使用DCAT测量粉末的表面自由能--二氧化硅粉末研究

-dataphysics- DataPhysics Instruments GmbH· RaiffeisenstraBe 34·D-70794 Filderstadtphone++49 (o)711770556-0·fax++49(0)711-770556-99 e-mail info@dataphysics.de. internet http：//www.dataphysics.de Application Note 10 Determination of surface free energy on powders with DataPhysics DCAT 11 Determination of wetting properties of powdersaccording to Washburn Problem The knowledge of the wetting properties of powderwith liquids is important for numerous applicationfields. Not only in the pharmaceutical or cosmeticindustry， but also for many technical applications， thewettability of powders is of essential importance. Anecessary criterion here is the knowledge of thesurface free energy， as well as its polar and dispersivecontributions. The so called Washburn-method willbe introduced by the example of silica powder inorder to measure the surface free energy. TheDataPhysics DCAT 11 and the software module SCAT11have been used. Silica powder was chosen as it showsgood absorption characteristics and due to itsimportance in the analytic chemistry， as well as in theindustry. Silica powder is needed as a raw material formany different products. The presented procedure is of fundamental interestand can be transferred onto every material in powderform， for which the wetting property is to bedetermined. 'E. W. Washburn， Phys. Rev. Ser. 2， 17， 1921， p. 273 Method The contact anglemeasurement according toWashburn is done by thecapillary rise of a liquid intoa porous medium impliesthe replacement of thesolid-air interface by asolid-liquid interface. The Washburn equationdefines the flow of aliquidthrough a capillary： where h is the distance travelled by the liquid front，ythe surface tension of the liquid， r the radius of capillary，@ the advancing contact angle， n the viscosity ofliquidand t the time of flow. A powder which is packed in acolumn can be described as a bundle of capillaries withsome mean capillary radius of r . A modified Washburnequation can be used for this system： where (c.r) is a constant to approximate the tortuouspath and the mean radius of the capillaries. However，this term is an empirical constant which depends onparticle size and degree of packing. In order to use the DataPhysics DCAT 11 for contact anglemeasurements on powders， the rise of the liquid isreplaced with mass gain. A modified version ofthe Washburn equation is used to calculate contact angle： where m is the weight of the penetrating liquid， pthedensity of measuring liquid， e the relative porosity and Rthe inner radius of the measuring tube. The term insquare brackets is known as the geometric factor C(mm). It can be determined experimentally in the samemanner as the packing constant. The wicking rate is defined as the speed of a liquid frontas it moves up a packed column of powder. Ifthis quan-tity is determined for a particular powder using a totallywetting liquid (which has contact angle of o°againstthe powder) than the packing constant C obtained fromthe modified Washburn equation. Once the packingconstant is determined， it may be substituted into themodified Washburn equation and the advancingcontact angle for other liquids can be calculated. Procedure To determine the surface free energy of a powder with its polar and dispersive portions，the contact angle is measured with a number oftest liquids and evaluated according to themethod of Owens-Wendt， respectively Wu.The first step is the packing technique for thepowder. The powder must be packedreproducibly for each measurement. It is alsoimportant to keep the glass tube and filter cleanand dry. When measuring with several testliquids a different tube should be used witheach liquid to ensure smooth progression of theexperiment and reduce the risk ofcontamination. The bottom of each tube isclosed with a porous ground glass filter or frit. Asmall circle of filter paper should be placed ontop of this frit before filling the tube in order toavoid clogging the pores of the glass filter.Weigh precisely a quantity of the powder to beacked. Add it to the tube in one portion，tapping the tube manuallyon a hard surfaceuntil it has settled. A good general rule forpowder packing is that higher reproducibility isobtained with a tighter packing.In general， when determining the geometricfactor C for a particular powder， a liquid with aow surface tension is chosen and the contactangle against the powder is assumed to be o°.Hexane is generally used as a test liquid. After this measurement， the powder columnmust be cleaned and packed again (or severalcolumns must be used). With the now knownpacking constant C， determinations of contact angles with other test liquids of different polarityhave to be carried out. Very often water (WA) is also used. Alternativelyethylene glycol (EG)， diiodomethane (DI) anddimethylsulfoxide (DMSO) are polar replacementliquids or have been used for its own contribution inthe surface energy analysis. Results The silica powder (mesh 6o) sample was examined.In Table1 the measured contact angles are summedup. Table 1. Contact angle of silica powder with different liquids Diiodomethane Water contact angle [] 50.92±2.90 47，17±2.58 When evaluating the contact angle results accordingto Owens-Wendt， the values as shown in Table 2 areobtained. Table 2. Surface free energies and their dispersive and polarcontributions in mN/m. Surface freeenergy (total)[mN/m1 Dispersivepart[mN/m1 Polarpart[mN/m1 silica powdermesh 60 52.06 21.37 30.69 Summary The DCAT 11 with the software module SCAT 11 offersan easy way to determine the surface energy ofpowders. The technique described above shouldprovide accurate contact angles and the surface freeenergy of powders within the range of 30 -一6o mN/m. For powders outside this range， itprovides useful data for comparing wettingcharacteristics of liquids on powders.