Hydration mediated interfacial transitions on mixed hydrophobic/hydrophilic nanodroplet interfaces

Interfacial phase transitions are of fundamental importance for climate, industry, and biological processes. In this work, we observe a hydration mediated surface transition in supercooled oil nanodroplets in aqueous solutions using second harmonic and sum frequency scattering techniques. Hexadecane nanodroplets dispersed in water freeze at a temperature of similar to 15 degrees C below the melting point of the bulk alkane liquid. Addition of a trimethylammonium bromide (C(X)TA(+)) type surfactant with chain length equal to or longer than that of the alkane causes the bulk oil droplet freezing transition to be preceded by a structural interfacial transition that involves water, oil, and the surfactant. Upon cooling, the water loses some of its orientational order with respect to the surface normal, presumably by reorienting more parallel to the oil interface. This is followed by the surface oil and surfactant alkyl chains losing some of their flexibility, and this chain stretching induces alkyl chain ordering in the bulk of the alkane phase, which is then followed by the bulk transition occurring at a 3 degrees C lower temperature. This behavior is reminiscent of surface freezing observed in planar tertiary alkane/surfactant/water systems but differs distinctively in that it appears to be induced by the interfacial water and requires only a very small amount of surfactant. (c) 2018 Author(s).

Freezing effects of oil-in-water emulsions studied by sum-frequency scattering spectroscopy

Sylvie Roke, Nikolay Smolentsev

Temperature-dependent sum-frequency scattering spectroscopy is used to study the properties of hexadecane and dodecane oil droplets in water. The sum-frequency scattering spectra contain vibrational bands that correspond to the symmetric and antisymmetric CH stretching vibrations of the methylene (CH2) and methyl (CH3) groups of the alkane molecules. The relative amplitudes of the vibrational bands provide information on the surface structure and the shape of the oil droplets. We study the sum-frequency scattering spectra over a temperature range of -48 to 24 degrees C, including the freezing transitions of the water matrix and the oil droplets. Hexadecane oil droplets freeze at a higher temperature than the surrounding water, whereas dodecane oil droplets freeze at a lower temperature than the surrounding water. This allows us to independently study the freezing effect of oil and water on the surface structure of the oil droplets. In both cases, freezing leads to a change in the polarization dependencies that are valid in the case of the spherical-symmetric shapes that the oil droplets assume when both water and oil are liquid. We find that the freezing of water leads to a strong distortion of the liquid dodecane surface but has little effect on the surface of already solidified hexadecane. For completely frozen emulsions a further decrease in temperature is observed to lead to a further distortion of the surface of the solid oil particles, which might be caused by increasing hardness of the ice matrix encapsulating the particles. Published by AIP Publishing.

Amer Inst Physics2016

Tuneable PTC effect in polymer-wax-carbon composite resistors

Ludivine Ammon, Caroline Jacq, Thomas Maeder, Peter Ryser

Purpose: The resistivity dependence on temperature of composite resistors made of carbon fillers dispersed in an organic matrix are known to be strongly affected by the matrix thermal expansion. High positive temperature coefficient (PTC) effects, i.e. essentially switching from resistive to quasi-insulating behaviour, can be caused by phase changes in the matrix and the assorted volume expansion, a behaviour that has been previously shown with both simple organic waxes and semicrystalline polymers. However, waxes become very liquid upon melting, possibly resulting in carbon sedimentation, and tuneability of semicrystalline polymers is limited. Approach: We therefore study a ternary polymer-wax-conductor (ethylcellulose-octadecanol-graphite) composite resistor system, where polymer and wax fuse to a viscous liquid upon heating, and re-solidify and separate by crystallisation of the wax upon cooling. Findings: It is shown that with appropriate formulation, the resulting resistors exhibit strong PTC effects, linked with the melting and crystallisation of the wax component. The behaviour somewhat depends on sample history, and notably cooling speed. Limitations: The phase equilibria and transformation kinetics of the polymer-wax system (including possible wax polymorphism), as well as the exact mechanism of the conductivity transition, remain to be investigated. Originality/value: As many compatible polymer-wax systems with different melting/solidification behaviours are available, ternary polymer-wax-conductor composite PTC resistors allow a high tuneability of properties. Moreover, the high viscosity in the liquid state should largely avoid the sedimentation issues present with binary wax-conductor systems.

Emerald2014

From Hydrophobic to Hydrophilic: The Structure and Density of the Hexadecane Droplet/Alkanol/Water Interface

Yixing Chen, Kailash Chandra Jena, Sylvie Roke

The molecular structure of the hexadecane droplet/alkanol/water interface has been investigated using vibrational sum frequency scattering and second harmonic scattering. This combination of methods allows us to investigate the interfacial alkyl chain conformation of the oil and several alkanols, ranging from 1-pentanol to 1-dodecanol, the orientational distribution of the methyl groups, the surface density of the alkanols, as well as the orientational alignment of water. For the hexadecane/1-dodecanol/water interface, dodecanol alkyl chains form a fluid layer with a wide distribution of tilt angles of the terminal CH3 groups. Indistinguishable spectra are recorded for the alkanol alkyl chains of 1-pentanol, 1-hexanol, 1-octanol, and 1-dodecanol, and alkanols with chain length longer than 6 C atoms all form films with similar densities. In contrast, the alkyl chains of the oil phase are relatively more distorted with respect to the pure oil/water interface for alkanols with shorter chain lengths. The projected surface area of a saturated film of hexanol is 29 +/- 5 angstrom(2), which requires a free energy of adsorption of Delta G = -26.3 +/- 0.7 kJ/mol. In addition, with increasing alkanol density the interfacial water structure loses its initial orientational alignment, which matches with the added number of interfacial 1-hexanol molecules. The found structures differ significantly from those reported on the alkanol/water and alkanol/air interface and charged surfactant/oil/water interface.

American Chemical Society2015