Nonionic mixtures

Small micelles in dilute solution close to the CMC are generally beheved to be spherical. Under other conditions, micellar materials can assume stmctures such as oblate and prolate spheroids, vesicles (double layers), rods, and lamellae (36,37). AH of these stmctures have been demonstrated under certain conditions, and a single surfactant can assume a number of stmctures, depending on surfactant, salt concentration, and temperature. In mixed surfactant solutions, micelles of each species may coexist, but usually mixed micelles are formed. Anionic-nonionic mixtures are of technical importance and their properties have been studied (38,39). 

The micellization and adsorption properties of industrial sulfonate/ ethoxylated nonionic mixtures have been assessed in solution in contact with kaolinite. The related competitive equilibria were computed with a simple model based on the regular solution theory (RST). Starting from this analysis, the advantage of adding a hydrophilic additive or desorbing agent to reduce the overall adsorption is emphasized. 

The value of the characteristic interaction parameter of these systems (30° C), adjusted from the CMC measurements in Figure 1, was calculated by means of RST and taken equal to -2.5. This value is effectively in the range of the ones found by Graciaa for similar anionic/nonionic mixtures (8). 

Chen [8] studied mixtures of the pure surfactants Ci2(EO)4 and sodium dodecyl sulfate (SDS) at 30 °C. At this temperature the former is a liquid which does not dissolve in water (see Fig. 3), and the latter is a solid. The SDS was doubly recrystallized from ethanol to remove n-dodecanol and other impurities. The solubility of SDS in pure Ci2(EO)4 at 30 °C was found to be approximately 9 wt. %. When small drops of an 8 wt. % mixture were injected into water at 30 °C, complete dissolution was observed, the time required being a linear function of the square root of initial drop radius. For instance, a drop having an initial radius of 70 (xm required approximately 100 s to dissolve, significantly more than the 16 s cited above for a slightly larger drop of pure Ci2(EO)6. Behavior was similar to that of nonionic mixtures below their cloud points discussed previously in that most of the drop dissolved rapidly, but the final small volume dissolved rather slowly with some observable emulsification. 

Bai [2] performed similar drop dissolution experiments with sodium oleate (NaOl) and Ci2(EO)4. For drops initially containing 7 and lOwt. % NaOl (particle size < 38 jim) the behavior was similar to that described above for drops having 8 wt. % SDS. However for drops with 15 and 17 wt. % NaOl dissolution was faster—comparable to that of the pure nonionics—and neither a surfactant-rich liquid immiscible with water nor emulsification was seen. Instead a concentrated liquid crystalline phase transformed directly into a micellar solution, as seen for the pure nonionics and nonionic mixtures well below their cloud points. 

Fig. 10 Effect of surfactant concentration on the optimum formulation (minimum tension position) for anionic mixtures (/e/t), pure anionic surfactant center) and ethoxylated nonionic mixtures (right)...

Penfold et al. [62] have also used neutron reflectivity to study the adsorption (structure and composition) of the mixed anionic/nonionic surfactants of SDS and C12E6 at the hydrophilic silica-solution interface. This is rather different case to the cationic/nonionic mixtures, as the anionic SDS has no affinity for the anionic silica surface in the absence of the Ci2E6. The neutron reflectivity measurements, made by changing the isotopic labelling of the two surfactants and the solvent, show that SDS is coadsorbed at the interface in the presence of the Ci2E6 nonionic surfactant. The variations in the adsorbed amount, composition, and the structure of the adsorbed bilayer reflect the very different affinities of the two surfactants for the surface. This is shown in Fig. 7, where the adsorbed amount and composition is plotted as a function of the solution composition. 

U.S. 4,430,237 (1984) Pierce et al. (Colgate-Palmolive) Nonionic mixture of alkyl glyceryl esters Improved grease cleaning and foam stability... 

The interaction between the two surfactants is mainly due to electrostatic forces. The strength of attractive electrostatic interaction decreases in the order anionic-cationic > anionic-zwitterionic capable of accepting a proton > cation-zwitterionic capable of losing a proton > anionic-POE nonionic > cationic-POE nonionic. Mixtures of surfactants of the same charge type (anionic-anionic, cationic-cationic, nonionic-nonionic, zwitterionic-zwittenonic) show only very weak interaction (negative p values of 1 or less) at the aqueous solution-air interface, although they can show significant interaction at other interfaces. 

Interaction between two surfactants in aqueous solution producing synergism in foaming and decreased adsorption onto solid surfaces has been used to advantage in the separation of minerals. An alkyl sulfosuccinate-POE nonionic mixture that shows synergism in foaming and whose interaction results in decreased adsorption onto scheelite and calcite surfaces produced enhanced selectivity and recovery of scheelite by the flotation process (von Rybinski, 1986). 

The text is kept as pedagogical and as simple as possible, and the literature review is not exhaustive, particularly on theoretical aspects, but rather selective of important fundamental developments, historical milestones, and technological breakthroughs. The scope is limited by editorial choice to systems containing ionic surfactants and some ionic-nonionic mixtures such as those involving pH-sensitive systems. 

The PIT system of surfactant evaluation theoretically applies only to nonionic materials. However, it is often found that for a given oil-water system, a combination of two or more surfactants (e.g., a nonionic and an ionic) will produce better results than either surfactant alone, at the same (or less) total surfactant concentration. Ionic surfactants usually have the normal temperature-solubility relationship—higher temperature means greater solubility—and in mixtures can often swamp out the phase inversion effect of a nonionic material. However, if the ionic/nonionic mixture is used with an aqueous phase of relatively high ionic strength, the HLB/S/F value of the molecule will be reduced and the phase inversion effect may reappear and become a useful tool again. 

FIGURE 4.9 CMCs in binary anionic/nonionic mixtures of SDS and QE4. The data are fitted to the regular solution theory model with p = -3.1, and the dashed line is the prediction from ideal solution theory. Reprinted with permission from Holland [Holland and Rubingh (1982)]. Copyright 1982 American Chemical Society. 

In Fig. 5 we report the advancing and receding contact angles of ionic-nonionic mixtures at supramicellar concentration. A similar behaviour is observed for monodisperse molecules such as CnDMPO and CiEj but not for Triton X-100. Again this can be related to the presence of smaller size molecules in this commercial surfactant. This progressive adsorption phenomenon is clearly evidenced by Fig. 6a and 6b where the variation of L-S area fraction, fl, at each cycle is reported for SDS, CTAB and their mixtures with non-ionic species. 

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