Peptide nanotube membranes represent an emerging class of biomimetic materials for water desalination, offering unique advantages in ion rejection and salt selectivity. These self-assembled nanostructures are formed through the precise arrangement of cyclic peptides, creating uniform sub-nanometer channels that enable selective transport of water molecules while excluding ions. The mechanism of ion rejection in peptide nanotubes differs fundamentally from conventional polymeric membranes and even advanced graphene-based systems, relying on a combination of steric exclusion, electrostatic interactions, and dehydration energy barriers.

The design of peptide sequences governs the nanotube's structural parameters and subsequent filtration performance. Cyclic peptides with alternating D- and L-amino acids form stable β-sheet-like stacks through backbone hydrogen bonding, creating nanotubes with precisely tunable pore diameters ranging from 0.7 to 1.2 nm. The internal pore diameter is determined by the number of amino acid residues in the cyclic peptide, with octapeptides typically forming channels around 0.8 nm in diameter. This dimension is critical for ion rejection, as it approaches the hydrated diameter of common salt ions (Na+ hydrated diameter ~0.72 nm, Cl- ~0.66 nm), creating a steric barrier to their passage. The pore size can be systematically adjusted by varying the peptide ring size, with hexapeptides producing smaller pores and decapeptides larger ones.

Beyond steric exclusion, the peptide nanotube's interior surface chemistry plays a crucial role in ion selectivity. Functional groups lining the pore lumen can be precisely engineered through amino acid side chain selection. Cationic residues like lysine create positively charged pores that repel Na+ ions through Donnan exclusion, while anionic residues like aspartate achieve chloride rejection. Aromatic residues such as tryptophan provide hydrophobic regions that disrupt ion hydration shells, increasing the energy penalty for ion dehydration and subsequent pore entry. The combination of these effects results in salt rejection rates exceeding 90% for NaCl solutions at optimal peptide designs.

Pore alignment and membrane density represent significant challenges in practical implementation. Unlike graphene oxide laminates where stacking creates natural 2D channels, peptide nanotubes require careful orientation to form continuous permeation pathways. Techniques such as interfacial polymerization on porous supports or electric field alignment during self-assembly have achieved up to 10^11 pores per cm^2 with predominantly vertical orientation. Membrane performance scales directly with nanotube density, as higher densities increase water flux while maintaining selectivity. Current fabrication methods yield water permeabilities in the range of 5-20 L m^-2 h^-1 bar^-1, comparable to commercial reverse osmosis membranes but with potentially better salt rejection.

Pressure tolerance of peptide nanotube membranes depends on the stability of non-covalent interactions maintaining the tubular structure. While hydrogen bonds between peptide subunits are relatively strong (8-40 kJ/mol), the membranes typically operate best below 20 bar to prevent structural deformation. Crosslinking strategies using glutaraldehyde or genipin have improved mechanical stability, allowing operation up to 50 bar with less than 5% flux decline over 100 hours. The biological origin of these materials makes them inherently more susceptible to pressure-induced damage than graphene-based systems, but also more amenable to self-repair mechanisms through dynamic subunit exchange.

Comparing with graphene-based desalination reveals fundamental differences in transport mechanisms. Graphene membranes rely on precisely engineered nanopores (often 0.5-1 nm) created through physical or chemical etching, with water molecules passing through in a single-file fashion. The atomically thin graphene sheet (0.34 nm) enables extremely high water fluxes (up to 1000 L m^-2 h^-1 bar^-1) but requires perfect pore uniformity to maintain selectivity. Graphene's hydrophobic surface creates frictionless water flow but also necessitates energy-intensive ion desolvation at pore entry. In contrast, peptide nanotubes provide a longer transport path (membrane thickness typically 100-500 nm) with more gradual dehydration and rehydration of water molecules, potentially reducing energy requirements.

Salt selectivity profiles differ markedly between the two systems. Graphene membranes show sharp cutoff behavior based strictly on hydrated ion size, with nearly complete rejection above a threshold diameter but minimal selectivity between similarly sized ions. Peptide nanotubes exhibit more nuanced selectivity due to their functionalized pores, enabling discrimination between ions of similar size but different charge or hydration energy. For example, some peptide designs show Na+/K+ selectivity ratios up to 3:1 despite nearly identical hydrated diameters, attributed to differences in dehydration energy and cation-π interactions with aromatic side chains.

Environmental stability presents another key distinction. Graphene membranes maintain performance across wide pH ranges (2-12) and temperatures (0-100°C), while peptide nanotubes are generally limited to pH 4-9 and temperatures below 60°C to prevent denaturation. However, peptide membranes show superior resistance to organic fouling due to their biomimetic surfaces, with flux declines of less than 15% after exposure to humic acid solutions that cause 40% declines in graphene membranes. The self-assembling nature of peptide systems also allows for in situ repair of damaged pores through subunit exchange, whereas graphene pores require external intervention for repair.

Future developments in peptide nanotube membranes will likely focus on three areas: enhanced mechanical stability through hybrid organic-inorganic composites, improved production scalability using continuous flow synthesis methods, and multi-functional pores capable of simultaneous desalination and contaminant degradation. Computational modeling suggests that incorporating non-natural amino acids could create pores with precisely tuned dipole moments for enhanced ion selectivity, while maintaining the biocompatibility that makes peptide systems attractive for certain applications.

The unique combination of biomimetic design, tunable selectivity, and potential for self-repair positions peptide nanotube membranes as a complementary technology to graphene-based systems, particularly in applications where chemical specificity or environmental compatibility are prioritized over maximum flux. As understanding of structure-function relationships in these systems deepens, peptide nanotubes may enable a new class of smart membranes capable of adaptive selectivity and self-optimization in response to changing feedwater conditions.