Hydrophobic pores

As has been described in Ref. 70, this approach can reasonably account for membrane electroporation, reversible and irreversible. On the other hand, a theory of the processes leading to formation of the initial (hydrophobic) pores has not yet been developed. Existing approaches to the description of the probability of pore formation, in addition to the barrier parameters F, y, and some others (accounting, e.g., for the possible dependence of r on r), also involve parameters such as the diffusion constant in r-space, Dp, or the attempt rate density, Vq. These parameters are hard to establish from first principles. For instance, the rate of critical pore appearance, v, is described in Ref. 75 through an Arrhenius equation ... 

It follows from previous discussion that the destabilizing electrostatic contribution grows in absolute value with x (with increasing A.). But the influence of the nonuniform electrical force is overwhelmed by the stabilizing bending and stretching contributions. As a result, the traditional smectic model cannot explain how a small transmembrane voltage can lead to membrane breakdown. The obvious solution is to abandon this approach and to develop an alternative, such as the pore formation model. However, as we noticed before, this approach postulates rather than proves the appearance of hydrophobic pores. 

It is theoretically predicted [10] and experimentally proven [11] that the existence of a network of very tiny hydrophobic pores in the carbon-catalyst grains play an important role for the effective operation of the electrodes and is of prime importance for their long exploitation life. 

After testing a number of DLs with and without MPLs, Lin and Nguyen [108] postulated that the MPL seemed to push more liquid water back to the anode through the membrane. Basically, the small hydrophobic pores in the MPL result in low liquid water permeability and reduce the water transport from the CL toward the DL. Therefore, more liquid water accumulated in the CL is forced toward the anode (back diffusion). This reduces the amount of water removed through the cathode DL, decreases the number of blocked pores within the cathode diffusion layer, and improves the overall gas transport from the DL toward the active zones. 

Fairweather et al. [204] developed a microfluidic device and method to measure the capillary pressure as a function of fhe liquid water saturation for porous media wifh heferogeneous wetting properties during liquid and gas intrusions. In addition to being able to produce plots of capillary pressure as a function of liquid wafer safuration, their technique also allowed them to investigate both hydrophilic and hydrophobic pore volumes. This method is still in its early stages because the compression pressure and the temperatures were not controlled however, it can become a potential characterization technique that would permit further understanding of mass transport within the DL. 

For the membrane interior, independent sources of informahon exclude the existence of vapor and of hydrophobic pores ... 

The structural model of GebeF and its recent overhaul by Schmidt-Rohr o show no indicahon of hydrophobic pores these models correspond to hydrated cylindrical fibrils or water-containing inverted cylinders with rather uniform distribution of charged surface groups at polymer-water interfaces. 

The values of the effective permeabilities vary over orders of magnitude, and this corresponds to the different results of the models. Furthermore, as discussed in various papers,the effective permeability of Natarajan and Nguyen (curve e) varies significantly over a very small pressure range, although they state that their capillary-pressure equation mimics data well. With respect to the various equations, the models that use the Leverett J-function all have a similar shape except for that of Berning and Djilali (curve a), who used a linear variation in the permeability with respect to the saturation. The differences in the other curves are due mainly to different values of porosity and saturated permeability. As mentioned above, only the models of Weber and Newman (curve d) and Nam and Kaviany (curve f) have hydrophobic pores, which is why they increase for positive capillary pressures. For the case of Weber and Newman, the curve has a stepped shape due to the integration of both a hydrophilic and a hydrophobic pore-size distribution. 

Using a similar approach, Notman et al. [81], determined the free energy for pore formation in bilayers composed of ceramide, as a model for the stratum corneum of the skin, both in the presence and in the absence of DMSO. Without DMSO, the bilayer was in the gel phase, and interestingly, a hydrophobic pore was observed with a high free-energy barrier ( 60 kj/mol). In the presence of DMSO, the bilayer was more fluid, and the more typical hydrophilic pore was observed, with a much smaller activation energy of 20kJ/mol. This work provided a thermodynamic and structural explanation for the enhanced permeability of skin by DMSO. 

Figure 19.3 Schematic model of biomimetic material M3 for amine signaling. Changes in color are due to the reaction between the pyrylium ring and the primary amine. Short amines are too hydrophilic and do not enter into the hydrophobic pores, whereas long-chain amines tend to clog in the pore openings.

Separate fractions (nanophases) of near-surface water attached probably by hydrophobic surface segments and hydrophobic pores as well as by polar chemical groups have been revealed in water fullerene and shungite carbon nanoparticles... 

Within the micropores both hydrophilic and hydrophobic pores, of mineral and carbon origin, could be detected. The rieh earbonaeeous materials (gas - coking coal, cellulose) enriched the adsorbents structure in the hydrophobic micropores. 

As we have mentioned in the Introduction, the location of the critical point of the lowest density liquid-liquid transition of real water is unknown and both scenarios (critical point at positive or at negative pressure) can qualitatively explain water anomalies. Recent simulation studies of confined water show the way, how to locate the liquid-liquid critical point of water. Confinement in hydrophobic pores shifts the temperature of the liquid-liquid transition to lower temperatures (at the same pressure), whereas effect of confinement in hydrophilic pores is opposite. If the liquid-liquid critical point in real water is located at positive pressure, in hydrophobic pores it may be shifted to negative pressures. Alternatively, if the liquid-liquid critical point in real water is located at negative pressure, it may be shifted to positive pressures by confinement in hydrophilic pores. Interestingly, that it may be possible in both cases to place the liquid-liquid critical point at the liquid-vapour coexistence curve by tuning the pore hydrophilicity. We expect, that the experiments with confined supercooled water should finally answer the questions, concerning existence of the liquid-liquid phase transition in supercoleed water and its location. 

Figure 7.8 log-log plot (solid lines) of the collective orientational correlation functions Qoii for water confined in pores of different diameters (inserted number) and power-law fits (dashed lines). Lower and upper traces are for hydrophilic and hydrophobic pores, respectively [16],... 

---