Adsorption surfactants on solids

Surfactant adsorption on solids from aqueous solutions plays a major role in a number of interfacial processes such as enhanced oil recovery, flotation and detergency. The adsorption mechanism in these cases is dependent upon the properties of the solid, solvent as well as the surfactant. While considerable information is available on the effect of solid properties such as surface charge and solubility, solvent properties such as pH and ionic strength (1,2,3), the role of possible structural variations of the surfactant in determining adsorption is not yet fully understood. 

This overview will outline surfactant mixture properties and behavior in selected phenomena. Because of space limitations, not all of the many physical processes involving surfactant mixtures can be considered here, but some which are important and illustrative will be discussed these are micelle formation, monolayer formation, solubilization, surfactant precipitation, surfactant adsorption on solids, and cloud point Mechanisms of surfactant interaction will be as well as mathematical models which have been be useful in describing these systems,... 

The increase in the hydrophilic head group size reduces the amount of adsorbed surfactant at surface saturation. On the other hand, increasing the hydrophobic tail length may increase, decrease or maintain the surfactant adsorption. If the surfactant molecules are not closely packed, the increase in the chain length of the tail increases surfactant adsorption on solid surfaces. If the adsorption of surfactant on the solid surface is due to polarisation of tc electrons, the amount of surfactant adsorbed on the surface reduces at surface saturation. If the adsorbed surfactants are closely packed on the solid surface, increasing the chain length of the surfactant tail will have no effect on the surfactant adsorption. 

The nature of surfactant adsorption on solid surfaces depends on the polarity and solubility of the surfactant. Thus, when an aqueous surfactant solution is in contact with non-polar coal particles, adsorption layers are formed which have polar groups oriented towards the aqueous phase. In contrast, surfactant solutions in oils (hydrocarbons, vegetable oil oxidation products etc.) in contact with polar materials or powders (carbonates, silicates) the polar groups are on the solid phase surface. 

Surfactant adsorption on solid-liquid and liquid-vapour interfaces changes the corresponding interfacial tensions. Liquid motion caused by surface tension gradients on... 

In simimary, the adsorption of surfactants on colloidal particles is influenced by the EDL and it affects the EDL. Surfactant molecules compete with simple ions from the background electrolyte for adsorption in the Stern layer (Dimov et al. 2002), yet the interaction between adsorbed surfactant molecules introduces additional complexity to the problem of EDL formation. Detailed introduction into the surfactant adsorption on solid surfaces is, e.g., given by Myers (1999, Chap. 9) and Holmberg et al. (2002, Chap. 17). 

IV. Literature Survey on Calorimetric Studies of Surfactant Adsorption on Solid Substrates... 

IV. LITERATURE SURVEY ON CALORIMETRIC STUDIES OF SURFACTANT ADSORPTION ON SOLID SUBSTRATES... 

The properties (e.g. cleaning and stabilizing capabilities) of surfactants depend on both solution properties (temperature, time, presence of salts and cosurfactants) and their own characteristics, especially CMC, the Krafft point and their chemistry. The surfactant chemistry and especially the balance between hydrophobic and hydrophilic parts is quantified using tools like the CPP or HLB (critical packing parameter, hydrophilic-lipophilic balance, respectively). For example, it is often observed that detergency increases with concentration especially up to CMC and is often best at CPP values around 1. We will meet the important concept of CPP again in Chapter 7 where we will see that surfactant adsorption on solid surfaces is connected to CPP. 

Practical applications of surfactants usually involve some manner of surfactant adsorption on a solid surface. This adsorption is always associated with a decrease in free-surface energy, the magnitude of which must be determined indirectly. The force with which the adsorbate is held on the adsorbent may be roughly classified as physical, ionic, or chemical. Physical adsorption is a weak attraction caused primarily by van der Waals forces. Ionic adsorption occurs between charged sites on the substrate and oppositely charged surfactant ions, and is usually a strong attractive force. The term chemisorption is applied when the adsorbate is joined to the adsorbent by covalent bonds or forces of comparable strength. 

Various hypotheses were conceived for the study of elementary processes during adsorption on solids. For surfactant adsorption, the most important ones are described by Richter and Schneider74). 

Maxima, often followed by minima, were observed on adsorption isotherms of surfactants adsorbed on solids by many authors86,87,107-113 . It is not, however, possible to say that one and the same surfactant causes maxima above the CMC on all kinds of solid substances. The surface properties of minerals are one of the chief factors determining the behavior of surfactants at concentrations greater than CMC. 

Surfactants adsorb on solid surfaces due to hydrophobic bonding, electrostatic interaction, acid-base interaction, polarisation of rr electrons and dispersion forces. Hydrophobic bonding occurs between the hydrophobic surfactant tail and the hydrophobic solid surface (tail down adsorption with monolayer structure) or between the hydrophobic tails of the surfactant adsorbed on the hydrophilic solid surface and the hydrophobic tails of the surfactant from the liquid phase (head down adsorption with bilayer structure) [54, 55]. 

The formation of multilayer structure can be carried out by several ways 1) adsorption on liquid/liquid interface - Langmuir-Blodgett films [5,6] 2) adsorption on solid/liquid interface - alternate adsorption of oppositely charged polyelectrolytes (PE) and surfactants on flat surfaces or spherical particles [7,8], To control the process of multilayer systems formation, it is necessary to under-... 

The order of increased surfactant adsorption on the solid produced by the different alkyl pyrrolidinones parallels the order of their enhancement of superspreading. In addition, it was shown (Wu, 2002) (1) that the change in the spreading coefficient (equation 6.1) parallels enhancement of superspreading and (2) that the order of increased attractive molecular interaction between the different alkylpyr-rolidinones and the trisiloxane surfactant at the hydrophobic solid-aqueous solution interface, as measured by the interaction parameter Psl° (Chapter 11) n-butyl < n-cyclohexyl < -octyl < n-hexyl < 2-ethylhexyl, is exactly the same order as that of their enhancement of the superspreading. 

Adsorption from solutions onto solid surfaces is important in many industrial practices, such as dye or organic contaminant removal, edible oil clarification by activated carbon, and ion exchange, where the adsorption of ions from electrolyte solutions is carried out. Adsorption from solution is also used in analytical chemistry in various chromatography applications. On the other hand, surfactant, polymer and biological material adsorption on solids, to modify the surface of solid particles in stabilizing dispersions, are also very important industrial fields. 

Surfactant adsorption on saltlike minerals, such as calcite and dolomite, is a more complex process and is less understood than adsorption on oxide surfaces. These minerals are relatively soluble and when in contact with an aqueous medium develop an interfacial region of complex composition (41—43). In addition to the two mentioned mechanisms of adsorption, covalent bonding, salt formation between surfactant and lattice ions at the solid surface, ion exchange of surfactant with lattice ions, and surface precipitation have been suggested as adsorption mechanisms (36, 43—47). The dissolution products of sparingly soluble minerals may interact with the surfactant, precipitate or adsorb at the solid surface, or lead to mineral transformations that affect surface composition and electrochemical properties (46, 48—52). All these factors can be expected to influence surfactant adsorption. 

Petroleum reservoirs can exhibit the full range of wettabilities from water-wet to oil-wet (53). Adsorption of crude oil heavy ends modifies solid surface properties and is thought to change reservoir wettability toward more oil-wet. Surfactant adsorption on hydrophobic surfaces takes place by hydrophobic interactions between surfactant hydrocarbon chains and the solid surface (35, 54—58). At low surfactant concentrations, surfactant molecules are oriented parallel to the surface. As the surfactant concentration increases, hydrophobic interactions between surfactant hydrophobes become significant. The surfactant molecules become oriented vertically to the surface with the polar groups toward the aqueous phase. 

Surfactant—solid and surfactant—surfactant hydrophobic interactions lead to minimization of solid—water and surfactant-chain—water contact and are energetically favorable. Unlike hydrophilic surfaces, hydrophobic surfaces do not lead to significant structuring of interfacial water, and the interfacial water is displaced from the surface relatively easily by the surfactant molecules. Consequently, surfactant adsorption on hydrophobic surfaces has often been found to be higher than adsorption on the corresponding hydrophilic surfaces (39, 54, 56, 57, 59—62), provided aqueous phase salinity is low. 

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. 

The presence of copolymer and surfactant together alters the rheological properties of solutions, adsorption on solid particles, solubility, and stability of colloidal dispersions. The solution properties are mainly influenced by... 

The second reason for the anomalous change of surface tension of a solid polymer containing surfactant lies in the structural and conformational conversions of the polymer itself under the influence of the surfactant. Such factors as the increase of the polymer surface tension when surfactant is added cannot be explained by the surfactant adsorption on the polymer surface only (see Fig. 2.13). Later we will consider this in detail. As was noted above, if the rate of aggregation of the surfactant molecules is higher than or equal to the rate of polymerization, the system surface tension alters during polymerization in the same way as in the coiu-se of the equilibrium process. At a high rate of polymerization, the formation of micelles of the maximum possible size can be hindered by the rapid increase of the system viscosity. In this case, when an IS substance is applied the split into two phases is not observed and the system appears to be more oversaturated by surfactant than in the first case. 

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