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AbstracLIn this anicle we present a method for alculating the chemical potential of arbitrary chain molecules in a mmpuier simulation. The method is based on a generalization of Siepmann's method tor mlculaling the chemical potential of chain molecules with a finite number of conformations. Next. we show that i t is also possible 10 atend the configurational-bias Monte Carlo scheme developed recenty by Siepmann and

6933dynamics. Also, the rotational collisional efficiency is expected to increase as the expansion cools the gas, and therefore the assumption of a constant efficiency is unrealistic.We have obtained excellent agreement between the generalized Boltzmann flow theory and the experimental distribution measurements for stagnation pressures to approximately 400 Torr.Above this pressure higher terminal rotational temperatures than predicted with F = 0.17 were observed. Such discrepancy in the expansion at high pressure could be due to mechanisms such as molecular clustering which release energy to the flow. In regard to the rotational distributions observed off the central streamline, they remain Maxwellian across a 30° span from the center and Trot was constant. It thus appears that, at modest stagnation pressures, molecular free jet flows of pure N2 can now be accurately modeled within a central streamline cone of 60'. ConclusionWe have demonstrated an alternative method for studying the rotational evolution of molecular free jet expansions. Our observations show that for N2 the evolution of the highly populated lower rotational levels remains Maxwellian. From the observed signal to noise ratio, that is, our minimum detectable signal, we find less than 0.2% of the total population can reside in any particular unobserved rotational level. Using this value and the errors in the fit we can conservatively state that better than 95% of the N2 population is described by a Maxwell-Boltzmann distribution function with a rotational temperature, T,,(z). For expansions with low stagnation pressure, 400 Torr or less with the nozzles employed in this work, the evolution of Tw agrees well with a numerical solution of the generalized Boltzmann equation. Further measurements on rotational cooling with a variety of molecules and the connection of these results with the existing studies on translational distributions will provide further evidence useful in mapping the total distribution function of a molecular free jet. In this letter a simple oil/water/surfactant model is presented. This model allows us to study systematically, using the molecular dynamics technique, the relation between the structure of surfactants and various thermodynamic properties. Specifically, the effect of increasing the hydrophobic tail length on the interfacial tension has been studied. Experimental results on various model surfactants are also presented and compared with molecular dynamics calculations. The experimental data as well as the simulations indicate that increasing the tail length makes the surfactants more effective in decreasing the interfacial tension at low concentration. Acknowledgment. This work was supported by the

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