Stratification transitions

M. Schoen, D. J. Diestler, J. H. Cushman. Stratification-induced order-disorder phase transitions in molecular confined films. J Chem Phys 707 6865-6872, 1994. 

L. M. Martyushev, V. D. Seleznev, S. A. Skopinov. Reentrant kinetic phase transitions during dendritic growth of crystals in a two-dimensional medium with phase stratification. Tech Phys Lett 25 495, 1997. 

To illustrate how stratification works in the context of free energy calculations, let us consider the transformation of state 0 into state 1 described by the parameter A. We further assume that these two states are separated by a high-energy barrier that corresponds to a value of A between Ao and Ai. Transitions between 0 and 1 are then rare and the free energy estimated from unstratified computer simulations would converge very slowly to its limiting value, irrespective of the initial conditions. If, however, the full range of A is partitioned into a number of smaller intervals, and... 

Film studies in the past decade have revealed the existence of another film stability mechanism If the continuous phase contains not only a small amount of surface active substances but also a "sufficient amount" of "small particles", these particles can form layers inside the draining film (see Fig. 6, right)[9,22-32]. As a result, such films thin step-wise, by several step-transitions (also called stratification) when at a step transition a layer of small particles leaves the film. 

Figure 11. Thinning pseudoemulsion film, a) Thick film with dimple, b) Film undergoing stratification - two thickness transitions - and, c) Enhanced image of film undergoing stratification. The film has three discrete thicknesses resulting from the first two transitions.

In more complicated cases, different combinations of phase transitions of stratification and ordering are possible. For example, a system may pass over to the disordered and ordered phases or to two different ordered phases. The type of the phase transitions and the regions of their realization are determined by the concentrations of the components, the temperature, and the potentials of particle interaction. Similar transitions also occur in multicomponent solutions. An increase in the number of components increases the number of different combinations of the phase transitions [29]. 

Some thickness transitions occurring in the foam films, such as CBF/NBF were considered so far and estimated from the h(Cei), /i(pH) and TT(/i) dependences. These are transitions in the equilibrium thickness from the thicker CBF to the thinner NBF. The reverse thickness transitions were also realised experimentally, for instance NBF/CBF (see Fig. 3.57) in the Tl(/x) isotherm of NaDoS films at Cei = 0.165 - 0.18 mol dm 3. Similar reverse transition was found in the h(Cei) dependence of lyso PC films in the presence of CaCl2. In this case there occurs a specific adsorption of the Ca2+ ion and the films transfer from CBF to NBF (Fig. 3.50). Along with transition from one equilibrium state into another, non-equilibrium thickness transitions also exist. This is the phenomenon known as stratification, i.e. a consecutive stepwise film thinning. During this process the initially formed films thin to... 

As already mentioned, stepwise transitions in foam films are observed, as a rule, at thicknesses less then 60 - 70 nm. The number of transitions increases with the increase in surfactant concentration. Manev et al. [351] have observed up to 10 transitions when the NaDoS concentration in the initial aqueous solution was raised to 0.5 mol dm 3 (in the absence of additional electrolyte). Upon increasing the ionic strength (addition of electrolyte or ionic surfactants) the differences in the transition thicknesses decrease. In some cases [351-353] electrolyte inhibits stratification. 

Stratification of asymmetric aqueous films from NaDoS, CTAB and a commercial surfactant (alpha-olefin sulphonate) solutions at C > Ccmc on a decane substrate have been studied by Bergeron and Radke [236], They found three transitions by thickness in the metastable multilayer films. They observed also stratified CTAB aqueous films on glass. The n(/i) isotherms of stratified films were analysed considering also the oscillatory structural component of disjoining pressure. 

We have seen evidence of a similar structuring of micelles in thin foam and emulsion films containing C AOS, in the form of stepwise transitions or stratification phenomena (see Figure 5). 

Numerous authors have observed a similar phenomenon, Bruil and Lyklema (24), Kruglyako et al ( ), Manev et al. (16,26) and Wasan et al. (23,28). Some of these authors have established that the thickness of the transitions and the number of transitions are a function of surfactant concentration. The stratification phenomena have been observed for foam (16,24-29) and emulsion (23-24) films with both anionic (10,19,22) and nonionic (28) surfactants. 

Wasan et al. (27-28) explained the process of stratification on the basis of a micelle-latticing structure model. In Figure 8 a schematic of the latticing model for film thinning is provided. By fluctuations in the structure of the micellar lamellae (i.e. the individual rows of micelles in Figure 8), the film can change its thickness by stepwise transitions, each of which are equal to the micellar-lamellae thickness. According to this model the number of transitions will depend upon the micelle concentration. 

Furthermore, the recognition of variations across the different scales of spatial and temporal dimensions would enable the identification of shifting therapeutic targets to address both of the individual and the time variances in personalized medicine (see Fig. 1). Accurate and robust biomarkers can also be useful for the stratification of diseases and classification of patient subgroups for more effective prevention and therapy. The prediction of drug responses would in turn help avoid adverse events for better clinical outcomes. In addition, the construction of dynamic disease predictive networks derived from the analyses of omics data would allow for the transition from reactive treatments to holistic and proactive care. With the transformation from disease-centered to human-based care, the systems and dynamical models would provide patient-centric information to enhance the participation of individuals, the goal of participatory medicine. 

Epithelial cells are classified by their shape, stratification and specialization. The cell shape is subdivided into squamous, cuboidal, columnar and transitional ... 

This argiimcntation is further supported by the predictions of a simple mean-field theory of the ferroelectric transition, which was originally presented in Ref. 257. Within this theory, we neglect any stratification (i.e., inhomogeneities of the local density) as well as any oscillations in the order parameter (which are indeed observed in the computer simulations). We also neglect nontrivial interparticle correlations. Our system can then be viewed as a system composed of N uncorrelated dipolar particles individually interacting with the mean field... 

The behavior and characteristics of confined fluids is more complex than that of bulk liquids or of simple solvated systems described in the chapters mentioned above. One must consider the complexities of the interface between confining walls and the fluid along with confinement-induced phase transitions, critical points, the stratification of the fluid near the confining walls, the idea that confined fluids may sustain certain shear stress without exhibiting structural features normally associated with solid-like phases, and... 

In the beginning of the 20th century, Johnnott and Perrin observed that soap hhns decrease their thickness by several stepwise transitions. The phenomenon was called stratification. Bruil and Lyklema and Friberg et al. studied systematically the effect of ionic surfactants and electrolytes on the occurrence of the stepwise transitions. Keuskamp and Lyklema anticipated that some oscillatory interaction between the Him surfaces must be responsible for the observed phenomenon. Kruglyakov and Kruglyakov and Rovin reported the existence of stradficaHon with emulsion films. 

The experimental observations show that stratification is always observed when spherical colloidal particles are present in the film at a sufficiently high volume fraction therefore, a realistic explanation can be that the stepwise transitions are manifestations of the oscillatory structural forces. The role of the hard spheres this time is played by the colloidal particles rather than by the solvent molecules. The mechanism of stratification was studied theoretically in Reference 346, where the appearance and expansion of black spots in the stratifying films were described as being a process of condensation of vacancies in a colloid crystal of ordered micelles within the film. 

Lobo and Wasan (81) observed the drainage and stability of pseudoemulsion films from nonionic surfactant solutions (Enordet AE1215-30 ethoxylated alcohol) at concentrations much above the CMC. They observed that, for a 4 wt% surfactant system, the film thinned stepwise by stratification (Figure 27), in a fashion similar to the foam films from micellar solutions (Figure 14). Three thickness transitions were observed (81) at 4 wt% concentration with n-octane as oil, which was the same number of steps as observed by Nikolov et al. (54) in foam films at the same concentration. This observation on the micellar layering in the pseudoemulsion film confirms, again, the universality of the stratification phenomenon. 

Plate 3. Development of authigenic illite. a Chlorite b-d development of elongate and lamellar crys- tals at right angles to the stratification (b-axis) e-h authigenic illite with fibrous crystal habit developed by transition from I/S mixed-layer minerals... 

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