A Fourier transform infrared spectroscopy study of the effect of external pressure on water penetration in micelles

A keto surfactant (sodium 7-oxooctanoate) was incorporated into aqueous micelles of sodium octanoate. Infrared spectra of this system were recorded as a function of pressure up to 43 kbar. The extent of water penetration into these micelles is detected by the existence of hydrogen bonding to the carbonyl group at position 7 of the keto-surfactant molecule. The spectra show that there is a micelle-coagel transition at {approximately} 18 kbar. At this transition, water is removed from the micellar hydrophobic core. In contrast, the spectra of 5-nonanone dissolved in sodium octanoate micelles show that relatively low pressures (up to 5 kbar) favor solubilization of the guest ketone into the dry micellar core. Higher pressures force a translocation of the guest inducing a fixed immobilized bent {intersection} conformation in the lamellar phase.