An experimental study about the imbibition of aqueous solutions of low concentration of a non-adsorbable surfactant in a hydrophilic porous medium
Department of Physics, University of Extremadura, Campus Universitario, Av. Elvas s/n, 06071 Badajoz, Spain.Journal of Colloid and Interface Science (Impact Factor: 3.37). 10/2006; 301(1):323-8. DOI: 10.1016/j.jcis.2006.04.076
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 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.
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ABSTRACT: We report on dye deposition patterns obtained when printing dye-labeled protein solutions onto a macroporous membrane. A typical area of application would be the manufacturing of immunochromatographic tests, but the results bear significance for a wide range of printing processes. As the liquid is taken up by the membrane, the protein adsorbs non-specifically to the wetted membrane surface. The transport of protein to the pore surface involves spreading of a drop on top of the membrane, imbibition of liquid into the membrane, adsorption of protein at the pore surface, liquid redistribution from large to small pores, and liquid redistribution from the center of the wetted area to the rim.Two techniques were employed to monitor the transport of protein, which are, firstly, conventional optical imaging of the wet membrane, and, secondly, fluorescence microscopy on vertical cuts through dried samples. The latter technique visualizes protein deposition. It is conceptually related to the acquisition of breakthrough curves in chromatography. Comparison of the deposition patterns obtained with different amounts of proteins allows to reconstruct the flow pattern.We frequently find a brightening of the rim, which can be traced back to redistribution of liquid from the center to the edge during drying and to a preferential uptake of liquid across the rim of the drop. Small pores and slow drying are beneficial for the achievement of sharp printing patterns.
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ABSTRACT: It has been shown that talc powder can be compacted into tablets with a preferred orientation of the platelets. The tablets can be obtained with different controlled porosity depending on pressing methods and applied pressure. The tablets can be obtained with or without additives, which may, in turn, be adsorbed. The orientation of the high aspect ratio platy talc, the surface chemistry imparted by the additives and the transported fluid influence the imbibition and permeation rates. Non-polar hexadecane displays a higher imbibition and permeability than water for all particulate orientations during short timescale absorption, likely due to the oleophilic nature of talc, and thus a more complete filling of the pores for non-polar liquids is to be expected. At longer timescales water is imbibed either at a similar rate to hexadecane or faster depending on the surface chemistry generated by additives leading to hydrophilicity. The swelling of the added polymers used to create wettability leads to break-up of the structure and exposure of hydrophilic surfaces for more rapid imbibition. It is not possible, therefore, to measure reliably the water uptake parameters when talc is fully dispersed with surfactants. Furthermore, dispersing agents tend to contribute to the blocking of pores and throats in the swollen state, and so a limitation in total imbibed volume occurs. The permeability under pressure is also inhibited by additives, which supports the suggestion of partial blockage of the pores and throats. When the individual talc crystal c axes, defining the perpendicular to the  planes, are oriented 90\textdegree \ to the primary average liquid flow direction, i.e. are oriented in a planar configuration to the flow, imbibition and permeation of wetting liquid are increased. This is assumed to be due to decreased tortuosity, provided the liquid is wetting in respect to the oriented edge surface. However, non-wetting liquids in respect to all, or geometrically dependent, orientations are subject also to the surface chemistry presented by the orientation, i.e. whether the talc is primarily displaying OH-groups or not, or adsorbed species aiding wetting by the liquid. Measurements where additives are not strongly adsorbed are complicated by the solubility of some wetting and dispersing agents.
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