Solute rotational relaxation at liquid interfaces

Solute rotational energy and orientation relaxation have been heavily studied in bulk liquids experimentally using a variety of techniques and also theoretically mainly with continuum models and In contrast, 

The many studies in bulk liquids have clarified contributing factors that influence rotational dynamics in condensed media. A natural question is how do these factors play out at liquid interfaces In particular, liquid density and polarity have been identified as two of the most important factors affecting rotational energy and reorientation relaxations in the bulk, and, as discussed earlier in this chapter, both are modified significantly at liquid interfaces. 

The reduced density at a liquid/vapor interface is expected to lower the collision frequency and to reduce the rate of rotational energy relaxation. At the same time, fewer collisions enable faster scrambling of molecular orientations and thus are expected to increase the rate of orientational relaxation (as long as 

In polar liquids, a polar solute experiences an additional friction, called the dielectric friction, produced by a lag in the electrostatic forces as the solute dipole rotates away from its equilibrium orientationT ° ° The reduced polarity at the liquid-vapor and water-organic liquid interfaces is thus expected to slow energy relaxation and speed up reorientation. However, surface roughness, capillary fluctuations, and the ability of an ionic solute to keep its hydration shell can complicate this picture. 

The reorientation dynamics can be studied theoretically by computing the equilibrium orientational correlation function, defined as  

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