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...

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We have examined the early stages of surfactant dissolution and mesophase formation using a dimer-solvent model with phase behaviour representative of surfactant-water systems. We use an orientational order parameter to characterize systematically the appearance of mesophases. We find the process is diffusion controlled, and the appearance of mesophases is governed adiabatically by the equilibrium phase diagram from the earliest point at which the orientational order parameter can reliably distinguish between mesophases, when only a few repeat spacings of the mesophase microstructures are present. In real systems, such a stage would correspond to times of the order of a few microseconds after the initial contact.

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