The phase behaviour of surfactant systems is decisive for the formulation of liquid and solid products and the mode of action of the surfactants in soil removal during the washing and cleaning processes. Due to the different phases of surfactant systems at higher con-centrations, e.g. the flow properties can vary greatly depending on the concentration and type of surfactants.
This is of crucial importance for the production and han-dling of liquid products. In addition to this, the phase behaviour influences the dissolution properties of solid detergents when water is added, forming or preventing high-viscous phases. One can distinguish between the phase behaviour of surfactant-water systems and mul-ticomponent systems including an additional oil phase which occurs when soil is released from the surfaces.
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
T (0C)
Figure 3.28. The effect of the phase behaviour of C12E9 on detergency (21)
with the mesophase region may result, as depicted in Figure 3.27. At low surfactant concentrations in such systems, several two-phase areas are observed in addition to the single-phase isotropic Li range, namely two co-existing liquid phases ( W + Li), a dispersion of liquid crystals (W + La) and a two-phase region of water and a surfactant liquid phase (W + L2).
The phase behaviour can have an significant impact on the detergency (21). If there is no phase change for the surfactant-water system, a linear dependence of the detergency on temperature is observed (Figure 3.28).
The surfactant exists in an isotropic micellar solution at all temperatures. The cloud point of the surfactant used here is 850C at the given concentration (2 g/1), i.e.
above the highest washing temperature.
Tests with other pure ethoxylated surfactants have revealed that a discontinuity is observed with respect to oil removal versus temperature in cases of the existence of dispersions of liquid crystals in the water-surfactant binary system. Figure 3.29 shows that the detergency
R(%)
2 g/l Surfactant
Olive oil
Mineral oil
T(0 C) R(%)
Olive oil
Mineral oil
C12E5 (mass%)
Figure 3.27. Phase diagram of the binary system water-penta-oxyethylene rc-dodecanol (C12E5) (20)
7"(0C)
Figure 3.29. The effect of the phase behaviour of the poly-oxyethylene alcohol C12E3 on detergency, using 2 g/l surfac-tant (21)
values for mineral oil and olive oil, i.e. two oils with significantly different polarities, are at different levels.
This also demonstrates that in both cases a similar reflectance versus temperature curve exists. In the region of the liquid crystal dispersion, i.e. between 20 and 400C, the oil removal increases significantly. Above the phase transition W + La -> W + L3, between 40 and 70°C, no further increase in oil removal takes place. For olive oil, a small decrease in detergent performance is observed.
The interfacial tensions between aqueous solutions of Ci2E3 and mineral oil lie at about 5 mN m"1 at 30 and 50°C. These relatively high values indicate that in this system the interfacial activity is not the decisive factor in oil removal from fabrics. The macroscopic properties of the liquid crystal dispersion seem to be responsible for the strong temperature dependence. It can be assumed that fragments of liquid crystals are adsorbed on to fabric and oily soil in the W + La region during washing. The local surfactant concentration is therefore substantially higher in comparison to the molecular surfactant layer that forms when surfactant monomers adsorb. As the viscosity of liquid crystals in the single-phase range is strongly temperature-dependent, it can be assumed that the viscosity of a fragment of a liquid crystal deposited on a fabric also significantly decreases with increasing temperature. Thus, the penetration of surfactant into the oil phase and removal of oily soil are both promoted.
Technical-grade surfactants are of specific interest for applications. As in the case of pure nonionic surfactants, definite ranges exist in which there is only a slight dependence of oil removal on the temperature (Figure 3.30). For C12/18E5, this is in the range of the two co-existing liquid phases ( W + Li), while for C12/18E4 it is in the range of the surfactant liquid phase (W + L2). An unusually strong increase of oil removal with increasing temperature occurs in the region of the
liquid crystal dispersion (W + La). At 30 and 50°C, the interfacial tensions between aqueous surfactant solutions and mineral oil and the contact angles on glass and polyester were determined for Ci2/I8E4. Whereas the values of interfacial tensions are practically identical (approximately 10"1 mN rrr1), the contact angles on both substrates are slightly less advantageous at higher temperatures. Hence, the increased oil removal between 30 and 500C cannot be attributed to an increase in the adsorbed amounts of surfactants. Rather, in both cases, the decisive part is probably played by the macroscopic properties of the liquid crystal dispersion and their temperature dependence.
During the oil removal from fabrics or hard surfaces, ternary systems occur where three phases co-exist in equilibrium. These systems are also referred to as three-phase microemulsions. These effects have been studied in detail for alkyl polyglycol ethers (22). Depending on the temperature, different phases exist, having a three-phase region between the temperature 7] and Tu
(Figure 3.31). When these three phases are formed,
Nonionic
Figure 3.30. The effects of the phase behaviours of the poly-oxyethylene alcohols Ci2/isE4 and Ci2/isE5 on detergency (21)
Figure 3.31. Schematic phase diagram of a ternary system consisting of water, oil and an ethoxylated nonionic surfactant (22)
T(0C)
R(%)
extremely low interfacial tensions between two phases are observed. Because the interfacial tension is generally the restraining force, with respect to the removal of liquid soil in the washing and cleaning process, it should be as low as possible for optimal soil removal. Other parameters such as the wetting energy and the contact angle on polyester, as well as the emulsifying ability of, e.g. olive oil, also show optima at values of the same mixing ratio at which the minimum interfacial tension is observed.
Figure 3.32(b) represents the three-phase tempera-ture intervals for C12E4 and Q2E5 versus, the num-ber n of carbon atoms of several rc-alkanes, while Figure 3.32(a) shows the detergency of these surfactants for hexadecane. Both parts of this figure indicate that the maximum oil removal is in the three-phase interval of the oil used (n -hexadecane) (23). This means that not only the solubilization capacity of the concentrated sur-factant phase, but probably also the minimum interfacial
tension existing in the range of the three-phase body, are responsible for the maximum oil removal. Further details about the influence of the polarity of the oil, the type of surfactant and the addition of salt are summarized in the review by Miller and Raney (24).
Studies of diffusional phenomena have direct rele-vance to detergency processes. Experiments have been reported which investigate the effects of changes in tem-perature on the dynamic phenomena, which occur when aqueous solutions of pure nonionic surfactants con-tact hydrocarbons such as tetradecane and hexadecane.
These oils can be considered to be models of non-polar soils such as lubricating oils. The dynamic - contacting phenomena, at least immediately after contact, are repre-sentative of those which occur when a detergent solution contacts an oily soil on a synthetic fabric surface. With C12E5 as the nonionic surfactant at a 1 wt% level in water, quite different phenomena were observed below, above, and well above the cloud point when tetradecane or hexadecane were carefully layered on top of the aque-ous solution. Below the cloud point temperature of 31°C, very slow solubilization of oil into the one-phase micel-lar solution occurred. At 35°C, which is just above the cloud point, a much different behaviour was observed.
The surfactant-rich L1 phase separated to the top of the aqueous phase prior to the addition of hexadecane.
Upon addition of the oil, the Li phase rapidly solubilizes the hydrocarbon to form an oil-in-water microemulsion containing an appreciable amount of the non-polar oil.
After depletion of the larger surfactant-containing drops, a front developed as smaller drops were incorporated into the microemulsion phase.
Unlike the experiments carried out below the cloud point temperature, appreciable solubilization of oil was observed in the time-frame of the study, as indicated by upward movement of the oil-microemulsion interface.
Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. When the tempera-ture of the system was raised to just below the phase-inversion temperatures of the hydrocarbons with Ci2E5
(45°C for tetradecane and 500C for hexadecane), two intermediate phases formed when the initial dispersion of Li drops in the water contacted the oil. One of these was the lamellar liquid crystalline phase L0, (probably containing some dispersed water). Above this was a middle-phase microemulsion. In contrast to the studies carried out below the cloud point temperature, there was appreciable solubilization of hydrocarbon into the two intermediate phases. A similar progression of phases was found at 35°C when using rc-decane as the hydrocarbon.
At this temperature, which is near the phase-inversion temperature of the water-C^Es-decane system, the T(0 C)T(0 C)
R (%)
n
Figure 3.32. Detergency effects of C12E4 and C12E5 against hexadecane as a function of temperature (a) and the corre-sponding three-phase ranges for these surfactants as a function of the number n of carbon atoms of various rc-alkanes (b) (23)
existence of a two-phase dispersion of La and water below the middle-phase microemulsion was clearly evi-dent. These results can be utilized to optimize surfactant systems in detergents, and in particular to improve the removal of oily soils. The formation of microemulsions is also described in the context of the pretreatment of oil-stained textiles with a mixture of water, surfactants and cosurfactants.
Besides the effects on detergency the liquid crys-talline phases of surfactant systems at higher concen-trations are of crucial importance for the processing of concentrated surfactant systems and the formulation, as well as the application, of liquid products. This will be demonstrated with the help of the phase diagram of anionic surfactants for the example of fatty alcohol sul-fates. Figure 3.33 shows the complete phase diagram
of sodium dodecyl sulfate (25). At higher concentra-tions of the surfactant, a multitude of different liquid crystalline phases occur. These liquid crystalline phases significantly influence the rheological properties of the surfactant systems (26). This is demonstrated by a com-parison of the simplified phase diagram of hexadecyl sulfate and the viscosity at a constant shear rate and the yield point (Figure 3.34). With increasing surfac-tant concentration and a transition from the micellar solution to the hexagonal phase, a strong increase in viscosity is observed. At even higher concentrations, a lamellar liquid crystalline phase occurs which leads to a decrease in the viscosity. This high-viscous region of many surfactants in the medium-concentration range has a strong impact on the formulation and production of concentrated surfactant systems. The same is valid for
SDS (wt%) T(0 C)
Figure 3.33. Phase diagram of sodium dodecyl sulfate(SDS) (25)
Hexagonal Temperature 700C1 pH 11.5
Lamellar
C16-FAS Viscosity D= 307 s
C16-FAS Yield point
Y (mN/m) Yield point (Pa)
Time (s)
Figure 3.34. Comparison of the liquid crystalline phases with viscosities and yield points for Ci6-fatty alcohol sulfate (FAS) as a function of concentration (26)
the dissolution of concentrated solid detergents where intermediate high-viscous phases have to be avoided.
The addition of nonionic surfactants to the anionic sur-factants may have a strong influence of the rheological behaviour (Figure 3.35). A decrease is observed both in viscosity and yield point, which leads to improved flow properties.