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The properties and behavior of mixed surfactant systems are discussed in the context of experimental techniques and modeling. The overview begins with a general description of mixed surfactant solutions, followed by a more detailed examination of mixed micelles, surfactant mixtures at interfaces, and the phase behavior of mixed systems. Topics include experimental measurements, approaches to modeling, nonideality, unusual surfactant types, micellar demixing, adsorption at various interfaces, chemical reactions in micelles, precipitation, cloud point phenomena and perspectives on the direction of future research on mixed surfactant systems.Mixed surfactant systems are encountered in nearly all practical applications of surfactants. These mixtures arise from several sources. First is the natural polydispersity of commercial surfactants which results from impurities in starting materials and variability in reaction products during their manufacture. These are less expensive to produce than isomerically pure surfactants and often provide better performance. Second is the deliberate formulation of mixtures of different surfactant types to exploit synergistic behavior in mixed systems or to provide qualitatively different types of performance in a single formulation (e.g. cleaning plus fabric softening). Finally, practical formulations often require the addition of surfactant additives to help control the physical properties of the product or improve its stability.

Gas phase ion clustering reactions of the form A+(L)n+L⇄A +(L)n+1 were studied using high pressure mass spectrometry. Equilibrium constants together with enthalpy, entropy, and free energy changes were determined for stepwise clustering reactions of water about the Ag+ ion with n=0 to5 and the Cu+ ion with n=2 to4, ammonia about the Ag+ and Cu+ ions from n=1 and n=2 to4, respectively, and pyridine about the Ag+ and H+ ion from n=2 and n=1 to  3, respectively. These results, together with those of other studies, show evidence for the existence of structure in some cluster ions. Considerations of bonding and entropy, together with comparisons with the results of ion chemistry in the liquid phase, suggest that such structures can be attributed to several types of cluster ion systems, including gas phase analogs of traditional ’’coordination complexes’’ known in the aqueous phase.

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there are initial concentration gradients in both phases at the beginning of the ripple experiments. We, therefore, first solved eq 10, 12,13, 17, 20, and 21, respectively, for a flat interface starting with a fresh interface at t = 0, i.e., Ci(x,y,0) = C0 and C2(x,y,0) = 0. We then assumed that quantities like C^Cy.t), which now depend on time and location because of evaporation, do not change significantly on the time scale of ripples and calculated the perturbed concentration profile by an appropriate linearization procedure. So we determined Ct(1) from the equation [see (12)] dCi(1)(x,y,0 _ Dll 77 + 77 |c1(1)(x,y,t) -v{x,y,t)^-Cxm{y,te) (22) \ dx2 dy2 4J oy where te is the time of observation.For all practical situations, the initial concentration gradients lead to negligible deviations from the equilibrium results presented above. This can easily be understood: according to ( 14), perturbations of solute concentration decay on a length scale ^ * 1 1~( 1/ )1/2 while Cif0)(t0) changes on a length scale of ^)1/2. Hence, initial gradients are of little importance for ( ^)1/2 » 1 which holds for all experimental situations. ConclusionsOur main result is expression 19 for the dilational index e. We have shown that diffusion in the liquid dominates evporation as the mechanism for restoring gradients in solute concentration. But even diffusion is of minor importance: Lucassen and Hansen5 6have shown that it is already difficult to determine the diffusion coefficient from ripple experiments. Hence, our results suggest that it will be almost impossible to get reliable data on evaporation from studies of the ripple spectrum.Similar equations can be applied to ripples at a liquidliquid interface when discussing transfer of surfactant between the liquids. In that case, the additional terms in the expression for the dilational modulus will probably be more important, and the effect may be larger.