The characteristic effect of surfactants is their ability to adsorb on to surfaces and to modify the surface properties. At the gas/liquid interface, this leads to a reduction in surface tension. Figure 3.1 shows the dependence of surface tension on the concentration for different surfactant types (5). It is obvious from this figure that the nonionic surfactants have a lower surface
Y (mN/m)
c(mol/l)
Figure 3.1. Surface tensions of several surfactants with the same chain length as a function of concentration (5) Table 3.3. Major components of powder detergents
Ingredients
Surfactants Foam boosters Foam depressants Builders
- Sodium triphosphate - Mixed or non-phosphate - Sodium carbonate Antiredeposition agents Anticorrosion agents Optical brighteners Bleach
Enzymes Water Fillers
United States, Canada, Australia
8-20 0-2
25-35 15-30 0-50 0.1-0.9
5-10 0.10-0.75
6-20 20-45
Composition (%) South America, Middle East, Africa
17-32
20-30 25-30 0-60 0.2-1.0
5-12 0.08-0.50
6-13 10-35
Europe 8-20
0-3 0.3-5.0
20-35 20-45 0.4-1.5 5-9 0.10-0.75
15-30 0-0.75 4-20 5-45
Japan 19-25 1-4 0-15 0-20 5-20 1-2 5-15 0.1-0.8
0-5 0-0.5
5-10 30-45
tension for the same alkyl chain length and concentration than the corresponding ionic surfactants. The reason for this is the repulsive interaction of ionic surfactants in the adsorption layer, which leads to a lower surface coverage than for the nonionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. The second effect which can be seen from Figure 3.1 is the discontinuities of the surface tension-concentration curves, with constant values for the surface tension above these point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc), above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic of the ability of surfactants to solubilize hydrophobic substances in aqueous solution. Therefore, the concentration of surfactant in the washing liquor has at least to be just above the cmc.
The presence of electrolytes increases the adsorption of anionic surfactants at the gas/liquid interface, as already mentioned. This leads to a reduction of the surface tension at an equal solution concentration (5) and to a strong decrease of the cmc (Figure 3.2). This effect can be of the magnitude of several decades in order. Similar to this are the effects of mixtures of surfactants with the same hydrophilic group and different alkyl chain lengths or mixtures of anionic and nonionic surfactants (6). Such mixtures follow the mixing rule (equation (3.1)) in the ideal case, as follows:
1 a 1 -a
= + (3.1) cmcmiX cmc i cmc2
where cmcm[X, cmc\ and cmci are the critical micele concentrations of the mixture, surfactant 1 and
surfactant 2, respectively, and a is the mole fraction of the surfactant in the bulk solution.
According to a theory, which is itself based on the regular solution theory, the deviation from ideal behaviour can be described by the introduction of the activity coefficients, f\ and /2, as follows:
1 a 1 — a
j (2 2) cmcmix ficmci ficmc2
/ ! ^ e x p / K l - x O2 (3.3) h = expGS*?) (3.4) AH1n = PRTx1(I-X1) (3.5) where / i and /2 are the activity coefficients of compo-nents 1 and 2, respectively, /3 is the interaction param-eter, Xi is the mole fraction of component 1 in the micelle, and AHm is the micellization enthalpy.
The interaction parameter /3 characterizes the devia-tion from ideal behaviour. If P has negative values, there is an attractive interaction between the surfactants, and the cmc of the mixture is lower than that expected for ideal behaviour. For /3 > 0, there is a repulsive inter-action and the cmc is higher than for ideal behaviour.
For highly negative values of P and cmc values for the surfactants which are quite similar, the cmc of the mix-ture is even lower than those of the single surfactants.
The strongest interactions are observed for mixtures of anionic and cationic surfactants, due to the electrostatic forces between the head groups. An example of the influ-ence of the interaction of the surfactant molecules on the cmc is shown in Figure 3.3. The interaction between the surfactants has not only an influence on the cmc, but also on various other properties which are relevant to washing and cleaning. Thus, a synergistic effect has
c (mol/l)
Figure 3.2. Influence of counterions on the surface activity of a typical anionic surfactant as a function of the surfactant concentration (5)
c (mol/l)
Figure 3.3. Critical micelle concentrations of mixtures of sodium /2-dodecyl sulfonate and rc-octylnonaglycol ether (5)
cM (mol/l)
Y (mN/m)
Calculated (ideal behaviour)
Measured No additional
electrolyte + 0.2 mol/l Na2SO4
been observed for foaming, emulsification and dispers-ing properties, and even washdispers-ing and cleandispers-ing efficiency for negative /3 parameters (6).
An aspect which has been underestimated for a long time regarding the mechanisms of washing and clean-ing is the kinetics of surface effects. Particularly at lower concentrations, there might be a strong influence of time on the surface and interfacial tensions. Figure 3.4 shows the dependence of time on the dynamic surface tensions of both a pure anionic and a pure nonionic surfac-tant at different concentrations (7). For both surfacsurfac-tants, the time dependence of the surface tension is greatly reduced when the concentration increases. This effect is especially pronounced when the critical micelle con-centration is reached. The reason for this dependance is the diffusion of surfactant molecules and micellar aggre-gates to the surface, which thus influences the surface tension at newly generated surfaces. This dynamic effect of the surface tension can probably be attributed to the observation that an optimum of the washing efficiency usually occurs well above the critical micelle concen-tration. This effect is an important factor for cleaning and institutional washing where short process times are common.
Connected with the surface tension parameter is the wetting process of the surface, e.g. fabrics or hard surfaces. This wetting can be described by the Young equation (see Figure 3.5):
Ks = Ys\ + Y\cosO (3.6)
Figure 3.5. Schematic of the wetting of solid surfaces
where ys and ysi are the solid/gas and solid/liquid interfacial tensions, respectively, y\ is the liquid/gas surface tension, and 0 is the contact angle.
The so-called wetting tension j can be defined by the following equation:
j = Ys-Ysi = Yi cos 0 (3.7) A complete wetting of a solid is only possible for spontaneous spreading of a drop of the liquid at the surface, i.e. for 6 = 0 or cos 0 = 1. For a specific solid surface of low surface energy, a linear correlation is observed between cos 0 and the surface tension. This is demonstrated for polytetrafluoroethylene in Figure 3.6.
The limiting value, i.e. cos 0 = 1, is a constant for a solid and is called the critical surface tension of a solid, yc. Therefore, only liquids with y{ < yc are able to spontaneously spread on surfaces and to wet them completely.
Y (mN/m)
f(s)
Figure 3.4. Dynamic surface tensions of (a) C^SC^Na {cms = 11 mmol/1) and (b) C12E6 (cmc = 0.07 mmol/1) as a function of concentration at 40° C (7)
f(s)
Y (mN/m)
KL (mN/m)
Figure 3.6. Influence of the surface tensions of various fluids on the wetting of polytetrafluoroethylene according to Fox and Zismann (cf. ref. 4)
Table 3.4. Critical surface tension values of a number of polymer solids (8)
Polymer yc at 20° C (mN/m)
Polytetrafluoroethylene 18 Polytrifluoroethylene 22 Poly(vinyl fluoride) 28 Polyethylene 31 Polystyrene 33 poly(vinyl alcohol) 37 poly (vinyl chloride) 39 poly/(ethylene terephthalate) 43 poly(hexamethylene adipamide) 46
Table 3.4 gives an overview of the critical surface tension values of different polymer surfaces (8). From these data, it is obvious that polytetrafluoroethylene surfaces can only be wetted by specific surfactants with a very low surface tension, e.g. fluoro surfactants.
Figure 3.7 shows the wetting tensions of two all-purpose cleaners for different surfaces (9). As these tensions are in very good agreement with the surface tensions of the cleaners, a spreading of the cleaner solution on the surfaces and therefore a good wetting can be assumed.
It is only on polytetrafluoroethylene surfaces that an incomplete wetting is observed.
In cleaning and washing, the situation becomes more complicated due to the presence of oily or fatty soils on the surface. In this case, there is a competition between the wetting by the surfactant solution and that of the oily soil (Figure 3.8). When two droplets - one of the surfactant solution and one of the oily soil - are set on a solid surface, the two wetting tensions jA and jB
will act on the basal plane (3). When the two droplets approach each other, so that a common interface is formed, the difference between the wetting tensions will act at the contact line. This parameter is called the oil
Solid
Figure 3.8. Two liquids, A (detergent) and B (oily soil), on a solid surface, shown for (a) separated and (b) in-contact situations: J'A and JB = wetting tensions; / A B = interfacial tension; R = interfacial wetting tension (3)
displacement tension, as follows:
N = JA +JB (3.8)
By the adsorption of the surfactant from the phase A, jA is increased and thus Aj becomes larger. In addition to this, a fraction of the interfacial tension yAB acts in the basal plane with a value of /AB COS G9 with G being the contact angle in B, i.e. the oily phase. The resulting force R is called the contact tension and is defined as follows:
R = A y + XAB cos© (3.9) When R becomes equal to zero, equilibrium is reached.
For the washing and cleaning processes, the complete
cos# Wetting tension (mN/m)
Teflon Steel Glass China clay
Solid Air
Figure 3.7. Wetting tensions of two all-purpose cleaners (A and B) on different surfaces (9)
Figure 3.9. Schematic of the displacement phases of an oily drop B by a cleanser A (10)
removal of the oil B by the surfactant solution A is the important step. This process is shown schematically in Figure 3.9 (10). The interfacial tension yAB for 90° >
0 > 0° supports the contraction of the oil drop in the first step. For a contact angle 0 > 90°, this will change and the interfacial tension acts in an opposite way.
Dependent on Ay and /AB > a complete removal of the oil can occur. In practice, the rolling-up is never complete, so that support of the removal of the oil drop from a solid surface by mechanical forces is necessary for the washing and cleaning step.