Mesoscale Simulations of Surfactant Dissolution and Mesophase Formation

Abstract

The evolution of the contact zone between pure surfactant and solvent has been studied by mesoscale simulation. It is found that mesophase formation becomes diffusion controlled and follows the equilibrium phase diagram adiabatically almost as soon as individual mesophases can be identified, corresponding to times in real systems of order 10 µs.When pure surfactant comes into contact with water, mesophases appear at the interface. This is an important process not only for the practical use of surfactants, but also from the point of view of fundamental surfactant phase science. Indeed, contact 'flooding' or penetration scan experiments can yield quantitative information on mesophases as a function of composition [1]. The phenomenon is invariably diffusion controlled in the sense that the widths of the mesophases follow t 1/2 growth laws, and adiabatic in the sense that the local composition determines the mesophase boundaries according to the equilibrium phase diagram [2,3,4]. But penetration scan experiments are restricted to observation times of minutes to hours: what happens on time scales shorter than this? How early does diffusion control set in, and how soon can one expect the mesophase boundaries to track the local composition adiabatically? Such questions are not just of scientific interest since time scales of seconds or less are important in modern processing and the everyday use of surfactants, for instance determining how rapidly washing powder dissolves. Experimentally, this regime is very difficult to access because of the short time scales and the relatively small amounts of mesophase involved. To probe these questions therefore, we have therefore undertaken novel mesoscale simulations of surfactant dissolution. We find adiabatic diffusion control is established remarkably rapidly in our model, on time scales in which only a few repeat units of the growing mesophases have appeared, corresponding to times in the real systems of order 10 µs.The model we have used is a 'minimalist' particlebased model of a binary surfactant / water mixture, based on the dissipative particle dynamics (DPD) method [5,6]. In DPD, the particles are soft spheres, interacting with pairwise soft potentials of the form U = 1 2 A(1 − r/r c ) 2 (r < r c ) where r is the particle separation, r c is the range of the interaction, and A the amplitude. In the model we have three species of particles: A, B and C. The A and B particles are bound together in pairs as dimers with a fixed separation r d , and represent the surfactants. The C particles are monomers representing the solvent (water). The different species are distinguished by their interaction amplitudes, and the trick is to find a set of amplitudes which recover suitable phase behaviour. Following earlier work [6], we use A AA = A BB = A CC = 25, A AB = 30, A AC = 0, A BC = 50 and r d = 0.5 which gives phase diagram features lying in a convenient temperature range around k B T ∼ 1 (we fix units by choosing m = r c = 1 where m is the mass of the particles). Fig. 1 shows the...

Effect of surfactant structure on interfacial properties

“…The dissipative and random forces are chosen such that a proper Boltzmann distribution of configurations is sampled corresponding to the intermolecular interactions from which the conservative forces are derived [12]. In analogy with previous simulations of surfactants using the DPD technique [13], we use softrepulsive interactions to mimic the mesoscopic interactions between the oil, water, and surfactant molecules. In our model, we distinguish four types of particles, o, w, h, and t, to mimic the oil, water, and the head and tail molecular groups of a surfactant, respectively.…”

“…1. This model maps typically 3-6 CH 2 groups onto one tail bead [13][14][15]. We simulated a system with approximately 8000 particles at temperature T = 1.0 and pressure P = 23.6 corresponding to a bulk density of 3.0.…”

Swelling and shrinking kinetics of a lamellar gel phase

“…In everyday life and many industrial processes, materials swell by absorption of solvent, e.g., washing powder, 1 foodstuff, 2 diapers, 3 eyeballs, 4 and clay. 5 Conversely, if the solvent flow is reversed materials shrink, as for a hypertonic cell with a lower solute concentration than its environment.…”

From molecular dynamics to hydrodynamics: A novel Galilean invariant thermostat

“…15 At the same value of interaction parameters and system composition, DPD and NHLAT are leading to a similar phase structures. Since NHLAT allows bigger time steps and the diffusion coefficient is higher, the equilibrium phase is reached about 1.5 times faster when compared to DPD.…”