Foaming ability and foam stability

Foaming Ability and Foam Stability. Foaming ability and foam stability were determined in triplicate on 100-mg/mL solutions of freeze-dried raw foam and humic substances from stream, foam, and foam-extract samples. Measurements were made at 25 at pH 3.0, 7.0, and 10.0, with sample pH adjusted using O.IN sodium hydroxide using the Ross Miles method (D1173-53 ASTM standards, 1991). Briefly, a 200-mL... 

Figure 5. Measurements of foaming ability and foam stability for samples of raw foam, and humic substances from stream, foam, and foam extract samples from Como Creek and Suwannee River. Final foam heights were recorded after 5 minutes.

In summary, the chemical characterization data show that humic substances in foam are an aliphatic, carboxyl-poor fraction, that are enriched in humic acid compared to stream humic substances. The carboxyl moiety appears to be the dominant ionic species affecting the surface activity of the humic substances, and foaming ability and foam stability are strongly dependent on pH as the average pKa of the organic acids is 4.2 (25). 

FIGURE 4.28 Effect of addition of 0.01 wt.% of various emulsified alcohols on the foam-ability and foam stability of 2.6 mM aqueous saline solution of a branched chain sodium dodecylbenzene sulfonate blend (using the dynamic Ross-Miles technique at 25-27°C). (Reprinted with permission from Amaudov, L. et al. Langmuir, 17, 6999. Copyright 2001 American Chemical Society.)... 

A foam is a dispersion of a gas in a liquid or a solid. The formation of foam relies on the surface activity of the surfactants, polymers, proteins, and colloidal particles to stabilize the interface. Thus, the foamability increases with increasing surfactant concentration up to critical micelle concentration because above critical micelle concentration, the unimer concentration in the bulk r ains nearly constant. The structure and molecular architecture of the foam is known to influence foam-ability and its stability. The packing properties at the interface are not excellent for very hydrophilic or very hydrophobic drug. The surfactant promoting a small spontaneous curvature at interface is ideal for foams. Nonionic surfactants are the most commonly used one. The main advantage with foams is its site-specific delivery and multiple dosing of the drug. ... 

The main factors, which determine the foam formation ability and physical stability of the foams in mixed Lys-MR solutions, are surface activity and complex dilatation modulus. With increase of MR concentration in certain range a surface activity and complex dilatation modulus of interfacial layers increased and phase angle decreased. It means that viscoelasticity of interfacial layers became higher. In these conditions the foam volume and mutiplicity as well as stability of foams were growing. This effect may be used for creation of mixed protein-MR system foam type for pharmaceutical applications with improved physical stability and wide range of antibacterial actions. 

Asymmetrical triesters of phosphoric acid of the general formula ROPO (OR,)2 (R = C8 i4 alkyl R, = C, 3 alkyl) were obtained in approximately 70% yield by treatment of a higher fatty alcohol and a Ci 3 alcohol with P0C13 in hexane or pyridine at <0°C. The products were soluble in nonpolar organic solvents and partially soluble in polar organic solvents and water. But the foamforming ability and foam stability of the compounds in water were low [11]. 

It is essential to consider the physico-chemical properties of each WPC and casein product in order to effectively evaluate their emulsification properties. Otherwise, results merely indicate the previous processing conditions rather than the inherent functional properties for these various products. Those processing treatments that promote protein denaturatlon, protein-protein Interaction via disulfide interchange, enzymatic modification and other basic alterations in the physico-chemical properties of the proteins will often result in protein products with unsatisfactory emulsification properties, since they would lack the ability to unfold at the emulsion interface and thus would be unable to function. It is recommended that those factors normally considered for production of protein products to be used in foam formation and foam stabilization be considered also, since both phenomena possess similar physico-chemical and functionality requirements (30,31). 

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 common understanding of foam stability usually refers to the ability of a foam to maintain its main parameters constant with time, i.e. bubble size, liquid content (expansion ratio) and total foam volume. Foam lifetime is most often used as the simplest measure of foam stability. 

Chapter 7 will treat the role of black foam films on foam stability and the stabilising ability of surfactants. 

Definition of the Term Foam Stability and Foaming Ability of Solutions... 

The kinetics of foam collapse, i.e. the process of gas and liquid separation, is characterised by the rate of reduction of foam volume with time or by the rate of decline in its height, if the cross-sectional area is equal along the whole foam column. The stability of the foam as a whole can be characterised quantitatively at any moment by the reciprocal quantity of the rate of foam column destruction. Most often, however, the estimation of the stability of the foam column, is expressed by an integral characteristic time of decay of the whole foam column or a part of it. The relation between the internal foam collapse and the destruction of the foam column is discussed in Section 6.5. Sometimes foam stability is considered in terms of foaming ability of the solution. In general the latter characteristic involves the easiness of foam formation, foam volume and stability. Such an interpretation, however, makes this characteristic rather indefinite. For example, Abramson [12] indicates that for the estimation of the foaming ability of surfactants it is necessary to know the quantity and stability of the foam obtained from a particular surfactant as well as the conditions under which the surfactant acts as a foam stabiliser. That is why it has been repeatedly emphasised that foaming ability... 

In order to develop efficient techniques for the preparation and application of foams in industry, agriculture, firefighting, etc., it is necessary to know the physicochemical parameters of surfactants and their relationship with the foam stabilising ability of the surfactant solutions. Usually the criterion of the surfactant foaming ability is the adsorption of these compounds at the solution/air interface and the related to it properties, such as decrease in surface tension, adsorption work, maximum adsorption T. [13,39,43]. CMC is often used as a characteristic of a foaming agent (if micellisation is possible in the surfactant solution). Parameters related to foam stability, such as foam lifetime and foam column height, are also employed [12,13,39],... 

It seems promising to estimate the stabilising ability of surfactant mixtures analysing their phase diagrams [76,77]. The comparison between the surfactant state in the bulk phase and the foam stabilising ability allows not only to choose the most suitable foaming agents but also to clarify the main reasons for foam stability. 

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