Surfactants, pure nonionic

Both commercial grade and pure nonionic and anionic surfactants have been evaluated by phase inversion and optimal salinity screening procedures to establish relationships to their molecular structures. 

In recent studies, Friberg and co-workers (J, 2) showed that the 21 carbon dicarboxylic acid 5(6)-carboxyl-4-hexyl-2-cyclohexene-1-yl octanoic acid (C21-DA, see Figure 1) exhibited hydrotropic or solubilizing properties in the multicomponent system(s) sodium octanoate (decanoate)/n-octanol/C2i-DA aqueous disodium salt solutions. Hydrotropic action was observed in dilute solutions even at concentrations below the critical micelle concentration (CMC) of the alkanoate. Such action was also observed in concentrates containing pure nonionic and anionic surfactants and C21-DA salt. The function of the hydrotrope was to retard formation of a more ordered structure or mesophase (liquid crystalline phase). 

R. Aveyard, B.P. Binks, T.A. Lawless, and J. Mead Interfacial Tension Minima in Oil -I- Water - - Surfactant Systems. Effects of Salt and Temperature in Systems Containing Nonionic Surfactants. J. Chem. Soc. Faraday Trans. 1 81, 2155 (1985). R. Aveyard and T.A. Lawless Interfacial Tension Minima in Oil-Water-Surfactant Systems. Systems Containing Pure Nonionic Surfactants, Alkanes, and Inorganic Salts. J. Chem. Soc. Faraday Trans. 1 82, 2951 (1986). 

In the remainder of this article, discussion of surfactant dissolution mechanisms and rates proceeds from the simplest case of pure nonionic surfactants to nonionic surfactant mixtures, mixtures of nonionics with anionics, and finally to development of myehnic figures during dissolution, with emphasis on studies in one anionic surfactant/water system. Not considered here are studies of rates of transformation between individual phases or aggregate structures in surfactant systems, e.g., between micelles and vesicles. Reviews of these phenomena, which include some of the information summarized below, have been given elsewhere [7,15,29]. 

Table 1 Effective diffusion coefficients in the various phases for pure nonionic surfactants...

Provided that the temperature remains below Tc, where the micellar solution Li separates into and L" phases (see Fig. 1), the rate of dissolution of pure nonionic surfactants increases with increasing temperature. For example, dissolution time fo for a drop of Ci2(FO)6 with Ro = 73 pm was 11 s at 35 °C. As indicated above, to is proportional to Rq, so that to would be about 13 s at this temperature if Ro were 78 pm. As indicated in the preceding paragraph, a drop with Ro = 78 pm dissolved more slowly, taking 16 s, when the temperature was reduced to 30 °C. 

Eurther videomicroscopy investigations were conducted for model systems consisting of binary mixtures of pure nonionic surfactants [10]. The results indicated that the common source of the intriguing phenomena described above is that the more hydrophilic species dissolve faster, causing the remaining drop to become enriched in the less hydrophihc species, which are less soluble and dissolve more slowly. Indeed, the drop can achieve compositions... 

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. 

The explanation for this behavior is similar to that given in the preceding section for nonionic surfactant mixtures. Adding a hydrophihc anionic surfactant raises the temperature at the cloud point and other phase transitions above those for pure Ci2(EO)4. If the amount of anionic added exceeds only slightly that needed for complete solubility, the final stages of the dissolution process are slow because preferential dissolution of the anionic causes the remaining drop to rise above its cloud point and nucleate small droplets of surfactant-rich liquid. But if the amount added is sufficiently large, drop composition remains below the cloud point in spite of preferential dissolution, with the result that dissolution is fast as with pure nonionic surfactants below their cloud points. 

As indicated above, miscibiUty gaps are small and intermediate lamellar liquid crystalline phases dissolve rapidly into the aqueous phase if the surfactant or surfactant mixture is rather hydrophihc with a high spontaneous curvature (low (v/la)), for instance at temperatures below Tq for pure nonionic surfactants. In this case dissolution, which converts lamellae of zero curvature to aggregates with significant curvature as surfactant concentration decreases, occurs spontaneously because it reduces system free energy. 

Nonionic surfactants can be mixed as well, but in practice most of them, at least the ethoxylated ones, are already a mixture because of the polycondensation mechanism in ethylene oxide adduction. Isomerically pure ethoxylates are extremely expensive and are exclusively reserved for research work. Little work has been carried out with mixture of isomerically pure nonionics and the bulk of the work on mixture deals with mixtures of commercial products, i.e., mixture of mixtures, which obey a linear mixing rule on EON, provided that the base mixtures are not too different [8,35]. 

The cloud point of a mixture of nonionic surfactants is intermediate between the pure nonionic surfactants involved (95.99) The cloud point of a dilute nonionic surfactant solution increases upon addition of ionic surfactant (95.98—104). The coacervate phase forms because of attractive forces between the micelles in solution. The incorporation of ionic surfactant into the nonionic micelles introduces electrostatic repulsion between micelles, causing coacervate phase formation to be hindered, raising the cloud point. 

In Figures 3.9 and 3.10 of Ref. (17) we show how the calculated cmc varies when Pi is constant and P2 is varied from 0.0 to 1.0 and for the case when P2 is constant and Pi is varied. For fixed Pi or P2, the calculated cmc decreases as the other counterion binding parameter decreases for the ionic/ionic cases. The cmc s calculated for the nonionic/ionic systems are larger than those calculated for the ionic/ionic systems. For the nonionic/ionic systems, a maximum occurs in the cmc-versus-a curve at values of a close to that of the pure nonionic surfactant. The maximum becomes greater as the value of a for the ionic surfactant increases. 

I. For 4-NH9 dye. First, into several 100 ml glass-stoppered Erlenmeyer flasks, 25 ml portions of a given concentration of pure surfactant solution were placed. Next a measured amount of 4-NH (5.0 X 10 mol/1) was added to each solution. The mixture were stirred ultrasonically for 5 min and then agitated with a shaker (Model SS-82D type of Tokyo Rlkaklkal Co., Ltd, Tokyo, Japan) for 24 hr and allowed to stand for 24 hr in a thermostat at 30 C in order to established a solubilization equilibrium. After the equilibrium had been established, these pure anionic surfacant solutions Including 4-NH2 were mixed with pure nonionic surfactants solutions Involving one. 

The cloud point phenomena as a lower consolute solution temperature is becoming better understood in terms of critical solution theory and the fundamental forces involved for pure nonionic surfactant systems. However, the phenomena may still occur if some ionic surfactant is added to the nonionic surfactant system. A challenge to theoreticians will be to model these mixed ionic/nonionic systems. This will require inclusion of electrostatic considerations in the modeling. 

Abe, M., H. Uchiyama, T. Yamaguchi, T. Suzuki, and K. Ogino. 1992. Micelle formation of pure nonionic surfactants and their mixtures. Langmuir 8, 2147-2151. 

Selective solubilization can also occur in mixtures of polar and nonpolar oils. Using their oil drop method described previously, Chen et al. measured solubilization rates of mixtures of triolein and oleic acid in solutions of pure nonionic surfactants. As Figure 9.9 shows for a drop initially containing 85/15 triolein/oleic acid by weight injected into 2 wt% Tergitol 15-S-7 at 35°C, they observed that the solubilization process consisted of two stages. In the first stage, the drop radius... 

Mixtures containing 1 wt% of the pure nonionic surfactant C,2E5 in water were contacted with pure n-hexadecane and n-tetradecane at various temperatures between 25 and 60°C using the vertical cell technique. Similar experiments were performed with C,2E4 and n-hexadecane between about 15 and 40°C. In both cases the temperature ranged from well below to well above the phase inversion temperature (PIT) of the system, i.e., the temperature where hydrophilic and lipophilic properties of the surfactant are balanced and a middle phase microemulsion forms (analogous to the optimal salinity for ionic surfactants mentioned above). The different intermediate phases that were seen at different temperatures and the occurrence of spontaneous emulsification in some but not all of the experiments could be understood in terms of known aspects of the phase behavior, e.g., published phase diagrams for the C12E 5-water-n-tetradecane system, and diffusion path theory. That is, plausible diffusion paths could be found that showed the observed intermediate phases and/or spontaneous emulsification for each temperature. 

As an example of the different phases of surfactants. Figure 3.27 shows the phase diagram of a pure nonionic surfactant of the alkyl polyglycol ether type (20). In particular, the phase behaviour of nonionic surfactants with a low degree of ethoxylation is very complex. As the lower consolute boundary is shifted to lower temperatures with a decreasing EO (ethylene oxide) number of the molecule, an overlapping of this boundary... 

FIGURE 4.32 Detergency versus Tfor pure nonionic surfactants and hexadecane. 

Another interfacial rate process that has been invoked to explain dynamic surface tension data for some pure nonionic surfactants is reorientation of adsorbed surfactant molecules between a state in which they he nearly flat along the surface, which is favored at low values of T, and a state in which their orientation is nearly vertical, which is favored at high values of E (Fainerman et al., 1996 Miller et al., 1999). Neutron reflection data are eonsistent with sueh an interpretation (Lu et al., 1993). Further discussion and references dealing with... 

Equation 6.87 predicts that the time tp until liquid crystal formation begins is proportional to the square of the initial drop radius and inversely proportional to the bulk surfactant concentration. These predictions were in agreement with experiments for systems containing pure nonionic surfactants, n-hexadecane, oleyl alcohol, and water (Lim and Miller, 1991a). Moreover, for a hydrocar-bon alcohol ratio of 3 1 by weight and for solutions of at 30°C, the phase diagram was determined and K calculated as 0.52. When the data were htted to Equation 6.87, D2 was found to be 1.3 x 10" ° m /sec. The Stokes-Einstein equation was then used to estimate micelle radius r. 

Some work has been reported on the kineties of dissolution of pure detergents. This rate for a commercial solid detergent has been found to be diffusion limited by Figge and Neogi (1996). Chen et al. (2000) also found in thdr studies on dissolution of pure nonionic surfactants that rates were limited by diffusion. 

Dissolve 1.000 g of pure nonionic surfactant of the type to be determined in water, ensuring complete dissolution, transfer to a 250 ml volumetric flask, dilute to volume and mix. 

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