Characterisation Of Particle Surface Energy And Surface Energy Component Changes Due To Surface Modification By Thin Layer Wicking Technique

The possibility of characterisation of particle surface energy and surface energy componentchanges due to surface modification by thin layer wicking technique is studied. The particlesconcerned include silica gel, aluminium oxide, and magnesium hydroxide with or withoutsurface modification. The contact angle of liquids on the particles is first studied and it isfound that the differentiation of particles with or without sub-pores is very important. Forparticles with sub-pores like silica gel capillary condensation takes place during pre-contactof the particle with volatile liquids, which strongly affects wicking velocity and need to beindependently corrected before calculating contact angle by typical Washburn equation. Thecapillary condensation effect can be quantitatively corrected by introducing a parameter,tortuosity, into typical Washburn equation, which can be calculated based on the saturatedadsorption of wicking liquid vapour on the particles. With commercial silicagel plate for TLC analysis by using wicking technique andapplying modified Washburn equation it is obtained that n-alkanes and ethylene glycol showzero contact angles, while 1-bromonaphthalene, diiodomethane, formamide and water shownon-zero contact angles. The difference between dynamic advancing and receding angles isfound to be so small that their average could be taken as the equilibrium or Young's contactangle. With this approximation the measured contact angles of 1-bromonaphthalene anddiiodomethane on the silicagel plate are 16.1° and 35.8 ° respectively, well agreeing withliterature reported, and closed nonpolar component of the surface tension, 42.7 and41.7mN/m, are calculated correspondingly. The polar component of the surface tensionobtained based on the wicking of three pair of probe liquids, formamide-water, ethyleneglycol-water and formamide-ethylene glycol, are 7.03, 7.04, and 7.30mN/m, respectively,quite in good agreement with each other.The adsorption tests showed that, both aluminium oxide and magnesium hydroxide haveno sub-pores and their wicking can be deal with by typical Washburn equation. The totalsurface tension and the surface tension components of aluminium oxide magnesiumhydroxide obtained by wicking with different probe liquid are in good agreement with eachother, respectively.The surface energy of magnesium hydroxide decreases by modification with 0.5% stearicacid so greatly that the spontaneous wicking of typical polar probe liquids become impossible,or the contact angle of these liquid on the surface is higher than 90°. The wicking of1-bromonaphthalene is still possible and the nonpolar component of the surface free energy iscalculated to be 22.5mN/m, greatly reduced compared with that without surface modification.By wicking with methanol and ethanol the total polar component are successfully calculatedto be 1.08mN/m, also decreases compared to the 5.88mN/m before surface modification, andit is found that the γ +s tendts to be zero(0.012mN/m),while γ ?s increases. Increasingstearic acid amount to 1% leads to a very small further decrease of both nonpolar and polarsurface tension component and is not suggested.Compared with other methods such as wet adsorption and dispersing, wicking techniquecan provide not only the reliable results but also the detail change of surface tensioncomponents, which is significant for the selection of modifying agents and their addingconcentration and for the characterisation of their adsorption styles on particle surface.In addition,the surface tension components of n-butanol is calculated based on thewicking technique, but the accuracy is waiting for further confirming with other systems.