Adsorption of nonionic surfactants

Bashforth, F. and Adams, J.C. (1883) An Attempt to Test the Theories of Capillary Action, Cambridge University Press, Cambridge. 

Aveyard, R. and Haydon, DA. (1973) An Introduction to the Principles of Surface Chemistry, Cambridge University Press, Cambridge. 

and Randall, H.M., in Adsorption from Solution at the Solid/Liquid Inteiface, (eds G.D. Parfitt and C.H. Rochester), London, Academic Press, 1983, p.247. 

Fuerstenau, D.W. and Healy, T.W., in Adsorptive Bubble Separation Techniques, (ed. R. Lemlich), London, Academic Press, 1972, p. 91. 

Somasundaran, P. and Hannah, H.S., Improved Oil Recovery by Surfactant and Polymer Flooding, (eds D.O. Shah and R.S. Schechter), London, Academic Press, 1979, p.205. 

As the surfactant concentration approaches the c.m.c., the alkyl groups tend to aggregate. This will cause vertical orientation of the surfactant molecules (stage IV) 

Adsorption of Polymeric Surfactants at the Solid/Liquid Interface 

The most convenient polymeric surfactants are those of the block and graft copolymer type. A block copolymer is a linear arrangement of blocks of varying composition (5.1) [16]. 

A graft copolymer is a nonlinear array of one B block on which several A polymers are grafted (5.2). 

Two types of investigations are essential to unravel the behaviour of block and graft copolymers (1) their properties in a solvent in which both the A and B blocks are soluble, giving information on their conformation (2) properties in a solvent which is a non-solvent for one of the blocks but a good solvent for the other. 

---

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

In a study of the adsorption of soap and several synthetic surfactants on a variety of textile fibers, it was found that cotton and nylon adsorbed less surfactant than wool under comparable conditions (59). Among the various surfactants, the cationic types were adsorbed to the greatest extent, whereas nonionic types were adsorbed least. The adsorption of nonionic surfactants decreased with increasing length of the polyoxyethylene chain. When soaps were adsorbed, the fatty acid and the aLkaU behaved more or less independently just as they did when adsorbed on carbon. The adsorption of sodium oleate by cotton has been shown independently to result in the deposition of acid soap (a composition intermediate between the free fatty acid and the sodium salt), if no heavy-metal ions are present in the system (60). In hard water, the adsorbate has large proportions of lime soap. 

Shalaby, M.N. El-Feky, A.A. Adsorption of nonionic surfactant from its aqueous solution onto commercial rubber. J. Dispersion Sci. Tecbnol. 1999, 20 (5), 1389-1406. 

We commence with the adsorption of nonionic surfactants, which does not require the consideration of the effect of the electrical double layer on adsorption. The equilibrium distribution of the surfactant molecules and the solvent between the bulk solution (b) and at the surface (s) is determined by the respective chemical potentials. The chemical potential /zf of each component i in the surface layer can be expressed in terms of partial molar fraction, xf, partial molar area a>i, and surface tension y by the Butler equation as [14]... 

In recent papers (1-2), we have shown how the thermodynamics of adsorption of nonionic surfactants on latex surfaces can be described in terms of a few simple parameters that may be used to predict the relative strength of adsorption of surfactants with different hydro-philic/hydrophobic balance on surfaces of different polarity. 

The adsorption of nonionic surfactants on polar and nonpolar surfaces also exhibits various features, depending on the nature of the surfactant and the substrate. Three types of isotherms may be distinguished, as illustrated in Fig. 7. These isotherms can be accounted for by the different surfactant orientations and their association at the solid/liquid interface as illustrated in Fig. 8. Again, bilayers, hemimicelles, and micelles can be identified on various substrates. 

FIGURE 8 Model for the adsorption of nonionic surfactants showing the orientation of the molecules at the surface. 

The inner part of the double layer may include specifically adsorbed ions. In this case, the center of the specifically adsorbed ions is located between the surface and the Stem plane. Specifically adsorbed ions (e.g., surfactants) either lower or elevate the Stem potential and the zeta potential as shown in Figure 4.31. When the specific adsorption of the surface-active or polyvalent counter ions is strong, the charge sign of the Stem potential will be reversed. The Stem potential can be greater than the surface potential if the surface-active co-ions are adsorbed. The adsorption of nonionic surfactants causes the surface of shear to be moved to a much longer distance from the Stem plane. As a result, the zeta potential will be much lower than the Stem potential. 

Adsorption of Nonionic Surfactants onto Hydrophilic Surfaces... 

Although the same model was applicable for each of the different substrates, the adsorbed amount or fractional coverage varied between 40 and 75%. There was an implication in the data that this correlated with the surface roughness of the substrates, but the evidence is not conclusive. Subsequently, Penfold et al. [25] have considered the consequences of the different surface treatments on the adsorption of nonionic surfactants at the hydrophilic silicon-solution interface. The delicate nature of the cooperativity of the adsorption results in variations in the adsorbed amount, which depend strongly upon... 

Adsorption of Nonionic Surfactants on Quartz in the Presence of Ethanol, HCl, or CaCl2... 

Regarding the surfactant type and rock type, nonionic surfactants have much higher adsorption on a sandstone surface than anionic surfactants (Liu, 2007). However, Liu s initial experiments indicated that the adsorption of nonionic surfactant on calcite was much lower than that of anionic surfactant without the presence of NaaCOs and was of the same order of magnitude as that of anionic surfactant with the presence of Na2C03. Thus, nonionic surfactants might be candidates for use in carbonate formations from the adsorption point of view. The role of salinity is much less important, but the temperature effect is much more important for nonionics than for anionics (Salager et al 1979a). More factors that affect adsorption were discussed by Somasundaran and Hanna (1977). 

The adsorption of nonionic surfactants, such as the alcohol ethoxylates, are in many cases Langmuirian, like those of most other highly surface active solutes adsorbing from dilute solutions, and the adsorption is generally reversible. However, several other adsorption types are produced [27] giving several steps that may be explained in terms of the various adsorbate-adsorbate, adsorbate-adsorbent and adsorbate-solvent interactions forming bilayers, hemi-miceUes, and micelles on the particle surface. 

An illustration of some of the various states that may be produced is provided in Figure 9.10. These states may be described in terms of three different energy-distance curves as (i) electrostatic, produced for example by the presence of ionogenic groups on the surface of the particles, or the adsorption of ionic surfactants (ii) steric, produced for example by the adsorption of nonionic surfactants or polymers and (iii) electrostatic -I- steric (electrosteric), as for example produced by polyelectrolytes. 

Fig. Ill-18. Change in the shape of the electrocapillary curve due to the adsorption of nonionic surfactants...

The measurement of the variation of A(AV(t)) often does not give us a direct information on changes in the surfactant or polymer adsorption. So far the models are more or less qualitative. A quantitative model should be able to describe the complex structure of the electric double layer in the interfacial region and to explain peculiarities, such as the surface potential of a bare interface and changes in the potential due to adsorption of nonionic surfactants. 

Adsorption of nonionic surfactants on porous solids has been studied by Huinink et al. in a series of p ers [ 149,150]. They elaborated a thermodynamic approach that accounts for the major features of experimental adsorption isotherms. It is a very well known fact that during the adsorption of nonionic surfactants there is a sharp step in the isotherm. This step is interpreted as a change from monomer adsorption to a regime where micelle adsorption takes place. Different surfactants produce the step in a different concentration range. The step is more or less vertical depending on the adsorbate. The thermodynamic analysis made by Huinink et al. is based on the assumption that the step could be treated as a pseudo first order transition. Their final equation is a Kelvin-like one, which shows that the change in chemical potential of the phase transition is proportional to the curvature constant (Helmholtz curvature energy of the surface). 

FIGURE 3.6 A schematic representation of the adsorption of nonionic surfactants onto membranes (a) Triton X-100 and (b) Pluronic F108. (Reprinted from Journal of Membrane Science, 209, Maartens, A., et al., 81-92, Copyright (2002), with permission from Elsevier.)... 

---