Surfactant adsorption on solid surfaces

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. 

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). 

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. 

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. 

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]. 

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. 

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

The adsorption of surfactants at the interface of solid and liquid phase is one of the most important phenomena that have captured the interest of surfactant research [1]. In the case that the substrates are constrained to metallic surfaces, the surfactant adsorption on metal surfaces is extremely... 

The extent of adsorption of eommereial surfaetants developed for use in reservoir recovery proeesses ean vary from near zero to as high as 2.5 mg/g. Surfactant adsorption on rock surfaces is usually measured by either static (batch) or dynamic (coreflood) experiments. The static adsorption method, employing crushed rock samples, is essentially the classical method for determining adsorption isotherms at the aqueous solution/solid interface and involves batch equilibrations of particles in solutions of different initial surfactant concentration. The dynamic coreflood method is more involved but employs a greater solid to liquid ratio and is therefore more sensitive, see references [J69-J7J]. Temperature, brine salinity and hardness, solution pH, rock type, wettability, and the presence of a residual oil phase have all been found to influence the extent of adsorption of different surfactants [116,152,172],... 

This section will deal with the above interfacial aspects starting with the equilibrium aspects of surfactant adsorption at the air/water and oil/water interfaces. Due to the equilibrium aspects of adsorption (rate of adsorption is equal to the rate of desorption) one can apply the second law of thermodynamics as analyzed by Gibbs (see below). This is followed by a section on dynamic aspects of surfactant adsorption, particularly the concept of dynamic surface tension and the techniques that can be applied in its measurement. The adsorption of surfactants both on hydrophobic surfaces (which represent the case of most agrochemical solids) as well as on hydrophilic surfaces (such as oxides) will be analyzed using the Langmuir adsorption isotherms. The structure of surfactant layers on solid surfaces will be described. The subject of polymeric surfactant adsorption will be dealt with separately due to its complex nature, namely irreversibility of adsorption and conformation of the polymer at the solid/liquid interface. 

Adsorption is a universal phenomenon in colloid and surface science We arc talking about the adsorption of high molecular weight amphiphilic compounds for monolayer creation, the adsorptirxi of gases on sohds, adsorption of surfactants, polymas or proteins (biomolecules). We have adsorption on solid surfaces and/or... 

The process of adsorption of polyelectrolytes on solid surfaces has been intensively studied because of its importance in technology, including steric stabilization of colloid particles [3,4]. This process has attracted increasing attention because of the recently developed, sophisticated use of polyelectrolyte adsorption alternate layer-by-layer adsorption [7] and stabilization of surfactant monolayers at the air-water interface [26], Surface forces measurement has been performed to study the adsorption process of a negatively charged polymer, poly(styrene sulfonate) (PSS), on a cationic monolayer of fluorocarbon ammonium amphiphilic 1 (Fig. 7) [27],... 

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. 

Mineral flotation is a method for selective separation of mineral components out of polymineral dispersions of ground ores in water (ca. 5-35 vol.% of the solid) by using dispersed gas (usually air) bubbles. The method consists in the different adhesion of hydrophobized and hydrophilic mineral particles to an air bubble. Hydrophobized mineral particles adhere to the air bubble and are carried out as a specifically lighter aggregate to the surface of the mineral dispersion where they form a foam (froth) layer. This foam, called concentrate, is mechanically removed (Fig. 1A). A mineral is hydrophobized by adsorption of a suitable surface-active compound (surfactant, collector) on the surface of the mineral component to be flotated. All other nonhydrophobized particles remain dispersed in the mixture (Fig. IB). 

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. 

Adsorption of surfactant on solid surfaces is generally described by adsorption isotherms. For this purpose, a simple adsorption experiment can be performed at a constant temperature by dispersing known amounts of solid adsorbent into a constant volume of dilute surfactant solution at which the initial surfactant concentrations are varied and shaking the mixture until equilibrium is reached. The moles of surfactant adsorbed per unit mass of the solid (Ns) for each solution can be determined from ... 

The characteristics of surfactant adsorption isotherm on solid surface are generally analysed by the plot of log Ns versus log Ce based on eqn 2.24 or the plot of log T versus log Ce based on eqn 2.25. These plots show four region isotherms as shown in Figure 2.4. 

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. 

In general, the adsorption of ionic surfactants follows the Langmuir isotherm, as discussed in Section 4.1. The adsorption of the surfactants onto the solid surfaces is dependent on the orientation and the packing efficiency of the solid surfaces. The onset of the adsorption plateau may occur at the critical micelle concentration (c.m.c.) of the surfactant in water, as shown in Figure 4.28. If the adsorption isotherm... 

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