Indirect foaming

Other methods attempt to probe the stmcture of the foam indirectly, without directly imaging it. Eor example, since the Hquid portion of the foam typically contains electrolytes, it conducts electrical current, and much work has been done on relating the electrical conductivity of a foam to its Hquid content, both experimentally (15) and theoretically (16). The value of the conductivity depends in a very complex fashion on not only the Hquid content and its distribution between films and borders, but the geometrical stmcture of the bubble packing arrangement. Thus electrical measurements offer only a rather cmde probe of the gas Hquid ratio, a quantity that can be accurately estimated from the foam s mass density. 

Ammonia is used in the fibers and plastic industry as the source of nitrogen for the production of caprolactam, the monomer for nylon 6. Oxidation of propylene with ammonia gives acrylonitrile (qv), used for the manufacture of acryHc fibers, resins, and elastomers. Hexamethylenetetramine (HMTA), produced from ammonia and formaldehyde, is used in the manufacture of phenoHc thermosetting resins (see Phenolic resins). Toluene 2,4-cHisocyanate (TDI), employed in the production of polyurethane foam, indirectly consumes ammonia because nitric acid is a raw material in the TDI manufacturing process (see Amines Isocyanates). Urea, which is produced from ammonia, is used in the manufacture of urea—formaldehyde synthetic resins (see Amino resins). Melamine is produced by polymerization of dicyanodiamine and high pressure, high temperature pyrolysis of urea, both in the presence of ammonia (see Cyanamides). 

Finding F Either Eq. (22-45) or Eq. (22-46) can be used to find the surface excess indirectly from experimental measurements. To assure a close approach to operation as a single theoretical stage, coalescence in the rising foam should be minimized by maintaining a proper gas rate and a low foam height [Brunner and Lemhch, Ind. Eng. Chem. Fundam. 2, 297 (1963)]. These precautions apply particularly with Eq. (22-45). 

It is important to note that it is common in the reticulated foam industry to use air flow-through to define pore size. Manufacturers calculate average pore sizes from air flow measurements. Presumably, one would do this via a microscope either by counting pores or using an optical scanning device, but given the problems of sampling a bulk material with a microscope, it has become standard to use an empirically derived correlation between air flow and pore size. This serves as an indirect confirmation of the effect of the quality of the foam and its dynamic... 

The surface or interfacial phenomena associated with colloidal systems such as emulsions and foams are often studied by means of experiments on artificially prepared flat surfaces rather than on the colloidal systems themselves. Such methods provide a most useful indirect approach to the various problems involved. 

Good descriptions of practical experimental techniques in conventional electrophoresis can be found in Refs. [81,253,259]. For the most part, these techniques are applied to suspensions and emulsions, rather than foams. Even for foams, an indirect way to obtain information about the potential at foam lamella interfaces is by bubble electrophoresis. In bubble microelectrophoresis the dispersed bubbles are viewed under a microscope and their electrophoretic velocity is measured taking the horizontal component of motion, since bubbles rapidly float upwards in the electrophoresis cells [260,261]. A variation on this technique is the spinning cylinder method, in which a bubble is held in a cylindrical cell that is spinning about its long axis (see [262] and p.163 in Ref. [44]). Other electrokinetic techniques, such as the measurement of sedimentation potential [263] have also been used. 

In devolatilization with viscous polymeric melts, it is difficult, of course, to carry out similar experiments and prove indirectly that free-streaming nuclei may play a similar role, but microscopic particles originating from the monomers and catalyst systems are likely to be found in the polymeric product. Moreover, it is well known that the addition of fine powders and solid particles induces foaming. Therefore, the Biesenberger-Lee proposition seems plausible. 

The gradual reduction in cellular activity, culminating in culture death a few days after cultures were stoppered, was clearly the result (directly or indirectly) of changes in dissolved gas concentrations. Otherwise, the cells would have continued actively metabolizing as long as did the conventional unstoppered cultures. The conventional cultures, capped with permeable foam and foil, con-... 

Another form of clouding that may be caused indirectly by the perfume is sometimes observed in water-detergent systems such as foam baths, shampoos, or dishwashing detergents. This, paradoxically,... 

Adsorption can be measured by direct or indirect methods. Direct methods include surface microtome method [46], foam generation method [47] and radio-labelled surfactant adsorption method [48]. These direct methods have several disadvantages. Hence, the amount of surfactant adsorbed per unit area of interface (T) at surface saturation is mostly determined by indirect methods namely surface and interfacial tension measurements along with the application of Gibbs adsorption equations (see Section 2.2.3 and Figure 2.1). Surfactant structure, presence of electrolyte, nature of non-polar liquid and temperature significantly affect the T value. The T values and the area occupied per surfactant molecule at water-air and water-hydrocarbon interfaces for several anionic, cationic, non-ionic and amphoteric surfactants can be found in Chapter 2 of [2]. 

The conductometrical technique for the study of foam films allows to measure directly their lateral electrical conductivity. It is widely used for indirect determination of film thickness study of kinetics of film thinning and rupture as well as study of other processes. Fig. 2.16 presents a scheme of one of the devices for measurement of the lateral electrical conductivity of foam films [16]. 

In conclusion it is worth noting that the method of equilibrium foam film proved to be very appropriate for the determination of the equilibrium diffuse electric layer potential at the solution/air interface. Though it is an indirect experimental technique, it provides reliable results about the appearance of a negative surface charge in the case of surfactant-free solutions as well as in the case of non-ionic surfactant solutions. The existence of an isoeletric point and the re-charging of the interface can be considered as a direct evidence. 

The techniques employed in foam drainage investigations involve the determination of liquid quantity that drains from the foam per unit time. Different foam numbers were introduced as indirect drainage characteristics of low expansion ratio foams produced by the... 

The analysis as to whether coalescence or diffusion play a decisive role, using the variation in the distribution function is much more complicated. The curves in Fig. 6.6 show that the polydispersity of the foam rises considerably with time but from the curve type it is not clear which process regulates the course and what is the contribution of each process in the foam collapse. Some indirect data as well as the observations presented in [9], allow to assume that in the initial stage of collapse of polydisperse foams the diffusion transfer is the decisive process, since at the moment of foam formation the films are rather thick and cannot rupture spontaneously. 

Certain antifoams, for example, methyl or ethyl alcohols, when introduced in the foaming solution, affect indirectly the foam stabilising properties of the adsorption layers because they change either the surfactant solubility or its CMC. It has been established [39] that addition of antifoams (2-ethylhexanoI and tributyl phosphate) increases CMC. Schick and Fowkes [40] have observed a certain change in CMC of NaDoS when tetradecanol is added. If the compounds added improve the foaming ability (such as dodecylglycol ether and (3-hydroxyethyl laurylamide), CMC decreases. 

The concentration of black spot formation in microscopic films Cm characterises not only the threshold concentration of the surfactant at which stable foams and emulsions can be obtained but it can also be used as an indirect measure of film stability. The relations between film stability and Cm of the emulsifier depend on the polarity of the organic phase of the emulsion films (aqueous and hydrocarbon) [58], on the hydrophilic-lipophilic balance of the surfactant mixture [59] as well as on other properties. 

Environmental Aspects. Airborne particulate matter (187) and aerosol (188) samples from around the world have been found to contain a variety of organic monocarboxylic and dicarboxylic acids, including adipic acid. Traces of the acid found in southern California air were related both to automobile exhaust emission (189) and, indirectly, to cyclohexene as a secondary aerosol precursor (via ozonolysis) (190). Dibasic acids (eg, succinic acid) have been found even in such unlikely sources as the Murcliison meteorite (191). Public health standards for adipic acid contamination of reservoir waters were evaluated with respect to toxicity, odor, taste, transparency, foam, and other criteria (192). Bio degradability of adipic acid solutions was also evaluated with respect to BOD/theoretical oxygen demand ratio, rate, lag time, and other factors (193). 

As with cell shapes of a real foam, cell sizes in this material can also be characterized only by nominal (effective) values. The actual effective values depend, first, on the observation method (whether direct — macroscopic, or indirect — adsorption, volumetric, picnometric, etc.). Secondly, they depend on the particular simplified model of the structure and cell shape and thirdly on the method of processing the measured data. 

---