where P0 is the partial pressure and Ptotal is the total pressure.
Hence, high humidity creates high vapor pressure, according to
Eq. (9). The hydrophilic silica films contain –OH groups on their
surfaces, which react with the atmospheric moisture and the film
structure gets detoriated. For long-term durability of the water
repellent silica films, the surface of the film should be chemically
modified with non-polar groups as represented in Eq. (4). The
effect of humidity on the wetting properties of unmodified and iso-
BTMS modified (M = 0.965) silica films was carried out at relative
humidity of 85% at 30 8C temperature over 45 days. The
unmodified silica film (M = 0) shows a decrease in contact angle
(u = 138) after 45 days exposure to humidity. However, it was
observed that there was no significant change in the contact angle
for the iso-BTMS modified silica films even up to 45 days. The effect
of humidity on the water contact angle of the films with respect to
number of days is shown in Table 2. This reveals that the iso-BTMS
modified silica film maintained high contact angle during the
humidity test periods as compared to the unmodified silica film.
Chemical aging tests were conducted by immersing the
samples into a solution of acetic acid with a pH value of 3 at
room temperature for 2 h. Prior to the contact angle measurement;
the samples were taken out from the solution and thoroughly
cleaned with double distilled water. It is found that unmodified
film shows water contact angle of 308, whereas the iso-BTMS
modified film shows decrease in water contact angle value of 1238.