Moderately charged surface

Merrison, J.P., Charlton, M., Deutch, B.I. and Jorgensen, L.V. (1992). Field assisted moderation by surface charging of rare gas solids. J. Phys. 

For particles, the electrophoretic mobility pe is related to their surface charge density, which is best expressed in terms of the -potential. For moderately charged particles ( -potential < 25 mV), this relationship is given by Eq. (75), where the function f(KDdH) varies smoothly between 1.0 and 1.5 as kd varies between very small and very large values [264] ... 

Moderation efficiency could be greatly enhanced by drifting a larger fraction of thermalised positrons to the exit surface. Attempts to realise field-assisted moderation have to date largely foundered because of the interactions of positrons with the interfaces between the material across which an electric field is maintained and the conductive coatings to which the potentials are applied. One reported observation of the enhancement of positron emission by an electric field has been that from a solid gas moderator whose surface was charged by electron bombardment [44]. 

The same qualitative conclusions can be drawn from the analysis of curves in Fig.2b. At free pH (pH 6.5 before adsorption), alumina sample represents a moderately-charged hydrophilic substrate because the pH value is close to the pzc. The maximum quantity of adsorption corresponds to the area of 0.52 nm per one adsorbed molecule (cf. 0.35 nm at the water-air interface). For pH 3 surfactant ions achieve a close-packed arrangement in the adsorbed bilayer and the density of bilayer adsorption at the plateau (0.11 nm /molecule) is even less than the air-water interfacial density. At the same time, the cmc is markedly diminished by a decrease in the pH. Both effects can be attributed to the appearance of a non-ionized surfactant species in a solution. The neutral form of the surfactant is less soluble in water and thus exhibits a greater affinity for a hydrophobic surface of alumina modified with grafted aliphatic chains. The decresed repulsion between uncharged heads causes a closer packing of the adsorbate in a mixed surface structure. 

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