Polydisperse smooth and spherical biocolloidal particles were suspended in aqueous media and allowed to consolidate via evaporation-induced self-assembly. The stratification of the particles at the solid–air interface was markedly influenced, but not monotonically, by the drying rate. Cross-sectional imaging via electron microscopy indicated a structured coating morphology that was distinctive from that obtained by using particles with a mono- or bimodal distribution. Segregation patterns were found to derive from the interplay of particle diffusion, interparticle forces, and settling dynamics. Supporting our experimental findings, computer simulations showed an optimal drying rate for achieving maximum segregation. Overall, stratified coatings comprising nano- and microparticles derived from lignin are expected to open opportunities for multifunctional structures that can be designed and predicted on the basis of experimental Péclet numbers and computational order.

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Here, we examine a well-characterized model system of phospholipids in cyclohexane via molecular dynamics simulations using a force field known for reproducing both phospholipid behavior in water and cyclohexane bulk properties to a high accuracy, CHARMM36, with the aim of evaluating the transferability of a force field parametrization from an aqueous environment to an organic solvent. We compare the resulting reverse micelles with their expected experimental shape and size, and find the model struggles with reproducing basic, experimentally known reverse micellar structural characteristics for common phosphadidylcholine lipids such as 1,2-dipalmitoyl-snglycero-3-phosphatidylcholine (DPPC), 1,2-dioleyl-sn-glycero-3-phosphatidylcholine (DOPC), and 1,2-dilinoleyl-sn-glycero-3-phosphatidylcholine (DLPC) in cyclohexane solvent. We find evidence that the deviation from the experimental behavior originates from an underestimation of the lipid tail-cyclohexane interaction in the model. We compensate for this, obtain reverse micellar structures within the experimentally expected range, and characterize these structurally in molecular detail. Our findings indicate extra caution and verification of model applicability is warranted in simulational studies employing standard biomolecular models outside the usual aqueous environment. C 2015 AIP Publishing LLC. [http://dx

We have studied adsorbed layers of cetyltrimethylammonium bromide (CTAB) at air-water interfaces in the presence of added electrolyte. Fast bubble compression/expansion measurements were used to obtain the surface equation of state, i.e., the surface tension vs CTAB surface concentration dependence. We show that while a simple model where the surfactant molecules are assumed to be noninteracting is insufficient to describe the measured response of the surfactant layer, a modified Frumkin equation where the local interactions between the molecular components depend on their surface concentration captures the response. The variation of the effective interactions in the surfactant layer in the model shows that the interactions in the surfactant layer change from effectively repulsive to attractive with increasing surface concentration. Molecular dynamics simulations are performed to probe the origins of the change in the interactions. The simulations indicate that already at low surface concentrations the surfactants aggregate as highly dynamic rafts with surfactant orientation parallel to the interface. Increasing the concentration leads to a change in the assembly morphology at the interface: the surfactant layer thickens and the surfactants sample a range of tilted orientations with respect to the interfacial plane. The change from transient raftlike assemblies to dynamical aggregates at the interface involves a clear increase in the degree of counterion binding: we speculate that the flip of the effective interaction parameter in the model used to interpret the experimental results could result from this. The work here presents basic steps toward a proper understanding of the molecular organization and interactions of surfactants at an air-water interface. This is crucially important in understanding macroscopic properties of surfactant-stabilized systems such as foams, emulsions, and colloidal dispersions.

In this work, we employ all-atom molecular dynamics simulations to examine the hydration response of phospholipid reverse micelles in cyclohexane. This ternary phospholipid-water-cyclohexane system is an important organogel forming system and the focus of this study is on gaining insight on the factors governing the gelation transition. We map the contributions rising from specific lipid-lipid and lipid-water interactions, and their response to increasing aggregate size and changes in water-to-lipid ratio. We find that, opposed to phospholipid-heptane organogels, in cyclohexane, lipid bridging and hydrogen bond driven stabilization of the lipid head group packing is at minor role in dictating the reverse micelle structural transitions corresponding to the organosol-organogel phase transition in this system. Instead, increasing the lipid head hydration changes the lipid packing factor directly which leads to gelation through the formation of long, wormlike micelles. Furthermore, the confined environment in the reverse micellar cores slows down the water dynamics significantly in comparison to fully hydrated phospholipid bilayers and at low water-to-lipid ratios this slow-down is even more significant. The findings map the role of hydration at microscopic level in these systems and could enable tailoring reverse micellar systems for applications relying on the structure and dynamics of the reverse micelles. Examples include such as drug transport, nanotemplating, or confined chemistry in the reverse micelle core water space, e.g., in catalysis.