Surface energy anisotropy, defined

The surface tension defined above was related to an interface that behaved mechanically as a membrane stretched uniformly and isotropically by a force which is the same at all points on the surface. A surface property defined this way is not always applicable to the surfaces of solids and the surface energy of planar surfaces is defined to take anisotropy into account. The surface energy is often in the literature interchanged with surface tension without further notice. Although this may be useful in practice, it is strictly not correct. 

Tnterfacial phenomena play a fundamental role in biological systems. It A is important to know if surface energy and anisotropy affect the conformation of biological macromolecules. Well defined physicochemical models might simplify this problem (1- 8) spread monolayers at the air-water interface exemplify this kind of model. For polypeptides which are introduced as simple models of proteins, no surface denatura-tion of the spread macromolecules occurred (9, 10, 11). Protein structures are too complex to yield direct information about eventual changes of conformation, but one can detect the presence or the disappearance of biological activity—e.g., enzymic activity. The enzyme would be denatured if the conformation were modified by the anisotropy of the interface. 

Fig. 15 - Typical polygonal texture of smectic A phase containig ellipses of FCD-I located at the cell boundary. In between the large FCD-I one observes clear gaps that are filled with spherically curved layers. The macroscopic size of the smallest domains is defined by the balance of the elastic constant K and the anisotropy of the surface energy when one proceeds with temperature from the deep smectic A phase (a) towards the smectic B ph (b) the critical size increases and the smallest domains disappear. 

First of all, a FCD-I with conical shape has volume L x a and extends through the whole sample (length L). For example, in Figs. 14-16, each FCD-I has its base located at the sample surface. This is why the anisotropy of the surface energy is so important for the scenario with FCD-I s. In contrast, the FCD-II has a closed shape with characteristic volume a, where a might be much smaller than L. Thus the physical limit for the iteration process is defined solely by the bulk properties of the lamellar phase. 

For molecular van der Waals crystals of non-polar molecules, surface dipoles and work function anisotropy have only recently been explored [5], While variations of several tenths of an eV in the ionization energy (IE the molecular analog to a metal s <(>) depending on the molecular orientation on a surface have been reported several times [6-8], a consistent picture was lacking. An explanation for the intriguing observation that one and the same molecule can have different - still well-defined - IEs in ordered thin films is that the electrostatic potential above a molecular crystal surface is determined by the orientation of the molecules and their intramolecular charge distribution. 

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