Effects of chain length of surfactants on the interfacial tension: molecular dynamics simulations and experiments

Abstract

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

Mesoscopic models of biological membranes

“…Concerning the nonbonded interactions, they can be represented by Lennard-Jones like potentials, of by soft-repulsive potentials. These lipid models have many points in common with the earlier models developed to study the self-assembly of micelles and surfactant monolayers [69][70][71][72][73]. It is worth mentioning that, despite their simplicity, these models show a remarkable ability to reproduce the structural, thermodynamic and mechanical properties of lipid aggregates (e.g., bilayers and vesicles).…”

Investigation of Surfactant Efficiency Using Dissipative Particle Dynamics

“…These give very detailed information on the monolayers formed, and the effect of for example increased chain length has been successfully studied. 3 However, the time scales accessible to ordinary MD simulations are too short to observe diffusion to the interface and formation of micelles. Spontaneous formation of a micelle takes a few nanoseconds and could be done using fully atomistic models.…”

Systems Involving Surfactants

“…In addition, a bending potential is sometimes introduced which favors chain stretching and makes the chains stiffer. A model of this kind has first been introduced by Smit et al [135] and later employed to explore the interplay of micelle formation and amphiphile adsorption at an oil/water interface [136,137], the self-assembly of micelles in general [138,139,140,141], and that of bilayers [141]. In the case of binary systems, the model can be further simplified by ignoring the solvent particles.…”