6.7
渗透压
渗透作用是指溶剂分子通过半透膜向溶液移动的过程。随着溶剂的进入，溶液被稀释并发生膨胀。这种膨胀会增加溶液的静水压。当静水压等于渗透压时，渗透作用停止。
渗透压（用 Π 表示）是指为阻止溶剂通过渗透作用进入溶液所需的最小压力。范特霍夫方程用于计算理想溶液的渗透压，其中溶质与溶剂之间的相互作用与溶剂分子之…
渗透作用是指溶剂通过半透膜向溶质浓度较高的溶液移动的现象。
由于加入溶剂，溶液被稀释并发生膨胀，其静水压（即流体在重力作用下处于平衡状态时所施加的压力）随之升高，最终会阻止渗透作用的进行。
渗透压（Π）是指为阻止溶剂流入溶液而需要施加在溶液上的压力。
理想溶液的渗透压（Π）可通过范特霍夫方程计算，该方程将渗透压（Π）与溶质浓度相关联。
然而，由于排除体积效应和聚合物-聚合物相互作用，聚合物等大分子溶液属于非理想溶液。因此，其摩尔质量 M 需通过引入渗透第二维里系数 B 的扩展范特霍夫方程来计算。
为简化该方程，等式两边同时除以聚合物 J 的摩尔浓度，该浓度等于质量浓度 cmass,J 与 M 的比值。
在不同 J 浓度下绘制 Π/cmass,J 与 cmass,J 的关系图，可通过截距估算 M 值，再通过斜率确定 B 值。
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Q1: What is osmotic pressure and how does it form?
Osmotic pressure, denoted Π, is the minimum pressure required to prevent solvent from moving across a semipermeable membrane into a solution. It develops as solvent molecules move toward higher solute concentrations, diluting the solution and increasing its hydrostatic pressure until equilibrium is reached and osmosis stops.
Q2: How does the van't Hoff equation calculate osmotic pressure?
The van't Hoff equation correlates osmotic pressure with solute concentration for ideal solutions, where solute-solvent interactions match those among solvent molecules. This equation provides a straightforward method to compute osmotic pressure from known concentrations, making it fundamental for understanding colligative properties.
Q3: Why do polymer solutions require a modified van't Hoff equation?
Polymer solutions are non-ideal due to excluded volume effects, where polymer chains cannot occupy certain spaces due to unfavorable overlapping, causing them to spread out more than ideal chains. Additionally, polymer-solvent interactions differ from solvent-solvent interactions, necessitating an expanded van't Hoff equation with the osmotic virial coefficient B.
Q4: How is polymer molar mass determined from osmotic pressure data?
By plotting osmotic pressure divided by mass concentration (Π/cmass,J) versus mass concentration (cmass,J) at various polymer concentrations, the molar mass M is determined from the y-intercept of the resulting line. The osmotic virial coefficient B is then calculated from the slope.
Q5: What role does hydrostatic pressure play in stopping osmosis?
As solvent enters the solution through the semipermeable membrane, the solution expands and its hydrostatic pressure increases. When hydrostatic pressure equals osmotic pressure, the driving force for solvent movement ceases, halting osmosis and establishing equilibrium across the membrane between solutions.
Q6: What is excluded volume effect in polymer solutions?
Excluded volume effect describes the space a polymer chain cannot occupy due to unfavorable chain overlapping. This causes polymer molecules to be more spread out than ideal polymer chains would be, affecting their osmotic behavior and requiring corrections to the van't Hoff equation for accurate molar mass calculations.
Q7: How does osmosis differ between ideal and nonideal two component liquid solutions?
Ideal solutions follow the van't Hoff equation directly because solute-solvent interactions match solvent-solvent interactions. Nonideal two component liquid solutions like polymer systems deviate due to excluded volume effects and different intermolecular interactions, requiring the expanded van't Hoff equation with the osmotic virial coefficient for accurate pressure calculations.