Bancroft point

Given Psatl(T) = Psat2(T) T = Find(T) The Bancroft point for this system is ... 

It is clear that if p then the smallest deviation from ideality will be sufficient to introduce a maximum or minimum into the total vapour pressure curve. Hence if the vapour pressure curves of two pure liquids intersect at a temperature then a mixture of them will exhibit azeotropy at this temperature, and in its vicinity. The point of intersection of the vapour pressure curves is often called the Bancroft point, In the absence of a Bancroft point there is no general criterion for predicting whether two liquids will form an azeotrope. 

From Eq. (5.46), it can be seen that azeotropic behavior can easily occur in a binary system if the vapor pressures of the two components are very similar, since in this case already very small deviations from Raoult s law are sufficient to fulfill the equation and to create an azeotropic point either with positive or negative deviation from Raoult s law. If the vapor pressures are identical (e.g., at the Bancroft point), the binary system shows the azeotropic behavior. 

As mentioned before, azeotropic behavior always occurs if the compounds to be separated have identical vapor pressures (Bancroft point). Since the slope of the vapor pressure curve, following the Clausius-Clapeyron equation, depends on the value of the enthalpy of vaporization, a low boiler may become the high boiler with rising temperature, if the enthalpy of vaporization is smaller than the one for the second compound. This is shown for ethanol and benzene in Figure 5.55. While benzene is the low boiler at low temperatures, the opposite becomes true at higher temperatures, since the molar enthalpy of vaporization of the polar component ethanol is larger than the molar enthalpy of vaporization of benzene (see Appendix A). 

Even the system water-deuterated water shows a Bancroft point, since deuterated water with a higher normal boiling point (101.4 ""C instead of 100 " C) shows a larger enthalpy of vaporization than water. For example, at 25 the enthalpy... 

But these byzantine discussions on mechanism are beyond the point. This book is based on experiments. It does not claim to solve all problems (e.g what is thereat origin of Bancroft s rule) but it presents them with common sense and precision. I am convinced that it will be of great help. 

W. D. Bancroft and H. B. Weiser point out that the blue luminescence of sodium is obtained without the yellow luminescence (i) when sodium salts are introduced into a flame of hydrogen in chlorine (ii) when metallic sodium bums slowly in oxygen, chlorine, or bromine (iii) when a sodium salt is fused (iv) when cathode rays act on sodium chloride (v) when anode rays first act on sodium chloride (vi) when one heats the coloured residue obtained by the action of anode rays or cathode rays on sodium chloride and (vii) when sodium chloride is precipitated rapidly from aq. soln. with hydrochloric acid or alcohol. The yellow luminescence of sodium is obtained, accompanied by the fainter blue luminescence (i) when a sodium salt is introduced into the Bunsen flame (ii) when sodium burns rapidly in oxygen, chlorine, or bromine and (iii) when canal rays act on sodium chloride. It is claimed that the yellow luminescence is obtained when sodium vapour is heated but it is very difficult to be certain that no burning takes place under these conditions. 

In a previous paper Bancroft and Richter3 have confirmed the hypothesis of Claude Bernard that anesthesia is due to the reversible coagulation of the colloids of the sensory nerves. It appeared that the proteins were the important factors rather than the lipoid materials. If we have different proteins in different parts of the body we shall expect them to be flocculated reversibly to different extents by different drugs. Claude Bernard pointed out that the anesthetics affect the brain first but the sensory nerves very soon after, while the narcotics affect the brain much more readily than the sensory nerves. Morphine will, therefore, tend to drive chloroform from the brain to the sensory nerves, thereby increasing the anesthetic effect of the chloroform. 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

An empirical generalization used to predict which phase in an emulsion will be continuous and which dispersed. It is based on a physical picture in which emulsifiers are considered to have a wedge shape and will favour adsorbing at an interface, such that most efficient packing is obtained that is, with the narrow ends pointed toward the centres of the droplets. A useful starting point, but there are many exceptions. See also Bancroft s Rule, Hydrophile-Lipophile Balance. 

As pointed out by Bancroft and Gesser,412 bromates may decompose by two competing reactions ... 

The polarized spectra of staurolite, Fe2Al9Si4023(0H), accomodating tetrahe-drally coordinated Fe2+ ions (point symmetry Cm mean Fe-0 = 200.8 pm) and consisting of absorption bands spanning the 5,000 to 7,000 cm-1 region (Bancroft and Bums, 1967a Dickson and Smith, 1976), are illustrated in fig. 4.6 and discussed in 4.4.3. These bands completely mask any contributions from Fe2+ ions that might be present in centrosymmetric octahedral sites in the staurolite structure. The A, and CFSE parameters of tetrahedral Fe2+ ions in staurolite are estimated to be about 5,300 cm-1 and 3,700 cm-1, respectively. The spectra of the cobaltian staurolite, lusakite, illustrated in fig 4.7, indicates that tetrahedrally coordinated Co2+ ions have A, and CFSE values of about 6,500 cm-1 and 7,800 cm-1, respectively. 

In a few instances, emulsions can be stabilized by solid particles as shown by S. U. Pickering in 1907 (see Petrowski, 1976). Examples are mustard seeds in traditional mayonnaise (Anon., 1968) or crystals of high melting-point triglycerides in margarine or butter (Precht and Buchheim, 1980 Heertje et al., 1987). The solid particles are considered to act as a barrier, which prevents the coalescence of droplets. Bancroft (1913) observed that the phase that wets the solid more easily will become the continuous phase. 

As a prachcal point, it may be mentioned that the chief difficulty in setting up such gas cells is due to the fact that the platinum has already dissolved some oxygen from the atmosphere, and this m contact with OH necessanly present in aqueous solutions, causes an oxygen concentration potential to be set up It is necessary, therefore, to remove the oxygen as completely as possible For details a textbook on Electrochemistry must be consulted A very complete list of various sorts of cells, classified under seven heads, is given by W D Bancroft Journal Physical Chemistry, 12, 103, 1908)... 

The presence of alkaloids was discovered in many other Queensland plants by Dr. T.L. Bancroft, who continued the work of his father Joseph. Although the techniques and facilities available in Australia at that time were hardly adequate for the isolation, purification and structural determination of new alkaloids, or even for the identification of ones already known, the pioneering investigations by the Bancrofts, Petrie and others nevertheless formed a starting point for subsequent studies. T.L. Bancroft, for example, obtained impure samples of alkaloids from certain Daphnandra species (Monimiaceae) [11, 12], and studied their pharmacology. His work was later extended by Pyman in England 113, who isolated crystalline bases from D. micrantha that proved eventually [14] to belong to the same biscoclaurine series as berbamine (1). 

Satellites may also be observable in photoelectron spectra if a 2-hole final state, with similar total energy and the same total angular momentum and parity as the original core hole state, can occur. Bancroft et al. (18) pointed out that there are several energetically favorable examples for such (configuration interaction) satellites in light actinides. Their computed satellite... 

Figure 21.1 points out the key parameters which determine the nature of the dispersion [3]. These are the emulsifier concentration (horizontal axis) and the hydrophile-lipophile balance (HLB) of the emulsifier (vertical axis). According to Bancroft s rule, HLB values higher than 7 favour the formation of oil-in-watCT... 

---