(456c) Thermophysical and Structural Properties of Electrolyte Solutions Confined in a Carbon Nanotube

Recently, single-wall carbon nanotubes (SWCNT) have found use as sensing materials where, for example, a particular nanotube chirality will bond to certain strands of DNA and, through further electronic measurements, can identify particular base sequences in a DNA strand [1]. When this type of technology has been sufficiently developed, solutions of SWCNTs could, in principle, be used for lab-on-chip biometric applications for gene identification. A key barrier at present is the satisfactory identification of SWCNTs in solution. It is difficult to separate SWCNTs by conventional analytical techniques and most significant progress has only occurred recently [2]. Furthermore, SWCNTs do not readily solubilize in water and a surfactant is typically used to disperse nanotube clusters.

Recent efforts to develop centrifugation techniques that efficiently and reliably separate SWCNTs by chirality have yielded a puzzling result: preliminary measurements have suggested that the density of the surfactant solution in a SWCNT exceeds the bulk density of water by nontrivial amounts for some SWCNT species [3]. If this effect is genuine, it must be a consequence of cation solvation, since the anionic surfactant itself is unable to enter the SWCNT due to size constraints. It is speculated that the alteration of the fluid density is a response to cation-induced restructuring of water in the nanotube.

We address this issue via grand-canonical transition-matrix Monte Carlo simulation, in an effort to eliminate effects that may obscure a clear measurement of the fluid density in experiment. Using the venerable SPC/E model of water [3], we compute solution density as a function of pressure and electrolyte content and compare our results to existing simulations of pure water in SWCNTs [4]. Our results present a clearer view of the phase behavior of electrolyte solutions in SWCNTs which will aid future efforts in reliably separating solutions of SWCNTs.

[1] Tang et al., Nano. Lett., 6:1632 (2006), Tu et al., J. Am. Chem. Soc., 133:12998 (2011)

[2] Fagan et al., Langmuir, 24:13880 (2008), Fagan et al., ACS Nano, 5:3943 (2011)

[3] Berendsen et al., J. Phys. Chem., 91:6269 (1987)

[4] Kyakuno et al., J. Chem. Phys., 134:244501 (2011), Pascal et al., Proc. Natl. Acad. Sci. USA, 108:11794 (2011)