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Van der Spoel and coworkers (15) have recently shown that, in simulations, a number of ions in small water droplets actually follow the opposite trend, with negative enthalpies and entropies of adsorption.
Here we present a measurement of temperature-dependent adsorption free energy for a prototypical inorganic ion at the liquid water surface, which also yields considerably negative enthalpy and entropy of adsorption.
The resulting adsorption enthalpy and entropy changes of this prototypical chaotrope were both determined to be negative.
This surprising result is supported by molecular simulations, which clarify the microscopic origins of observed thermodynamic changes.
The general approach and apparatus used in our resonance-enhanced UV second harmonic generation (SHG) experiments are described elsewhere in connection with our previous studies of thiocyanate as a prototype for the behavior of chaotropic ions (8, 22, 23). Changes in SHG signal with temperature could in general be due to either a change in number of chromophores in the interface, variation in the SHG oscillator strength due to a spectral shift in the resonant (charge transfer to solvent) transition frequency, or a change in interfacial susceptibility corresponding to altered statistics of molecular orientation (23).
Measuring the SHG signal as a function of concentration allows us to distinguish among these possibilities.
Calculations reveal energetic influences of adsorbed ions on their surroundings to be remarkably local.
A significant percentage of these infections are polymicrobial containing both fungal and bacterial species, with Candida being the most prevalent fungal species associated with these infections.
The signs and magnitudes of these changes provide important constraints on the adsorption process and indicate that the competition between enthalpic and entropic forces underlying adsorption mechanism is resolved in an unanticipated way.
Accompanying calculations indicate the generality of this finding and reveal underlying driving forces that have been overlooked in theories of interfacial solvation. 1 shows the SHG signal collected as a function of bulk concentration of Na SCN (sodium thiocyanate) at five temperatures.
This debate is of quite general interest because of the established similarity of ion adsorption to the air/water interface and hydrophobe/water interfaces of proteins, and the corresponding relevance to vital biological phenomena (e.g., protein folding and solubility, enzyme activity) (16, 17), as embodied in the famous Hofmeister series of relative lyotropic ion strength (1, 2, 16, 17).
In this ongoing debate, ion polarizability has been invoked as the essential ionic property by several authors (5, 17, 18).