Secondary films

The magnetite film may, under some circumstances, be layered. For example, in power boilers, under conditions of high steaming rate and low steam-water velocity, a secondary film of precipitated particulate iron oxide may form over the original magnetite film. 

Spreading may occur by a process of surface solution or by vaporisation from the lens and condensation on the water surface. This latter, indeed, is the only method of spreading on a solid. The adsorption of vapours from a liquid onto a second liquid surface to the point of equilibrium results in the formation of a primary (unimolecular) film and this is doubtless followed in many cases by secondary film formation or a banking up of the layers on the primary film to a thickness which may be several hundred molecules thick. The conditions which have to be fulfilled are two (1) the surface tension of the film whether primary or secondary o- must attain the value... 

If the specific rate of evaporation v be very small and p large, evidently multimolecular layers or secondary film formation may readily be obtained. 

Similar conclusions may be drawn from the experiments of Hardy on lubrication. Hardy has obtained very convincing data in support of the hypothesis that on the adsorption of a vapour such as octyl alcohol by a metal surface, whilst the first layer is held very tenaciously the thickness of the film of vapour condensed on the metal surface which is in equiUbrium with the free surfeice of the liquid is certainly multimolecular in character and those layers forming the secondary film may be squeezed out by application of sufficient pressure. 

It must be concluded that although the first layer of adsorbed molecules is held extremely tenaciously and that a large decrease in free energy occurs on the adsorption of a unimolecular layer of gas, a further but smaller decrease in free energy may take place on the further adsorption of gas in the secondary films. We would anticipate that a surface of feeble adsorptive power such as diamond would be almost completely saturated with a unimolecular layer, but active surfaces such as of metals are not completely saturated. 

It is well-known that free films of water stabilized by surfactants can exist as somewhat thicker primary films, or common black films, and thinner secondary films, or Newton black films. The thickness of the former decreases sharply upon addition of electrolyte, and for this reason its stability was attributed to the balance between the electrostatic double-layer repulsion and the van der Waals attraction. A decrease in its stability leads either to film rupture or to an abrupt thinning to a Newton black film, which consists of two surfactant monolayers separated by a very thin layer ofwater. The thickness of the Newton black film is almost independent of the concentration of electrolyte this suggests that another repulsive force than the double layer is involved in its stability. This repulsion is the result of the structuring of water in the vicinity of the surface. Extensive experimental measurements of the separation distance between neutral lipid bilayers in water as a function of applied pressure1 indicated that the hydration force has an exponential behavior, with a decay length between 1.5 and 3 A, and a preexponential factor that varies in a rather large range. 

As already discussed in Section 3.4, these two states of black films are, respectively, common black (CBF) and Newton black (NBF) films. Initially, bilayer films were named Perrin films by Scheludko later Jones, Mysels and Scholten called them primary and "secondary films. It was not until the issuing of IUPAC nomenclature that they were termed CBF and NBF. It is rather arguable, however, whether it is fairer to name them after the scientist who observed them first or after the one that characterised them quantitatively. In many cases NBF are also called amphiphile bilayers . 

FIGURE 5.13 Sketch of a disjoining pressnre isotherm of the DLVO type, O vs. h. The intersection points of the n(/t)-isotherm with the line H = correspond to equihhrium films /j = /j, (primary film), /j = /jj (secondary film). Point 3 corresponds to unstable equilibrium. 

With oxygen gas (4) a primary layer had a mean heat of adsorption of 139 kcal. and contained 4X10 molecules. A secondary film was also detected with a heat of 48 kcal./mole oxygen which contained 0.9X10 molecules. The method has also been used to detect adsorbed nitrogen (12). 

Group I. Liquids which spread upward, exhibiting a primary film and a secondary film with nearly constant slope at the leading edge (n-hexadecane, pristane, squalane, squalene, polychlorobiphenyl, and the polymer liquids poly methyl siloxane, polyisobutylene, and polytrichloroethylene). 

A systematic explanation of the spreading phenomena observed requires consideration of mechanisms which will account for the invariable advance of a primary film, and relate the presence of volatile impurities to the spreading of secondary films and the frequent development of a ridge at their leading edge, account for the distinct recession of some liquids from a boundary at which they exhibit a zero contact angle, and explain the upward transport of significant amounts of liquid in films a micron or more thick. 

R. Frankenthal, On the passivity of iron-chromium alloys I. Reversible primary passivation and secondary film formation, J. Electrochem. Soc. 114 (1967) 542—547. 

Surface tension gradients can induce drainage rates in excess of that expected for simple viscous flow if the low MW components have higher surface tensions than the undistilled liquid, e.g., alkyl-substituted aromatics This reverse flow is illustrated in Fig. 16 for a liquid mixture of isomeric amylnaphthalenes. In this case, evaporation of low MW components from the primary film results in a lower surface tension and a surface flow from the primary film onto the secondary film. Distillation of the amylnaphthalene essentially eliminated this reverse flow (Fig. 16). 

The equilibrium state at A = 2 (Fig. 8a) corresponds to a very thin secondary film, which is stabilized by the short-range Bom repulsion. The secondary film represents a bilayer of two adjacent surfactant mono-layers its thickness is usually about 5 run (slightly greater than the doubled length of the surfactant molecule) (77). Secondary films can be observed in emulsion floes and in creamed emulsions. 

Let us first consider a quiescent emulsion film, say the film between two drops within a floe or cream. At a given sufficiently small thickness of the film, termed the critical thickness (92—97, 115), the attractive surface forces prevails and causes growth of the thermally excited capillary waves. This may lead to either film rupture or transition to a thinner secondary film. Two modes of film undulation have been distinguished symmetric (squeezing, peristaltic) and antisymmetric (bending) modes it is the symmetric mode which is related to the film breakage/transition. The critical thickness, h = h, of a film having area nR can be estimated from the equation (94) ... 

The rate of this process in aprotic electrolytes is rather high the exchange current density is fractions to several mA/cm. As pointed out already, the first contact of metallic lithium with electrolyte results in practically the instantaneous formation of a passive film on its surface conventionally denoted as solid electrolyte interphase (SEI). The SEI concept was formulated yet in 1979 and this film still forms the subject of intensive research. The SEI composition and structure depend on the composition of electrolyte, prehistory of the lithium electrode (presence of a passive film formed on it even before contact with electrode), time of contact between lithium and electrolyte. On the whole, SEI consists of the products of reduction of the components of electrolyte. In lithium thionyl chloride cells, the major part of SEI consists of lithium chloride. In cells with organic electrolyte, SEI represents a heterogeneous (mosaic) composition of polymer and salt components lithium carbonates and alkyl carbonates. It is essential that SEI features conductivity by lithium ions, that is, it is solid electrolyte. The SEI thickness is several to tens of nanometers and its composition is often nonuniform a relatively thin compact primary film consisting of mineral material is directly adjacent to the lithium surface and a thicker loose secondary film containing organic components is turned to electrolyte. It is the ohmic resistance of SEI that often determines polarization of the lithium electrode. 

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