The cloud point of high-molecular-weight poly(ethy1ene oxide), PEO, in aqueous salt solution has been determined as a function of the salt concentration for all potassium halides, alkali-metal chlorides and alkali-metal hydroxides. Our theoretical model for the pure PEO +water system (J. Chem. Soc., Faraday Trans. I, 1981, 77, 2053) has been extended to include the effects of salt on the phase separation. Basic features of the present model are a hydration shell with enhanced structuring of water as well as a zone with decreased salt concentration surround each chain. Overlaps of such regions are involved in polymer-polymer contacts and imply transfer of water molecules and ions from the proximity of the chains to the bulk solution, which gives important contributions to the free energy of interaction. The existence of the salt-deficient zone is explained as a consequence of asymmetric hydration of the ions near the polymer. The effects of the zone are large enough to account for the influence of salts on the clouding. The experimental differences found for the alkali-metal halides have been rationalized mainly in terms of varying degrees of salt penetration into the region around the chain.
Through recent surface force measurements it has been convincingly demonstrated that strong and amazingly long-range, attractive interaction forces act between hydrophobic surfaces immersed in water. Upon separating two such surfaces from molecular contact a vapour/gas cavity normally forms. This is not the case, however, when gradually diminishing the surface separation. The hydrophobic attraction forces have been recorded in this latter, metastable regime.A mean-field theory based on a square-gradient assumption is presented in this paper which is shown to account reasonably well for the surface forces found experimentally for two cylindrically shaped, hydrophobic surfaces interacting in water. The order parameter/variational approach taken is closely related conceptually to earlier theories of repulsive hydration forces by Marcelja et al. and Cevc et a/. The present theory implies that rather minor, hydrogen-bond-propagated molecular ordering effects, in the contact layers of water molecules next to the hydrophobic surfaces and in the core of the thin water film, give rise to the attraction observed. However, it does not fully address the intriguing question as to how it comes about that the hydrophobic attraction forces extend over such a wide range as nm. It merely points in the direction that surface-induced structural changes in the core of the thin water film (so far not captured by molecular dynamics simulations) which demand minimal free-energy expense may generate an interaction of a long-range nature.It has become firmly established that a long-range attractive force acts between hydrophobic surfaces immersed in water.'-' The most recent investigations by Claesson a n d Christenson,' Rabinovich a n d Derjaguin6 a n d Claesson a n d Christenson' have shown that this attractive force is measurable for surfaces which are free of charges even at separations of 70-90 nm, a n d that the long-range tail of the surface force curve decays exponentially. However, at distances less than ca. 10 nm, the hydrophobic attraction is further amplified. These observations are substantially different from those first reported
Synergistic effects in mixed binary surfactant systems have been investigated by analyzing the main contributions to the free energy of forming a mixed surfactant aggregate. We show that a nonlinear behavior of the critical micelle concentration (cmc) with respect to the surfactant composition of the aggregates is determined by a nonlinear behavior of the free energy per aggregated surfactant. It appears that synergistic effects are due mainly to entropic free energy contributions related with the surfactant headgroups. For a mixture of a monovalent ionic and a nonionic surfactant in the absence of added salt we obtain, entirely because of electrostatic reasons, a negative deviation from ideal behavior of the cmc vs the aggregate composition corresponding to an interaction parameter β ≈ -1, whereas β values on the order of -5 or even less can arise for mixtures of two ionic surfactants with the same charge number but with different hydrocarbon moieties. Moreover, we introduce a novel expression for the free energy of mixing aggregated surfactant headgroups with surrounding solvent molecules. Accordingly, synergistic effects arise as a result of different headgroup cross-section areas in mixtures of two nonionic surfactants with rigid headgroups. These effects are found to be rather small, with 0 > β > -1, when the difference in headgroup size is modest but can become more significant when the size difference is larger. In mixtures of an ionic and a nonionic surfactant with different headgroup cross-section areas the two contributions to synergistic effects always enhance one another and, hence, β values below -1 are obtained. Generally, the synergistic effects tend to increase with increasing asymmetry between the two surfactants.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.