P -casein) and they are mixed with Tween 20, respectively. This is a low molecular weight surfactant used in the food industry, which is water soluble and nonionic. The different behavior of these two mixed systems is again discussed on the basis of fundamental magnitudes such as surface tension and foam film thickness. 

Fig. 3. Two-dimensional schematic illustrating the distribution of Hquid between the Plateau borders and the films separating three adjacent gas bubbles. The radius of curvature r of the interface at the Plateau border depends on the Hquid content and the competition between surface tension and interfacial forces, (a) Flat films and highly curved borders occur for dry foams with strong interfacial forces, (b) Nearly spherical bubbles occur for wet foams where...

The stabihty of a single foam film can be explained by the Gibbs elasticity E which results from the reduction ia equiUbrium surface concentration of adsorbed surfactant molecules when the film is extended (15). This produces an iacrease ia equiUbrium surface tension that acts as a restoring force. The Gibbs elasticity is given by equation 1 where O is surface tension and is surface area of the film. 

In general, organic contaminants induce foaming and inorganics increase surface tension, although clearly there are exceptions. For example, sugar increases surface tension, while tannins, lignosulfonates, car-boxymethyl cellulose (CMC), phosphinocarboxylic acids (PCAs), and other dispersants reduce surface tension and help destabilize foams. 

Schulze [51] described an extensive study on C12-C14 ether carboxylic acid sodium salt (4.5 mol EO) in terms of surface tension, critical micelle concentration (CMC), wetting, detergency, foam, hardness stability, and lime soap dispersing properties. He found good detergent effect compared to the etho-xylated C16-C18 fatty alcohol (25 mol EO) independent of CaCl2 concentration, there was excellent soil suspending power, low surface tension, and fewer Ca deposits than with alkylbenzenesulfonate. 

Monoamidotriphosphate compounds have been evaluated for their combined detergent-sequestrant action [65,66]. Good surfactant properties are also attributed to organoaminodialkylenephosphonic acids. Typical compounds of this kind are the tetra- and trialkali salts of decyl-, dodecyl-, and tetradecylaminodi (methylphosphonate). Values of surface tension and detergency are given in Refs. 118 and 216-219. Wash test results, foam behavior, wetting performance, and surface tensions of aqueous solutions of phosphate esters have been tabulated [12,17,18,33,37,50,52,56,90,220]. 

It is common observation that a liquid takes the shape of a container that surrounds or contains it. However, it is also found that, in many cases, there are other subtle properties that arise at the interface of liquids. The most common behavior is bubble and foam formation. Another phenomena is that, when a glass capillary tube is dipped in water, the fluid rises to a given height. It is observed that the narrower the tube, the higher the water rises. The role of liquids and liquid surfaces is important in many everyday natural processes (e.g., oceans, lakes, rivers, raindrops, etc.). Therefore, in these systems, one will expect the surface forces to be important, considering that the oceans cover some 75% of the surface of the earth. Accordingly, there is a need to study surface tension and its effect on surface phenomena in these different systems. This means that the structures of molecules in the bulk phase need to be considered in comparison to those at the surface. 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

High Viscosity and Surface Tension Bravo (Paper presented at the AIChE Spring National Meeting, Houston, Tex., 1995) studied a system that had 425-cP viscosity, 350 mN/m surface tension, and a high foaming tendency. He found that efficiencies were liquid-phase-controlled and could be estimated from theoretical HTU models. Capacity was less than predicted by conventional methods which do not account for the high viscosity. Design equations for orifice distributors extended well to the system once the orifice coefficient was calculated as a function of the low Reynolds number and the surface tension head was taken into account. 

For example, in mineral flotation, surfactant can be added to adsorb on metal ore particles, increasing the contact angle, so they attach to gas bubbles, but the surfactant does not adsorb much on silicates, so these do not attach to gas bubbles. The surfactant may also stabilize a foam containing the desired particles facilitating their recovery as a particle-rich froth that can be skimmed. Flotation processes thus involve careful modification of surface tension and wettability. 

Although many factors, such as film thickness and adsorption behaviour, have to be taken into account, the ability of a surfactant to reduce surface tension and contribute to surface elasticity are among the most important features of foam stabilization (see Section 5.4.2). The relation between Marangoni surface elasticity and foam stability [201,204,305,443] partially explains why some surfactants will act to promote foaming while others reduce foam stability (foam breakers or defoamers), and still others prevent foam formation in the first place (foam preventatives, foam inhibitors). Continued research into the dynamic physical properties of thin-liquid films and bubble surfaces is necessary to more fully understand foaming behaviour. Schramm et al. [306] discuss some of the factors that must be considered in the selection of practical foam-forming surfactants for industrial processes. 

The effect of temperature satisfies the Arrhenius relationship where the applicable range is relatively small because of low and high temperature effects. The effect of extreme pH values is related to the nature of enzymatic proteins as polyvalent acids and bases, with acid and basic groups (hydrophilic) concentrated on the outside of the protein. Finally, mechanical forces such as surface tension and shear can affect enzyme activity by disturbing the shape of the enzyme molecules. Since the shape of the active site of the enzyme is constructed to correspond to the shape of the substrate, small alteration in the structure can severely affect enzyme activity. Reactor s stirrer speed, flowrate, and foaming must be controlled to maintain the productivity of the enzyme. Consequently, during experimental investigations of the kinetics enzyme catalyzed reactions, temperature, shear, and pH are carefully controlled the last by use of buffered solutions. 

The gas holdup in a slurry reactor depends upon superficial gas velocity, power consumption, the surface tension and viscosity of the liquids, and the solids concentration. For the first three parameters, the relationship cg oc yO.36-o.75pO.26-o.470.o.36-o.65 holds. For low solids concentration and waterlike liquids, the relationship eg = f(P/V, ug) is useful, although the nature of such a relationship depends upon the foaming characteristics of the liquids. An increase in solids concentration decreases gas holdup, whereas an increase in viscosity first increases and then decreases the gas holdup. A decrease in surface tension and an increase in stirrer speed increases the gas holdup. 

The best fit between experimental results and theory is achieved when both the change in hydrostatic pressure along the height of the forming bubble at the moment of its detachment from the capillary orifice and the expansion of bubble during its rising are taken into account. Surface tension and density of foaming solution (see Eq. (1.9)) determine the size of bubbles when they are formed slowly. The surfactant kind and concentration affect both the rate of formation of adsorption layers at bubble surface and the stability of foam obtained. 

There are several works [1,14,23,61] treating qualitatively the influence that foam structure and foaming solution properties, such as viscosity, surface tension and temperature, exert on the drainage rate. A possibility to treat quantitatively the role of various factors on drainage rate of low expansion ratio foams in gravitational field is given by Eq. (5.46). 

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