Bile salt structural effect on the thermodynamic properties of a catanionic mixed adsorbed monolayer

MARCOS D. FERNÁNDEZ LEYES; PAULA V. MESSINA; PABLO C. SCHULZ

COLLOID AND POLYMER SCIENCE

SPRINGER

Lugar: Berlin; Año: 2011 vol. 289 p. 179 - 191

0303-402X

The interfacial effects of two bile salts (sodium deoxycholate (NaDC) and sodium dehydrocholate (NaDHC)) in a catanionic mixed adsorbed monolayer have been investigated at 25 °C. The surfactant interfacial composition, the interfacial orientation of the molecules and the energy changes are analysed to show a thermody- namic evidence of the hydrophobic BSs effect during its intercalation into interfacial adsorbed didodecyldimethyl ammonium bromide (DDAB) molecules. Both mixed systems (NaDC-DDAB and NaDHC-DDAB) have analogous adsorption efficiencies, which are similar from a pure DDAB monolayer and superior to that obtained for both bile salts molecules. Nevertheless, their adsorption effectiveness is different: NaDC causes an increment of Gamma while NaDHC produces the opposite effect. The adsorption efficiency in surface tension reduction is due to the existence of interfacial synergistic interactions. Maximum synergistic interaction is seen for alpha_(BSs) = 0.4. The hydrophobic steroid backbone of NaDHC molecule presents a deep interfacial penetration than NaDC. This fact causes a great disturbance of DDAB hydrocarbon tails and conduces to a large separation of molecules (high Am values) which explains the reduction of adsorption effectiveness (low  γ_m values).