LETTER pubs.acs.org/NanoLett
Polymeric Nanopore Membranes for Hydrophobicity-Based Separations by Conformal Initiated Chemical Vapor Deposition Ayse Asatekin and Karen K. Gleason* Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States
bS Supporting Information ABSTRACT: High-aspect ratio hydrophobic, cylindrical nanopores having diameters as low as 5 nm are rapidly fabricated using conformal vapor deposition of fluorinated polymeric layers into porous track-etched polycarbonate membranes. The resultant selectivity of these membranes for pairs of small molecules of similar size, but of different hydrophobicity, arises from solute-pore wall interactions emphasized by confinement. Increasing selectivity was observed as pore diameter decreased and as the surface of the pore became more hydrophobic. Cylindrical pores provided higher selectivity than bottleneck-shaped pores having the same minimum diameter. A maximum selectivity of 234 was achieved between mesitylene and phloroglucinol by the best performing membrane. Membranes with small fluorinated pores exhibited an effective cutoff based on the polar surface area of the molecules, with limited correlation with solute size. This technology could lead to a new generation of membrane separations based on specific interactions. KEYWORDS: Nanopores, membranes, CVD, hydrophobicity, conformal coatings, selectivity
M
embranes offer a scalable, energy efficient, easy to operate separation method for many industries.1 While certain membrane applications such as size-based sieving of particles and desalination by reverse osmosis have become relatively mainstream processes, separation of small molecules by membranes is difficult to accomplish.1,2 This is especially of interest in pharmaceutical and biological applications, where such separations are performed by chromatography, which is difficult to achieve in industrial scale.3,4 Furthermore, membranes usually fractionate molecules either by sieving or by electrostatics.2 Membranes that can perform separations based on other chemical parameters, such as hydrophobicity and other chemical properties, can expand the use of this technology to a wider range of applications. One method of performing such separations takes inspiration from biological pores that regulate permeation through the cell membrane.5 These structures consist of through-pores of controlled surface composition whose diameter is comparable to the size of the molecules passing through them. Permeation through such a pore is dominated by the interaction of solutes with the pore walls. Diffusion through the “bulk solvent”, without interaction with the walls, is very limited due to the small diameter. Hence, solutes that adsorb onto the pore surface more strongly are partitioned more into the pore and permeate faster.6 The result is regulation of permeation based not solely on size but on chemical structure. The basis of separation can be various factors that affect surface-solute interactions, including hydrophobicity,7-12 charge,10,13 and even chirality.14,15 Membranes that incorporate such functional nanopores can be used to perform separations of small molecules based on their chemical r 2010 American Chemical Society
structure. For example, a hydrophobicity-based separation can be performed using nanopores with highly hydrophobic pore walls. In this case, of two molecules of similar size, the more hydrophobic one will interact more strongly with the membrane, partition into its pores, and diffuse through the membrane faster. A recent report of models describing this type of separation shows that the separation mechanism is strongly related with that of facilitated transport, where molecules are carried on the pore walls rather than through interactions with functional groups distributed throughout the membrane.6 For fractionating small molecules, usually 24 h
bottleneck
highest
chemicals used
compatible substrates
hydrophobicitybased selectivity
heavy metal
polymer/
200
salts
inorganic
ALD16
high
2 nm
bottleneck
low
3-4 h
silanes
inorganic
polymer self-assembly20
very limited
6 nm
cylindrical
very high
