Solution Behavior and Gel Properties of Long-chain Dialkyl Phosphates in Nonpolar Solvents

Some Aluminum salts of long-chain alkylphosphates (Al-DHP) form stiff gels in the presence of nonpolar oils such as isooctane, n-heptane, cyclohexane, p-xylene, benzene and n-hexadecane. The mechanism of gelation of the Al-DHP-hydrocarbon system and association state of Al-DHP in this system were investigated by measurement of gel hardness, differential scanning calorimetry (DSC), scanning electron microscopic observation (SEM) and polarizing technique. Above the gel/jelly transition temperature (Tc), the gel converted into a highly viscous solution, indicating the formation of linear polymeric molecules of Al-DHP. Cooling down under Tc caused this viscous solution to become a hard and staff gel. Uunder Tc, a flexible Al-DHP polymer changes to linear micro crystals. The stabilization of the gel is considerd due to formation of a well dispersed three dimensional network structure consisting of linear microcrystals. SEM observation of the gel indicated formation of a fiber network, thus supporting the above assumption. The hardness of the gel varied according to the solubility parameter (S.P.) of the solvent. The gel became hardest when the S.P. of the solvent was 8.2 to 8.75. Tc was lowest when the gel was hardest. It is thus evident that gelation is closely related to the dissolution of Al-DHP to the solvent.