Sodium methallyl sulfonate (SMAS), with the chemical structure CH₂=C(CH₃)-CH₂-SO₃Na, is an anionic monomer characterized by a hydrophilic sulfonate group (-SO₃Na) and a hydrophobic methallyl moiety (CH₂=C(CH₃)-CH₂-). In supercritical CO₂ (scCO₂) — a fluid state defined by temperatures and pressures above its critical point (31.1°C, 7.38 MPa), here at ~90°C and 20 MPa — the unique solvent properties (low dielectric constant, high diffusivity, and adjustable density) profoundly influence molecular interactions, driving distinct conformational changes in SMAS. Below is a detailed analysis of these changes:
1. Solvent-Solute Interactions in scCO₂: A Driving Force for Conformational Adjustment
Supercritical CO₂ exhibits a dielectric constant (ε) typically between 1.5–2.5 under the specified conditions (~90°C, 20 MPa), far lower than that of water (ε ≈ 78) but higher than nonpolar organic solvents like hexane (ε ≈ 1.9). This low polarity creates a weakly solvating environment that prioritizes hydrophobic interactions over hydrophilic ones, reshaping SMAS’s conformation:
- Hydrophobic Segregation: The methallyl group (a short alkyl chain with a vinyl substituent) is inherently hydrophobic. In scCO₂, this moiety experiences reduced solvent-solute repulsion compared to polar solvents, favoring its extension or relaxation. Unlike in water, where hydrophobic groups aggregate to minimize contact with the polar solvent, scCO₂’s low polarity allows the methallyl chain to adopt a more extended conformation, reducing intramolecular crowding.
- Hydrophilic Solvation Challenges: The sulfonate group (-SO₃Na⁺) is highly polar and ionic, making it poorly solvated in scCO₂. ScCO₂ cannot effectively screen the electrostatic charge of the sulfonate head, leading to strong intramolecular or intermolecular ionic interactions. This drives the sulfonate group to minimize exposure to the nonpolar solvent, often by folding toward the hydrophobic segment to form a “self-shielded” conformation.
2. Conformational Adaptations: Folding and Ionic Aggregation
The interplay between the hydrophobic methallyl chain and the hydrophilic sulfonate group leads to specific conformational shifts in SMAS under scCO₂ conditions:
- Chain Folding: To reduce the energy penalty of exposing the polar sulfonate group to scCO₂, the flexible CH₂ linker between the methallyl group and the sulfonate (CH₂=C(CH₃)-CH₂-SO₃Na) tends to fold. This folding brings the sulfonate group into closer proximity to the methallyl chain, allowing the hydrophobic segment to partially shield the polar head from the nonpolar solvent. The vinyl group (CH₂=) in the methallyl moiety, with its slight polarity, may interact weakly with the sulfonate oxygen atoms via dipole-dipole forces, stabilizing this folded state.
- Ionic Pairing and Aggregation: The sulfonate’s strong electrostatic charge (due to poor solvation) promotes association between SMAS molecules. Sodium cations (Na⁺), which are also poorly solvated in scCO₂, bridge sulfonate groups (-SO₃⁻) of adjacent molecules, forming dimers or small aggregates. Within these aggregates, individual SMAS molecules adopt compact conformations to maximize ionic interactions, with folded chains minimizing solvent exposure. Even in monomeric form, the sulfonate group may coordinate with Na⁺ to form intramolecular ion pairs, further reducing polarity and stabilizing the molecule in scCO₂.
3. Effects of Temperature and Pressure on Conformation
The specified conditions (~90°C, 20 MPa) amplify these conformational trends:
- Temperature (90°C): Higher temperatures increase molecular motion, weakening intramolecular interactions (e.g., van der Waals forces between the methallyl chain and sulfonate group). This slightly reduces the stability of the folded conformation but enhances the tendency for ionic groups to associate (since thermal energy disrupts weak solvation shells around Na⁺, promoting ion pairing). The net effect is a dynamic equilibrium: molecules oscillate between partially folded and slightly extended states but remain predominantly compact to shield polar groups.
- Pressure (20 MPa): At 20 MPa, scCO₂ has a higher density (~0.7–0.8 g/cm³) than at lower pressures near the critical point. Higher density increases solvent “compressibility” and weakens the hydrophobic effect slightly, as closer CO₂ molecules can interact more strongly with the methallyl chain via dispersion forces. This reduces the need for extreme folding, allowing the methallyl chain to adopt a more relaxed (but still partially folded) conformation compared to lower-pressure scCO₂. However, the sulfonate group remains poorly solvated, so folding is retained to minimize contact with the solvent.
4. Comparison to Conformations in Aqueous or Organic Solvents
- Aqueous Environment: In water, SMAS is highly solvated: the sulfonate group forms strong hydrogen bonds with water, and the methallyl chain is extended to minimize hydrophobic interactions. No folding occurs, and ionic aggregation is negligible due to effective charge screening by water.
- Nonpolar Organic Solvents (e.g., hexane): Similar to scCO₂, but with even lower polarity, SMAS exhibits extreme folding and strong aggregation, as the sulfonate group is nearly insoluble. The methallyl chain extends fully, but the molecule is primarily stabilized by intermolecular ionic bonds.
- scCO₂: Occupies a middle ground: partial folding balances the need to shield the sulfonate group with the moderate solvation of the methallyl chain by dense CO₂. Aggregation is less pronounced than in hexane but more significant than in water.
Conclusion
In supercritical CO₂ at ~90°C and 20 MPa, sodium methallyl sulfonate undergoes conformational adjustments dominated by the need to shield its polar sulfonate group from the low-polarity solvent. This results in partial folding of the alkyl linker, bringing the sulfonate group closer to the hydrophobic methallyl chain, and ionic aggregation (via Na⁺ bridging) to stabilize poorly solvated charged groups. Temperature and pressure modulate these effects: higher temperature increases molecular motion, while higher pressure slightly relaxes folding due to denser CO₂-solute interactions. The resulting conformation is a compact, dynamically fluctuating structure that balances solvation of hydrophobic segments and shielding of polar groups — distinct from both the extended conformation in water and the highly folded state in nonpolar solvents.
