Adsorption of oil into surfactant monolayers and structure of mixed surfactant+oil films |
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| Institution: | 1. Surfactant Science Group, Department of Chemistry, University of Hull, Hull HU6 7RX, UK;2. Physical and Theoretical Chemistry Lab., University of Oxford, South Parks Road, Oxford OX1 3QZ, UK;3. Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK;1. Department of Chemical and Materials Engineering, Hefei University, 99 Jinxiu Road, Hefei 230601, PR. China;2. Department of Mechanical Engineering, Hefei University, 99 Jinxiu Road, Hefei 230601, PR. China;3. Institute of Tribology, Hefei University of Technology, 193 Tunxi Road, Hefei 230009, PR. China;4. Mechanical Engineering, School of Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom;1. Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, CNBS and CSGI, s.s. 554 bivio Sestu, I-09042 Monserrato (CA), Italy;2. Department of Agricultural, Environmental and Food Sciences (DIAAA) and Center for Colloid and Surface Science (CSGI), Università degli Studi del Molise, Via De Sanctis, I-86100 Campobasso, Italy;3. Department of Medicine and Health Sciences, Università degli Studi del Molise, Via De Sanctis, I-86100 Campobasso, Italy;1. Key Laboratory on Deep GeoDrilling Technology of the Ministry of Land and Resources, School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China;2. State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China |
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| Abstract: | We describe a tensiometric method for determining the adsorption isotherm of an oil on a surfactant monolayer adsorbed at the air–water surface. The method involves measuring the surface pressure of oil, πoil, as a function of its activity, aoil, varied by changing the relative vapour pressure. We compare the isotherm of dodecane adsorption onto a C12E5 monolayer determined in this way with that measured directly using neutron reflectivity. The agreement between the two allows us to conclude that, at least for this system, addition of oil does not result in a change in the chemical potential of the surfactant. Structural analysis of the dodecane+C12E5 mixed film has been performed with neutron reflectivity using two contrasts. In one, only the surfactant chain region is highlighted, whilst in the other only the oil film is visible. We document, for the first time, changes in thickness and packing density of both the oil and surfactant chains in the mixed surfactant+oil layer upon increasing oil content. For a diverse range of other oil+surfactant systems, we have determined the initial (Si) and equilibrium (Se) spreading coefficients of oil by measuring πoil following addition of liquid oil (at unit activity) to surfactant solution surfaces. Those systems in which Se is close to zero display a repulsive van der Waals component of the disjoining pressure-oil film thickness isotherm, whilst with toluene as oil the calculated isotherm is attractive, consistent with non-spreading observed for this oil. |
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