Physical and Theoretical Chemistry Laboratory, Oxford University, Oxford, UK.
J Colloid Interface Sci. 2012 Mar 1;369(1):287-93. doi: 10.1016/j.jcis.2011.12.002. Epub 2011 Dec 11.
The adsorption of the non-ionic surfactants tetraoxyethylene glycol monododecyl ether (C(12)EO(4)), pentaoxyethylene glycol monododecyl ether (C(12)EO(5)), and hexaoxyethylene glycol monododecyl ether (C(12)EO(6)) to single crystal sapphire substrates has been studied using specular neutron reflection for solutions at the critical micelle concentration. The effects of temperature and pH of the solutions were studied as well as the differences between two different crystal faces, the C and the R planes. At neutral pH, significant adsorption was only observed when the temperature was raised above the cloud temperature. This adsorption was reversible and surfactant was displaced on cooling. Reducing the pH to 3 results in significantly increased adsorption of C(12)EO(5) at 25°C with a central layer consisting mainly of surfactant (about 90%) on the C-plane substrate. A slightly smaller surface excess was observed for the R-plane. This contrasts with the significantly lower density observed even at high temperatures at neutral pH on both substrates. The results suggest that for neutral solutions surfactant association above the cloud point is the primary driving force for adsorption. At low pH, specific interactions with protonated surfaces are important. The structures of the highly hydrated layers are similar to those found for the surfactants at hydrophilic silica surfaces.
使用镜面中子反射法研究了在临界胶束浓度下，非离子表面活性剂四氧乙烯二醇单十二醚（C(12)EO(4)）、五氧乙烯二醇单十二醚（C(12)EO(5)）和六氧乙烯二醇单十二醚（C(12)EO(6)）在单晶蓝宝石衬底上的吸附。研究了溶液温度和 pH 值的影响以及两种不同晶面（C 面和 R 面）之间的差异。在中性 pH 值下，只有在温度升高到浊点以上时，才观察到明显的吸附。这种吸附是可逆的，冷却时表面活性剂会被置换。将 pH 值降低至 3 会导致在 25°C 时 C(12)EO(5)的吸附显著增加，C 面衬底上的中心层主要由表面活性剂（约 90%）组成。在 R 面观察到的表面过剩略小。这与在中性 pH 值下即使在高温下在两种衬底上观察到的密度明显较低形成对比。结果表明，对于中性溶液，在浊点以上的表面活性剂缔合是吸附的主要驱动力。在低 pH 值下，与质子化表面的特异性相互作用很重要。高度水合层的结构与在亲水二氧化硅表面的表面活性剂相似。
