Vietnam's E10 fuel: limits of water‑separation additives

E10 gasoline in Vietnam is a blend of about 90% conventional gasoline and 10% ethanol. Because ethanol is polar, it can absorb moisture from the atmosphere. When water exceeds roughly 0.5% of the fuel volume, the mixture may separate into an ethanol‑water layer that settles at the bottom of the tank.

Additives marketed to prevent "phase separation" do not eliminate water from the fuel. They contain surfactants or co‑solvents that keep small amounts of water dispersed, extending the stability window of the blend. They cannot bind ethanol permanently, restore fuel that has already separated, or guarantee complete protection if moisture levels become too high.

Tests in Vietnam and the United States show that E10 stored in sealed containers typically does not separate for at least three months, even under humid conditions. The risk rises for fuel kept long‑term in open or poorly sealed containers, especially in boats, generators, or rarely used vehicles. Authorities advise users to avoid extended storage, keep containers tightly closed, and periodically run the engine to circulate the fuel.