Unassisted transport across

Computer simulations of both equilibrium and dynamic properties of small solutes indicate that the solubility-diffusion model is not an accurate approximation to the behavior of small, neutral solutes in membranes. This conclusion is supported experimentally [57]. Clearly, packing and ordering effects, as well as electrostatic solute-solvent interactions need to be included. One extreme example are changes in membrane permeability near the gel-liquid crystalline phase transition temperature [56]. Another example is unassisted ion transport across membranes, discussed in the following section. 

Fluctuations of interfaces are directly relevant to a number of interfacial phenomena. One example, ion transfer across a liquid-liquid interface, will be discussed in Section 6.1. Another example is the behavior of monolayers of surfactants on water surfaces. Surface fluctuations are also fundamental to several processes in water-membrane systems, such as unassisted ion transport across lipid bilayers and the hydration forces acting between two membranes. Here, however, the problem is more complicated because not only capillary waves but also bending motions of the whole bilayer have to be taken into account. Furthermore, the concept of the surface tension is less clear in this case. This topic is discussed in Molecular Dynamics Studies of Lipid Bilayers. 

The molecular mechanism of ion transfer across liquid-liquid interfaces is remarkably similar to the mechanism of unassisted ion transport across membranes, recently studied by molecular dynamics simulations. The study revealed that permeation of ions into the membrane is accompanied by the formation of deep, asymmetric thinning defects in the bilayer, whereby polar lipid head groups and water penetrate the nonpolar membrane interior. As can be seen in Figure 14, these defects are quite similar to water fingers observed during... 

To help answer this question, a detailed, molecular simulation of the unassisted ion transport of Na+ and Cr across a GMO bilayer was performed using molecular dynamics [25]. Due to the large free energy barrier, however, the transfer does not occur on the timescales accessible to computer simulations. Therefore, a series of molecular dynamics trajectories were generated in which the ion was restricted to overlapping regions along the interface normal (c-direction). 

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