Investigation of Structural Properties of n-Hexane and Decane under Different Cooling Regimes by Raman Spectroscopy

The glass-forming ability of short-chain alkanes remains a fundamental challenge in condensed matter physics. This study investigates the structural properties of n-hexane (C₆H₁₄) and decane (C₁₀H₂₂) under two distinct cooling regimes using Raman spectroscopy: fast cooling (~50–100 K/s via contact freezing on a copper substrate at 77 K) and conventional cooling (~1–5 K/s). Despite employing rapid cooling protocols, both alkanes underwent crystallization without forming amorphous phases. n-Hexane formed a defective crystalline structure characterized by broad spectral bands (FWHM ~40–45 cm⁻¹) and diffuse phase transitions in the 180–200 K range, while decane exhibited highly ordered crystalline structures with sharp spectral features (FWHM ~15–20 cm⁻¹) and abrupt transitions at 220–240 K. Quantitative analysis of characteristic Raman bands (skeletal deformations, C-C stretching, and C-H stretching vibrations) revealed fundamental differences in crystallization mechanisms related to molecular chain length. The study demonstrates that contact freezing methods are fundamentally incapable of achieving the extreme cooling rates (>10⁴ K/s) and ultra-thin film conditions (<1 μm) necessary for alkane vitrification. These findings establish spectroscopic diagnostic criteria for distinguishing between defective and well-ordered crystalline structures and define the limitations of conventional cryogenic techniques for glass formation in alkanes.