Pores and drops: Made to order.

Porous solids are mundane materials.  Well, by making such a statement we are not certainly undermining their importance in the realm of science and technology.  From the very simple pumice stone for pedicure to the highly sophisticated zeolites , the utility of porous solids can never be undermined.  A recent paper in Nature not only announces the possibility of porous liquids, but also demonstrates the concept.   Another paper discusses self-shaping oil droplets. In both cases  simplicity of the concepts and approach are commendable.  

First about the porous liquids.  According to well established theories, dissolution of  a solute in a liquid is the process of carving out cavities to house the solute.

There are several physical and  chemical forces at play. Giri et al asked the question why not have pre-fabricated pores/cavities in the solvent ? There  are indeed conditions. The molecular dimensions of the pore  can't be  bigger than that of the  solvent molecule. Why not? Because then the solvent molecules will reside inside these pores happily ever after.  So the team chose 15 Crown 5 ether, a liquid at room temperature, as the solvent. They fabricated  the cage molecule from crown ether functionalized diamine.  This cage  has a cavity size of about 5 angstrom diameter. The solvent could dissolve  up to 44 wt % of the cage molecules   and still retain fluidity.  While the molecular dynamics simulation studies provided theoretical and structural information about the nature of the porous liquid, actual experiments demonstrated the ability of the pores to absorb and desorb  gases such as nitrogen, methane, carbon dioxide and xenon. With appropriate competitors these gas molecules could be forcibly evicted too.  

Denkov et al. demonstrate  that they can  shape oil droplets, and freeze those shapes. Again the experimental set up is extremely simple.  Recipe calls for water as the medium,  surfactant (ionic or non-ionic)  as an additive, and linear  long chain hydrocarbons with 14-20 carbon atoms, (as the  oil, the droplet former). It is imperative that the chemistry of the surfactant and the hydrocarbon should match, that is the alkyl chain length of the surfactant should be equal to or longer than the hydrocarbon chain length. Other decisive factors include the initial drop size and temperature. Or to be more precise the rate of cooling. In a typical experiment with hexadecane (C16H34) in aqueous medium containing 1.5 wt% Brij 58 (a non ionic surfactant with the formula C16H33(CH2CH2O)20OH)  the team captured the shape transformations of the droplet under varying rates of cooling. These shape transformations are induced by the phase transitions that occur within the oil droplet. At any stage these shapes can be selectively frozen. 

References 

1.  Liquids with permanent porosity : Giri et al. Nature Vol. 527, 12 Nov. 2015, pp 216-220

2. Self-shaping of oil droplets via formation of intermediate rotator phases upon cooling  Denkov et al Nature Vol 528, 17 Dec. 2015 pp 392-395