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Abstract
The reduction in strength of porous Vycor glass caused by adsorption of water vapor has been related to the lowering of the surface free energy, which has been obtained from measurements of dimensional changes. Deviations from this simple relationship have been explained by capillary condensation, which causes a negative pressure under the concave menisci of completely filled pores. It has been shown that the Griffith equation accounts quantitatively for the observed weakening effect if the negative pressure under the menisci is taken into account. The data permit an estimate of γ0=700∓100 erg/cm2 for the surface free energy of the Vycor glass in a vacuum. At complete saturation with water vapor the surface free energy is reduced to ∼200 erg/cm2. It is shown that adsorbates that are not freely mobile on the surface do not cause expansion of the porous glass, although they do reduce the strength. The significance of this result is discussed.
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