Foamability, foam stability

Ten aspects of the Maillard reaction will be considered in this chapter the effect on a,, on pH, on redox potential, on solubility, on texture, on foamability and foam stability, on emulsifying power, on volatile formation on storage, on binding of volatiles, and on other functionalities. The third of these has received by far the most attention and this is reflected here. 

Early researchers turned to foamability or foaminess" measurements to screen surfactants for flow experiments (51). In one variation of this test, a long, vertical glass cylinder with a frit at the bottom was filled with the test solution, and gas was forced through the frit. The height of foam formed in the column was then measured, or, the foam was collected and the amount of liquid in the foam determined (51). In his screening of some 200 materials, Raza measured interfacial tensions of aqueous solutions with respect to air and to oil, and the foamability and foam stability in the absence and presence of oil. The latter experiment consisted simply of shaking the solution in a test tube and measuring the volume of foam at various times (60). 

A foamable compound is produced by compounding the obtained prepolymer mixture with a physical blowing agent, a foam stabilizer, and further additives (e.g., adhesion promoters, catalysts, etc.). 

For the evaluation of the foamability of a surfactant the bulk concentration is used at which the relative rate of foam collapse is equal to 50% of its formation (cw °). The cw ° values determined from foam formation isotherms of a number of products are given in Table 6.1. As it is seen, typical representatives of anionics (sodium dodecyl sulphate), cationics (cetyl trimethyl ammonium bromide) and nonionics (ethoxylated alkylphenols) give bubble foams at very low concentrations, and the foam stability of ionic surfactants does not differ much from that of nonionics. For anionics, the highest concentrations are required for soaps of higher carboxylic acids. 

Amphoterics are excellent foamers and foam stabilizers under a variety of conditions. Their foamability is essentially insensitive to water hardness. Capitalizing on these properties, amphoterics were among the many specialty surfactants commercialized after World War II [2]. Since then, these surfactants have been used in personal care products because of their unsurpassed mildness and low eye-sting properties. Today, most mild skin cleansers and shampoos contain an amphoteric surfactant as a major component. 

The second report [6] involves a similar study with n-alkyl fatty adds (Cs-Cio). Here Malysa et al. show that the fatty adds follow exactly the same trends as the n-alcohols. There is a maximum in foam stability that corresponds with a maximum in effective elasticity as the homologous series is ascended (for short time scales, 0.02—0.64 s). The authors conclude from the first two studies that foamability does not relate simply to the... 

Our results also proved the correlation between foamability and surface tension gradient for aqueous nonionic surfactant solutions. Foam formation was estimated from a dynamic surface tension using the maximum bubble pressure method, and foam stability was estimated from a transfer distance of lamella using a laminometer. Laminometer measurements were made using the Du Noiiy ring method [1,78,96]. Force profile during the expansion of lamella was monitored using an electronic-balance with... 

The effect of the antifoam on the foam is best estimated if the total volume of liquid and gas is measured and the volume of air in the foam is used as a measure of either foamability or stability. The method is surprisingly reproducible even when comparing results obtained by different individuals. This is exemplified by a plot of F (= volume in air in the presence of antifoam/volume of air in the absence of antifoam) against logio(antifoam concentration) shown in Figure 2.1. This plot was prepared by three different individuals using a predispersed antifoam [4],... 

We may conclude by noting that, in general, antifoam effects with oils appear to require that the antifoam be undissolved in the foaming medium. Solubilization of the antifoam oil in micelles largely restores the foamability. However, there is some evidence to suggest that the some solubilized antifoam oils may have a weak adverse effect on the foamability or foam stability of some aqueous micellar surfactant solutions. We should note, however, that dissolved antifoam can even enhance foamability [5,6], at least in the absence of surfactant micelles. 

FIGURE 4.5 Effect of on foamability ( ) and foam stability (O) (after 10 min) of 2 mM sodium dodecyl 4-phenyl sulfonate in aqueous 17 mM NaCl solution at 25°C. (After Ran, L. Foaming of anionic surfactant solutions in the presence of calcium ions and triglyceride-based antifoams, PhD Thesis, University of Manchester, 2011.)... 

FIGURE 4.29 Effect of concentration of alcohols on foamability and foam stability of 2.6 mM aqueous saline solntion of branched chain sodium dodecylbenzene sutfonate blend (using the dynamic Ross-Miles technique at 25-27°C). (a) 2-Butyloctanol. (b) n-Heptanol. (Reprinted with permission from Amaudov, L. et al. Langmuir, 17, 6999. Copyright 2001 American Chemical Society.)... 

FIGURE 4.38 Foamability ( ) and foam stability ( , after 300 s) by Ross-Miles technique as function of temperature for 0.02 M aqueous solutions of commercial nonyl phenol ethoxyl-ate (NP.EO9). (With kind permission from Springer Science+Business Media J. Surfactants Deterg., 6, 2003, 345, Chaisalee, R., Soontravanich, S., Yanumet, N., Scamehorn, J., figure 1.)... 

FIGURE 4.39 Foamability ( ) and foam stability ( , after 60 s) by hand shaking of cylinders as function of temperature for 1 wt.% of Triton X-100 containing 0.3 wt.% of a PDMS-EOPO copolymer (of mol.wt. 29,000, EO-PO ratio 0.66, PDMS-EOPO ratio 0.26). Open symbols give foam behavior of dilute conjugate solution formed by centrifuging the turbid dispersion at 50°C foamability (O), foam stability ( , after 60 s). (After Nemeth, Z. et al., J. Colloid Interface Sci., 207, 386, 1998.)... 

Nemeth et al. [132] also report the effect of polyethoxy-polypropoxy block copolymers (EO .PO .EO , where m = 33 and 2.5 foam behavior of a protein—bovine serum albumin (BSA). These block copolymers reduced both the foamability and stability of the foam of BSA solutions at temperatures below the relevant cloud point where the solution was homogeneous. Filtration of the mixed solutions at the tanperature of these foam experiments produced essentially no enhanconent of foamability or foam stability. However, the filtration was done before foam generation so that the possibility of the decomposition of any putative metastable state during foam generation was not examined. Unlike with the PDMS-EOPO copolymer/Triton X-100 system, the possibility that antifoam effects at temperatnres above a measured cloud point for this EO-PO-EO -i- BSA system, which could be eliminated by removal of the relevant conjugate phase, was not explored. 

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