Adsorption of non-ionic surfactants

Carberry, J.B. Geyer, A.T. Adsorption of non-ionic surfactants by activated carbon and clay. Proceedings of the 32nd Industrial Waste Conference, Purdue University, Lafayette, IN, 1977, Vol. 32, 867. 

It is also shown that the adsorption of non-ionic surfactants at a vinyl acrylic latex/water interface that exhibit a saturation type isotherm can be related to the polarity of the polymer surface, in agreement with earlier sufactant adsorption studies. 

The increase in temperature increases adsorption of non-ionic surfactants on solid surfaces since the solubility of non-ionic surfactants in water decreases with increased temperature. On the other hand, increasing temperature decreases the adsorption of ionic surfactants on solid surfaces because the solubility of ionic surfactant increases with increased temperature. Furthermore, the presence of electrolytes increases the adsorption of ionic surfactants if the solid surface has the same charge as the surfactant head groups. 

As a case study we discuss some aspects of the adsorption of non-ionic surfactants, non-ionics for short, from aqueous solution. Such surfactants have Invariably long molecules and strongly associate In solution to form micelles. The latter aspect is beyond the confines of the present chapter. Here we shall briefly Introduce some main features of the adsorption of non-ionics. Ionic surfactant adsorption belongs to the domain of electrosorption see sec. 3.12d. 

J.S. Clunie, B.T. Ingram, Adsorption of non-ionic surfactants, in Adsorption from Solution at the Solid-Liquid interface (see sec. 2.10b), p. 105. 

In summary, this case study demonstrates that adsorption of non-ionic surfactants, an interesting theme in its own right, exhibits not only definite characteristics belonging to the present theme, but also involves aspects of polymers and association colloids. 

Adsorption from Solution at the Solid/Liquid Interface, G.D. Parfitt, C.H. Rochester, Eds., Academic Press (1983). (Contains chapters on adsorption of smEill molecules (G.D. Parfitt and C.H. Rochester), adsorption from mixtures of miscible liquids (J.E. Lane) and adsorption of non-ionic surfactants (J.S. Clunle,... 

Elworthy PH, Guthrie WG. Adsorption of non-ionic surfactants at the griseofulvin-solution interface. / Pharm Pharmacol 1970 22 (SuppL) 114S-120S. 

P. Levitz, H. Van Damme, D. Keravis, Fluorescence decay study of the adsorption of non ionic surfactants at the solid-liquid in-ter ce. 1. Structure of the adsorption layer on a hydrophflic soHd, /. Phys. Chem. 1984, 88, 2228 2235. 

We restrict ourselves in the current paper to a simple, yet rather general case. A sharp, flat interface is assumed to separate an aqueous surfactant solution from another fluid, non-polar phase. The solution is assumed to be below the critical micelle concentration, i.e., it contains only monomers. We start in Section 2 by considering the adsorption of non-ionic surfactants, for which previous theories yield satisfactory results. We then proceed in Section 3 to discuss salt-free ionic surfactant solutions, where strong electrostatic interactions exist and interesting time dependence has been observed in experiments [13]. In Section 4 the effect of added salt to ionic surfactant solutions is examined. 

The extent of adsorption of non-ionic surfactants decreases with increase in the length of the polyoxyethylene chain [43,45,46] and, as seen in Fig. 1.9, increases with increase in the length of the alkyl chains [43]. 

The adsorption of non-ionic surfactants (polyoxyethylene glycol nonyl-phenols) is decreased by water-structure breaking anions while structure making anions increase it [73]. 

The same approach was used by Kong et al. in their study [10] of the adsorption of non-ionic surfactant on emulsion drops stabilized with SDS. They studied the same... 

The change in surface wettability (measured by the contact angle) with concentration for the three surfactants is plotted in Fig. 2.54 (Zhang and Manglik 2005). The contact angle reaches a lower plateau around the CMC where bilayers start to form on the surface. Wettability of non-ionic surfactants in aqueous solutions shows that the contact angle data attains a constant value much below CMC. Direct interactions of their polar chain are generally weak in non-ionics, and it is possible for them to build and rebuild adsorption layers below CMC. The reduced contact an-... 

Trathnigg, B., Rappel, C., Rami, R., Gorbunov, A. (2002b). Liquid exclusion-adsorption chromatography a new technique for isocratic separation of non-ionic surfactants V. Two-dimensional separation of fatty acid polyglycol ethers. J. Chromatogr. A 953(1-2), 89-99. 

Carberry, J.B. Clay adsorption treatment of non-ionic surfactants in wastewater. J. Water PoU. Control Fed. 1977, 49, 452. 

The presence of mixed surfactant adsorption seems to be a factor in obtaining films with very viscous surfaces [411]. For example, in some cases the addition of a small amount of non-ionic surfactant to a solution of anionic surfactant can enhance foam stability due to the formation of a viscous surface layer, which is possibly a liquid crystalline surface phase in equilibrium with a bulk isotropic solution phase [25,110], In general, some very stable foams can be formed from systems in which a liquid crystal phase is present at lamella surfaces and in equilibrium with an isotropic interior liquid. If only the liquid crystal phase is present, stable foams are not produced. In this connection foam phase diagrams may be used to delineate compositions that will produce stable foams [25,110],... 

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]. 

This adsorption probably takes place with the hydrocarbon end of the Triton molecule toward the oleate, and the ethylene oxide groups toward water. The amount of non-ionic surfactant adsorbed was 10-2(j)g based on the stoichiometric amount of magnetite formed. 

The non-steady diffusion of surfactant ions is a problem similar to the non-steady diffusion of non-ionic surfactant, which was described in Chapter 4. There is a specific distinction caused by the electrostatic retardation effect. The non-steady transport of ionic surfactants to the adsorption layer is a two-step process, consisting of the diffusion outside and inside the DL. 

The theoretical models proposed in Chapters 2-4 for the description of equilibrium and dynamics of individual and mixed solutions are by part rather complicated. The application of these models to experimental data, with the final aim to reveal the molecular mechanism of the adsorption process, to determine the adsorption characteristics of the individual surfactant or non-additive contributions in case of mixtures, requires the development of a problem-oriented software. In Chapter 7 four programs are presented, which deal with the equilibrium adsorption from individual solutions, mixtures of non-ionic surfactants, mixtures of ionic surfactants and adsorption kinetics. Here the mathematics used in solving the problems is presented for particular models, along with the principles of the optimisation of model parameters, and input/output data conventions. For each program, examples are given based on experimental data for systems considered in the previous chapters. This Chapter ean be regarded as an introduction into the problem software which is supplied with the book an a CD. 

There are some informations about monotonous decrease of the equilibrium surface tension, dilatational elasticity, and adsorption of lysozyme for non-ionic surfactant decyl dimethyl phosphine oxide (Cj DMPO) as the concentration of surfactant increases in the mixture. However, in the case of mixtures of non-ionic surfactants with more flexible proteins like P-casein, the elasticity of the interfacial layer decreases before passing through a maximum as the concentration of surfactant increases [7], Possibly, the weaker interfacial network formed by P-casein as compared to globular proteins determines the dilatational response of the mixtures. The same picture was shown for the system P-casein mixed with dodecyl dimethyl phosphine oxide (C,2DMPO). For all studied frequencies (0.005-0.1 Hz) the elasticities for adsorption layers have a maximum about 4x10" mol/1 Cj2DMPO concentration. It was shown the obtained values are very close to those measured for the surfactant alone. Thus, in this concentration region the surfactant dominates the surface layer. In our case we have... 

In the case of CnDMPOs, a class of non-ionic surfactants characterized by a small hydrophilic head, with reduced steric influence, even with a semipolar character [22], the more hydrophobic tail of C12DMPO enhances the hystheresis. With CiEjs the situation is more complicated the steric effect of polyoxyethylene oxide chains is not negligible, producing a significant antagonistic effect in the adsorption on the solid. 

In order to specify these effects, it will be useful to return to the simple case of non-ionic surfactants. We shall consider the various parameters governing competition between preferred solubilisation in one or other of the phases (possibly as micelles) and adsorption onto the interface. 

This analysis leads us to the conclusion that ionic surfactants in salt-free solutions undergo kinetically limited adsorption. Indeed, dynamic surface tension curves of such solutions do not exhibit the diffusive asymptotic time dependence of non-ionic surfactants, depicted in Fig. 1. The scheme of Section 2, focusing on the diffusive transport inside the solution, is no longer valid. Instead, the diffusive relaxation in the bulk solution is practically immediate and we should concentrate on the interfacial kinetics, Eq. (21). In this case the subsurface volume fraction, t, obeys the Boltzmann distribution, not the Davies adsorption isotherm (15), and the electric potential is given by the Poisson-Boltzmann theory. By these observations Eq. (21) can be expressed as a function of the surface... 

The adsorption of surfactants on solids is affected by the surface properties (hydrophilicity/hydrophobicity, surface charge) and by the surfactant properties (ionic/non-ionic, CMC, HLB). Figure 3.9 illustrates the possible configurations of non-ionic surfactants on hydrophobic (A) and on hydrophilic (B) solid surfaces as well as those of cationic surfactants on negatively charged oxide layers (C). The surface concentrations increases successively from row I (ideal gas) to row IV or V (saturated surface). The picture reveals a broad variety including monolayers. 

The Langmuir equation (Equation 1.18) has been v ridely used in the interpretation of adsorption data of non-ionic surfactants... 

The presence of an adsorbed surfactant layer on the surface of solid particles dispersed in an aqueous medium can affect the stability of the dispersion in several ways [95]. The adsorption of ionic surfactants on to non-polar surfaces imparts a surface charge to the solid surface which may increase stability through the repulsion of the electrical double layers. One of the ways in which the stability of a dispersion is increased by the adsorption of non-ionic polyoxyethylated surfactants is thought to be associated with the polyoxyethylene chains which extend into the solution. Interaction of the polyoxyethylene chains of the adsorbed layers on neighbouring particles would result in restriction in their movement and hence a decrease of the entropy - a process referred to as entropic stabilization. Stabilization of dispersions by surfactants is discussed in detail in Chapters 8 and 9. 

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