Untitled - Technische Universiteit Eindhoven

7.4. Results and discussion 105the initial load W 0 . The bulk shear modulus G p is subsequently derived from E p usingPoisson’s ratio. The fit also yields τ R from which the permeability D is calculated usingEq. 7.2. The viscosity η L of ethanol is about 1.07 mPa s at room temperature [67]. Themeasured moduli and permeabilities for the gel rods are listed in Table 7.1 and are furtherdiscussed below.Acid-catalyzed gelsIn series I (pH = 3.1, total reaction and aging time of 43 hours, and with varying φ Si ) themodulus E p increases from 5.9 ± 0.2 MPa for the lowest (approximate) silica fraction,φ Si = 0.113 ± 0.007, to 22.8 ± 0.9 MPa for the highest fraction, φ Si = 0.149 ± 0.009.The gels of series II were cured and aged for 91 hours and contain a varying amountof additional sodium-chloride. The modulus of the first gel with C S = 0 is 18.6 ± 0.7MPa, which is high compared to the moduli of the gels in series I. Due to aging thesilica network becomes more consolidated leading to enhanced elastic moduli. Additionof sodium-chloride to the gelling solution resulted in lower values of E p of about 15.5 MPabut with no clear correlation to C S within the range considered. According to Martin andOdinek [136] the presence of small amounts of salt has little influence on the structureand growth of acid-catalyzed silica alcogels. The little variation of E p for series II is inagreement with this finding, though the difference in E p between the gel without extrasalt (E p is about 20% higher) and the gels with extra salt cannot be explained at thispoint.The permeability D of the acid-catalyzed gels is on the order of 1 nm 2 . In series Ithe permeability decreases from 1.04 ± 0.06 nm 2 for the lowest silica fraction to 0.38 ±0.02 nm 2 for the highest fraction. The average permeability of the gels in series II is 0.72± 0.04 nm 2 , which corresponds well with the permeability results of series I, given thesilica fractions φ Si . The additional salt slightly increases the permeability, but this is nota clear function of concentration.Base-catalyzed gelsIn series IV (pH = 8.8, total reaction and aging time of 72 hours, and varying φ Si ) themodulus E p increases from 1.4 ± 0.1 MPa for the lowest silica fraction, φ Si = 0.078 ±0.004, to 4.0 ± 0.2 MPa for the highest fraction, φ Si = 0.100 ± 0.005. The moduli of thegels in series III (pH = 9.25) have an average value of 7.1 ± 0.3 MPa and appear to beindependent of C S within the range considered. The average modulus is high with respectto the moduli of the gels in the other series, given the approximate silica densities φ Si .The faster gelation of the sol-gel solutions in series III compared to the gels in series IVimplies that the effective aging time was longer for the gels in series III, so that highervalues for E p are expected. In general, the modulus E p for the acid and base-catalyzed gelsis a strong function of φ Si and there is no significant difference in the order of magnitudeof E p for both types of gels, given the different values of φ Si .For the gels in series III the permeability increases from 14.8 ± 0.7 nm 2 for C S = 0mMolal to 26 ± 2 for C S = 60 mMolal. Although the additional sodium-chloride did notaffect E p , the permeability is strongly affected when C S increases from 0 to 60 mMolal.