Binks Bernard P, Fletcher Paul D I, Roberts Noel A, Dunkerley John, Greenfield Hannah, Mastrangelo Antonio, Trickett Kieran
Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull, HU6 7RX, UK.
Phys Chem Chem Phys. 2015 Feb 14;17(6):4107-17. doi: 10.1039/c4cp04329d.
We have investigated how four different pour point depressant (PPD) polymers affect the pour point transition in mixtures of a single pure wax in a solvent. We used either n-eicosane (C20), CH3(CH2)18CH3, n-tetracosane (C24), CH3(CH2)22CH3 or n-hexatriacontane (C36), CH3(CH2)34CH3 as the wax component with either n-heptane or toluene as the solvent component. For all wax-solvent combinations, the measured variation of wax solubility with temperature is well predicted by ideal solution theory. The variation of pour point temperature as a function of the overall wax concentration is quantitatively modelled using the idea that, for each overall wax concentration, the pour point occurs at a temperature at which a critical volume fraction ϕ* of wax crystals has precipitated. Close to the pour point temperature, extraction and examination of the wax crystals show they consist of polydisperse, irregularly-shaped platelets with axial ratios (h/d, where h is the plate thickness and d is the plate long dimension) in the range 0.005-0.05. It is found that the measured ϕ* values corresponding to the pour point transitions are weakly correlated with the wax crystal axial ratios (h/d) for all wax-solvent-PPD polymer combinations. These results indicate that the pour point transition occurs at a volume fraction larger than the value at which the volumes of rotation of the platelet crystals overlap, i.e., 2.5(h/d) < ϕ* < 11(h/d). PPD polymers work, in part, by increasing the wax crystal axial ratio (h/d), thereby increasing ϕ* and reducing the pour point temperature. Since the PPD's ability to modify the wax crystal shape relies on its adsorption to the crystal-solution surface, it is anticipated and observed experimentally that optimum PPD efficacy is correlated with the difference between the wax and the polymer solubility boundary temperatures. This finding and the mechanistic insight gained here provide the basis for a simple and rapid screening test to identify candidate species likely to be effective PPDs for particular wax systems.
我们研究了四种不同的降凝剂（PPD）聚合物如何影响单一纯蜡在溶剂中的混合物的凝点转变。我们使用正二十烷（C20），CH3(CH2)18CH3、正二十四烷（C24），CH3(CH2)22CH3或正三十六烷（C36），CH3(CH2)34CH3作为蜡组分，以正庚烷或甲苯作为溶剂组分。对于所有蜡 - 溶剂组合，理想溶液理论能很好地预测蜡溶解度随温度的测量变化。凝点温度随总蜡浓度的变化通过以下理念进行定量建模：对于每个总蜡浓度，凝点出现在蜡晶体的临界体积分数ϕ沉淀的温度。接近凝点温度时，对蜡晶体的提取和检查表明它们由多分散、不规则形状的薄片组成，轴比（h/d，其中h是薄片厚度，d是薄片长尺寸）在0.005 - 0.05范围内。研究发现，对于所有蜡 - 溶剂 - PPD聚合物组合，与凝点转变对应的测量ϕ值与蜡晶体轴比（h/d）弱相关。这些结果表明，凝点转变发生在体积分数大于片状晶体旋转体积重叠的值时，即2.5(h/d) < ϕ* < 11(h/d)。PPD聚合物部分通过增加蜡晶体轴比（h/d）起作用，从而增加ϕ*并降低凝点温度。由于PPD改变蜡晶体形状的能力依赖于其在晶体 - 溶液表面的吸附，预计并通过实验观察到，最佳PPD效果与蜡和聚合物溶解度边界温度之间的差异相关。这一发现以及在此获得的机理见解为一种简单快速的筛选测试提供了基础，以识别可能对特定蜡体系有效的PPD候选物种。
