Simulasi Monte Carlo sistem Zn2+ dalam campuran amoniak-air

BARROROH, Himmatul, Dr. M. Utoro Yahya, MSc

2003 | Tesis | S2 Ilmu Kimia

Simulasi Monte Carlo terhadap sistem Zn2+ dalam campuran amoniak-air 41,43%-molekul telah dilakukan dengan menggunakan model interaksi potensial pasangan dan potensial badan tiga untuk mempelajari sistem solvasi preferensial. Potensial interaksi (Rumus) diambil dari literatur sedangkan potensial badan tiga Zn2+-H2O-NH3 disusun dalam penelitian ini. Fungsi potensial Zn2+-H2O-NH3 disusun dengan pengumpulan 2212 titik energi interaksi antara molekul amoniak dan air pada ruang sekitar ion Zn2+. Energi interaksi dihitung dengan metode ab initio pada tingkat teori Restricted Hartree Fock (RHF) menggunakan himpunan basis (rumus) augmented untuk N. Hasil fitting persamaan potensial badan tiga dengan metode Simplex -quasi Newton dinyatakan sebagai: (Rumus) Sistem simulasi terdir i dari 1 ion Zn2+, 123 molekul H2O dan 87 molekul NH3. Kondisi simulasi dibuat pada temperatur 263,15 K agar sistem tetap berada dalam fasa cair. Hasil simulasi menunjukkan bahwa solvasi preferensial kulit pertama Zn2+ terdiri atas rerata 1 molekul H2O dan 5 molekul NH3. Hasil simulasi dengan hanya menggunakan potensial pasangan menunjukkan adanya rerata 3 molekul H2O dan 5 molekul NH3 di sekitar ion Zn2+. Sifat solvasi Zn2+ sangat mudah mengalami pertukaran. Data distribusi sudut, jejari dan bilangan koordinasi menunjukkan struktur geometri solvasi kulit pertama segitiga antipiramida. Potensial badan tiga menggeser nilai maksimum energi interaksi pasangan Zn2+-H2O dan Zn2+-NH3 sekitar 25,10-29,29 kJ/mol. Energi interaksi badan tiga Zn2+-H2O-NH3 dengan kontribusi rerata 21,80 kJ/mol, lebih berpengaruh terhadap energi interaksi badan tiga (rumus) dibanding (rumus). Terdapat kelebihan kelimpahan molekul NH3 pada kulit solvasi pertama sebesar 41,57% dibanding fasa ruah.

Monte Carlo Simulation of Zn2+ in 41.43 mol% water-ammonia mixed system was carried out using pair potential including three-body potential in order to study preferential solvation system. Interaction potential of (formula) was constructed in this study. Potential function of Zn2+-H2O-NH3 was constructed by collecting 2212 interaction energy points between ammonia and water molecules around Zn2+ ion. The energy of the system was calculated by ab initio method at Restricted Hartree Fock (RHF) level of theory using LANL2DZ ECP basis set for Zn2+, SBKJC DZP ECP for O and H atoms, and SBKJC VDZ ECP augmented for N atom. Fitting function result of three-body potential by Simplex-quasi Newton method was written as: (Formula) Simulation system consisted of 1 ion Zn2+, 123 H2O molecules, and 87 NH3 molecules. Temperature of the system was set at 263.15 K in order to keep the system in its liquid state. The result showed that first solvation shell of Zn2+ consist of average 1 H2O molecule and 5 ammonia molecules. Applying pair potential only led to first solvation shell of average 3 H2O molecules and 5 NH3 molecules around Zn2+ ion. Ligand exchange reaction was particularly easy to occur between first and second solvation sphere of Zn2+. Distribution data of angle, radial, and first shell coordination number show that the geometry structure of first solvation shell was a trigonal antipyramida. Three-body potential affects to shift pair interaction energy maximum of (formula) about (formula). Three-body potential energy of Zn2+- H2O-NH3 with everage contribution of 21.80 kJ/mol favourably affects three-body potential of (formula) compare with (formula). There was an excess amount of NH3 molecules in the first solvation shell about 41.57 % compare with the bulk.

Kata Kunci : Amoniak Cair, Sistem Zn, Simulasi Monte Carlo