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INTERFACIAL AND CONFINED WATER Part 3

Nếu các hồ sơ mật độ chất lỏng ở các điểm quan trọng là gần ngang (không hiển thị hấp phụ, suy thoái), bề mặt là trung lập và không thích giai đoạn chất lỏng hoặc hơi. | 54 Interfacial and confined water Figure 29 Density profiles of liquid water in cylindrical pores Rp 25 A with smooth surfaces of various strengths U0 of the water-surface interaction. profiles approach each other and become identical at T Tc. If the fluid density profile at the critical point is close to horizontal one do not show adsorption or depletion the surface is neutral and does not prefer liquid or vapor phase. From the analysis of the density profiles shown in Fig. 29 we may expect approximately horizontal density profiles for water at the critical point when U0 -1.0 kcal mol. Only for this unique level of the surface hydrophilicity hydrophobicity neither wetting nor drying transition occurs in the system and we cannot expect distortion of the liquid density profiles due to the developing drying layer or distortion of the vapor density profiles due to the developing wetting layer. At all other water-surface interactions these distortions may noticeably affect fluid density profiles. Near a hard wall liquid-vapor coexistence occurs above the temperature of a drying transition. This situation is unrealistic as the long-range interactions between fluid molecules and solid surface typical of real systems are absent near a hard wall. However it is useful to use this model surface as a reference one to study the effect of the weak attraction on the density profiles. In Fig. 30 density profiles of liquid water near a hard wall and near a weakly attractive wall with U0 -0.39 kcal mol are compared at various temperatures. Even in the case of a hard wall a drying layer cannot be detected at T 300 K despite the fact that the Surface transitions of water 55 r Ả r  r Ả Figure 30 Density profiles of liquid water in cylindrical pores Rp 25 A with hard wall and with weakly attractive wall. system is definitely above the temperature Td of a drying transition and a macroscopic vapor layer should be present near hard wall in semiinfinite liquid coexisting with a vapor. .