The aim of this thesis is prediction of structure of factor S(Q) in low-Q region for confined CO 2 in aerogel and comparison of it with macroscopic CO 2 . To do so, at first we obtained I(0) and S(0) values of macroscopic and confined CO 2 via extrapolation of the experimental data, the known equations. Obtained value of S(0) for confined CO2 was larger than macroscopic CO 2 . We could conclude that correlations in confined fluid is larger than macroscopic fluid. We used RPA theory that had predicted S(Q) for macroscopic fluids in low-Q region. Our results showed that this theory was just usable for macroscopic fluid so we used models for c(r) to predict S(Q) for confined fluid in this low-Q region. These models should show the larger correlation in confined fluids than macroscopic fluids so we offered PYMFF theory. This theory showed unknown fluctuations for S(Q) in this low-Q region. According to importance of attractions in low-Q region this incompatibility was due to low positive correlation contribution in c(r) than negative correlation contribution. To improve negative to positive correlations in DCF we used perturbation in that in PYMFF theory. This model first used by Keshavarzi – Nikoofardthat had predictedS(Q) for macroscopic fluids in low-Q region. Calculated S(Q) for confined fluid in this model had good agreement with experimental results. To improve results we offered Keshavarzi Khani I model for c(r). This model had better results than Keshavarzi – Nikoofard model for confined fluid. Keshavarzi –Khani II was final model for c(r) that had the best agreement with experimental results for confined fluid in this low-Q region. This agreement was due to better positive to negative correlation in this method.
