Semi-permeable vesicles composed of natural clay
The authors report a way to produce semi-permeable vesicles from montmorillonite, a natural plate-like clay mineral that occurs widely in the environment. Simple shearing forces can assemble an aqueous suspension of montmorillonite clay plates onto air bubbles producing clay-armoured bubbles. Replacing the water with organic liquids will displace the air pocket inside the clay-armour with the liquid forming vesicles. Clay vesicles are microporous, exhibit size-selective permeability. This is the first time that any group has demonstrated self-assembly of inorganic minerals into structured compartments.
Clay armoured bubbles are produced by sandwiching a suspension of clay plates and air bubbles between two glass slides, and sliding them. The trapped air bubble will gather sufficient clay plates to form clay-armoured bubbles. A schematic is shown in the figure below. The bubbles range from 5 to 100 microns in radius. The clay armoured bubbles can created by sliding this bubbly clay suspension between other hydrophilic suspensions such as aluminum, copper and stainless steel.
Thin-shell vesicles of clay can then be produced by exposing the clay-armoured bubbles to certain water-miscible organic liquids. The picture below shows a detailed image of the clay vesicles. Note that there are pores on the vesicles which allow for it to act as a semi-permeable membrane.
The formation of clay vesicles can be rationalized from partial to complete wetting of clay aggregates by the outer fluid. The parameter that determines whether the air bubble will be displaced by the outer fluid is S, the spreading coefficient. If S > 0, clay vesicles will be formed, whereas if S < 0, there will be no transition from clay-armoured bubbles to vesicles. This is illustrated in figure 8. The nano-plates are stabilized by van der waal's interactions, i.e. they are in a potential minimum. But there are always defects where the nano-plates are not at a minimum. These defects turn into pores when the nano-plates are dislodged as the outer fluid enters the clay armoured bubbles. This is depicted in figure 6.
As mentioned, the clay vesicles exhibit size-selectivity. They are able to exclude 110 nm polystyrene particles (See red channel, figure 9), while allowing the fatty acid salt sodium oleate to diffuse in and out freely (green channel, figure 9). Sodium oleate can self-assemble into liposomes, and liposomes inside the vesicles are segregated from those outside the vesicles. More interestingly, over the course of several hours, more complex lipid mesophases form inside the vesicles. (Figure 9H).
The most interesting question that crops up in my mind is if life could have evolved out of these semi-permeable vesicles. These clay vesicles can easily be formed in nature, perhaps in gaps between sliding rocks. These vesicles can provide sequestered environment for some inorganic precursors to organic proto-cells. This view is supported by an increasing body of evidence that clay can easily act as catalyst for spontaneous assembly of fatty acid vesicles, RNA formation from nucleotides and many others. There is no hard proof that this is the origin of life, but this increases the plausibility of the formation of proto-cells in some inorganic soup during early history of earth.
1. A.B. Subramaniam, J. Wan, A. Gopinath and H.A. Stone, "Semi-permeable vesicles composed of natural clay", Soft Matter, 2011, 7, 2600-2612
2. M.M. Hanczyc, S.M. Fujikawa and J.W. Szostak, "Experimental Models of Primitive Cellular Compartments: Encapsulation, Growth, and Division", Science, 2003, 302 5645, 618-622