Does TiO2 coating form denser pore size?

Interesting question is raised because that result that what I have got. Interesting, in one side it is known that the coating of TiO2 has caused higher affinity of membrane surface. In my opinion higher affinity, more or less, means higher permeability, higher chance that more water passes the pore of membrane. But my experiment shows the opposite. I just did a simple permeability though, but I think despite of its simplicity, it offers a very powerful tool to gain some insight about what is going on after coating in terms of its ability to allow water pass through the membrane. This post is dedicated to find out is it really true that the coating of TiO₂ affect the pore size of membrane surface and make it denser.

In my experiment, the permeability after the TiO2 coating has shown a decrease value, simply means that the pore size is smaller after the coating. It must be noted though, different membranes were used between naked membrane and TiO₂ coated membrane. Assumption were made that since these membranes were manufactured by automatic machine from TAMI industries, thus higher chance that these membranes were the same regarding their properties including pore size. Another reason is that, it is not practical to first measure permeability of naked membrane without coating then measure permeability after coating since the experiments had to be conducted simultaneously, after permeability test, then directly running the feed water towards membrane module.

The results of my experiment shows that the TiO₂ coating gave more denser pore size within the membrane. Since I actually did not use the term of pore size, I will change it to MWCO, molecular weight cut off, terms of molecular weight of compound that 90% can be retained by the membrane. I will display the result of my 50 kD membrane. Permeability of 50 kD naked membrane is 0.0129 gr/(cm².min.psi), while on the other hand, 50 kD TiO₂ coated membrane shows 0.01 gr/(cm².min.psi). The same thing occur for the rest of the membrane (1 kD and 15 kD). I will not put the number here, but after my theses is over, I would consider to display them.

So, basically what I found is the presence of TiO₂ on the membrane surface did tighten the pore size (MWCO), in this experiment, both terms pore size and MWCO can be used. Now, what makes me courious is that was there any author found the same thing of mine? Well, this is actually basic question, but this is very important to confirm my result. Anyhow, qualitatively, by seeing through the SEM, Bideau et al (1995) stated that in most cases, the specks of photocatalysts are irregular in shape, dimension and thickness, well it seems confirm my SEM results, yet it has voids between particles. That is the comment qualitatively.

Another finding about this permeability is that, higher permeability tends only give higher flux in the short term, while on the long term, it will result the same steady flux. This has been proven by my experience using the same mass of TiO₂ coated on the membrane but different permeability. Two membranes were coated 0.1 gram and the other two were 0.08 gram. The result confirms that permeability, representing pore size, or MWCO of the membrane, will only affect within several hours, thus means short term, after 3 or 4 hours from total 8 hours, both of them reach the same steady flux. This interesting findings prove that permeability only affect the short term, only several hours, 2-3 hours. While on the long term, steady flux of higher permeability will reach the same steady flux on lower permeability membrane, and it affects rejection too. Lower permeability membranes give higher rejection. So which one is better, lower permeability of higher permeability?

On this case, better to get lower permeability membrane, because the flux will not decline as sharp as higher permeability, and the most important thing is the rejection is higher. However, it must be noted that we cannot control permeability, as it is solely depends on the manufacturing process or the coating process itself. In this experiment, permeability is only indicator, and will not be used as a main subject here to consider best or worse performance of membrane.