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    STUDIA CHEMIA - Issue no. 1 / 2005  
         
  Article:   LANGMUIR KINETICS AND ADSORPTION MECHANISM AT THE OIL/WATER INTERFACE.

Authors:  GHEORGHE TOMOAIA, MARIA TOMOAIA-COTISEL, CSABA RACZ, CRISTINA RAMONA ISPAS, CĂLIN FLOARE.
 
       
         
  Abstract:  The adsorption mechanism of some biocompounds, e.g. two local anesthetics, like dibucaine and tetracaine, and of stearic acid, from bulk solutions to the oil/water interface was studied by using the pendant drop and ring methods. The biocompounds are approaching the oil/water interface from the opposite directions, namely anesthetics from water phase and stearic acid from oil phase. On the other hand, anesthetics are charged species and stearic acid is uncharged under working conditions. The kinetic analysis shows that Langmuir kinetic approach describes the dynamic interfacial pressures within the limits of the experimental errors over a wide range of time and for different surfactant concentrations in bulk solutions. It is also concluded that this approach allows the calculation of the ratio of the adsorption and desorption rate constants of these biocompounds at the oil/water interface. Obtained results are in substantial agreement with similar earlier reported data for the adsorption of different surfactants at various oil/water interfaces as well as with their molecular structure. The driving force for the adsorption of anesthetics (water soluble molecules) is the hydrophobic interactions among the penetrated hydrophobic chains and the oil phase, which are accompanied by the increase of the entropy of the system because of the destruction of the ordered structure of water molecules formed around the hydrophobic chains in aqueous phase. On the contrary, the driving force for the adsorption of stearic acid (practically water insoluble compound) is the change in the enthalpy of the system due to the hydration of hydrophilic polar head groups when they immerse in the water phase.  
         
     
         
         
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