McBain system

Figure 4.6. Diagram of the McBain system for the measurement of extents of adsorption, that is, the isotherm. The extensions of the calibrated spring are converted to a weight, and then to (mmol g ) of the known adsorbate.

The McBain system is useful when working with new materials of unknown properties because rates of adsorption can be monitored and the process is under the control of the operator. An important parameter is the time taken to reach equilibrium, or rates of adsorption. By so doing, knowledge is gained about contents of the smallest of microporosity, <0.5 nm entrances. Automatic adsorption equipments, commercially available, are suitable for routine measurements (for quality control) of adsorption isotherms. For research purposes, when using such equipments, an isotherm must be printed out when an experienced researcher can provide an independent interpretation. It would be an advantage if all studies of porosity published their as-obtained experimental isotherms. 

One association of the above type—viz., acid-soap formation between a fatty acid and its neutral salt or soap—was postulated a century and a half ago by Chevreul (2). However, this concept was not generally accepted until much later, when studies of such workers as McBain (15, 16, 17), Ekwall (6), Malkin (18), Piper (19), and Levi (13) provided convincing evidence of its correctness. The most recently published data on sodium acid-soaps are those of Ryer (20) who, for the stearate system,... 

The composition data obtained for the series of mixed fatty acid-potassium soap systems, prepared by both the ethanol and petroleum ether routes, lend strong support to the formation of 1 to 1 acid-soap complexes. It is of interest to inquire into the phase relationships in these two-component systems. A phase diagram presented by McBain and Field (15) for the lauric acid-potassium laurate system shows that compound formation takes place between the two components at the 1 to 1 molar ratio, but the compound undergoes melting with decomposition at 91.3 °C. [A similar type of phase behavior has been reported by us for the sodium alkyl sulfate-alkyl alcohol (9) and sodium alkyl sulfonate-alkyl alcohol (12) systems, but in these cases the stoichiometry is 2 to 1]. 

Adsorption Measurement. The capacities of the molecular sieves to adsorb vapor phase 1,3,5-triisopropylbenzene (97 %, Aldrich) and 1,2,4-triisopropylbenzene (99%, Camegie-Mellon University) were measured at 373 K using a McBain-Bakr balance. The adsorption temperature was chosen such that no chemical reactions of the adsorbates were observed. Prior to the adsorption experiment, the NH4+-forms of the solids (except SAPO-37) were dehydrated at 573 K under a vacuum of 10" 2 Torr. The as-made SAPO-37 was calcined at 793 K in an oxygen flow of 6 L/h in-situ in the adsorption system for removal of organic species and dehydration. The vapor pressure at 296 K of 1,3,5- and 1,2,4-triisopropylbenzene is approximately 0.45 Torr. The adsorption experiments were conducted at this pressure. 

Figure 2.9. Schematic diagram of a two compartment diffusion cell. The experiments by Vinograd and McBain (1941) on diffusion in electrolyte systems were carried out in apparatus of this kind.

Another property of zeolites is the high conversion rates in the channel system. It was also observed that with different spatial configurations of channels, cavities, windows, etc, the catalytic properties are changed and the selectivity orientates toward less bulky molecules due to limitation in void volume near the active sites or to resistance to diffusivity. This feature termed shape-selectivity, was first proposed by McBain (20) demonstrated experimentaly by Weisz et al (21) and reviewed recently (22). For instance CaA zeolite was observed to give selective dehydration of n-butanol in the presence of more bulky i-butanol (23) while CaX non selective zeolite converted both alcohols. In a mixture of linear and branched paraffins, the combustion of the linear ones was selectively observed on Pt/CaA zeolite (24). Moreover, selective cracking of linear paraffins was obtained from petroleum reformate streams resulting in an improvement of the octane number known to be higher for branched paraffins and for aromatics than for linear paraffins. Shape selectivity usually combines acidic sites within... 

Both adsorption measurements and TPD of NH3 were performed under vacuum conditions. About 150 mg of a sample was charged in a quartz basket attached to a standard vacuum system(1xl0- Pa). A McBain-type quartz spiral spring was hung down to the sample basket. The sample was evacuated for 5 h at 773 K. The weight change before or after NH3 adsorption at 373 K was determined by the... 

Adsorptive capacities for several calcined TS-NU-1 samples were measured at room temperature (298 K) on a McBain type gravimetric unit connected to a vacuum/gas handling system. Prior to adsorption the samples were outgassed in vacuum at 673 K for... 

Liquid crystalline products are formed in aqueous soap systems at high concentrations. The existence of different mesophases in such systems was observed by McBain (17,18). Toward the end of the 1950 s Luzzati et al. (.11,1/+, 15,16) discovered by x-ray diffraction various mesomorphous structures in these two-component systems and regarded this as a proof that they contained different mesophases, which, however, they did not separate. 

The simplest lyotropic system is exemplified by the behavior of a fatty acid salt in water thus, using potassium palmitate, McBain and Marsden (24) identified three phases an isotropic micellar solution, a turbid, viscous coagel, and an intermediate region. These phases exist because of the amphiphilic property of the fatty acid salt which has a hydrophilic polar group and a water-insoluble hydrocarbon chain any substance of similar structure can behave as an amphiphile. If water is used as the solvent, a considerable number of substances capable of forming equilibrated phases can thus be identified predictably from their molecular structure salts of fatty acids, ionic and nonionic detergents, phosphatides, and many others. 

Fig. 1. The fatty acid soap-water phase diagram of McBain (58) modified (1) to show the molecular arrangement in relation to aqueous concentration (abscissa) and temperature (ordinate). Ideal solution, i.e., true molecular solution, is to the left of the vertical dashed line, indicating the critical micellar concentration (CMC), which varies little with temperature. At concentrations above the CMC, provided that the temperature is above the critical micellar temperature (CMT), a micellar phase is present. At high concentrations, the soap exists in a liquid crystalline arrangement, provided that the solution is above the transition temperature of the system, i.e., the temperature at which a crystalline phase becomes liquid crystalline. The Krafft point is best defined (D. M. Small, personal communication) as the triple point, i.e., the concentration and temperature at which the three phases (true solution, micelles, and solid crystals) coexist, but in the past the Krafft point has been equated with the CMT. The diagram emphasizes the requirement for micelle formation (a) a concentration above the CMC, (b) temperature above the CMT, and (c) a concentration below that at which the transition from micelles to liquid crystals occurs. Modified from Hofmann and Small (1).

When only one of the components is volatile most of the static methods previously described can be used. An alternative is to use a McBain balance in a closed system. The involatile component is placed on the balance in a thermostatted evacuated chamber and the mass of the sample is determined by the... 

Fio. 2.18. The system sodium oleatc-ben-zene-propylene glycol at 20°C. [PalU and McBain, Ind. Eng, Chem, 38, 741 (1946).]... 

In view of the difficulties in the preparation of aluminium tricarboxylates from aqueous systems, the first attempt to prepare a metal carboxylate from its alkoxide appears to have been made in 1932 by McBain and McLatchie from the reaction... 

Antimicrobial products have been the main weapons used to control unwanted biofilms. Although this strategy is widespread in biofilm control, there are no standardized antimicrobials with reliable efficacy. Strategies to remove unwanted biofilms must take into account the system characteristics, such as the biofihn colonizer species and the EPS composition (SimOes et al. 2009b). It is expected that an effective and wide spectrum biofihn control strategy will overcome the resistance and cross-resistance problems (Gilbert and McBain 2003). 

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