Non-surface active electrolyte

FIGURE 4.6. The interface profile (relative to the interface position at r = 0) for an alkane drop in water with a silica probe at an axial separation distance Do of 6.5 and 4.5 nm. The dots denote the coiresponding disjoining pressure value at the marked radial distance. The inset is the disjoining pressure (electrostatic and van der Waals) for a water/aUtane/silica system with a univalent (non-surface-active) electrolyte ( " = 30 nm ijfo = 50 mV). Reprinted from Ref. [67], with permission from Elsevier. 

For ionic surfactants, note must be taken of the adsorption not only of the surfactant ion and its counterion but also of the ions of any added electrolyte. The simplest ionic system to treat theoretically is that of an ionic surfactant in the presence of excess of a non-surface-active electrolyte with a common counterion. Thermodynamic treatment [19,20] shows that the Gibbs equation as expressed by Equation 1.3 should apply. Thus for an anionic surfactant, e.g. sodium dodecyl sulphate (NaDS), in the presence of an excess of sodium chloride, the surface excess of the surface-active dodecyl sulphate anion r s- in dilute solution is given by... 

Sodium / -naphthalenesulfonate was chosen as the surface-active electrolyte because its structure is simple and rigid. It does not form micelles, so there is no question as to the species adsorbed on the surface. It is a strong electrolyte and is expected to be essentially completely ionized at saturation coverage. SNS stabilized dispersions flocculate over periods of minutes to months depending on the concentration of SNS. Sterling FTG has a non-polar, non-ionic, hydrophobic surface. The ultimate particles have large, flat, polyhedral surfaces. The particle size distribution of the dry carbon is narrower than that of most colloidal carbons (2). 

The final class of adsorbent surface is the most complex of the three for several reasons. From the standpoint of the nature of the surface, these materials are capable of undergoing adsorption by all the previously mentioned mechanisms. Possibly more important, however, is the fact that adsorption involving charge-charge interactions is significantly more sensitive to external conditions such as pH, the electrolyte content of the aqueous phase, and the presence of non-surface-active cosolutes than are the other mechanisms. 

The Gibbs equation in this form could be applied to a solution of a non-ionic surfactant. For a solution of an ionic surfactant in the absence of any other electrolyte, Haydon and co-workers3,151 have argued that equations (4.20) and (4.21) should be modified to allow for the fact that both the anions and the cations of the surfactant will adsorb at the solution surface in order to maintain local electrical neutrality (even though not all of these ions are surface-active in the amphiphilic sense). For a solution of a 1 1 ionic surfactant a factor of 2 is required to allow for this simultaneous adsorption of cations and anions, and equation (4.21) must be modified to... 

For the ionic surfactant solution in the presence of electrolyte containing non-surfactant counterion, the surface activity can be quantified with eqn 2.4. The more complicated Gibbs... 

The overall results show that electrostatic factors lead to the formation of first black films from non-ionic surface active agents in electrolytes and that zeta-potentials as low as 18 mV are adequate to stabilize the film assuming the film potentials are comparable with those of the emulsion drop. As shown in more detail elsewhere, the DLVO theory can be used... 

Unlike the cases of non-ionic surfactants or ionic surfactants in the presence of electrolyte excess, when calculations are made according to Eq. (3.41), one should choose the concentrations (activities) of the individual solutions such that the products of the concentrations of the corresponding surface-active ions and counterions in the individual solutions and in the mixture of surfactants are equal. For example, assuming that the RiX concentration in the mixture is ci, and the concentration of R2X is C2, then the surface pressure in the individual solutions should be determined for the concentrations coi=[ci(ci+C2)] and co2=[c2(ci+C2)], respectively. This means, the increase of the counterion concentrations in the surfactant mixture due to the addition of the second surfactant s counterion has to be taken into account. In a similar way the influence of small additions of indifferent electrolyte with a common counterion has also to be considered. 

Non-ionic surfactants do not exhibit Krafft points. Rather the solubility of non-ionic surfactants decreases with increasing temperature and the surfactants begin to lose their surface active properties above a transition temperature referred to as the cloud point. This occurs because above the cloud point, a separate surfactant rich phase of swollen micelles separates the transition is visible as a marked increase in dispersion turbidity. As a result, the foaming ability of, for example, polyoxyethylenated non-ionics decreases sharply above their cloud points. The addition of electrolyte usually lowers the cloud point while the addition of ionic surfactant usually increases the cloud points of their non-ionic counterparts, this increase being dependent on the composition of the mixed micelle. 

Up to this point the surfaces between pure water and its vapor (or air), or another liquid, or a solid substrate have been dealt with. The situation at the free surface of an aqueous solution is also relevant in the present context. The behaviour of simple ions at the surface is dealt with first, and that of non-electrolytes and of surface active agents, ionic or non-ionic, is dealt with in the following sections. 

Contfary to electrolytes consisting of small ions that generally increase the surface tension of water, solutions of non-electrolytes tend to have surface tensions lower than that of water. Liquid non-electrolytes at ambient conditions, i.e., solvents that are immiscible with water but have a limited solubility in it, were already dealt with in Sect. 4.2. The surface tensions of aqueous organic solutes were reported in Adamson s book (Adamson 1990). The molecules of organic molecules tend to concentrate at the solution surface, they are surface active. 

This entire phenomenon is affected by the fact that the surface tension and viscosity vary all the way from that of a relatively pure liquid phase through all types of situations with dissolved chemicals, either electrolytes or non-electrolytes and other types of surface-active agents. 

P. Becher, Non Ionic Surface Active Compounds. V. Effect of Electrolytes, J. Colloid Science, 17 325 (1962). 

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