Contact Angle Measurement and Its Application To Sol-Gel Processing
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0 =(1)
Where -yIvis the liquid vapor surface tension, 0 is the contact angle, and r, is the capillary pore radius. For small pores, less than 100 A, capillary pressures created by wetting fluids can reach into the hundreds of atmospheres. This is why the typical method of drying a gel to create an aerogel is super critical extraction'. 643 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
Previous work by Mitsyuk, Vysotskii, and Polyakov2 has shown that the bulk density of gels dried from different solvents is not directly proportional to the liquid vapor surface tension of the drying solvent. They attributed this effect to the differences in the contact angle of the drying solvent to the gel surface and supported this hypothesis with heat of wetting experiments although the contact angle was never measured. Our own work has confirmed this trend in bulk densities. No previous work has measured the solvent gel contact angle or compared both the strength of the gel and the solvent gel contact angle to the dried bulk density. From this information it will be possible to demonstrate role of contact angles in gel shrinkage. Experimental
Microscope slides were used as both a clean glass reference and a support for a two step acid/acid catalyzed silica gel (designated as A2) coating for use in measuring contact angles. The coating was made by preparing an A2 sol in the usual way5 , and allowed to age three hours at 50'C. It was then diluted one part A2 sol to two parts 200 proof ethanol. The clean glass slides were dip coated with a velocity of approximately 1 cm/minute. The coated plate was then dried at 150 'C for 6 hours. Other work has shown this method of coating an A2 sol will result in a near zero porosity thin film'.
After heat treatment, some slides were washed repeatedly in hexane at 50 'C for at least three days before proceeding with the contact angle experiments. Other slides underwent two hexane washes at 50 'C in 1 day, followed by liquid phase silylation by a five volume percent mixture of chlorotrimethylsilane in hexane at 50 'C. The silylation was allowed to proceed for a day, after which the samples underwent at least two hexane washes in at least one day at 50 'C. The Wilhelmy Plate Method is a common contact angle measurement procedure6 . A slide or plate is suspended above a solvent bath such that its lower edge is parallel with the surface of the solvent. The bath is constructed such that the solvent to bath menisci are far from the plate. For static measurements, the liquid is raised slowly until the base of the slide first contacts the liquid. A force
measurement from the scale is read, and the static contact angle can be determined from a force balance on the slide. Young's equation shows the contact angle is6: cos0-
fg
(2)
.pylv
Where f is the measured force (grams), p is the perimeter of plate or slide, and y,, is the liquid vapor surface tension. Figure 1 contains a diagram of a typical Wilhelmy plate experiment. A more detailed description of contact angle measur
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