Interfacial phase transitions in conducting fluids

Abstract

We present a review, largely based on recent experimental work of our group, on phase transitions at interfaces of fluid metals, alloys and ionic liquids. After a brief analysis of possible experimental errors and limitations of surface sensitive methods, we first deal with first-order wetting transitions at the liquid/vapour and liquid/wall interface in systems such as Ga-based alloys, K-KCl melts, and fluid Hg. The following chapter refers to surface freezing or surface induced crystallization in different metal alloys. The respective surface phase diagrams are discussed in comparison with their bulk counterpart. In the last part we present very recent investigations of ionic liquid interfaces, including order-disorder transitions at the liquid/vapour interface and examples of two-dimensional phase transitions at the electrified ionic liquid/metal interface. Finally, a simple electrowetting experiment with an ionic liquid droplet under vacuum is described which gives new insight into the contact angle saturation problem. The article ends up with a few perspective remarks on open problems and potential impact of interfacial phenomena on applied research.

Double Layer in Ionic Liquids: Overscreening versus Crowding

“…Thousands of RTILs have been synthesized with large organic cations and similar organic or smaller inorganic anions. Nonvolatile and capable of withstanding up to ±4-6 V without decomposition, RTILs also hold promise as solventfree electrolytes for super-capacitors, solar cells, batteries and electroactuators [2][3][4][5][6][7][8][9][10].For such applications, it is crucial to understand the structure of the RTIL/electrode double layer. The classical Gouy-Chapman-Stern (GCS) model for dilute electrolytes was used to interpret RTIL capacitance data until recently, when a mean-field theory for the crowding of finite-sized ions [11] suggested bell or camel shapes of the differential capacitance versus voltage, decaying as C ∼ V −1/2 .…”

Structural Transitions at Ionic Liquid Interfaces

“…The figure shows the free energy landscape for the system as a function of surface charge density σ and potential ΔΨ, in units of Boltzmann's constant, k B , times near ΔΨ = 0.9 V and the other near ΔΨ = these critical voltages on the free energy surfa shows that they correspond to the location o where fluctuations sample two adjacent free Similar features in the differential capacitan reported in numerous experimental studies. 27,45 The peaks in capacitance arise from correlat interfacial layer of fluid. This is evident from surface charge distributions that exhibit non-G characteristic of a first-order phase transitio conditions away from phase coexistence an distribution suggesting incipient bimodality (lim rather small system size) at conditions of coexis relevant structures in the fluid comprise corr that are small compared to the net surface area, will be Gaussian because the net surface-char reflect many uncorrelated contributions (the us theorem argument).…”

The anatomy of the double layer and capacitance in ionic liquids with anisotropic ions: Electrostriction vs. lattice saturation

“…Hundreds of them are already known [4]; the structure and dynamics of many of them is currently under investigation [5]. The wave of interest to RTILs has reached electrochemistry and electrochemical engineering [6][7][8][9][10][11], because RTILs as electrolytes are nonvolatile, can sustain substantially higher voltages in electrochemical cells without decomposition, and many RTILs are environmentally friendly. At ambient temperatures, however, RTILs have high viscosity, and the diffusion and conductivity of ions is normally lower in RTILs than in aqueous electrolyte solutions.…”