Carlos Larriba

The mobility-volume relationship below 3.0 nm examined by tandem mobility-mass measurement

The validity of the Stokes-Millikan equation is examined in light of mass and mobility measurements of clusters of the ionic liquid 1-ethyl-3-methyl-imidazolium tetrafluoroborate (EMI-BF4) in ambient air. The mobility diameter dZ based on the measured mobility and the Stokes-Millikan law is compared with the volume diameter dv , which generalizes the mass diameter for binary substances such as salts. dv is based on the sum of anion and cation volumes in the cluster corrected for the void fraction of the bulk ionic liquid. For dv > 1.5 nm, d Z is within 1.4% of dv + 0.3 nm. For smaller clusters 3.84 and 14.3% deviations are observed at dv = 1.21 nm and 0.68 nm, respectively. These differences are smaller than expected due to a cancellation of competing effects. The increasing difference seen for dv < 1.5 nm is due primarily to the interaction between the cluster and the dipole it induces in the gas molecules. Other potential sources of disagreement are non-globular cluster geometries, and departures of the cluster void fraction from the bulk value. These two effects are examined via molecular dynamics simulations, which confirm that the volume diameter concept is accurate for EMI-BF4 nanodrops with dv as small as 1.6 nm.

Taylor cones of ionic liquids from capillary tubes as sources of pure ions: The role of surface tension and electrical conductivity

The emissions of Taylor cones from a wide range of ionic liquids (ILs) have been tested in vacuo in an attempt to identify what physical properties favor the purely ionic regime (PIR). This regime is well known in the case of Taylor cones of liquid metals. For nonmetallic liquids, it has been previously observed in conventional (capillary tube) electrospray sources at room temperature only for the room temperature molten salt (ionic liquid) EMI–BF4 (EMI=1-ethyl-3-methylimidazolium). A large number of other ILs and their mixtures have been studied here, most of which (but not all) are unable to reach the PIR at room temperature. Based on these results and additional theoretical considerations, strong support is assembled for the notion that the PIR is favored by ILs not only of high electrical conductivity but also of high surface tension. This hypothesis is confirmed by tests with three recently synthesized ILs, EMI–GaCl4, EMI–C(CN)3, and EMI–N(CN)2, all of which combine exceptional surface tension and el...

The gas phase structure of coulombically stretched polyethylene glycol ions.

Prior ion-mobility mass-spectrometry (IMS-MS) studies of polyethylene glycol (PEG) ions have identified only two out of many sharply different observed structures: Linear shapes with several individually solvated singly charged cations at high charge states z (beads on a string), and single multiply charged globules at low z. The present study is devoted to assign all other existing structures of PEG ions, for the first time reaching masses of 100 kDa and charge states up to z = 10. There are at most z different structures at charge state z. All involve a single globule carrying n charges, tied to one or several appendices bearing z - n separate charges in a beads-on-a-string configuration. All sharp shape transitions observed at decreasing ion mass involve ejection of one elementary charge (sometimes two) from the shrinking globule into the growing linear appendage. This picture is supported by molecular dynamics simulations and approximate calculations of electrical mobilities for computed structures.

Monoenergetic source of kilodalton ions from Taylor cones of ionic liquids

The ionic liquid ion sources (ILISs) recently introduced by Lozano and Martinez Sanchez [J. Colloid Interface Sci. 282, 415 (2005)], based on electrochemically etched tungsten tips as emitters for Taylor cones of ionic liquids (ILs), have been tested with ionic liquids [A+B−] of increasing molecular weight and viscosity. These ILs have electrical conductivities well below 1S∕m and were previously thought to be unsuitable to operate in the purely ionic regime because their Taylor cones produce mostly charged drops from conventional capillary tube sources. Strikingly, all the ILs tried on ILIS form charged beams composed exclusively of small ions and cluster ions A+(AB)n or B−(AB)n, with abundances generally peaking at n=1. Particularly interesting are the positive and negative ion beams produced from the room temperature molten salts 1-methyl-3-pentylimidazolium tris(pentafluoroethyl) trifluorophosphate (C5MI–(C2F5)3PF3) and 1-ethyl-3-methylimidazolium bis(pentafluoroethyl) sulfonylimide (EMI–(C2F5SO3)2N)....

Correlation between surface tension and void fraction in ionic liquids.

An effort to systematize published and new data on the surface tension gamma of ionic liquids (ILs) is based on the hypothesis that the dimensionless surface tension parameter gamma V v (2/3)/ kT is a function of the void fraction x v = V v/ V m. The void volume V v is defined as the difference between the liquid volume V m occupied by an ion pair (known from cationic and anionic masses and liquid density measurements) and the sum V (+) + V (-) of the cationic and anionic volumes (known from crystal structures), while kT is the thermal energy. Our hypothesis that gamma V m (2/3)/ kT = G( x v) is initially based on cavity theory. It is then refined based on periodic lattice modeling, which reveals that the number N of voids per unit cell (hence the dimensionless surface tension) must depend on x v. Testing our hypothesis against data for the five ILs for which surface tension and density data are available over a wide range of temperatures collapses all of these data almost on a single curve G( x v), provided that slight (4%) self-consistent modifications are introduced on published crystallographic data for V (+) and V (-). An attempt to correlate the surface tension vs temperature data available for inorganic molten salts is similarly successful, but at the expense of larger shifts on the published ionic radii (8.8% for K; 3.3% for I). The collapsed G( x v) curves for ILs and inorganic salts do not overlap anywhere on x v space, and appear to be different from each other. The existence of a relation between gamma and x v is rationalized with a simple capillary model minimizing the energy. Our success in correlating surface tension to void fraction may apply also to other liquid properties.

Electrospray Ionization Mechanisms for Large Polyethylene Glycol Chains Studied Through Tandem Ion Mobility Spectrometry

AbstractIon mobility mass spectrometry (IMS-MS) is used to investigate the abundance pattern, nz(m) of Poly-(ethyleneglycol) (PEG) electrosprayed from water/methanol as a function of mass and charge state. We examine nz(m) patterns from a diversity of solution cations, primarily dimethylammonium and triethylammonium. The ability of PEG chains to initially attach to various cations in the spraying chamber, and to retain them (or not) on entering the MS, provide valuable clues on the ionization mechanism. Single chains form in highly charged and extended shapes in most buffers. But the high initial charge they hold under atmospheric pressure is lost on transit to the vacuum system for large cations. In contrast, aggregates of two or more chains carry in all buffers at most the Rayleigh charge of a water drop of the same volume. This shows either that they form via Dole’s charge residue mechanism, or that highly charged and extended aggregates are ripped apart by Coulombic repulsion. IMS-IMS experiments in He confirm these findings, and provide new mechanistic insights on the stability of aggregates. When collisionally activated, initially globular dimers are stable. However, slightly nonglobular dimers projecting out a linear appendix are segregated into two monomeric chains. The breakup of a charged dimer is therefore a multi-step process, similar to the Fenn-Consta polymer extrusion mechanism. The highest activation barrier is associated to the first step, where a short chain segment carrying a single charge escapes (ion-evaporates) from a charged drop, leading then to gradual field extrusion of the whole chain out of the drop. Figureᅟ

Effect of liquid properties on electrosprays from externally wetted ionic liquid ion sources

Ionic liquid ion sources (ILISs) are externally wetted and electrochemically etched and sharpened tungsten tips used as electrospraying sources for ionic liquids in a vacuum. They have recently shown an ability to operate as emitters of pure ion beams (no drops), even with ionic liquids of moderate surface tension (γ<40dyn∕cm) and electrical conductivity (K<1S∕m) that had in all prior reports (all based on conventional internally fed capillary tips) always operated in the mixed ion-drop regime. The present study uses time of flight mass spectrometry to analyze full ion beams emitted from ILISs for a diversity of ionic liquids with properties in the wide range 0.26<K(S∕m)<2.8, 39.3<γ(dyn∕cm)<48.6. Remarkably, all liquids tested achieve the purely ionic regime. The main effect of reducing electrical conductivity is a reduction of the emission current from 180to10nA.

Taylor cones of ionic liquids from capillary tubes as sources of pure ions for electrical propulsion

The emissions of Taylor cones of a wide range of ionic liquids have been tested in vacuo in an attempt to identify what physical properties favor the purely ionic regime (PIR). This regime is well known in the case of Taylor cones of liquid metals. It has been previously observed in conventional (capillary tube) electrospray sources at room temperature only for the room temperature molten salt (ionic liquid) EMI-BF4 (EMI = 1-ethyl-3methylimidazolium). A large number of other ionic liquids and their mixtures have been studied here, most of which are unable to reach the purely ionic regime at room temperature. Based on these results and additional theoretical considerations we conclude that the purely ionic regime is favored not only by ionic liquids of high electrical conductivity, but also by those having relatively high surface tension.

Production of monodisperse submicron drops of dielectric liquids by charge-injection from highly conducting liquids

When ions or electrons are injected into an insulating liquid, they migrate towards its free surface, destabilize it, and form a charged jet. The jet then breaks into uniform drops charged at an approximately constant fraction of the Rayleigh limit, which relates the drop diameter DD to the flow rate of dielectric liquid QD and the injected current I as DD ∼ (QD/I)2/3. We have previously studied the analogous problem where the ions are substituted by nanodrops produced by a Taylor cone of a highly conducting ionic liquid (EMI-BF4) immersed in heptane or decane. This yielded hydrocarbon droplets with diameters as small as 4 μm [C. Larriba and J. Fernandez de la Mora, Phys. Fluids 22, 1 (2010)], with only incidental barriers to reaching smaller sizes. Here, we overcome these barriers via silica capillaries with smaller bores. These achieve substantially smaller QD and QD/I values, resulting in drops well below the ∼1-2 μm measurable with a phase Doppler anemometer. Extrapolating the DD ∼ (QD/I)2/3 scaling t...