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Oil/water interfacial tensions have been determined in systems containing alkane, aqueous NaCl and N-dodecyl polyoxyethylene non-ionic surfactants. The tensions are frequently very low (ca.mN m-l) and pass through a minimum as the salt concentration, temperature or alkane chain length is varied. By the use of thermodynamics it is shown that the minimum with respect to salt concentration can arise through the competing effects on tension of the positive adsorption of surfactant and the negative adsorption of salt at the oil/water interface. It is assumed that surfactant adsorption is independent of salt concentration; it is not necessary to suppose that the sign of the surface excess of surfactant changes as a tension minimum is traversed. It is shown that a minimum in tension with respect to temperature can occur when the entropy of micelle formation and of formation of the plane oil/water interface (both expressed per mole of surfactant) are equal. 2155 2156 TENSION MINIMA IN NON-IONIC SURFACTANT SYSTEMS O 0 4000 60 n/Hz 8 000

One area where water-based muds need improved performance is in shale inhibition. However, before existing mud syste'11s can be improved, the mechanisms by which water invades shales and how present-day inhibitive additives operate must be fully understood. An experimental technique has been developed that uses radioactive tracers to monitor the progress of water and selected dissolved ions through a shale core plug. By varying experimental parameters, such as water composition and applied pressure drop, the dominant mechanisms by which water is transported through shales have been identified. Under conditions of zero applied pressure, diffusion processes control water and ion movement through shales. Concentration gradients are the driving force for mass transfer of ionic species through shales. We observed no evidence to indicate that osmosis caused mass transfer of water. Applied pressure caused an increase in water and ion transport rates. Above a threshold pressure, water and dissolved ions travel at the same rate irrespective of the ion concentration.

. Can. J. Chem. 66, 000 (1988). Oillwater interfacial tensions are reported for systems containing pure alkane, aqueous sodium chloride, and a pure anionic surfactant, either Aerosol OT or p-dihexylbenzene sodium sulphonate (DHBS). Evidence is produced to support the claim that monolayer adsorption at the oillwater interface can produce ultralow tensions (-1 pN m-I), and that the presence at the interface of a third, surfactant-rich phase is not necessary. The aggregation of DHBS and its distribution between oil and aqueous phases of various salinities have been investigated. It has been confirmed that the behaviour of DHBS in these respects is similar to that of Aerosol OT, as might be expected from its molecular structure. The sizes of microemulsion droplets in equilibrium with planar adsorbed monolayers have been determined, and related to the tensions of the plane oil/aqueous phase interfaces using simple existing theory.ROBERT AVEYARD, BERNARD P. BINKS, THOMAS A. LAWLESS, et JEREMY MEAD. Can. J. Chem. 66, 000 (1988). On rapporte les tensions interfaciales des systkmes huileleau contenant un alcane pur, une solution aqueuse de chlorure de sodiumet un surfactant anionique a 1'Ctat pur: I'aCrosol OTou lep-dihexylbenzenesulphonate de sodiu-m (DHBS). On fournit des preuves qui confirment le fait que I'adsorption monocouche a I'interface huileleau peut produire des tensions trks faibles (-1 pN m-'), et qu'il n'est pas nCcessaire d'avoir a I'interface une troisikme phase riche en surfactant. On Ctudie l'aggregation du DHBS et sa repartition entre I'huile et les phases aqueuses dont on fait varier la salinitC. On confirme que dans ces conditions, compte tenu de sa structure molCculaire, le comportement du DHBS est analogue B celui de l'aCrosol OT. On a determine la taille des gouttelettes de microCmulsion en Cquilibre avec les monocouches planes adsorbees et on la relie aux tensions de l'interface plan huilelphase aqueuse en faisant appel B la thtorie existante.[Traduit par la revue]A number of studies have been made in recent years of systems which contain oil, aqueous electrolyte, and surfactant and which exhibit ultralow oillwater interfacial tensions (< lop2 mN m-'), e.g. refs. 1-7. These tensions can often be made to pass through a minimum as the temperature, electrolyte concentration, nature of the oil phase, or the concentration of a further surface-active material (e.g. cosurfactant) is varied (6-8). The minimum in tension occurs under conditions around which a surfactant-rich third or "middle" phase (intermediate in density between oil and aqueous phases) is formed. The tension between the middle phase and one of the other two ("outer") phases is lower than the oillwater tension and appears to be associated with a thick interface (3). Our concern here however is with the nature of the oillwater interface and of the surfactant aggregates which form in oil or aqueous phases at equilibrium. It has been claimed recently (9), on the basis of data obtained using sodium di-(2-ethylhexyl)sulphosuccinate (AOT) as t...

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