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.
3 ADSORPTION AT THE
Figure 3.10. Separation mechanisms of detergency (11)
a rise in the adsorbed amounts occurs. In this range of the adsorption isotherm, a fully covered monolayer or double layer is adsorbed on to the surface, hence making the surface either hydrophilic or hydrophobic.
< HMC > HMC > HMC
Depending on the type of the surface, in some cases micellar structures of the adsorbed surfactants have been postulated instead of flat double layers. Typical examples of adsorption isotherms of sodium dodecyl sulfate on to different surfaces are shown in Figure 3.12 (11). The adsorption isotherms for the carbon black and the graphitized carbon black (Graphon) are completely different. For graphitized carbon black, a step-like adsorption isotherm is observed which indicates the flat arrangement of the surfactant molecules on the surface at low concentrations with a perpendicular structure at higher concentrations (see Figure 3.11). The adsorption process is exothermic with an adsorption enthalpy of about —128 to —36 kJ mol"1. The adsorption of sodium dodecyl sulfate on titanium dioxide is an example of the specific adsorption via the hydrophilic group on to the polar pigment surface. A second adsorption layer is formed via hydrophobic interaction with the first adsorption layer, which thus makes the pigment surface hydrophilic again in the range of the plateau of the adsorption isotherm. Figure 3.12 also demonstrates the effect of the addition of electrolytes which are Figure 3.11. Adsorption models for surfactants: (a) model
of Fuerstenau; (b) model of Scamehorn and co-workers;
(c) model of Harwell and co-workers (12) (hmc, hemimicelle concentration)
(a) Electrostatic forces
(b) Disjoining pressure
(c) Rolling-up
Washing liquor Air
Air
Washing liquor
A = Detergent B = OiI
Substrate
present in the washing process. In the presence of ions, the amounts adsorbed of the anionic surfactant are increased. This is due to a decreased electrostatic repulsion of the negatively charged hydrophilic groups of the anionic surfactant in the presence of electrolytes.
Therefore, the adsorption density in equilibrium can be enhanced significantly. A similar effect can be observed in a comparison of an anionic and nonionic surfactant with the same alkyl chain length adsorbed on to a hydrophobic solid (Figure 3.13) (11). The nonionic surfactant gives higher adsorbed amounts than the anionic surfactants at the same concentrations. This is especially valid at low concentrations, whereas at very high concentrations both surfactants reach the same plateau value. For a hydrophilic solid surface, this effect can be almost opposite due to a higher affinity of anionic surfactant to the surface via specific interactions.
The electrolyte effect for the adsorption of anionic surfactants which leads to an enhancement of soil removal is valid only for low water hardness, i.e. low
concentrations of calcium ions. High concentrations of calcium ions can lead to a precipitation of calcium sur-factant salts and therefore to a reduction in concentration of the active molecules. In addition to this, the electri-cal double layer is compressed such an extent that the electrostatic repulsion between pigment soil and surface is reduced. Therefore, for many anionic surfactants the washing performance decreases with lower temperatures in the presence of calcium ions. This effect can be com-pensated by the addition of complexing agents or ion exchangers (see Section 4 below).
The characteristic change of the surface charge of the solid, which depends on the nature of the hydrophilic groups of the surfactant, is a consequence of the non-specific adsorption of the surfactants on pigments and fabrics or hard surfaces. This can be shown in aqueous solutions of different surfactants with the same alkyl chain length by the change of electrophoretic mobility of pigments, which is a measure of the surface charge (Figure 3.14) (11). The carbon black shown as an example has a negative surface charge in water at an
Carbon black Graphon and TiO2
cx103(mol/l)
Figure 3.12. Equilibrium adsorption isotherms of sodium rc-dodecyl sulfate on carbon black, TiC>2, and Graphon at room temperature (11)
Qx104 (mol/I) i/x1(T5 (cm2 s~1 v"1 )
cx103(mol/l)
Figure 3.13. Surfactant adsorption on to carbon black:
T = 298 K; surface area = 1150 m2 g"1 (BET) (11)
Figure 3.14. Electrophoretic mobility u of carbon black in solutions of different surfactants at 308 K (11)
alkaline pH value, as for most pigments present in the washing process the isoelectric point is below pH 10. The nonionic surfactant shows no influence on the electrophoretic mobility, whereas the anionic surfactant increases the negative surface charge of the pigment due to the adsorption. By the adsorption of the cationic surfactant the surface charge can be changed from a negative to a positive value during the adsorption process. This picture explains quite well the mode of action of different surfactant types for pigment removal in the washing process. As nonionic surfactants do not influence the electrostatic repulsion of pigment and fabric, their washing efficiency is mainly caused by the disjoining pressure of the adsorption layer. Anionic surfactants also increase the electrostatic repulsion, but usually have lower amounts adsorbed than the nonionic surfactants. Cationic surfactants show similar effects in the washing process as anionic surfactants, but in spite of this they are not suited for most washing processes due to their adverse effects in the rinse cycles. In these cycles, the positively charged surfaces (due to the adsorption of cationic surfactants) are recharged to negative values due to the dilution of the washing liquour and the consecutive desorption of cationic surfactants. As the different fabrics and pigment soils have different isoelectric points, positively and negatively charged surfaces are present in the washing liquor which leads to heterocoagulation processes and a redeposition of the already removed soil on to the fabric. Therefore, cationic surfactants are not used in the washing process, but only as softeners in the rinse cycle when no soil is present any more and a strong adsorption of cationic softener on the negatively charged fabric is desired.