研究目的
To study the growth-dependent surface chemistry of aspirin crystals, specifically identifying the surface terminations (carboxylic acid or acetoxy) on the <100> face and reconciling conflicting literature results by relating growth conditions to surface functionality.
研究成果
Polarized Raman spectroscopy combined with DFT modelling effectively identifies surface terminations of aspirin crystals, showing that growth conditions (e.g., polar solvents like acetone) lead to carboxylic acid termination with adsorbed water, while non-polar conditions favor acetyl termination. This resolves literature conflicts and demonstrates the method's utility for organic surface science, enabling better control of surface-derived properties.
研究不足
The study is limited by the penetration depth of the laser in Raman spectroscopy, which may capture bulk signals, and challenges in matching experimental and simulated spectra due to potential growth defects. Computational models may have errors in Raman activity calculations with semi-local DFT. ATR-MIR spectroscopy was not feasible for sublimed crystals due to fragility.
1：Experimental Design and Method Selection:
The study uses polarized Raman spectroscopy to characterize surface functional groups, supported by first-principles density-functional theory (DFT) modelling. Crystals were grown under various conditions to investigate solvent effects on surface termination.
2：Sample Selection and Data Sources:
Aspirin crystals were grown from acetone, methanol, acetonitrile, dichloromethane (DCM) by evaporative crystallization, and by vacuum sublimation. Samples were indexed and checked for polymorphism using X-ray diffraction.
3：List of Experimental Equipment and Materials:
Equipment includes a Renishaw InVia micro-Raman spectrometer with a 514 nm laser, Oxford Diffraction Xcalibur single-crystal X-ray diffractometer, Perkin Elmer Spectrum 400 for ATR-MIR spectroscopy, and computational tools like VASP for DFT calculations. Materials include aspirin (>98% purity) and various solvents.
4：Experimental Procedures and Operational Workflow:
Crystals were grown, face-indexed, and characterized using polarized Raman spectroscopy in specific Porto notation configurations (e.g., Z(X,X)Z, Z(Y,X)Z, Z(Y,Y)Z). Spectra were collected with controlled polarization, and solvent interaction experiments were performed by applying droplets to crystal surfaces.
5：Data Analysis Methods:
Raman spectra were analyzed for peak assignments, supported by computational simulations of vibrational frequencies and Raman activities using DFT. Surface energies were calculated with implicit and explicit solvent models.
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