Penetration of sodium cetylsulfate into monolayers of 1,2-dipalmitoyl- and 1,2-dimyristoyl-phosphatidylethanolamine

The penetration of sodium cetylsulfate into monolayers of dipalmitoyl- and dimyristoyl-phosphatidylethanolamine was studied by the measurement of surface and penetration pressures using the vertical plate method of Wilhelmy. The penetration isotherms in two systems were investigated at different initial molecular areasA_M:System I: Sodium cetylsulfate/1,2-dipalmitoyl-phosphatidylethanolamine atA_M= 0.85; 0.75; 0.65; 0.55; 0.50; 0.46 and 0.44 nm² · molecule^–1.System II: Sodium cetylsulfate/1,2-dimyristoyl-phosphatidylethanolamine atA_M= 0.85; 0.75; 0.60 and 0.55 nm² · molecule^–1.(T=295 K; substrate 0.1 M NaCl)The penetration isotherms (F_tvs. logc_s) increase linearly atF_t> 10 mN · m^–1 in system I and atF_t>25 mN · m^–1 in system II. The isotherms of both systems are shifted to lower surfactant concentrations with decreasing molecular area of spread monolayer. A maximum of the slopes (dF_t/d logc_s)occurs at A_M=0.50 nm² · molecule^–1. This behavior is also reflected in the dependenceG_p⁰(free standard penetration enthalpy) and_s(relative surface excess concentration of surfactant) onA_M. These changes are related to a different packing of the compounds in the binary penetrated monolayers.In the high pressure region both system are nearly identical. Differences in the low pressure region arise from the penetration into different monolayer states.Nomenclature _M effective cross sectional area of monolayer molecule - a_M partial molecular area of monolayer molecule - a_s partial molecular area of surfactant molecule in the penetrated film - a_s⁰ molecular area of surfactant molecule at definite film pressure (eq. (3)) - A_M molecular area of theF/A-isotherm - A_N constant in equation (2) - A_K collapse area - b penetration coefficient in equations (2); (2 a) - c_s bulk concentration of surfactant - logc_s relative shift of penetration isotherm with regard to the adsorption isotherm at constantF_t - F film pressure of monolayer component in absence of surfactant - F_t total film pressure - F_p film pressure change due to penetration - F_p,max constant in equation (1) - G_p⁰ free standard penetration energy - k Boltzmann constant - K constant in equation (1) - R gas constant - T temperature - x_s⁰ mole fraction of surfactant in the penetrated film - _M surface concentration of monolayer molecules - _s relative adsorption of surfactant - _w⁰ surface concentration of surfactant in monolayer-free surface - factor in equation (6 a) - surface tension