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Active, or microporous, carbons are established and effective adsorbents for toxic and other organic vapour-phase compounds. Their porosity results predominantly from slit-shaped pores of molecular dimensions (and hence widths < 2 nm) formed by spaces between graphene layers in a turbostratic structure. They are used in a wide range of separation and recovery processes where adequate performance is often routinely achieved using relatively cheap, generic active carbons (ACs) produced from precursors such as coal, nutshells and wood. However, many current, or new, applications require more refined properties, such as greater selectivity or specific surface chemistry, before the full potential of ACs can be realized. Typical examples occur in separations associated with toxic organic compounds, for example in industrial and military respiratory protection or effluent clean-up, where the removal of highly toxic or noxious vapour species, sometimes present at very low concentrations, is required from atmospheric air streams which contain near-saturation pressures of water vapour. In this instance, a hydrophobic carbon with a pore structure which is optimized to the target species is required to solve this problem, but other applications, for example to control other emissions to the environment (SO x /NO x /Hg) or in energy storage (H 2 /CH 4 ) and electrochemical double-layer capacitance (EDLC) usage, may require quite different properties. Against this background, it is not surprising that there is currently great interest in active carbons with controllable properties and increased adsorption performance to meet these new challenges. Research workers are now looking beyond conventional ACs to materials with very specific properties which are tailored to meet these more exacting high-performance applications. However, in turn, achieving these goals places greater emphasis on both accurate structure/property characterization and also upon methods by which adsorption properties can be manipulated. In this review, the basic structure and properties of ACs are presented in relation to their adsorption characteristics; this is accompanied by an overview of adsorption theories which underpin quantitative equilibrium isotherm analysis (Polanyi–Dubinin–Stoeckli, Sing-α S , DFT) and allow the derivation of detailed physical and chemical information about carbon adsorbents, and comparison and understanding of the adsorption behaviour of target vapour species. Emphasis is placed on the types of interactions which occur between vapour molecules and the carbon surfaces, and the relative energy of these interactions. Consideration is given to the non-specific (London-type) forces which occur between vapour molecules and graphene basal planes, and the enhancement of these forces, between these planes and within micropores (width < 2 nm), leading to the Type I isotherms characteristic of these carbons. The size of vapour molecules relative to the (slit) width of pores is also discussed, as is the effect of the presence of wider (> 2 nm) pores. In contrast, specific interactions occurring at polar sites, and therefore dominated by the surface chemistry of the carbon structure, are also reviewed; critical in this respect are the interactions which occur between surface oxygen-containing groups — which are mainly present on the edge sites of the carbon planes — and water molecules, the competitive adsorption of which may seriously limit the adsorption of target species in many separation applications which are carried out in humid environments. Discussion of these aspects is followed by some comments on adsorption kinetics and dynamic filtration and, to emphasize the theoretical points which arise, examples are given of how manipulation of carbon characteristics can begin to be used to control and optimize adsorption for a range of more challenging applications.

The main requirements for the adsorption of cigarette smoke vapours using active carbons and the methods currently being used to characterise and select carbons for this application are reviewed. Emphasis is placed on the total volume of smoke, the pulsed characteristics and relatively short challenge times, the compounds in the smoke and the environment in which adsorption takes place. Using experimental data, the types of carbons most suitable for cigarette filter application are considered, as are the adsorption kinetics required to meet this challenge. The concept of carbon ageing and the effects of water within the smoke are also reviewed.

The adsorption of water by the graphene surfaces of multi-wall carbon nanotubes (MWCNTs) in either the untreated (4.3 atom% oxygen) or oxidised (22.3 atom% oxygen) surface states has been studied. Different concentrations of surface oxygen groups, which have been directly measured using XPS, give rise to distinctly different shapes of water adsorption isotherms. Those from the untreated materials follow the pressure axis which lends them a Type III character in the BDDT classification. However, since they display a clear point of inflection at the lowest pressure, they are strictly speaking Type II isotherms but indicative of relatively few polar interactions and weak water adsorptivity. In sharp contrast, the isotherms from the oxidised MWCNTs are typically Type II and are characterised by a marked positive curvature in their low pressure region due to the increased numbers of specific interactions occurring between water molecules and the polar surface oxygen groups. The water adsorption data were modelled by the equation of D'Arcy and Watt with a direct correlation being observed between the surface polarity parameters (a mL and a 0 ) and also a s (the limiting water uptake) and the surface oxygen levels of the MWCNTs. The difference in polar surface energy was confirmed by measurements of the calorimetric enthalpies of immersion in water (Δh i ), which were −54 mJ/m 2 for the untreated and −192 mJ/m 2 for the oxidised materials. These values also reflect the difference in the integral net enthalpies of adsorption for the two hydrophilic surfaces: a value of ca. −35 mJ/m 2 being obtained for an oxygen-free (hydrophobic) surface. Water adsorption on these hydrophilic graphene surfaces was shown to occur by specific hydrogen bonding and was therefore strongly dependent on the numbers of oxygen-containing polar surface sites. This behaviour is well known for other types of porous and non-porous carbon materials and is also predicted for carbon nanotubes by molecular simulation studies. The work described herein therefore provides early experimental confirmation of the quantitative role of surface oxygen chemistry in determining the water adsorption character of MWCNT graphene surfaces; it also validates previous simulation studies.

The adsorp0tion of human serum albumin (HSA) and human plasma fibronectin (Fn) from solutions in the respective concentration ranges 1–20 μg/ml and 0.1–10 μg/ml by highly orientated pyrolytic graphite (HOPG) surfaces was investigated using atomic force microscopy. Adsorption from solutions of relatively low concentrations (HSA, 1 μg/ml; Fn, 0.1 μg/ml) allowed individual adsorbed protein molecules to be imaged. HSA appeared to take one of two forms (N or F) depending upon the adsorption energy and site, whilst Fn appeared to be adsorbed in the coiled, bent form. AFM analysis of films obtained by adsorption from solutions of higher protein concentrations, which were dried slowly over a, period of 24 h, indicated the formation of network structures with the surface coverage increasing with solution concentration. The precise structure, which was very similar for both proteins, appeared to result from dewetting of the hydrophobic HOPG by the protein solution. A grainy structure resulted when films were dried rapidly. In this case, evaporation occurred before dewetting and protein molecules became isolated in even clusters. The XPS nitrogen 1s signal from adsorbed protein was found to increase as the level of adsorbed protein increased, i.e. as a function of the concentration of the protein solution. Water contact angles decreased from 90° for HOPG to 60° after protein adsorption and then increased slightly to 70° as the level of adsorbed protein increased. Plots of the surface nitrogen concentration (atom %) versus the protein solution concentration were type I in the Brunauer, Deming, Deming and Teller isotherm classification. A linear fit was obtained when these data were plotted in the form of the Langmuir equation which resulted in a monolayer equivalent value of N m = 3.3 atom % protein nitrogen. The main mechanism of adsorption appeared to be dispersion- or hydrophobic-dominated, although electrostatic repulsions between polar surface oxygen groups and the negatively charged HSA may also occur.

The interaction of polar molecules at carbon surfaces is briefly reviewed within the framework of the Dubinin theory. A linear relationship is observed between the term a0 of the Dubinin-Serpinski equation and heats of immersion calculated from equilibrium isotherms for water vapour adsorption by a series of microporous carbons with increasing polarities. A quantitative relationship is proposed between surface polarity, characterised by the spectroscopically measured surface oxygen concentration [O]T and calorimetric heat of immersion in water for a series of non-porous carbon blacks.