An experimental study about the imbibition of aqueous solutions of low concentration of a non-adsorbable surfactant in a hydrophilic porous medium.
ABSTRACT The imbibition of aqueous solutions of Triton X-100 in calcium fluoride columns has been studied in order to determine the influence of the interfacial adsorption of the surfactant in the capillary rise of the solutions. This system has been chosen because this surfactant behaves as non-adsorbable at the surface of this solid when it is in aqueous solution. The experiments have consisted of the measurement of the increase in the weight of the porous columns caused by the capillary rise of the solutions. The analysis of the results has been made through a modified expression of Washburn's equation that takes into account that the experimental increase in the weight is caused by the imbibition as well as by the development of a liquid meniscus around the bottom base of the columns. From this analysis, it has been deduced that the surfactant concentration does not influence on the imbibition rate, it being equal to the observed for water. However, it has been also proved that the contact angle depends on the surfactant concentration, taking decreasing values as the surface tension of the solutions decreases. In order to justify these findings, a study about the influence of the interfacial adsorption on the imbibition has been carried out. By means of them, it has been proved that the absence of adsorption at the solid-liquid interface is the reason that explains both the independence of the imbibition rate from the surfactant concentration and the decrease of the contact angle. Moreover, this fact indicates that the depletion of the surfactant molecules from the advancing meniscus, which has been normally adduced as the phenomenon causing the observed behaviour, has to be ruled out as the physical cause that justifies the behaviour found from the analysis of the imbibition experiments. As a corollary, it has been also stated that only if the adsorption at the solid interfaces happened, the imbibition of aqueous solution of surfactant in hydrophilic media could be influenced by the surfactant concentration.
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ABSTRACT: Capillary filling dynamics of liquid n-tetracosane (n-C24H50) in a network of cylindrical pores with 7 and 10 nm mean diameter in monolithic silica glass (Vycor) exhibit an abrupt temperature-slope change at Ts = 54 degrees C, approximately 4 degrees C above bulk and approximately 16 degrees C, 8 degrees C, respectively, above pore freezing. It can be traced to a sudden inversion of the surface tension's T slope, and thus to a decrease in surface entropy at the advancing pore menisci, characteristic of the formation of a single solid monolayer of rectified molecules, known as surface freezing from macroscopic, quiescent tetracosane melts. The imbibition speeds, that are the squared prefactors of the observed square-root-of-time Lucas-Washburn invasion kinetics, indicate a conserved bulk fluidity and capillarity of the nanopore-confined liquid, if we assume a flat lying, sticky hydrocarbon backbone monolayer at the silica walls.Physical Review Letters 10/2009; 103(17):174501. · 7.73 Impact Factor
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ABSTRACT: Capillary analogical experiments have been designed to simulate the impregnation of surfactant-contained Nafion® solution in porous PTFE (ePTFE) matrix. It is found that the gas pressure in the capillary (initial Pinner) is the most important factor for the solution impregnation although the capillary force has slight influence on the capillary rise. The Nafion® solution can occupy 98.2% (4.91 cm vs. 5 cm) of the end-sealed capillary when the Pinner is lowered to 5 × 102 Pa. Hence, the decrease of the gas pressure in the porous PTFE matrix is very important to obtain compact Nafion®/ePTFE composite proton exchange membranes. The PFSA/ePTFE polymer electrolyte membranes (PEMs) prepared in the conditions of initial Pinner of 5 × 102 Pa and 5 vol.% surfactant concentration are well impregnated and show a high resistance to the hydrogen gas permeability over 4500 dry/wet cycles, indicating a better durability and stability. These thinner and highly conductive composite membranes are significant to improve the fuel cell output voltage. The single cell with the PEM prepared at optimized conditions gives the open circuit voltages of 0.954 V and current density of 1 A cm−2 @ 0.6 V at 60 °C.Journal of Membrane Science 01/2007; · 4.09 Impact Factor