A.J. Howes
Graduate Student, Ph.D. Program
University of Virginia, 2002
B.S. Chemical Engineering
Thesis: Physiochemical Properties of Protein Systems
that Affect Solubility and Adsorption
Research Interest:
Thermodynamic Modeling of Equilibrium Surface Tension at Fluid-Fluid Interfaces
Molecular Simulation of Surfactrant Adsorption at Fluid-Fluid Interfaces
Interfacial Hydrodynamics and Film Stability
Research Summary:
See our publication in Langmuir 23 (2007): 1835 - 1844 (requires subscription)
Thesis Abstract:
Towards a Molecular Understanding of Surfactant Adsorption at Fluid-Fluid Interfaces Using Monte Carlo Simulation of Lennard-Jones Amphiphiles
We present Monte Carlo simulations of nonionic surfactant adsorption at the liquid/vapor interface of a monatomic solvent. All molecules in the system, solvent and surfactant, are characterized with the Lennard-Jones (LJ) potential using differing and judiciously chosen interaction parameters. Surfactant molecules consist of an amphiphilic chain with a solvophilic head and a solvophobic tail. Adjacent concerted atoms along the surfactant chain are connected by finitely extensible harmonic springs. Solvent molecules move via the Metropolis random-walk algorithm, whereas surfactant molecules move according to the continuum configurational bias Monte Carlo (CBMC) method.
We desire not only a description of the surfactant molecular architecture at the interface but also a quantitative assessment of the adsorption and surface-tension isotherms. To this end, we generate quantitative thermodynamic adsorption and surface-tension isotherms in addition to surfactant radius of gyration, tilt angles, and potentials of mean force. Surface-tension simulations compared to those calculated from the simulated adsorbed amounts and the Gibbs adsorption isotherm agree confirming equilibrium in our simulations. We find that the classical Langmuir isotherm is obeyed for our LJ surfactants over the range of head and tail lengths studied.
Although simulated surfactant chains in the bulk solution exhibit random orientations, surfactant chains at the interface orient roughly perpendicular and the tails elongate compared to bulk chains even in the submonolayer adsorption regime. At a critical surfactant concentration, designated as the critical aggregation concentration (CAC), we find aggregates in the solution away from the interface. At higher concentrations, simulated surface tensions remain practically constant. Using the simulated potential of mean force in the submonolayer regime and an estimate of the surfactant footprint at the CAC, we predict a priori the Langmuir adsorption constant, K L , and the maximum monolayer adsorption, G m . We find that the maximum area per molecule, 1/ G m , increases as the number of solvophobic tail molecules increases. This effect is accounted for by the presence of decreasing amounts of solvent in between interfacial surfactant molecules as the solvophobic tail number increases. Furthermore, adsorption is driven not by proclivity of the surfactant for the interface, but by the dislike of the surfactant tails for the solvent, that is by a “solvophobic” effect. Accordingly, we establish that a coarse-grained LJ surfactant system mimics well the expected equilibrium behavior of aqueous nonionic surfactants adsorbing at the liquid/vapor interface.
Using the aforementioned LJ surfactant system, we study the effect of intermolecular attraction amongst surfactants, mixtures of surfactants, and surfactant structure (e.g., single-tailed, double-tailed, and branch-tailed) on the adsorption and the surface tension isotherms. We find that increasing surfactant tail attractions decreases the critical aggregation concentration of surfactants, while also allowing for more surfactants to reside at the interface. Moreover, the adsorption and surface-tension isotherms with and without tail attractions have identical values at low concentrations, but deviate at high concentrations. Molecularly, the tail attractions are allowing the surfactant molecules to work together (i.e., cooperate) in order to adsorb closer together and adsorb closer to the vapor phase as compared to surfactants without tail attractions.
Mixtures of surfactants are shown to obey the Ideal Adsorbed Solution (IAS) theory for adsorption and surface-tension, requiring only the G m and K L already determined from simulations of single types of surfactants for successfully fitting. We find that theories such as Scaled Particle Theory (SPT) are unable to capture successfully the adsorption and surface- tension behavior of our mixtures. In addition, we discuss the thermodynamic consistency of some of the available mixture models in the literature.
Double- and branch-tailed surfactants are more efficient at the interface as compared to single-tailed surfactants, i.e., a smaller number of surfactants at the interface reduce the tension a given amount if they have two tails instead of one. Furthermore, double- and branch-tailed surfactants are more effective, i.e., they achieve a larger overall reduction in the tension for a smaller concentration as compared to single-tailed surfactants. We hypothesize that the increased effectiveness and efficiency is related to the molecule geometric structure and the presence of two terminal tail groups for double- and branch-tailed surfactants.
In this work, model LJ surfactant molecules are studied that are strongly surface active and yet nonvolatile. The system size is large enough that bulk liquid surfactant concentrations can reliably be ascertained permitting calculation of equilibrium adsorption and surface-tension isotherms. Ultimately, our simple LJ coarse-grained surfactant model proves very successful at mimicking nonionic-surfactant adsorption behavior at the liquid/vapor interface for a number of different systems.