Oxyethylated alcohols

At high surfactant concentrations the asymmetric films can be destabilised additionally as a result of the dissolution of the surfactant in the antifoam phase (for example, extraction of non-ionic surfactants such as oxyethyl alcohols, acids and aikylphenols) and its adsorption at the surface of the emulsion drops [66,67]. 

Plurafac [BASF], TM for a series of 100% active, nonionic biodegradable surfactants of straight-chain, primary aliphatic oxyethylated alcohols. Available in liquid, paste, flake, and solid form. 

For lower ratios CO1/CO2 the shape of the isotherms is less convex than for the Langmuir isotherm. For non-zero a, which are especially characteristic for oxyethylated alcohols [87], this difference becomes even more significant. For example, the isotherms calculated for various a values (0 to 5) and 1/0)2 = 4 are presented in Fig. 2.6, while the calculations performed for the same a range and / 2 = 8 are illustrated by Fig. 2.7. 

This is illustrated in Fig. 2.8, where the surface layer coverage is plotted vi the bulk concentration. Here the partial coverage FjOj was calculated for the two states of the adsorbed molecules (i = 1, 2) with isotherm parameters coi= lO m mol, C02= 2.5-1 O m mol and a = 3, which are typical for oxyethylated alcohols (see below). It is seen that the maximum coverage by the molecules in state 1 is reached for a concentration of about 2-10 mol/1, while for c = 5-10 mol/1, i.e., when the curve corresponding to this parameter set at Fig. 2.6 exhibits a sharp increase, the coverages for the two states of the adsorbed molecules become approximately equal to each other (see Fig. 2.8). It should be noted that at concentrations above 2-10 mol/1 the total adsorption increases only due to a decrease of the fraction of molecules adsorbed in the state with maximum partial molar area. 

The related questions were extensively discussed in the monograph [17]. We believe that all data obtained in our laboratories are in full compliance with these requirements. Also, the conditions of equilibrium establishment were controlled in studies performed in the groups ofJoos, Lucassen-Reynders, Lin and other authors. For surfactants with high adsorption activity, e.g., oxyethylated alcohols or ethers, the concentration in the solution is low, and the time necessary to attain the equilibrium is usually tens of hours. Therefore the data published for such systems are not necessarily equilibrium ones. To illustrate this, we present below some results published in the literature, which do not satisfy the requirement (ii). Hence, it is no surprise that these data do not agree with the general trends observed. 

Equilibrium surface and interface tension of oxyethylated alcohols (CnEOm) was studied by many authors. The values for CnEOm with various length of the hydrocarbon (n) and oxyethylene (m) chain were reported in a number of publications [46-55],... 

It is seen from these dependencies that, with the increase of m, the onset of surface tension decrease (or the surface pressure increase) corresponds to lower surfactant concentrations at the same time, the decrease of the isotherm slope at high concentrations takes place. The increase of surface activity with the increase of m for low pressure in the framework of reorientation model can be qualitatively explained by strong increase of the molar area of C EO, molecule in the state 1 (coj), while the decrease of the isotherm slope (and also the decrease of surface activity) at high concentrations can be ascribed to slight increase of the CO2 value. Therefore, the Intersection of the isotherms which is observed for the oxyethylated alcohols with different m values is the consequence of the fact that these two molar areas are increased with the increase of m, but the rate of this increase is different for CO2 and coj values. 

The significant increase in co, with increasing m is caused by the localisation of the oxyethylene chain in the surface layer. This result which is implied by the reorientation model also agrees with the neutron reflection data. It was shown in [14] that, with the increase of the area per C EO , molecule in the adsoiption layer, i.e., with surface pressure deerease, the thickness of the layer occupied by the oxyethylene groups of C EO, becomes lower. At the same time, a decrease in the tilt angle of the oxyethylene groups to the interface is observed. These results were discussed in [14] in the context of the adsorption of oxyethylene groups in the non-saturated adsorption layer of oxyethylated alcohols. The dependence of molar areas in the two states on n is shown in Fig. 3.32. 

We compare now the adsorption behaviour of oxyethylated alcohols (with C,oEOg as an example) at the water/air and water/hexane interfaces, with reference to the data reported in [57]. The experimental and theoretic isotherms at the water/hexane interface are shown in Fig. 3.33. It was mentioned above that the experimental data agree satisfactorily with the Frumkin model for a physically unrealistic value of a = -10.8. Comparing the reorientation model parameters for CiqEOj at the two interfaces (cf. Fig. 3.33 and Table 3.13), one can see that the molar areas are almost the same, while the value of a for the water/hexane interface is 2.5 times higher than that for the water/air interface. Thus the adsorption activity of the oxyethylene groups at the water/hexane interface is significantly higher than that at the water/air interface. 

The adsorption behaviour of polyethylene glycol octylphenyl ethers (Tritons X-45, X-100, X-165 and X-305) is similar to that of the oxyethylated alcohols. The experimental data [58-62] and the results of the calculations according to the Frumkin and reorientation models are presented in Figs. 3.36, 3.37 and Tables 3.14 and 3.15. 

Similarly to the oxyethylated alcohols, the adsorption behaviour of Tritons agrees better with the two-state model. It is seen from the analysis of the experimental results reported in [62] that only in the high concentration range (dashed line in Fig. 3.37) the results are consistent with those obtained for the Tritons with other degrees of oxethylation, cf Tables 3.14 and 3.15. Higher surface tension values exhibited by Triton X-165 for the concentration below 5-10 mol/1 can be possibly ascribed to the lack of adsorption equilibrium in the experiments [62]. Similar results were obtained in [62] also for Triton X-100 (not shown in Fig. 3.37). We do not believe that the data listed in Table 3.15 are sufficient to make unambiguous... 

Oxyethylated alcohol C EOs. This system is discussed in Section 3.3.7. 

Miller, R., Aksenenko, E.V., Liggieri, L., Ravera, E, Ferrari, M., and Fainerman, V.B., Effect of the reorientation of oxyethylated alcohol molecules within the surface layer on equilibrium and dynamic surface pressure, Langmuir, 15, 1328, 1999. Mulqueen, M., Datwani, S.S., Stebe, K.J., and Blankenstein, D., Dynamic surface tensions of aqueous surfactant mixtures experimental investigation, Langmuir, 17, 7494, 2001. 

Oxyethylated FAME exhibits the surface-active and nsage properties similar to typical nonionic surfactants, including oxyethylated alcohols [25,49-64]. It is obvious that there are some differences, connected with the somewhat different structure, that is, the presence of a terminal methyl group. As a result, oxyethylated FAME show lower foaming ability, lower clouding points, and different tendency for gelation. 

Oxyethylated FAME show higher biodegradability and lower aquatic toxicity compared to oxyethylated alcohols [65]. Thus, they have a high potential to become an important group of surfactants... 

Szymanowski, J., Miesiac, L, Jerzykiewicz, W. 1980. Synthesis and properties of esterification products of some oxyethylated alcohols and alkylphenols with fatty acids, Fett. Wiss. Technol. 82 244-249. 

Hreczuch, W., Szymanowski, J., Bekierz, G., Pyralski, K. 1999. Comparison of the synthesis and composition of directly oxyethylated fatty acid methyl esters and oxyethylated alcohols with narrow and broad range distribution of homologues. Proceedings XXIX Joumadas Anuales del CED. Barcelona, 351-358. 

Aqueous solutions of oxyethylated alcohols were selected as model lubricating substances. They are produced from vegetable raw materials on an industrial scale and are relatively inexpensive. Alcohol ethoxylates exhibit high surface activity [14-22]. They form micelles and liquid crystal structures in aqueous solutions [16, 23-26]. Aqueous solutions of oxyethylated alcohols can be used as inflanunable, ecological lubricating substances, particularly in the friction pairs that are in direct contact with the environment, people, foodstuffs, pharmaceuticals, and cosmetics. 

Oxyethylated alcohols are prepared at 130°C-180°C in the presence of a catalyst whose concentration is on the order of a few tenths of a percent. Various kinds of catalysts are used. Earlier, basic catalysts (sodium hydroxide) were commonly used, which yielded a wide distribution of ethoxymers. Currently, acidic catalysts with a considerably narrower distribution of ethoxymers and a lower content of unreacted alcohol are used. It follows from this comparison that products obtained by means of different syntheses may have different compositions due to the presence of both different ethoxymers and residual free alcohol, which is often hard to vaporize. Additionally, alcohol as a reactant may be a mixture of compounds with various alkyl chain lengths. Therefore, particularly in the case of ethoxylates prepared from natural sources on an industrial scale, one may have to deal with a mixture of compounds with various ethylene oxides (m) and alkyl (n) chain lengths. 

In the REO notation used here, R is the alkyl chain with n carbon atoms in the chain, while EO , is the number of moles (m) of the combined ethylene oxide (EO). As a result of oxyethylation, fatty alcohols become amphiphilic, where the ethylene oxide chain is the hydrophilic part and the alkyl chain is the hydrophobic part. The general formula and a spherical model of lauryl alcohol oxyethylated with 3 moles and 5 moles of ethylene oxide are presented, respectively, in figs. 17.2 and 17.3. Using fig. 17.3, it is possible to geometrically estimate a relative proportion of the hydrophilic part (ethylene oxide chain) and of the hydrophobic part (alkyl chain) in the oxyethylated alcohol molecule. 

FIGURE 17.1 Reaction scheme for preparation of oxyethylated alcohols. 

The contact of a lubricating substance with a solid is particularly significant from a tribological point of view. Oxyethylated alcohols are nonionic surfactants, and their interactions with the surface are basically quite specific (hydrogen bonds). The contribution of universal (electrostatic) interactions is considerably smaller, as these are very weak dispersion interactions. In the solution in contact with a solid, one can distinguish the surface phase and the bulk phase. Due to adsorption from solutions, the surface phase is enriched with the component that has a stronger affinity for the surface. It is a characteristic of adsorption from solutions on a solid surface that individual components compete for free sites on the surface. At this point, one should not confuse adsorption with absorption, the latter of which may lead to penetration of the components into the solid. 

In dilute solutions, oxyethylated alcohols are present as monomers whose characteristic structural components are a hydrophilic part (ethylene oxide chain) and a hydrophobic part (aliphatic chain). With an increase in concentration, the monomers undergo spontaneous self-aggregation, forming micelles in the surface phase (hemimicelles) at lower concentrations, while at higher concentrations forming micelles in the bulk phase. Figure 17.4 shows a schematic example of a spherical micelle. 

FIGURE 17.6 Changes in surface tension of 1 wt% oxyethylated alcohol solutions as a function of (a) oxyethylation degree and (b) length and nature of the alkyl chain. 

The scuffing load is the lowest load at which there occurs a pronounced increase in the friction force moment, which indicates breaking of a lubricant film. The value of scuffing load for water (200 N) is several times lower than that for oxyethylated alcohol solutions, which exceeds 1000 N. The highest value was observed for a 1 wt% solution of C12H25EO23 (ca. 1500 N). High antiseizure efficiency of compounds can be observed even at the lowest concentrations (0.1 wt%). The dependence of... 

FIGURE 17.10 Changes in scnffing load (P as a function of the nature and concentration of oxyethylated alcohols as a component of model water-based lubricating substances. (Data... 

The other factor that causes reduction in the cloud point is the increase of the alkyl chain length in oxyethylated alcohols. Following the assumed interpretation, one can expect a correlation between the seizure load and the increase in the hydro-phobic properties of oxyethylated alcohols. The dependences can be conveniently analyzed on the basis of the results shown in fig. 17.11b, which clearly point to a significant effect of the increase in hydrophiUcity of the compound. The differences... 

---