Enantiospecific Wetting

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References

1. M. Rapp, W.A. Ducker, "Enantiospecific Wetting", J. Am. Chem. Soc., 132, 18051-18053 (2010).


Keywords

chiral, contact angle, wetting, enantiomer


Summary

This paper describes simple experiments that reveal an enantiospecific equilibrium contact angle between a chiral liquid and a surface functionalized with a chiral surface group. It is well known that the energy of interaction between a chiral liquid and a chiral solid can be different for each enantiomer of the liquid. In fact, chiral chromatography is able to separate a racemic mixture of two enantiomers precisely because of this interaction energy difference. It is then straightforward to imagine that the equilibrium contact angle of a chiral liquid on a chiral surface, which depends on the energy of interaction between the two, should be different for the two enantiomers. However, surprisingly, the authors claim that an experimental verification of enantiospecific wetting has never been done before.

The experiment set up by the authors is very simple. They simply measured the equilibrium contact angles of both enantiomers of a pure chiral liquid on a chiral surface in a background medium of air and then repeated the experiment in a background of hexadecane. As a chiral liquid they chose leucinol (a derivative of the amino acid, leucine). To obtain a chiral solid, they functionalized an oxidized silicon wafer by chemisorbing enantiopure leucinol using annealing under an inert atmosphere. As a control experiment (to validate the absence of impurities that could skew the contact angle measurements) they measured the surface tensions and equilibrium contact angles with respect to an oxidized (achiral) silicon wafer of both S-leucinol and R-leucinol, and found them to be the same. On the chiral surfaces, they measured the advancing and receding equilibrium contact angles for each enantiomer combination of liquid and surface group, namely S-S, R-R, S-R, and R-S. The results, using both air and hexadecane as a background medium, are shown in the tables below. By symmetry, one expects S-S and R-R to produce the same contact angles, likewise with S-R and R-S, but S-S (R-R) and S-R (R-S) could be different. In air, the enantiospecificity is not resolved and all contact angles, within a range of error, fall in the same range. However, enantiospecific behavior is observed when the hexadecane is used as the background medium. The amplification of this specifity by changing the background medium is a direct consequence of Young's equation, which describes the relationship between the three interfacial tensions and the contact angle. The enantiospecific term in Young's equation is the solid-liquid interfacial tension (the liquid-medium and solid-medium interfacial tension are non-specific because the medium is achiral). However, when computing the contact angle, this component gets divided by the liquid-medium interfacial tension. Replacing air with hexadecane in this experiment serves to reduce the liquid-medium interfacial tension by a factor of 8. Therefore the enantiospecific difference in the cosine of the contact angle should increase by a factor of 8, making it resolvable by the measurement apparatus. In fact, this is waht is observed experimentally.


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Discussion

One potentially useful application of this type of measurement that the authors point out is that the difference between the S-S (R-R) and S-R (R-S) contact angles can be used to measure the energy of chiral discrimination using the equation below. The only additional information that is needed is the liquid-medium surface tension (can be easily measured) and the density of chiral surface groups. Both of these quantities can be straightforward to measure. However, curiously, this calculation is not computed for the leucinol-leucinol interactions measured by these experiments.

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