Industrially produced sodium water glasses were dried in climates with controlled temperature and humidity to transparent amorphous water containing sodium silicate materials. The water glasses had molar SiO 2:Na 2O ratios of 2.2, 3.3 and 3.9 and were dried up to 84 days at temperatures between 40°C and 95°C and water vapour pressures between 5 and 40 kPa. The materials approached final water concentrations which are equilibrium values and are controlled by the water vapour pressure of the atmosphere and the microstructure of the solids. The microstructure of the dried water glasses was characterized by atomic force microscopy. It has a nanosized substructure built up by the silicate colloids of the educts but deformed by capillary forces. In the final drying equilibrium, the water vapour pressure of the atmosphere in the drying cabinet is equal to the reduced vapour pressure of the capillary system built up by the silicate colloids. Their size scale can be explained by the deformation of colloidal aggregates due to capillary forces.
Sodium water glasses are commonly produced by dissolving either alkali silicate glasses in water or silica materials in NaOH solutions at hydrothermal conditions [
Colloids in water glass consist mainly of silica [
Commercially available sodium silicate solutions are characterized by a molar SiO2:Na2O ratio of 2 - 4 and SiO2 contents of 25 - 30 wt% [
Results on thermal analysis of dried water glasses were published by Dent Glasser and Lee [
The vapour pressure of a solvent in a homogeneous solution is reduced according to Raoult’s law by the dissolved matter itself. The reduction is a colligative property and depends on the number of dispersed or dissolved units. Thermodynamically, the vapour pressure reduction is due to the difference of chemical potential between water dispersed in the sol and pure water. A consequence of that difference is the increase of boiling temperature. However measurements show that this effect is limited [
Dried colloidal suspensions are heterogeneous on a colloidal scale [
with
As a consequence, the mass loss stops when
Three types of technical grade sodium water glasses with different Rm values were used as starting materials. Their compositions and some properties are listed in
The surface area of the samples exposed to the atmosphere stayed constant during the test runs. Volume reduction due to evaporation resulted only in reduction of sample height and not in reduction of the drying surface area. After drying the samples were stored in closed polystyrene containers without gas exchange with atmosphere. Three individual samples of 10 g liquid water glass were dried at each parameter variation of climate and type of water glass. The standard deviation of a single determination of water content was lower than 0.3 wt%. Drying times between 14 and 84 days were applied. Within this time period the samples were weighed in intervals, in the earlier experimental stage daily, later twice a week. The weight loss of the sodium silicate solutions was used to calculate the residual water content. Some of the samples were used for other investigations. Samples for atomic force microscopy (AFM) were cut with a low speed saw (Isomet, Buehler, Düsseldorf, Germany) with water free sawing liquid into appropriate sizes of about 1 × 1 cm2. The sawing liquid was removed with
NaSi2.2 | NaSi3.3 | NaSi3.9 | |
---|---|---|---|
Supplier | Woellner GmbH, Germany | Carl Roth GmbH, Germany | BASF, Germany |
Na2O content in wt% | 12.43 | 8.48 | 5.98 |
SiO2 content in wt% | 26.3 | 27.42 | 22.76 |
Rm | 2.18 | 3.33 | 3.92 |
pH | 12.7 | 12.2 | 11.96 |
Density in g/cm3 | 1.47 | 1.36 | 1.27 |
Refractive index | 1.407 | 1.390 | 1.366 |
Viscosity in mPa×s | 65 | 81 | 20 |
T in ˚C ¯ | p(H2O) in kPa ® | 4.62 | 5.02 | 10.04 | 20.07 | 40.14 |
---|---|---|---|---|---|---|
40 | A (1) | |||||
60 | B (3) | |||||
80 | C (3) | D (3) | E (2) | F (3) | ||
95 | G (2) |
ethanol. AFM measurements were performed with an Extended MultiMode (Digital Instruments, now Bruker, Santa Barbara, CA, USA) in contact mode applying a constant set-point force between the probing tip at a cantilever and the sample. The force acting between the tip and the surface, measured by the deflection of the cantilever, is kept constant by controlling the vertical displacement of the sample by means of a feedback-loop. During the scan the vertical displacements, needed to keep the force constant, are displayed as “height image” while the remaining deflection (error signal of the feedback-loop) is displayed as “deflection image”, which is very sensitive to small topographical changes.
After drying the materials were solid, transparent and amorphous, at least at room temperature. Only after very long drying times some materials became opaque. In former experiments e.g. [
with:
A least square refinement was used to fit the parameters of the rate law to the experimental data. In the first days of the drying process the fit is not optimal. This is probably due to the transition from the liquid state to the solid state which changes the kinetics. Therefore, the time law is applied for drying times exceeding three days. In later test runs drying times of 14 d were sufficient to calculate
Climate | Temperature T in ˚C | Water vapour pressure p in kPa | NaSi2.2 c¥ in wt% | NaSi3.3 c¥ in wt% | NaSi3.9 c¥ in wt% |
---|---|---|---|---|---|
A | 40 | 4.56 | 44.5 | 34.3 | 32.5 |
B | 60 | 5.07 | 28.2 ± 1.2 | 22.3 ± 0.8 | 19.5 ± 0.3 |
C | 80 | 5.07 | 21.1 ± 0.6 | 16.3 ± 0.6 | 15.5 ± 0.5 |
D | 80 | 10.01 | 21.0 ± 0.6 | 16.4 ± 0.5 | 15.9 ± 1.2 |
E | 80 | 20.03 | 27.6 ± 2.0 | 21.5 ± 1.0 | 17.6 ± 2.5 |
F | 80 | 40.53 | 52.7 ± 2.6 | 36.0 ± 1.5 | 33.2 ± 0.3 |
G | 95 | 5.07 | 18.1 | 15.0 ± 0.5 | 12.7 ± 1.5 |
The weight constancy after long drying times suggests that a thermodynamic equilibrium between gas phase and dried materials is reached: the vapour pressure of the dried sodium silicate materials is equal to the H2O vapour pressure of the drying climate.
The Thomson-Freundlich equation [
with
Values for surface tension were taken from [
If the vapour pressure of the applied climate is equal to the vapour pressure of the sodium silicate materials dried at 80˚C and 5.07 kPa, the vapour pressure of H2O is reduced from 47.3 kPa to 5.07 kPa which is a reduction by roughly 90%.
In many cases, structure models for sols and gels are based on the assumption that the colloids are rigid spheres [
Climate | gLV in J/m2 | Vmol in cm3/mol | d in nm | Pc in MPa |
---|---|---|---|---|
A | 0.0696 | 18.16 | 4.0 | −70 |
B | 0.06624 | 18.32 | 1.3 | −210 |
C | 0.0627 | 18.54 | 0.7 | −350 |
D | 0.0627 | 18.54 | 1.0 | −240 |
E | 0.0627 | 18.54 | 1.9 | −130 |
F | 0.0627 | 18.54 | 10.2 | −250 |
G | 0.05987 | 18.73 | 0.9 | −460 |
close packing of particles, the capillaries are the voids between the particles. If the particles are rigid spheres, the size of the voids is between 40% and 70% of the particles diameters, i.e. between 16 and 700 nm. This size range cannot explain the results obtained by the application of the Thomson-Freundlich equation. In earlier measurements ultrafiltration was applied [
with
The respective capillary pressures are reported in
by water is always equal to the pore volume. Under this assumption, the difference between water vapour pressure of the capillary system and the water vapour pressure of the drying atmosphere controls the drying rate at constant temperature, surface area of drying materials and atmospheric flow conditions. The water vapour pressure of the capillary system is reduced during drying so that a first order reaction rate is observed, although the drying morphology of the water glass is typical of the constant rate period of the drying process described in [
The later stages of the drying of water glasses can be explained by a first order rate law. Driving force of the drying process is the difference between water vapour pressure of the drying solid and the water vapour pressure of the atmosphere surrounding the drying solid. Drying stops when both vapour pressures become equal. The colloidal microstructure of the dried materials was confirmed by atomic force microscopy. The microstructure is transformed during drying first to an amorphous solid made up by spherical colloids which―upon further drying―are deformed due to capillary forces. Therefore, the observed microstructures fit to a drying process based on the following transitions:
sol « aggregate/colloid gel « capillary solid with deformed aggregates.
The capillary pores have sizes between 0.7 and 10 nm. The deformed microstructure can be visualized by atomic force microscopy and reveals aggregate sizes between 50 and 1300 nm. The drying in controlled climates enhances the accuracy of the measurements.
The experimental assistance of S. Brinke is gratefully acknowledged. Thanks are also due to the state Saxony- Anhalt for funding a new climate chamber.