A hydrophobic surface has the characteristic of not adsorbing water or be wetted by water. In other words, if the forces accompanied with the interaction of surface with water are higher than the cohesive forces accompanied with the bulk liquid water, this will result spreading of water into the surface and no drops of water will be formed. If the cohesive forces accompanied with bulk water are higher than the forces accompanied with the interaction of water with the surfaces, then water drops over the surface will be formed and the surface will form a hydrophobic layer.
The hydrophobic sand is obtained by coating normal sand with hydrophobic compounds. When hydrophobic sand surface is exposed to water, sand particles on the top layer will tend to adhere to each other as a result of hydrophobicity to water. This will cause minimizing the surface area of the sand which will reduce soil’s water requirements.
When the pure silica particles in normal sand are exposed to a volatile liquid of an organ silicon compound, for example, trimethylchlorosilane [(CH3)3SiCl] while using nanotechnology, this additive, which is a colorless volatile liquid that fumes in moist air, will react with the silica compounds (SiO2) in the normal sand, when it interacts with the surface of the sand, resulting a coating layer of hydrophobic methyl groups to the surface of normal sand grains. The trimethylsilane group [(CH3)3Si…] will replace the hydrogen atom H, which is a part of hydroxyl compound in the normal sand.
The result will be forming a silicone film (monolayer) of an organohalosilane called trimethylsilanol [(CH3)3(OH) Si-O] as a thin coating to the normal sand grains, (Katz, 2005). Some molecules are non- polar, others are polar. How different atoms share electrons is described by chemical polarity. These methyl groups, which are coating the sand grains, are non-polar compounds; they tend to act as a hydrophobic material to the polar molecules of water. This hydrophobicity will create a capillary breaking hydrophobic encapsulation which results a repellent material for water. When the hydrophobic sand is exposed to water, the surface area of the sand will minimize as a result of the particles’ tendency to approach to each other. Because water molecules are unable to bond to the hydrophobic non- polar groups coating, this will leave a gap of air between the water molecules and sand particles and keep water as drops on the surface of the hydrophobic sand.
History of hydrophobic sand
The earliest indication for the water proof sand was in a book titled “The boy mechanic book 2” which was published in 1915. It was mentioned in this book, which was written by a group of well-known mechanics, that the hydrophobic sand was invented for the first time in India and used by magicians. It was obtained by adding melting wax to heated sand. While the present combination, was invented by a group of scientists and researchers who are employed at Cabot Corporation in Massachusetts, USA. They invented a procedure to develop a reaction between trimethylchlorosilane [(CH3)3SiCl] and the grains’ surfaces of silicate materials, such as sand. The invention was manufactured for the markets at the beginning of the eighties. Since then a considerable number of studies were implied to investigate the progress of the practical applications for the hydrophobic sand and experiments were executed to reach a conclusion about the best usages of this treated sand.
Other studies were undertaken in UAE to study the effect of using hydrophobic sand for plantation and farming. For example, a two year study was conducted in UAE University about the impacts of using hydrophobic sand on the growth of palm trees and grass. The result of the study showed that the plants have 20 % denser roots and rise in harvest quantity by 33 %. These results have encouraged UAE University to measure the water quantity that is required for growing rice, which needs significant amounts of water. The results of these studies were published in the Water Resources Management Journal and in the Journal of Food Agriculture and Environment that are issued in UAE. Also, a study about water conservation and management with hydrophobic sand was conducted in Alain University in UAE. After the invention of the Nanotechnology, producing the hydrophobic sand has become one of its tremendous applications.
Hydrophobic sand applications
- Hydrophobic sand can be used in farms for agriculture purposes to reduce water seepage into the lower layers of soil and into the groundwater. Water quantity that flows through sand will be significantly less because of using hydrophobic sand which traps water allowing plants roots to obtain the ultimate benefit of water and reduce the irrigation duration and number of watering rounds per day. It could be used also in gardens and parks as a landscape tool for creating a natural looking stream when water flows over the hydrophobic sand. According to the studies held by the production company in UAE and studies conducted by UAE University, it could save up to 75 % of water usage.
- It can be used in foundations, infrastructure and underground utilities protection. In the Middle East grounds, the existence of salt is very common and it is the main cause for the weakness in foundations because of steel bars oxidation and the existence of salt in the concrete mix through the drying procedure. Using the multi-grain hydrophobic sand instead of the mono- grained normal sand in constructions will from a rigid foundation when it is stressed and compressed while the normal sand is uncompressible and soft. Salt and water will be trapped away when using hydrophobic sand under foundations which will ensure more sustainability for structures. Also, using hydrophobic sand for underground utility protection, when extending pipes or around laid cables will prevent water from affecting them and will ensure longevity of the pipes net. It is widely used around the pipes and cables laid underground in the Arctic area. When reparation and maintenance work is needed for the cable net, which are buried to protect them from the severe cold and harsh weather, it would be easier to work when using hydrophobic sand as a cap for the electrical junction boxes. Because it doesn’t freeze and icy water cannot drain through, it is always dry even during winter and this would facilitate maintenance and reparation work.
- For oil adsorption, as oil is a non-polar compound, it is attracted to the hydrophobic sand which is a non-polar material also and they both hydrophobic to the polar water molecules. When adding hydrophobic sand into oil contaminated water, oil will penetrate the sand and they will both form one unit and sink into the water where it could be easily collected. Coastal areas, which are polluted with floating petroleum spilled from oil tankers, could be easily cleansed by using hydrophobic sand to trap oil and create heavy clumps of sand and oil united together which are easily extracted from water. This was the main reason behind inventing the hydrophobic sand, cleaning polluted water, beaches and coasts.
- For flood and coastal protection, using hydrophobic sand in bags will help to prevent the flood from advancing towards residential areas and will act as efficient barriers. Using normal sand is a traditional action against floods and is considered as a quick solution but after a short period of time, normal sand will be saturated with water and its efficiency in repelling water is reduced so water will continue flowing through the bags after a period of time. Hydrophobic sand property in constant repelling of water will assist for a longer duration of preventing floods from advancing.
- For parks and playgrounds, usually normal sand is used in playgrounds, golf courses and horse racing tracks but since normal sand could be easily wetted in the event of rains for example and gets muddy, it would be wiser to use hydrophobic sand which remains dray even in rainy seasons. Hydrophobic sand could be used as a surface layer for playgrounds and parks because it does not clump in wet seasons.
- Protecting ground water from contamination with pollutants of heavy metals and other pollutants when used as a layer beneath landfills. Contaminated water with wastes from landfills will seepage creating its bath towards the groundwater unless there is a layer of hydrophobic sand that prevents the contaminated water from reaching the groundwater.
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