Surface-induced disordering

Fig. 67. Order parameter profiles m(z)/mt, associated with surface-induced disorder. The coordinate z measures the distance from the surface (z = 0). is the bulk correlation length and mu the bulk order parameter. If ease (a) persists up to the first-order transition temperature Tc, this means the surface stays ordered up to 7 c, while case (b) shows surface induced disordering a layer of thickness L gets disordered already at T <71, and as T — Tc the (delocalized) interface at mean position z = L from the surface advances into the bulk, fJ(7 ) -+ oo as T - Tc, and the surface order parameter mj = m(z = 0) then vanishes continuously, mj a (1 - T/Tc). From Dasch et al. (1988).

Another long range force that must be considered for surface-induced disordering of solids are elastic forces (Wagner, 1978), which tend to truncate... 

Some solid surfaces induce disorder in nematic liquid crystals. It means that the order parameter at the interface is lower than the bulk value. For instance, evaporated SiO layers of a certain thickness due to their roughness decrease the order parameter of MBBA from the bulk value Sb 0.6 down to So 0.1-0.2. In some cases, the surface order parameter may be equal to zero (surface melting). 

Real surfaces will always exhibit a certain number of defects at temperatures above 0 K (Fig. 8.13). This is true in spite of the fact that defects have a positive energy of formation compared to an ideal crystalline surface. What stabilizes these defects is the change in entropy connected with the induced disorder. Therefore, a certain average number of defects — that increases with temperature — will be present. 

For molecular systems at surfaces, one must take into account that STM may interfere with natural assembly. That is, the tip-molecule interaction may induce disorder during scanning operation. In other words, defined molecular 2D crystals can be distorted by this method, not allowing true geometric analysis. Again, low temperatures in UHV or saturated monolayer systems, in general, help to circumvent this problem. 

Effect of Partial Disorder. The oscillations of the polarization are a consequence of the long-range order along the direction normal to the surface. For planar surfaces, some disorder is, however, induced by the gradual decrease of the cluster alignment away from the surface. 

The results suggest that a gel might form above the surface of silica, as proposed by Vigil et al. [36], which induces disorder in the interfacial water layers. 

Hydroxyl radicals are probably the most toxic for microorganisms [30], They promote peroxidation of polyunsaturated phospholipid components of the lipid membrane and induce disorder in the cell membrane [31]. The damage of the outer membrane increases the permeability to ROSs. This process is possible thanks to a sufficient lifetime of ROSs generated at the Ti02 surface. ROS diffusion was studied by Fujishima et al. Their experiments demonstrated the bactericidal effect of irradiated Ti02 film on E. coli even at the distance of 50pm from the film [21], Furthermore, oxidative perforation of the cellular membrane allows the photocatalyst nanoparticles to penetrate the interior of the cell, causing severe, efficient oxidation of the cell content [27,32], ROSs are responsible for oxidation of amino acids, peptides [33], enzymes [34], and nucleic acids [32, 35-37]. Destruction of... 

A variety of defect formation mechanisms (lattice disorder) are known. Classical cases include the - Schottky and -> Frenkel mechanisms. For the Schottky defects, an anion vacancy and a cation vacancy are formed in an ionic crystal due to replacing two atoms at the surface. The Frenkel defect involves one atom displaced from its lattice site into an interstitial position, which is normally empty. The Schottky and Frenkel defects are both stoichiometric, i.e., can be formed without a change in the crystal composition. The structural disorder, characteristic of -> superionics (fast -> ion conductors), relates to crystals where the stoichiometric number of mobile ions is significantly lower than the number of positions available for these ions. Examples of structurally disordered solids are -> f-alumina, -> NASICON, and d-phase of - bismuth oxide. The antistructural disorder, typical for - intermetallic and essentially covalent phases, appears due to mixing of atoms between their regular sites. In many cases important for practice, the defects are formed to compensate charge of dopant ions due to the crystal electroneutrality rule (doping-induced disorder) (see also -> electroneutrality condition). 

So the description of surface-induced ordering at the surface of a semi-infinite lamellar block copolymer melt is the gradual unbinding of an interface between a thin ordered layer at the surface and the disordered bulk as %—>%t the distance of this interface from the surface diverges, Eq. (76). At this point, we note two generalizations ... 

The situation is fully analogous to complete wetting at the surfaces of fluids or fluid mixtures [220], of course. Perhaps the closest analogy occurs between surface-induced lamellar ordering and the surface melting [220] of crystals - the distinction being, of course, that in the latter case it is the disordered rather than the ordered phase that is stabilized by the surface. 

In the last decade an abundant literature has focused more and more on the properties of low-symmetry systems having large unit cells which render unwieldy the traditional description in terms of the Bloch theorem. Low-symmetry systems include compUcated ternary or quaternary compounds, man-made superlattices, intercalated materials, etc. The k-space picture becomes totally useless for higher degrees of disorder as exhibited by amorphous materials, microcrystallites, random alloys, phonon-induced disorder, surfaces, adsorbed atoms, chemisorption effects, and so on. 

Segregation/order factor Surface induced order/disorder or neither... 

Surface-induced ordering and disordering surface melting. 262... 

Ohta M, Buckton G (2004) Determination of the change in surface energetics of cefditoren pivoxil as a consequence of processing induced disorder and equilibration to different relative humidities. Int J Pharm 269 81-88. 

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