广泛的等电点(IEP)已在文献中报道了蓝宝石-c(α-氧化铝),也称为基面,(001)或(0001),单晶。有趣的是,现有数据表明,IEP的变化与粒子遇到的IEP范围相当,虽然单晶应该在表面结构方面更好地定义。IEP范围的一种解释可能是污染单晶样品的小表面区域同时将其暴露于相对大的溶液容器的明显危险。文献表明,样本来源等因素,样品处理或调查方法都对表面有影响,很难清楚地分开各自的,个体效应。在本研究中,我们调查因果关系以更好地理解个体效应.我们样品的参考IEP在4和4.5之间。与UV处理一样,高温处理倾向于降低蓝宝石-c的IEP。增加初始错切(即与晶体的预期取向的发散)倾向于增加IEP,等离子体清洁,这可以理解为假设表面由于存在更多和/或更大的台阶以及增加的错切或由于由等离子体清洁引起的表面的非晶化而变得疏水性较低。在非常高的pH下的预处理导致IEP的增加。导致IEP不同于参考样品的稳定值的表面处理通常导致表面受到随后暴露于水的强烈影响。在水暴露一段时间后,流动电位数据似乎对参考样品行为有所放松。zeta电位测量与AFM研究的结合支持了原子光滑表面表现出较低IEP的想法。而与参考样品相比,更粗糙的表面(纳米量级的粗糙度)导致更高的IEP。两项补充调查得出了令人惊讶或模棱两可的结果。在非常粗糙的表面(粗糙度为微米级)上,与参考样品相比,IEP降低,观察到粗糙表面的纳米级粗糙度和瞬态行为。此外,从流动电位和静态胶体粘附测量中获得的IEP差异可能表明流体动力学在流动电位实验中起作用。最后,我们将先前研究的表面衍射数据与我们的电动数据的可能解释联系起来,以证实水膜的存在可以解释低IEP。计算表明,表面衍射数据与水膜的存在一致,然而,他们不允许明确地解决这种膜的关键特征,这可能解释了观察到的表面化学特性,例如在总和频率生成研究中报道的悬挂OH键。有关相关表面性能的广泛文献综述表明,这种水膜的存在在许多情况下会影响界面性能。水膜的持久性或不持久性可能是至关重要的。水膜的存在原则上可以影响重要的过程,如冰成核,润湿行为,充电,等。
A wide range of isoelectric points (IEPs) has been reported in the literature for sapphire-c (α-alumina), also referred to as basal plane, (001) or (0001), single crystals. Interestingly, the available data suggest that the variation of IEPs is comparable to the range of IEPs encountered for particles, although single crystals should be much better defined in terms of surface structure. One explanation for the range of IEPs might be the obvious danger of contaminating the small surface areas of single crystal samples while exposing them to comparatively large solution reservoirs. Literature suggests that factors like origin of the sample, sample treatment or the method of investigation all have an influence on the surfaces and it is difficult to clearly separate the respective, individual effects. In the present study, we investigate cause-effect relationships to better understand the individual effects. The reference IEP of our samples is between 4 and 4.5. High temperature treatment tends to decrease the IEP of sapphire-c as does UV treatment. Increasing the initial miscut (i.e. the divergence from the expected orientation of the crystal) tends to increase the IEP as does plasma cleaning, which can be understood assuming that the surfaces have become less hydrophobic due to the presence of more and/or larger steps with increasing miscut or due to amorphisation of the surface caused by plasma cleaning. Pre-treatment at very high pH caused an increase in the IEP. Surface treatments that led to IEPs different from the stable value of reference samples typically resulted in surfaces that were strongly affected by subsequent exposure to water. The streaming potential data appear to relax to the reference sample behavior after a period of time of water exposure. Combination of the zeta-potential measurements with AFM investigations support the idea that atomically smooth surfaces exhibit lower IEPs, while rougher surfaces (roughness on the order of nanometers) result in higher IEPs compared to reference samples. Two supplementary investigations resulted in either surprising or ambiguous results. On very rough surfaces (roughness on the order of micrometers) the IEP lowered compared to the reference sample with nanometer-scale roughness and transient behavior of the rough surfaces was observed. Furthermore, differences in the IEP as obtained from streaming potential and static colloid adhesion measurements may suggest that hydrodynamics play a role in streaming potential experiments. We finally relate surface diffraction data from previous studies to possible interpretations of our electrokinetic data to corroborate the presence of a water film that can explain the low IEP. Calculations show that the surface diffraction data are in line with the presence of a water film, however, they do not allow to unambiguously resolve critical features of this film which might explain the observed surface chemical characteristics like the dangling OH-bond reported in sum frequency generation studies. A broad literature
review on properties of related surfaces shows that the presence of such water films could in many cases affect the interfacial properties. Persistence or not of the water film can be crucial. The presence of the water film can in principle affect important processes like ice-nucleation, wetting behavior, electric charging, etc.