Relative humidity hydrophilic glasses

Langmuir-style descriptions of adsorption at high relative humidities (several monolayers adsorbed) are somewhat strained. Nevertheless, they are probably reasonable for strongly hydrophilic substances like salt and glass. In these materials, the water/solid bonds are much... 

To study the effect of sorbed water on the dynamic viscoelastic properties of hydrophilic polymers, we previously developed a moisture control system [41] as shown in Fig. 38. The moisture control system consists of two heat exchangers, denoted I and II, which are held at temperatures Tj and T2, respectively, a hot-water reservoir, a glass-filled column, and a water pump. The water in the reservoir is kept at about 90°C. Steam is fed into heat exchanger II, which is held at the dew point temperature Tj necessary to obtain the desired relative humidity at (temperature of the test chamber). 

However, as experience shows, if we apply a detaching force equal to the capillary forces or even greater than these, then even for a relative humidity close to 100% not all the particles are removed. Thus, for an applied force of 4.26 dyn, about 78% of the total number of spherical glass particles 80-100 /x in diameter are detached. (The surfaces are hydrophilic and incomplete wetting... 

Thermal or thermomechanical processes (e.g. extrusion or hot press molding) are used to form materials under low moisture conditions. The glass transition temperature (Tg)—because of their hydrophilic nature (which varies between proteins)—is highly affected by moisture (160-200°C decrease in Tg in the dry state and around 60-100 C for material with 10% moisture content). In practice, when protein materials contain about 15% water (i.e. which generally occurs when they are at equilibrium with 85% relative humidity at ambient temperature), their Tg is close to the ambient temperature. This effect is even more obvious in the presence of plasticizers. 

Until now, we have only considered perfectly smooth surfaces. Several observations cannot be explained with such an assumption. One such observation is the dependence of adhesion between hydrophilic surfaces at different humidities. The flow behavior of powders can depend critically on the content of moisture [483,485,486). The adhesion force between hydrophilic particles either increases continuously or shows an increase, a maximum, and a decrease [491-493, 545-547]. This includes particles, which are used as carriers for pharmaceutical substances [492, 548, 549]. Similarly, the force between AFM tips and hydrophilic surfaces depends on humidity [545,550-555]. Two typical results of adhesion force versus humidity experiments are shown in Figure 5.9. The adhesion force was measured between a hydrophilic glass sphere interacting with a hydrophilic silicon wafer surface. It increases monoton-ically with relative humidity. In other cases, adhesion force versus humidity curves... 

Figure 5.9 Adhesion force versus relative humidity curves, (a) A hydrophilic glass sphere of 20 pm radius interacting with a naturally oxidized silicon wafer as measured by AFM [493]. (b) Force between a microfabricated silicon nitride AFM tip and a silicon wafer [553].

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