The interfacial structure of nano- and micron-sized oil and water droplets stabilized with SDS and Span80.

In this work, we provide a comparison between the stability and the interfacial structure of micrometer-sized and nanometer-sized droplets by employing a multi-instrumental approach comprised of the surface-sensitive technique of sum frequency scattering as well as dynamic light scattering and microscopy. We monitor the stability of oil-in-water and water-in-oil emulsions and the structure of surfactants at the oil/water nano-interface, when stabilized with an oil-soluble neutral surfactant (Span80), a water-soluble anionic surfactant (sodium dodecyl sulfate, SDS), or with a combination of the two. Micron-sized droplets are found to be stabilized only when a surfactant soluble in the continuous phase is present in the system, in agreement with what is traditionally observed empirically. Surprisingly, the nanodroplets behave differently. Both oil and water nanodroplets can be stabilized by the same (neutral Span80) surfactant but with different surface structures. A combination of SDS and Span80 also suffices, but for the case of water droplets, the strongly amphiphilic SDS molecules are not detected at the interface. For the case of oil droplets, both surfactants are at the interface but do not structurally affect one another. Thus, it appears that, in this study, empirical rules such as the Bancroft rule, the hydrophile-lipophile-balance scale, and the surfactant affinity difference predict the stability of the micrometer-sized droplets better than the nanometer-sized ones, probably due to a different balance of interactions on different length scales.

The Diverse Nature of Ion Speciation at the Nanoscale Hydrophobic/Water Interface.

Many biological systems are composed of nanoscale structures having hydrophobic and hydrophilic groups adjacent to one another and in contact with aqueous electrolyte solution. The interaction of ions with such structures is of fundamental importance. Although many studies have focused on characterizing planar extended (often air/water) interfaces, little is known about ion speciation at complex nanoscale biological systems. To start understanding the complex mechanisms involved, we use a hexadecane nanodroplet system, stabilized with a dilute monolayer of positively charged dodecyltrimethylammonium cations (DTA+) groups in contact with an electrolyte solution (NaSCN). Using vibrational sum frequency scattering, second harmonic scattering, ζ-potential measurements, and quantum density functional theory, we find DTA+-SCN- ion pairing at concentrations as low as 5 mM. A variety of ion species emerge at different ionic strengths, with differently oriented SCN- groups adsorbed on hydrophilic or hydrophobic parts of the surface. This diverse and heterogeneous chemical environment is surprisingly different from the behavior at extended liquid planar interfaces, where ion pairing is typically detected at molar concentrations and nanoscale system stability is no requirement.

The Molecular Mechanism of Nanodroplet Stability.

Mixtures of nano- and microscopic oil droplets in water have recently been rediscovered as miniature reaction vessels in microfluidic environments and are important constituents of many environmental systems, food, personal care, and medical products. The oil nanodroplet/water interface stabilized by surfactants determines the physicochemical properties of the droplets. Surfactants are thought to stabilize nanodroplets by forming densely packed monolayers that shield the oil phase from the water. This idea has been inferred from droplet stability measurements in combination with molecular structural data obtained from extended planar interfaces. Here, we present a molecular level investigation of the surface structure and stability of nanodroplets and show that the surface structure of nanodroplets is significantly different from that of extended planar interfaces. Charged surfactants form monolayers that are more than 1 order of magnitude more dilute than geometrically packed ones, and there is no experimental correlation between stability and surfactant surface density. Moreover, dilute negatively charged surfactant monolayers produce more stable nanodroplets than dilute positively charged and dense geometrically packed neutral surfactant monolayers. Droplet stability is found to depend on the relative cooperativity between charge-charge, charge-dipole, and hydrogen-bonding interactions. The difference between extended planar interfaces and nanoscale interfaces stems from a difference in the thermally averaged total charge-charge interactions in the two systems. Low dielectric oil droplets with a size smaller than the Debye length in oil permit repulsive interactions between like charges from opposing interfaces in small droplets. This behavior is generic and extends up to the micrometer length scale.

Interfacial Structure and Hydration of 3D Lipid Monolayers in Aqueous Solution.

Three-dimensional (3D) phospholipid monolayers at hydrophobic surfaces are ubiquitous and found in nature as adiposome organelles or in man-made materials such as drug delivery systems. However, the molecular level understanding of such monolayers remains elusive. Here, we investigate the molecular structure of phosphatidylcholine (PC) lipids forming 3D monolayers on the surface of hexadecane nanodroplets. The effects of acyl chain length, saturation, and number of acyl tails per lipid were studied with vibrational sum frequency and second harmonic scattering techniques. We find that 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC) lipids form tightly packed monolayers. Upon shortening the tail length to 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), more gauche defects are observed. Monolayers of unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and single acyl chained 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (lyso-PC) contain more disorder. Despite these variations in the packing, the headgroup orientation remained approximately parallel to the nanodroplet interface. Remarkably, the lyso-PC uniquely forms more diluted and "patchy" 3D monolayers.