Mechanisms of Halomethane Adsorption on Functionalized Carbons: How Surface Chemistry Governs Selectivity in Realistic Gas Mixtures

Abstract

Halomethanes (CH₃X, where X = F, Cl, Br) are potent atmospheric pollutants, and their removal via adsorption on activated carbons (ACs) is a critical remediation strategy. However, the molecular-level influence of AC surface chemistry on adsorption, especially under realistic environmental conditions, is not fully understood. This work utilizes Grand Canonical Monte Carlo (GCMC) simulations to investigate the adsorption of CH₃F, CH₃Cl, and CH₃Br on realistic carbon models, comparing unfunctionalized graphitic surfaces (AC0) with surfaces functionalized with alcohol (AC1), carbonyl (AC2), and carboxyl (AC3) groups. We analyze the process for both pure components and in realistic mixtures (Quarantine and Pre-Shipment concentrations). Our findings reveal a critical inversion in adsorption preference. For pure components, CH₃Br adsorption is highest on the unfunctionalized (AC0) surface, driven by strong adsorbate–adsorbate interactions leading to condensation, characterized by a rising isosteric heat of adsorption ($Q_{st}\approx 35–45$ kJ/mol) that matches the enthalpy of sublimation. Conversely, in realistic humid mixtures, the pristine surface suffers a capacity collapse (>90% loss). The functionalized surfaces (especially AC3) demonstrate superior performance, exhibiting a thermodynamic selectivity of $S_{C{H}_{3}Br/Air}>100$ (compared to $S\approx 15$ for AC0) and retaining approximately 60% of their dry-condition affinity. This study elucidates the distinct roles of surface chemistry and intermolecular forces, providing a molecular basis for designing carbon materials optimized for high selectivity in complex environmental gas streams.

1. Introduction

Halomethanes (CH₃X, where X = F, Cl, Br, I) constitute a significant class of volatile organic compounds (VOCs) with profound environmental implications, ranging from stratospheric ozone depletion to greenhouse gas effects [1,2,3]. Among these, methyl bromide (CH₃Br) presents a critical challenge; despite global phase-out initiatives under the Montreal Protocol due to its high ozone-depleting potential, it remains in use for critical agricultural applications such as quarantine and pre-shipment fumigation [1,4]. Consequently, the development of efficient abatement technologies to prevent the venting of these fumigants into the atmosphere is mandatory [5,6].

Adsorption onto porous carbons is the most widely adopted strategy for the capture and recovery of such volatile halides due to the material’s cost-effectiveness and textural versatility [7]. However, the majority of existing research on halomethane adsorption has been predominantly macroscopic or engineering-oriented, focusing on breakthrough curves and capacity metrics under idealized conditions [4,7,8]; while these studies provide essential operational data, they often rely on standard characterization metrics—such as BET surface area or pore volume derived from nitrogen isotherms—which may not accurately predict the performance of polar molecules like CH₃Br under complex conditions.

A significant gap exists in the literature regarding the fundamental thermodynamic behavior of these systems when transitioning from pure component adsorption to realistic operating conditions. Industrial effluents and ambient air streams invariably contain moisture and atmospheric gases (N₂, O₂), yet fundamental studies often extrapolate performance from single-component isotherms. This simplification is critical; for instance, the presence of water vapor is known to dramatically alter the performance of activated carbons, often through competitive adsorption or pore blockage, a phenomenon that simple capacity metrics fail to predict accurately [4,9,10]. Experimental evidence has hinted at anomalous behaviors for CH₃Br, where the correlation between standard textural properties and adsorption capacity breaks down, particularly in humid environments [11]. Furthermore, the prediction of mixture selectivity based solely on pure component data—often employing models like the Ideal Adsorbed Solution Theory (IAST)—has been shown to be unreliable when specific molecular interactions or steric constraints are present [12].

To address this deficiency, it is essential to move beyond macroscopic descriptions and investigate the adsorption mechanisms at the molecular level. The selectivity of activated carbons is not merely a function of pore size but is intricately governed by the energetic landscape provided by surface functional groups and the adsorbate–adsorbate interactions that drive phase transitions within the micropores [13]. Previous computational efforts have successfully modeled nanoporous carbons using polyaromatic units to describe simple gas mixtures [14,15]; however, the specific interplay between the polarity of halomethanes and the surface chemistry of functionalized carbons under competitive conditions remains underexplored.

The objective of this work is to elucidate the adsorption mechanisms of halomethanes (CH₃F, CH₃Cl, CH₃Br) on activated carbons, connecting anomalous macroscopic observations with molecular-level interactions. We employ Grand Canonical Monte Carlo (GCMC) simulations using realistic, atomistic models of activated carbons—both functionalized (phenolic, carbonyl, carboxyl) and non-functionalized—as developed in our previous studies [14]. By integrating the analysis of isotherm shapes, isosteric heats of adsorption ($Q_{st}$), and structural snapshots, we provide a comprehensive analysis of how surface functionalization affects the trade-off between total capacity and selectivity. This study demonstrates that for realistic mixtures involving air and humidity, particularly in the case of CH₃Br, the design of adsorbents must prioritize selectivity driven by specific surface chemistry rather than total adsorption capacity, providing a rational basis for the development of next-generation environmental filters.

2. Materials and Methods

2.1. Simulation Methodology

Adsorption isotherms and thermodynamic properties were computed using the Grand Canonical Monte Carlo (GCMC) simulation method, which is the standard statistical mechanics approach for simulating adsorption equilibria in open systems [16]. In this ensemble, the chemical potential ($\mu$), volume (V), and temperature (T) are held constant, allowing the number of adsorbed particles to fluctuate until thermodynamic equilibrium is established with a hypothetical bulk reservoir [17].

2.2. Adsorbent Models: Construction and Validation

The activated carbon (AC) models were constructed following the Basic Structural Unit (BSU) packing methodology, which has been extensively validated in our previous works to reproduce the pore size distribution and adsorption capabilities of real microporous carbons [14,18].

Unlike idealized slit-pore models, this approach generates realistic, three-dimensional amorphous frameworks that capture the inherent disorder and connectivity of the pore network. The models were built from 2D polyaromatic clusters (graphene sheets) of three distinct sizes, containing 48, 120, and 360 carbon atoms, respectively. These BSUs were stochastically packed into cubic simulation boxes to achieve a target bulk density of approximately $0.7$ g/cm³, a value chosen to represent the accessible microporosity of commercial carbons consistent with experimental densities [14].

To isolate the specific influence of surface chemistry on halomethane selectivity, four distinct families of carbon models were developed based on the same structural backbone (see Figure 1):

• AC0 (Pristine): Unfunctionalized graphitic surfaces, serving as the hydrophobic baseline.
• AC1 (Phenolic): Functionalized with hydroxyl groups (–OH).
• AC2 (Carbonyl): Functionalized with carbonyl groups (=O).
• AC3 (Carboxyl): Functionalized with carboxyl groups (–COOH).

Functional groups were substituted exclusively at the unsaturated edges of the polyaromatic clusters to mimic realistic chemical functionalization rather than artificial basal plane substitution. The degree of substitution was inversely correlated with cluster size to reflect the edge-to-surface ratio: 12% for 48-atom clusters, 8% for 120-atom clusters, and 5% for 360-atom clusters [18].

To validate the realism of the generated structures, the textural properties of the carbon models were characterized and compared with experimental benchmarks as established in our previous work [18]. The simulated properties, summarized in

Table 1. Textural properties of the simulated activated carbon models compared to experimental trends. Data for surface area and density taken from Ref. [18].

Model	Functional Group	BET Surface Area (m2/g)	Bulk Density (g/cm3)
AC0-360 (Pristine)	None	2774.6	0.70
AC1-360 (Phenolic)	Hydroxyl (–OH)	2359.4	0.70
AC2-360 (Carbonyl)	Carbonyl (=O)	2405.1	0.70
AC3-360 (Carboxyl)	Carboxyl (–COOH)	2237.2	0.70

, confirm that the BSU packing method reproduces the features of high-surface-area activated carbons. The pristine AC0-360 model exhibits a BET surface area of 2775 m²/g and a bulk density of $0.7$ g/cm³, consistent with super-activated carbons utilized in gas storage applications. Notably, the functionalization with carboxyl groups (AC3) results in a ≈20% reduction in accessible surface area (to 2237 m²/g), accurately capturing the pore blocking and steric exclusion effects observed experimentally upon oxidation [18].

2.3. Interaction Potentials

The total potential energy of the system includes adsorbate–adsorbate ($ff$) and adsorbate-adsorbent ($sf$) interactions, modeled as pairwise additive. The van der Waals interactions were described using a 12-6 Lennard-Jones (LJ) potential (Equation (1)):

$${\phi }_{ij}^{LJ}\left(r\right)=4{ϵ}_{ij}\left[{\left({\displaystyle \frac{{\sigma }_{ij}}{r}}\right)}^{12}-{\left({\displaystyle \frac{{\sigma }_{ij}}{r}}\right)}^6\right]$$

(1)

where ${ϵ}_{ij}$ is the potential well depth and ${\sigma }_{ij}$ is the collision diameter. Cross-interaction parameters were determined using the standard Lorentz–Berthelot mixing rules. This interaction scheme has been previously shown to accurately reproduce adsorption trends of simple and polar gases in similar nanoporous carbon models [15,18].

To account for the long-range electrostatic interactions inherent to the polar halomethanes and functionalized surfaces, a Coulombic potential was employed. To handle the computational cost and avoid artifacts in the non-periodic simulation box, a shifted potential method was used as described by [19], with a cutoff radius $R_c=4\sigma$:

$$V_{SP}\left(r_{ij}\right)=q_i{q}_j\left[{\displaystyle \frac{1}{r_{ij}}}+\left({\displaystyle \frac{1}{R_c^2}}\right)(r_{ij}-R_c)\right]\mathrm{for}{r}_{ij}\le R_c$$

(2)

where $q_i$ and $q_j$ are the partial charges of the interacting sites. The explicit surface groups (hydroxyl, carbonyl, carboxyl) were modeled using parameters consistent with the TraPPE-UA force field and previous validations [20]. The specific force field parameters used to describe the adsorbent frameworks are listed in

Table 2. Lennard-Jones (LJ) parameters and partial atomic charges (q) for the adsorbent atoms.

Model Group	Atom	$\mathit{\sigma }$ (nm)	$\mathit{ϵ}/{\mathit{k}}_{\mathit{B}}$ (K)	q (${\mathit{e}}^{-}$)
Pristine (AC0)	C (graphitic)	0.34	20.0	0.0
Phenolic (AC1)	C (graphitic, site)	0.34	28.0	0.20
	O (hydroxyl)	0.307	78.2	−0.64
	H (hydroxyl)	0.0	0.0	0.44
Carbonyl (AC2)	C (graphitic, site)	0.34	28.0	0.50
	O (carbonyl)	0.296	105.8	−0.50
Carboxyl (AC3)	C (graphitic, site)	0.34	28.0	0.08
	C (carboxyl)	0.375	52.0	0.55
	O (carbonyl)	0.296	105.7	−0.50
	O (hydroxyl)	0.3	85.6	−0.58
	H (hydroxyl)	0.0	0.0	0.45

.

2.4. Adsorbate Models

The halomethanes (CH₃F, CH₃Cl, CH₃Br) were modeled as fully atomic, rigid molecules to accurately capture their geometric and electrostatic asymmetry. Their force field parameters are detailed in

Table 3. Lennard-Jones (LJ) parameters and partial atomic charges (q) for the adsorbate molecules.

Molecule	Atom	$\mathit{\sigma }$ (nm)	$\mathit{ϵ}/{\mathit{k}}_{\mathit{B}}$ (K)	q (${\mathit{e}}^{-}$)	Ref.
	C	0.391	15.549	−0.218
CH3F	H	0.231	9.108	0.037	[22]
	F	0.236	229.971	0.107
	C	0.34	55.328	−0.407
CH3Cl	H	0.25	9.117	0.152	[23,24]
	Cl	0.35	133.51	−0.049
	C	0.382	55.052	−0.575
CH3Br	H	0.277	7.901	0.233	[25]
	Br	0.404	211.352	−0.125

. Partial atomic charges were calculated via Density Functional Theory (DFT) using the ORCA software package [21]. A BP86 generalized gradient approximation functional was used in conjunction with the ZORA approximation and a def2-TZVP basis set (with SARC/J auxiliary basis sets). For the realistic mixture simulations, air components were modeled using standard potentials for N₂ and O₂, and water was described using the rigid SPC/E model, which is widely accepted for reproducing water properties in confined carbon geometries [18].

2.5. Simulation Conditions

Adsorption isotherms were computed for each halomethane on all 12 generated adsorbent structures at five environmentally relevant temperatures: 263, 273, 283, 293, and 303 K. To simulate realistic fumigation scenarios, specifically Quarantine and Pre-Shipment (QPS) applications, competitive adsorption was studied using a multicomponent gas phase at a total pressure of $P_{total}=101.325$ kPa. The specific composition of the mixture at 293 K is detailed in

Table 4. Composition and partial pressures of the realistic gas mixture simulated at $T=293$ K.

Component	Mole Fraction (${\mathit{y}}_{\mathit{i}}$)	Partial Pressure (kPa)	Condition
Methyl Bromide (CH3Br)	0.0165	1.67	QPS Fumigation Level
Water (H2O)	0.0230	2.33	100% Relative Humidity
Air (N2 + O2)	0.9605	97.32	Background Gas

. The partial pressure of water corresponds to saturation conditions (100% RH), defined according to the vapor-liquid coexistence properties of the SPC/E water model [26].

2.6. Thermodynamic Quantities

The isosteric heat of adsorption ($Q_{st}$) was calculated directly from the GCMC simulations using the fluctuation method. This approach relates $Q_{st}$ to the cross-fluctuations of the number of adsorbed particles (N) and the total configurational energy (U) of the system, as detailed in our previous studies [14,18,27]:

$$Q_{st}=RT-{\displaystyle \frac{\langle U_{conf}N\rangle -\langle U_{conf}\rangle \langle N\rangle }{\langle N^2\rangle -{\langle N\rangle }^2}}$$

(3)

where R is the ideal gas constant, T is the temperature, N is the number of adsorbed molecules, $U_{conf}$ is the total configurational energy of the system including fluid–fluid and fluid–solid interactions, and the brackets $\langle \dots \rangle$ denote ensemble averages.

This rigorous calculation allows for the distinction between fluid–fluid and fluid–solid energetic contributions, which is essential for identifying the condensation mechanisms discussed in the results.

3. Results: Pure Component Adsorption

3.1. Baseline Adsorption: CH₃F and CH₃Cl

The adsorption behavior of fluoromethane (CH₃F) and chloromethane (CH₃Cl) serves as a thermodynamic baseline, exhibiting characteristics typical of micropore filling dominated by fluid–solid interactions. For CH₃F, simulated at temperatures significantly above its boiling point ($T\gg 194.7$ K), the isotherms display a quasi-linear profile (Henry’s law region) across the investigated pressure range (Figure 2). The isosteric heat of adsorption ($Q_{st}$) remains nearly constant with loading (Figure 3), indicating a lack of significant lateral interactions or cooperative effects [18]. This behavior is consistent with previous GCMC studies on small, weakly polar gases on graphitic substrates, where the adsorbate density is primarily governed by the available micropore volume rather than specific surface chemistry [14,15].

In the case of CH₃Cl, the isotherms exhibit a Type I curvature (Figure 4), reflecting a stronger affinity for the carbon surface due to the molecule’s higher polarizability. Notably, the adsorption capacity correlates positively with the degree of surface functionalization (AC0 < AC1 < AC2 < AC3). The isosteric heat for CH₃Cl exhibits an intermediate behavior between the constant profile of CH₃F and the rising profile of CH₃Br, as detailed in Figure S2 (Supplementary Materials). This trend aligns with established experimental observations for chlorinated VOCs, where surface oxidation enhances adsorption capacity at low relative pressures through electrostatic interactions with the permanent dipole of the adsorbate [28,29]. In both cases (CH₃F and CH₃Cl), the adsorption mechanism is continuous pore filling, adequately described by standard Langmuirian models.

Effect of Temperature: Temperature acts as a critical thermodynamic modulator for these supercritical fluids. As expected for an exothermic physisorption process, an increase in temperature from 263 K to 303 K results in a monotonic decrease in adsorption capacity for both CH₃F and CH₃Cl across all carbon models. This behavior confirms that the mechanism is governed by the entropic penalty of confining the gas molecules; higher thermal kinetic energy allows the adsorbate to overcome the fluid–solid potential well, reducing surface coverage. The complete dataset of maximum adsorption capacities ($N_{max}$) for all adsorbates and temperatures is provided in the Supplementary Materials (Figure S1). No phase transitions are observed in this temperature range, as thermal agitation effectively prevents cooperative ordering.

3.2. Effect of Cluster Size (BSU Size) on Adsorption Capacity

A critical parameter often overlooked in simplified carbon models is the size of the graphitic domains (BSU size). Our simulations reveal a divergent effect of cluster size depending on the adsorbate size. For the smaller CH₃F, increasing the BSU size from 48 to 360 carbon atoms consistently increases the adsorption capacity. This is attributed to the formation of larger, more accessible slit-pores between larger graphitic sheets, effectively increasing the specific surface area available for small molecule packing [14].

Conversely, for the bulky CH₃Br molecule, this trend is disrupted. On the pristine AC0 surfaces, increasing the cluster size does not proportionally enhance capacity; in fact, specific geometric configurations with larger clusters lead to a stagnation or decrease in adsorption capacity efficiency. This counter-intuitive result suggests that for larger adsorbates, the accessibility of the deep microporosity formed by large stacked clusters is kinetically or sterically limited, a phenomenon previously described in the context of “constriction effects” in disordered carbons [30]. This geometric constraint is a crucial design variable, indicating that maximizing graphitic domain size is not universally beneficial for bulky halides.

3.3. Isotherm Modeling and Evidence of Mechanism Transition (CH₃Br)

The adsorption of methyl bromide (CH₃Br) on pristine carbon (AC0) presents a fundamental deviation from the Langmuirian behavior observed for lighter halomethanes (Figure 5). Mathematical analysis of the AC0 isotherms reveals that a single-site Langmuir model fails to capture the adsorption profile, particularly in the low-pressure region (<5 kPa). Instead, the data exhibits a sigmoidal character that necessitates a step-wise description: a Freundlich-type behavior at low coverage followed by a rapid adsorption capacity and saturation.

This deviation is diagnostic of a change in the adsorption mechanism. The sigmoidal shape implies that the adsorption of initial molecules facilitates the adsorption of subsequent ones, a hallmark of cooperative adsorption [31]. This contrasts with the functionalized surfaces (AC1–AC3), where the isotherms revert to a Type I shape and are well-fitted by the Langmuir model ($R^2$ > 0.98). The functional groups effectively break the long-range order required for cooperativity, forcing the system back into a site-specific filling regime, as confirmed by the regression analysis shown in Figure 6.

Temperature as a Phase-Transition Switch: This cooperative mechanism is strictly dependent on the proximity of the system temperature to the boiling point of CH₃Br ($T_b=276.7$ K).

• At $T<T_b$ (263 K–273 K): The adsorbate–adsorbate lateral interactions dominate over thermal entropy. The isotherms are sigmoidal, and the isosteric heat ($Q_{st}$) rises to ≈35–45 kJ/mol (Figure 7), matching the enthalpy of sublimation. This confirms a surface-induced condensation mechanism.
• At $T>T_b$ (293 K–303 K): As temperature increases, the kinetic energy disrupts the formation of the ordered condensed phase. The sigmoidal “step” in the isotherm flattens, and the behavior reverts towards a standard Type I profile governed by fluid–solid interactions. Figure 8

This thermal sensitivity explains why standard capacity metrics (often measured at ambient temperature) fail to predict performance under colder conditions. On functionalized surfaces (AC3), the functional groups sterically disrupt this cooperativity regardless of temperature, forcing the system into a Langmuirian site-filling regime (Figure 5b).

3.4. Thermodynamic Signature: Isosteric Heat Analysis

To confirm the physical origin of the cooperative mechanism on AC0, we analyzed the isosteric heat of adsorption ($Q_{st}$). For CH₃F and CH₃Cl, the $Q_{st}$ curves are flat or slightly decreasing, typical of heterogeneous surface filling. However, for CH₃Br on AC0, the $Q_{st}$ increases with loading, reaching values between 35 and 45 kJ/mol.

Crucially, these peak $Q_{st}$ values approach the theoretical enthalpy of sublimation ($\Delta H_{subl}\approx \Delta H_{vap}+\Delta H_{fus}$) of methyl bromide. This thermodynamic signature strongly indicates that the mechanism has shifted from fluid–solid pore filling to a surface-induced condensation or 2D-solidification process [13]. The smooth, energetic homogeneity of the AC0 basal planes allows the adsorbate–adsorbate lateral interactions to dominate, leading to the formation of a condensed phase even at pressures below the bulk saturation pressure.

3.5. Spatial Configurations as Structural Evidence

Visual inspection of the equilibrium snapshots from GCMC simulations provides direct structural evidence for these competing mechanisms.

• On AC0 (Pristine): At intermediate pressures, CH₃Br molecules are observed to form highly ordered, dense monolayers on the graphitic basal planes (Figure 9). The molecules maximize fluid–fluid contact, corroborating the condensation mechanism suggested by the $Q_{st}$ data.
• On AC3 (Functionalized): The presence of carboxyl groups at the edges of the BSUs introduces steric and electrostatic heterogeneity. Snapshots reveal that while CH₃Br molecules bind strongly to these sites initially, the functional groups physically disrupt the packing arrangement on the adjacent basal planes (Figure 10).

This “steric disorder” prevents the formation of the high-density condensed phase, explaining why the functionalized carbons—despite having higher interaction energy sites—exhibit lower total saturation capacities for the pure gas.

The identification of this condensation mechanism resolves inconsistencies reported in the experimental literature, where standard metrics like BET surface area failed to predict CH₃Br breakthrough times [4,11]. Our results demonstrate that for polarizable molecules near their boiling point, the macroscopic adsorption capacity is not governed by total pore volume but by the ability of the surface to support cooperative ordering. This mechanism is often overlooked in standard screening because it requires the rigorous integration of isotherm shape analysis, thermodynamic heat signatures, and structural visualization, as presented here.

4. Results: Adsorption from Realistic Gas Mixtures

4.1. From Pure Gas to Realistic Mixtures: Collapse of Capacity-Driven Trends

When the adsorption system is expanded from a single component to a realistic gas mixture—comprising CH₃Br at fumigation concentrations ($y_{C{H}_{3}Br}\approx 0.0165$), air ($N_2/O_2$), and water vapor at saturation humidity—the thermodynamic landscape changes drastically. In the previous section, we established that under pure component conditions, the pristine, unfunctionalized carbon (AC0) exhibited the highest saturation capacity due to surface-induced condensation. However, our GCMC simulations of the realistic mixture reveal a fundamental inversion of this performance trend.

As illustrated in Figure 11, the hierarchy of adsorbent performance is fully reversed. The pristine AC0 model, previously the superior material, suffers a catastrophic loss in adsorption capacity. Specifically, at 293 K and a partial pressure of 1.67 kPa, the loading decreases from ≈4.1 mmol/g in the pure component system to <0.2 mmol/g in the realistic mixture (a loss of >95%). Conversely, the functionalized models (AC1–AC3), which appeared inferior in the pure component analysis due to steric constraints, demonstrate significantly higher resilience and capacity in the competitive environment. This result indicates that the macroscopic “capacity” measured in pure isotherms is a poor predictor of performance in complex environmental matrices [4].

4.2. Quantitative Evidence of the Inversion

The quantitative extent of this inversion is highlighted by comparing the adsorption isotherms of CH₃Br in the pure state versus the mixture (Figure 11). For the pristine AC0 adsorbent at 293 K, the transition to the mixture leads to a reduction in CH₃Br loading by over an order of magnitude in the low-pressure regime. The steep sigmoidal adsorption capacity characteristic of the condensation mechanism disappears entirely, replaced by a shallow, linear isotherm indicative of weak affinity.

In sharp contrast, the carboxyl-functionalized surface (AC3) maintains a high and stable adsorption capacity adsorption profile; while the absolute loading decreases due to the reduction in partial pressure (following fundamental thermodynamic principles), the functionalized surface retains approximately 60% of its low-pressure affinity relative to the pure component baseline. Consequently, at realistic fumigation partial pressures, AC3 adsorbs significantly more CH₃Br per unit mass than AC0, validating the hypothesis that surface chemistry dominates over pore volume in dilute, competitive systems.

4.3. Competitive Adsorption and Non-Specific Site Poisoning (AC0)

The dramatic loss of performance observed for the pristine AC0 model can be attributed to a mechanism of non-specific site poisoning. On the homogeneous graphitic basal planes of AC0, the adsorption potential is relatively uniform and lacks high-energy specific sites. In the realistic mixture, the abundant background gas molecules (N₂, O₂) and, crucially, water vapor, compete for the same available micropore volume.

Although water interacts weakly with pure graphite, at saturation conditions, the entropic driving force favors the occupation of the pore space by the dominant species. The dilute CH₃Br molecules are statistically outnumbered and cannot overcome the entropic penalty to form the ordered condensed phase observed in the pure system. The presence of humidity effectively “poisons” the graphitic surface by blocking access to the micropores, a phenomenon consistent with experimental observations of rapid breakthrough of volatile organic compounds (VOCs) on non-polar adsorbents under humid conditions [9,32].

4.4. Functional Groups as Selective Anchors

In the functionalized models (AC1–AC3), the adsorption mechanism shifts from volume-filling to site-specific anchoring. The oxygenated functional groups (hydroxyl, carbonyl, carboxyl) create a heterogeneous energetic landscape with localized high-affinity sites. Our simulations indicate that these groups act as electrostatic “anchors” that interact specifically with the permanent dipole and polarizability of the halomethane molecules [33].

Specifically, the carboxyl groups in the AC3 model provide the highest binding energy for CH₃Br, allowing the adsorbate to compete effectively against water and air. Unlike the AC0 surface, where water blocks access non-specifically, the functionalized surfaces exhibit a cooperative effect where the specific interaction with the halomethane is thermodynamically favored over water clustering at the active sites [28]. This “anchoring” effect prevents the displacement of the target pollutant, preserving the adsorbent’s efficacy even in the presence of moisture.

It is noteworthy that while water interacts strongly with oxygenated groups via hydrogen bonding, the bulky CH₃Br molecule can effectively compete for these sites. This is not due to the displacement of bulk water, but rather because the carboxyl group creates a local environment where the specific binding energy of the halomethane (combining electrostatic attraction with dispersion forces from the surrounding carbon backbone) exceeds the condensation energy of water in that specific geometry [33]. The functional group acts as an anchor, while the hydrophobic “tail” of the methyl bromide interacts with the graphene plane, a synergy that water clusters cannot utilize.

4.5. Selectivity Analysis: Thermodynamic Quantification

To rigorously quantify the efficiency of the functionalized carbons, we calculated the adsorption selectivity ($S_{i/j}$) defined as:

$$S_{C{H}_{3}Br/gas}={\displaystyle \frac{x_{C{H}_{3}Br}/y_{C{H}_{3}Br}}{x_{gas}/y_{gas}}}$$

(4)

where x and y are the mole fractions in the adsorbed and bulk phases, respectively. The calculated selectivities for CH₃Br relative to air components ($N_2+O_2$) and water are presented in Figure 12.

The selectivity values for the functionalized AC3 model are orders of magnitude higher (S > 100) compared to the pristine AC0 model (S ≈ 10–20). This high thermodynamic selectivity confirms that the functionalized surface actively discriminates between the fumigant and the background gases. Even with high humidity, the AC3 surface preferentially retains the halomethane, whereas the AC0 surface loses its selectivity, becoming a non-selective adsorbent governed merely by partial pressures [12,14].

The complete reversal of adsorption trends between pure and mixed conditions exposes a critical limitation in traditional adsorbent screening protocols. Standard characterization metrics, such as BET surface area or Iodine Number, rely on single-component adsorption capacity (typically N₂) to predict performance [7]. As demonstrated here, these metrics would erroneously identify the pristine AC0 as the superior material for CH₃Br capture due to its high theoretical capacity in the absence of competition.

However, the “condensation engine” that drives high capacity in AC0 is fragile and collapses under competitive pressure. In contrast, the “anchoring mechanism” of AC3 is robust. This finding underscores that for realistic applications, design strategies must prioritize thermodynamic selectivity (driven by surface chemistry) over total capacity (driven by pore volume), and that multicomponent simulations are indispensable for predicting the actual behavior of filters in the field [4,14].

4.6. Thermodynamic Mechanism of the Inversion

The dramatic loss of performance observed for the pristine AC0 model in mixtures can be rigorously understood through the competition between enthalpic and entropic contributions to the Gibbs Free Energy ($\Delta G=\Delta H-T\Delta S$).

On the pristine AC0 surface, the high pure-component capacity is driven by cooperative condensation, which provides a significant enthalpic gain ($\Delta H_{cond}$) via lateral interactions. However, initiating this ordered phase from a dilute mixture imposes a prohibitive entropic penalty ($-T\Delta S$) associated with demixing CH₃Br from the bulk and displacing pre-adsorbed solvent molecules. Consequently, $\Delta G$ becomes less favorable.

In contrast, on the functionalized AC3 surface, the specific electrostatic interaction between the CH₃Br dipole and the carboxyl group provides a large, localized negative enthalpy ($\Delta H_{ads}\gg \Delta H_{water}$ interaction). This strong enthalpic contribution dominates the free energy balance, effectively “anchoring” the molecule and allowing it to displace water clusters despite the entropic cost.

4.7. Comparison with Experimental Literature

Our simulation results regarding the “collapse” of pristine carbon performance in humid environments are strongly supported by experimental findings in the literature. Peterson et al. [4] reported that for standard activated carbons, the breakthrough time for methyl bromide decreases precipitously as relative humidity increases from 0% to 80%, effectively rendering the carbon non-functional. They observed that impregnation with amines (TEDA), which introduces polar sites similar to the functional groups in our AC3 model, was necessary to restore performance. Similarly, Leesch et al. [11] noted inconsistencies in correlating BET surface area with adsorption capacity under field conditions, a phenomenon we now explain via the failure of the condensation mechanism in competitive scenarios.

5. Discussion: Implications for Adsorbent Design

The contrast between the results obtained for pure component adsorption and realistic mixtures reveals a fundamental conflict between two distinct thermodynamic regimes: capacity-driven adsorption versus selectivity-driven adsorption.

In the case of pure CH₃Br, the process is capacity-driven, dominated by fluid–fluid interactions and cooperative packing. Here, the energetic homogeneity of the pristine graphitic basal planes (AC0) facilitates the formation of a high-density, condensed phase, characterized by a rising isosteric heat of adsorption [15,18]. Under these idealized conditions, any structural disorder—whether geometric or chemical—acts as a perturbation that reduces the maximum packing density.

However, this regime collapses in realistic environmental scenarios. The introduction of competing species ($N_2,O_2,H_{2}O$) shifts the system control to a selectivity-driven regime. As demonstrated by the inversion of the isotherms (Figure 11), the very features that maximized pure gas capacity (extended, smooth, non-polar surfaces) render the material vulnerable to non-specific displacement by moisture and air. In this competitive regime, the functional groups on AC3, previously energetic penalties for packing, become essential thermodynamic anchors that enforce selectivity [14].

This mechanistic dichotomy elucidates why standard engineering metrics—such as the Iodine Number, Butane Activity, or BET surface area—systematically fail to predict halomethane capture performance under humid conditions [4,7]. These metrics are essentially probes of the available micropore volume for non-polar or weakly polar adsorbates in the absence of competition.

Our simulations show that these metrics correlate well with the behavior of our AC0 model in pure systems, effectively measuring the “capacity-driven” potential. However, they are blind to the “selectivity-driven” requirements of a realistic filter. By relying on these parameters, material selection processes risk optimizing the wrong variable (pore volume) while neglecting the critical parameter (surface specificity), leading to the selection of adsorbents that show early breakthrough in the field despite high theoretical surface areas [9].

While surface oxidation is critical for selectivity, our results caution against a simplistic “more oxygen is better” approach. The functionalization of the carbon framework introduces a trade-off: it creates high-affinity sites for anchoring CH₃Br (enhancing selectivity), but simultaneously reduces the accessible pore volume and introduces steric disorder that disrupts efficient molecular packing (reducing capacity).

This is evidenced by the snapshots of the adsorbed phase: on AC3, molecules are strongly localized at the carboxylated edges but are prevented from forming the dense, ordered monolayers observed on AC0 [18]. Therefore, the optimal adsorbent design is not merely about maximizing functional group density, which could choke the pore network, but about strategically placing specific functionalities (like carboxyl groups) that provide a binding energy sufficient to overcome water competition ($S_{i/j}>100$) without excessively compromising the textural properties of the carbon [33].

5.1. The Critical Role of Cluster Size and Graphitic Domain Geometry

Beyond surface chemistry, our study highlights the often-overlooked influence of the underlying carbon topology, specifically the size of the graphitic domains (Basic Structural Units). Our data reveals a divergent response to geometric variations:

• For smaller molecules like CH₃F, increasing the BSU size (from 48 to 360 atoms) consistently enhances adsorption by creating larger, more accessible slit-pores, facilitating a volume-filling mechanism [14].
• For the bulkier CH₃Br, the effect is non-monotonic. The formation of extensive graphitic domains (large BSU) in AC0 favors condensation in the pure state but offers no advantage in the competitive mixture.

This suggests that for selective capture, maximizing the graphitic character (large, ordered domains) is counterproductive if it is not paired with functionalization. The distinct performance of the 48-atom vs. 360-atom models implies that the “aspect ratio” of the pore walls—defined by the cluster size—determines the accessibility of the active sites. An optimal design requires a carbon skeleton that is sufficiently open to allow diffusion of bulky halides to the functionalized edges, rather than deep, narrow slit-pores formed by large graphene sheets where steric hindrance dominates [30].

The validity of these design principles is supported by the internal consistency of our diverse data sets. The thermodynamic signature (rising $Q_{st}$ for CH₃Br on AC0 vs. constant $Q_{st}$ for CH₃F) diagnostic of the condensation mechanism aligns perfectly with the structural evidence from snapshots (ordered vs. disordered phases) and the mathematical modeling of the isotherms (sigmoidal vs. Langmuirian) [13,31].

This cross-validation confirms that the observed “inversion” in mixtures is not an artifact of the potential model, but a physical consequence of the transition from a cohesive-energy dominated process (condensation) to an adhesive-energy dominated process (anchoring).

From a thermodynamic perspective, this inversion represents a shift in the dominant term of the Gibbs free energy of adsorption ($\Delta G_{ads}=\Delta H_{ads}-T\Delta S_{ads}$).

• On AC0 (Pure): The process is enthalpy-driven by lateral interactions. The homogeneous surface allows CH₃Br to form dense clusters, maximizing the fluid–fluid enthalpic contribution ($\Delta H_{lat}$).
• On AC0 (Mixture): In the dilute mixture, the low partial pressure prevents cluster formation (entropic penalty). Without the cooperative lateral enthalpy, the weak fluid–solid interaction ($\Delta H_{sf}$) is insufficient to overcome the competitive displacement by water and air.
• On AC3 (Mixture): The process remains enthalpy-driven but via a different mechanism: strong, specific fluid–solid interactions ($\Delta H_{sf}$) with the carboxyl groups. This interaction energy is sufficiently high to maintain a favorable $\Delta G_{ads}$ even for isolated molecules, allowing the adsorbate to anchor despite the entropic penalties of the mixture.

5.2. General Design Rules for CH₃Br Capture from Humid Air

Based on the integration of these molecular insights, we propose the following design rules for developing next-generation adsorbents for volatile halides:

• Prioritize Selectivity Over Capacity: In realistic humid streams, the total pore volume (capacity) is irrelevant if the surface lacks specific affinity. Design criteria must shift from maximizing BET area to maximizing the density of specific adsorption sites.
• Target Specific Functionality: Carboxyl groups demonstrate superior anchoring capability for CH₃Br compared to phenolic or carbonyl groups. Synthesis efforts should focus on oxidation methods that selectively enhance –COOH density.
• Optimize Graphitic Domain Size: Avoid maximizing graphitic order blindly. Moderate cluster sizes (BSUs) provide a better balance between site accessibility and pore volume for bulky halides than highly graphitized, large-domain carbons.
• Evaluate Under Multicomponent Conditions: Pure component isotherms are insufficient and potentially misleading predictors of performance. Screening protocols must include water vapor competition to assess the robustness of the adsorption mechanism.
• Distinguish Mechanism by Adsorbate Size: Recognize that design rules for small probes (N₂, CH₃F) do not scale linearly to larger condensable halides (CH₃Br); the latter require geometric considerations to avoid steric exclusion from active sites.

6. Conclusions

This study demonstrates that the adsorption performance of methyl bromide under realistic environmental conditions cannot be inferred from single-component isotherms or standard porosity metrics. We identified a critical thermodynamic transition between two distinct adsorption regimes: a capacity-driven mechanism dominated by surface-induced condensation on pristine graphitic domains, and a selectivity-driven mechanism governed by specific electrostatic anchoring on functionalized sites. Consequently, the interplay between surface chemistry and carbon geometry was shown to be non-additive, where structural disorder acts as a steric penalty for pure gas storage but transforms into an essential asset for competitive separation against water and air.

By elucidating the molecular origins of non-specific site poisoning, this work provides a predictive framework that reconciles historical inconsistencies in experimental breakthrough data. We conclude that the rational design of adsorbents for environmental remediation must prioritize thermodynamic selectivity over total pore volume, rendering the evaluation of materials under multicomponent conditions indispensable. Ultimately, optimizing adsorbents for the capture of volatile halides from humid air requires abandoning intuition derived from ideal systems in favor of a rigorous, molecularly informed approach to selectivity in complex streams.