Critical Size for Bulk-to-Discrete Transition in 2D Aliphatic Layers: Abrupt Size Effect Observed via Calorimetry and Solid-State 13C NMR

Anomalous changes of physical properties are observed in an abrupt bulk-to-discrete transition in layered silver alkanethiolate (AgSCn, n = 1–16). A critical chain length of ncᵣ = 7 marks the sharp boundary between the bulk (uniform, n ≥ 7) and discrete (individualistic, n ≤ 6) forms of AgSCn. Solid-state ¹³C NMR analysis reveals that none of the carbons share identical chemical environment in the discrete range, making each AgSCn with n = 2–6 uniquely different material, even though the crystal structure is preserved throughout. Extraordinary changes of thermodynamic properties appearing at this bulk-to-discrete transition include ∼500% increases of melting enthalpy (ΔHₘ), ∼50 °C increases of melting point (Tₘ), and an atypical transition between size-dependent Tₘ depression and Tₘ enhancement. We develop a new comprehensive Gibbs–Thomson model with piecewise excess free energy (ΔGₑₓcₑₛₛ) to predict the nature of the abrupt size effect melting. A new 3D spatial model is constructed to divide the aliphatic chains of AgSCn into three bulk or discrete segments: (a) tail segment containing three carbons, (b) head segment containing two carbons, and (c) bulk mid-chain segment containing (n – 5) carbons. Odd/even effect of Tₘ and ΔHₘ is described by a constant ΔGₑₓcₑₛₛ over the entire chain length range of AgSCn and is exclusively attributed to the localized tail segment. Bulk-to-discrete transition occurs when material properties are dominated by the discrete head and tail segments at n < ncᵣ. Values of ncᵣ are independently measured by both calorimetry and ¹³C NMR. This analysis is generalized to other aliphatic layers including n-alkanes with ncᵣ ≈ 11. This work is seminal to the design of novel aliphatic layers with tailorable properties (e.g., Tₘ) and has applications in molecular electronics and biophysics.