Surfactant adsorption anionic

The adsorption isotherm of sodium dodecyl sulfate (SDS) on alumina at pH = 6.5 in 0.1 M NaCI (Fig. 4.11a) is characteristic of anionic surfactant adsorption onto a positively charged oxide. As shown by Somasundaran and Fuerstenau (1966) and by Chandar et al. (1987), the isotherm can be divided into four regions. These authors give the following explanation for the adsorption mechanism ... 

Figure 1. Typical Anionic Surfactant Adsorption Isotherm on a Positively Charged Mineral Oxide Surface.

Hirasaki and Zhang (2004) found that potential determining ions (COs ) can change the surface charge and reduce the anionic surfactant adsorption on calcite. Carbonate formations and sandstone-cementing material can be calcite... 

In low alkaline concentrations, as the alkaline concentration is increased, anionic surfactant adsorption is reduced, as discussed previously. However, if the alkaline concentration is high, as the concentration is increased, ionic strength is increased. Then flocculation of surfactant micelles may occur. Also, as ionic strength is increased, the counter ions in the diffusion layer may enter the adsorption layer to reduce the electrostatic repulsion between the anionic surfactant and sand surface. Consequently, surfactant adsorption may be increased with alkaline concentration. Also, cationic surfactant adsorption increases with pH (Yang et al., 2002a). 

The Mechanism of Anionic Surfactant Adsorption on Clay and Silica... 

A more plausible explanation for anionic surfactant adsorption on silica gel is found in the presence of about 0.2 wt.% alumina in the silica gel. The alumina pz-c is about pH=8.0, so it would be positively charged at the pH of the experiments. If the alumina is uniformly distributed through the silica, all of the adsorption could be accounted for provided a close-packed mono-layer of surfactant is formed on the alumina. This circumstance would also be consistent with the shape of the toe of the isotherm. Gaudin and Fuerstenau (25) advanced the idea of hemimicelle formation (two-dimensional micelles on a surface) to account for similar observations in flotation processes. 

The effect of divalent cations on surfactant adsorption is shown in Figure 14, which provides a comparison of adsorption levels on several solids measured in sodium chloride brine with those measured in brines containing sodium chloride and divalent cations. The ionic strength of all brines is constant at 0.403 mol/L, thus ionic strength effects are eliminated. Evidently, the dependence of surfactant adsorption on divalent ions varies with the type of surfactant and rock. In most cases, adsorption is increased by the presence of divalent cations. Adsorption of the sul-fobetaine is less sensitive to divalent cations than adsorption of the betaine and the anionic surfactants. Adsorption of three surfactants on dolomite is not influenced very strongly by divalent cations. 

There have been a number of studies concerning mixtures of anionic surfactants with either nonionics or cationics, but only a very few have addressed the kinetics of these complex systems [75, 76]. When looking at enhancement of anionic surfactant adsorption at the water-cellulose interface, Paria et al. [75] fonnd that the greatest rate increase could be achieved by pretreating the snrface with cationic surfactant, rather than using a mixed solution. 

In the 1990s, the thmst of surfactant flooding work has been to develop surfactants which provide low interfacial tensions in saline media, particularly seawater require less cosurfactant are effective at low concentrations and exhibit lower adsorption on rock. Nonionic surfactants such as alcohol ethoxylates, alkylphenol ethoxylates (215) and propoxylates (216), and alcohol propoxylates (216) have been evaluated for this appHcation. More recently, anionic surfactants have been used (216—230). 

The NDR and the oscillation appear only in the presence of cationic surfactant and not in the presence of anionic surfactant, suggesting that the NDR arises from electrostatic adsorption of a cationic surfactant on a (negatively polarized) Cu-Sn... 

Emulsions and Emulsion Technology (in three parts), edited by Kenneth J. Lissant Anionic Surfactants (in two parts), edited by Warner M. Linfieid see Volume 56) Anionic Surfactants Chemical Analysis, edited by John Cross Stabilization of Colloidal Dispersions by Polymer Adsorption, Tatsuo Sato and Richard Ruch... 

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 surfactant salts and reduce the concentration of active molecules. Therefore, for many anionic surfactants the washing performance decreases with lower temperatures in the presence of calcium ions. This effect can be compensated by the addition of complexing agents or ion exchangers. 

The partition coefficients for different LAS homologues (Table 5.4.2) are higher in the marine environment due to the higher ionic strength that promotes sorption of anionic surfactants [14] and an increase in the partition coefficient with the alkylic chain length has been observed (Table 5.4.2). The evolution of the concentration of the various homologues of LAS in solids in suspension (cf. Fig. 5.4.2) is similar to that found in water and, in the process of adsorption, an increase can be observed in line with the chain length, as commented on previously. 

Anionic surfactants are present in surface water, resulting in serious environmental pollution. Therefore, adsorption of surfactants, such as sodium dodecylsulfate [155,156], on Mg/Al LDHs has received considerable attention. Ulibarri et al. also published the results of sorption of an anionic surfactant (sodium dodecylbenzenesulfonate) from water by LDHs and calcined samples (773 K), focusing both on their potential application as a sorbent and on the possibility of their recycling [154,157]. They found that anionic exchange was complete when the interlayer anion in the LDH precursor was Cl", reaching 100 % of AEG, and calcined LDH-carbonates were better adsorbents than those derived from LDH-chloride samples, however. It was also claimed that an increase in the crystallinity of the LDH samples probably leads to better ordered calcined mixed oxides, facilitating reconstruction of the layers and enlarging the absorption capacity. 

Alkyl ether sulfates are/after alkyl benzene sulfonates(LAS),the group of technically important anionic surfactants with the largest production voluJne and product value. They have in comparison with other anionic surfactants special properties which are based on the particular structure of the molecule. These are expressed,for example,in the general adsorption properties at different interfaces, and in the Krafft-Point. Alkyl ether sulfates may be used under conditions, at which the utilization of other surfactant classes is very limited. They possess particularly favorable interfacial and application properties in mixtures with other surfactants. The paper gives a review of all important mechanisms of action and properties of interest for application. 

Washing and Cleaning Action. The properties of alkyl ether sulfates, due to the good solubility and the special hydrophilic/hydrophobic properties of the molecule, are of particular practical interest. From the investigations described in sections 2 and 3, it can be concluded that, in addition to the decrease in the Krafft Point, favorable properties for practical applications can be expected as a result of the inclusion of the oxyethylene groups into the hydrophobic part of the molecule. As is true for other anionic surfactants, the electrical double layer will be compressed by the addition of multivalent cations. By this means, the adsorption at the interface is increased, the surface activity is raised, and, furthermore, the critical micelle concentration decreased. In the case of the alkyl ether sulfates, however these effects can be obtained without encountering undesirable salting out effects. 

LDAO/SDS Interaction. Mixing of cationic and anionic surfactant solutions results In the formation of a mixed species that Is more surface active than the Individual species. The enhanced synergistic effect has been explained (2,3) by showing that a close-packed adsorption of electroneutral R R takes place (R" " and R represent the long chain cation and anion respectively). In the case of Ci2 and C14-DAO, a 1 1 LDAO/SDS molar ratio produces a minimum In surface tension and Is accompanied by an Increase In pH In the bulk solution the association seems to be of the type R R", and the absence of visible precipitate may be attributed to the solubilization of the R R" complex In the solution. In the region where LDAO Is In excess, the structure Is probably [cationic (LDAOH ) anionic (SDS)] nonlonlc (LDAO), while [cationic (LDAOH anionic (SDS)] anionic (SDS) Is formed when SDS Is In excess. Equal molar concentration results In cationic (LDAOH ) anionic (SDS) complex which should favor precipitation. However, at pH >9, there Is no Indication of precipitation (even when the total solute concentration Is 0.35 M). When the pH Is below 9, then precipitation will take place. 

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