FREE-ENERGY COST OF BENDING N-DODECANE IN AQUEOUS-SOLUTION - INFLUENCE OF THE HYDROPHOBIC EFFECT AND SOLVENT-EXPOSED AREA

Computer simulations of the free energy associated with different conf ormations of a single n-dodecane molecule in liquid water are reported . The alkane chain was monitored as a function of the end-to-end dista nce, essentially following the formation of a hairpin bend from an ini tially fully extended state. While elongated conformations are among t he most stable state in both gas and solvated states, the influence of the solvent is to favor more compact states. At the most compact conf ormation, the hairpin bend, the solvent imparts a stabilizing free-ene rgy contribution of 6 kJ/mol. The torsional strain of bending the mole cule into this hairpin conformation is partly relieved by the reductio n of entropically unfavorable water molecules associated with the redu ction in the alkane's surface exposed area. The resultant solvent cont ribution to the free-energy change is characterized by an unfavorable enthalpic and a competing favorable entropic (-T Delta S) contribution . The conformational change associated with the transition from an ext ended state to a compact state for n-dodecane is accompanied by a rela tively small, non-uniform change in surface exposed area. A simplified model of the free energy as a function of this area can account for t he gross effects but is unable pick up the molecular details inherent in the system.