Interlamellar distance

Fig. 21 Variation of the interlamellar distance as a function of the temperature for ZurAI/VBS and Ca2Al/VBS (Reprinted from [70] with permission from Elsevier)...

Using MC simulations Delville and co-workers have investigated the clay-water interface [83-87], The number of hydration layers (2-3) increases suddenly during the swelling process [85]. For hydrated montmorillonite with interlayer sodium counterions it was determined that the water content of the pore is a function of the interlamellar distance. Water molecules are layered in successive shells, whose number (1-4) depends on the available interlayer space [87]. The MD study of structure of water in kaolinite [88] has indicated two types of adsorbed water molecules according to different orientations with respect to the structure of clay sheets with HH vector parallel or perpendicular to the surface. 

An important result is the coincidence of the temperature of the main phase transition determined for the water-ethanol dispersion by DSC (see below) with the temperature of the steep change in the foam bilayer thickness (23°C). Within the range from 22 to 23°C the foam bilayer thickness variation is similar to that of the interlamellar distance in water dispersions of DMPC [443]. These facts show that both in the bulk phase and in the foam bilayer a chainmelting phase transition occurs which is characterised by a sharp shift in the number of gauche conformations of carbon-carbon bonds [430,444]. 

It is well known that the main phase transition is due to the melting hydrocarbon tails of amphiphile molecules [430,436], The average number of gauche conformations for each DMPC molecule is about 7 in the liquid-crystalline state [444], In view of this the thickness of the hydrocarbon layers of a foam bilayer can be estimated to be h = 1.13 nm. Then for the liquid-crystalline state of the foam bilayer, the thickness of the polar inner layers is (i2 = 3.5 nm and the total thickness of the foam bilayer is h = 5.7 nm. These values are relevant to the liquid-crystalline state of the foam bilayers (24-30°C) and are shown in Table 3.14. The value obtained for the thickness of the DMPC foam bilayer in the liquid-crystalline state seems reasonable when compared with the value of the interlamellar distance in the liquid crystalline DPPC-water-alcohol dispersions [445], extrapolated to high ethanol concentrations. 

The XRD patterns of S], S2 are shown in Fig.3. The peak near 2.9° changes to 3.4° for the TEOS-treated sample, which means the interlamellar distance decreases when treated with TEOS. And the strength of the peaks at low angles becomes less sharp than the salt. It is contrast to the increase surface area of the material. We can deduce that the addition of Si leads to the bending of the layers, and at last results in the transformation of the lamellar salt into a porous structured compound through the... 

Asphaltenes have also been subjected to x-ray analyses to gain an insight into their macromolecular structure (15) the method is reputed to yield information about the dimension of the unit cell such as interlamellar distance (c/2), layer diameter (LJ, height of the unit cell (Lc), and number of lamellae (Nc) contributing to the micelle (Figure 2). 

Fractionation of an asphaltene by stepwise precipitation with hydrocarbon solvents (heptane to decane) allows separation of the asphaltene by molecular weight. The structural parameters determined using the x-ray method (Table II) show a relationship to the molecular weight (16). For the particular asphaltene in question (Athabasca), the layer diameters (La) increase with molecular weight to a limiting value similar relationships also appear to exist for the interlamellar distance (c/2), micelle height (Lc), and even the number of lamellae (Nc) in the micelle. 

Figure 5 describes the evolution of the Tobermorite interlayer distance upon rehydration at room temperature and water vapour pressure equal to the saturating pressure value (j.e. relative humidity RH=100%, P/Po=l). No swelling phenomenon was observed after 48 hours hydration the interlamellar distance remains unchanged at 11.5 A. The dehydration process (14 A-11 A) is thus irreversible with these rehydration conditions. 

In order to further validate our in-situ approach, we have realised a dehydration/re-hydration cycle for Ca-Montmorillonite in the same conditions as for Tobermorite. As presented in Figure 6 , the clay dehydration occurs at constant atmospheric pressure with increasing temperature the interlayer distance shifts from 16 A to 11 A. The inverse process, rehydration, is easily obtained at 300 K by increasing the water vapour pressure the interlamellar distance first shifts from 11.5 A to 14 A for P/Po<0.2 at larger pressure the swelling is more progressive and easily detected as shown in Figure 6 . 

The doo3 reflection (at 20 10) corresponds to the interlamellar distance plus the thickness of a mineral sheet (4.8 A). In the case of normal exchange and exchange in the presence of a swelling agent, no change in interlamellar distance is observed. An interlamellar distance of about 3 A, is too small to enable intercalation of the complex, which has dimensions of approximately 16 A. So it can be concluded that for both catalysts, the complex is bound at the outer surface of the LDH, and not in the interlamellar space... 

Figure 8.17 Proposed general mechanism for ion-exchange involving the initial saturated magnesium vermiculite with divalent cations in aqueous solution, d is the interlamellar distance.

The structure of a lamellar phase L is characterized by two important parameters, the interlameUar distance d and the already-mentioned elasticity of the amphiphilic film. The interlamellar distance is directly computed from the position of the Bragg singularities of the structure factor S q) by ... 

First, we treat a model of discontinuous precipitation of binary polycrystalline supercooled alloys at low temperatures as a result of DIGM. In the proposed approach, we independently determine the main parameters interlamellar distance, maximum velocity of the phase transformation front, and residual supersaturation at the front. This is achieved by using a set of equations for... 

In the model considered below, the role of both grain boundary and bulk diffusion in the transformation front and close to it, respectively, is analyzed within the problem of unambiguous determination of the discontinuous precipitation parameters in the binary Pb-Sn system at room temperature [9]. In order to complete this, we use the principle of maximum rate of free energy release and balance of entropy fluxes for the description of discontinuous precipitation kinetics for binary polycrystaUine alloys and independent determination of three basic parameters interlamellar distance, rate of phase transformation front, and concentration profile close to the transformation front. While solving the problem, we also find the optimal concentration distribution of components both along the precipitation lamella behind the transformation front and close to it, as well as the degree of the components separation. 

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