Surfactant adsorption application

For the solid-liquid system changes of the state of interface on formation of surfactant adsorption layers are of special importance with respect to application aspects. When a liquid is in contact with a solid and surfactant is added, the solid-liquid interface tension will be reduced by the formation of a new solid-liquid interface created by adsorption of surfactant. This influences the wetting as demonstrated by the change of the contact angle between the liquid and the solid surface. The equilibrium at the three-phase contact solid-liquid-air or oil is described by the Young equation ... 

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. 

Wesson, L.L. and Harwell, G.H., Surfactant Adsorption in Porous Media in Surfactants, Fundamentals and Applications in the Petroleum Industry, Schramm, L.L. (Ed.), Cambridge University Press, Cambridge, UK,... 

Adsorption can be measured by direct or indirect methods. Direct methods include surface microtome method [46], foam generation method [47] and radio-labelled surfactant adsorption method [48]. These direct methods have several disadvantages. Hence, the amount of surfactant adsorbed per unit area of interface (T) at surface saturation is mostly determined by indirect methods namely surface and interfacial tension measurements along with the application of Gibbs adsorption equations (see Section 2.2.3 and Figure 2.1). Surfactant structure, presence of electrolyte, nature of non-polar liquid and temperature significantly affect the T value. The T values and the area occupied per surfactant molecule at water-air and water-hydrocarbon interfaces for several anionic, cationic, non-ionic and amphoteric surfactants can be found in Chapter 2 of [2]. 

Abstract A crucial problem in the manufacturing of high aspect ratio structures in the microchip production is the collapse of photoresist patterns caused by imbalanced capillary forces. A new concept to reduce the pattern collapse bases on the reduction of the capillary forces by adsorption of a cationic surfactant. The application of a cationic surfactant rinse step in the photolithographic process leads to a reduction of the pattern collapse. Physicochemical investigations elucidate the mechanism of surfactant adsorption... 

In the last 10-15 years, neutron reflectometry has been developed into a powerful technique for the study of surface and interfacial structure, and has been extensively applied to the study of surfactant and polymer adsorption and to determine the structure of a variety of thin films [14, 16]. Neutron reflectivity is particularly powerful in the study of organic systems, in that hydrogen/deu-terium isotopic substitution can be used to manipulate the refractive index distribution without substantially altering the chemistry. Hence, specific components can be made visible or invisible by refractive index matching. This has, for example, been extensively exploited in studying surfactant adsorption at the air-solution interface [17]. In this chapter, we focus on the application of neutron reflectometry to probe surfactant adsorption at the solid-solution interface. 

The scope of the chapter will include an introduction to the technique of neutron reflectometry, and how it is applied to the study of surfactant adsorption at the planar solid-solution interface, to obtain adsorbed amounts and details of the structure of the adsorbed layer. The advantages and limitations of the technique will be put in the context of other complementary surface techniques. Recent results on the adsorption of a range of anionic, cationic and nonionic surfactants, and surfactant mixtures onto hydrophilic, hydrophobic surfaces, and surfaces with specifically tailored functionality will be described. Where applicable, direct comparison with the results from complementary techniques will be made and discussed. 

The more recent neutron reflectivity studies have established that flattened surface micelle or fragmented bilayer structure in more detail and with more certainty, using contrast variation in the surfactant and the solvent [24, 31]. However, the extent of the lateral dimension (in the plane of the surface) and the detailed structure in that direction is less certain. From those neutron reflectivity measurements [24, 31] and related SANS data on the adsorption of surfactants onto colloidal particles [5], it is known that the lateral dimension is small compared with the neutron coherence length, such that averaging in the plane is adequate to describe the data. The advent of the AFM technique and its application to surfactant adsorption [15] has provided data that suggest that there is more structure and ordering in the lateral direction than implied from other measurements. This will be discussed in more detail in a later section of the chapter. At the hydrophobic interface, although the thickness of the adsorbed layer is now consistent with a monolayer, the same uncertainties about lateral structure exist. 

The other application of pore-level mechanisms exploits their dependence on dispersion type, wettability, capillary number, and capillary pressure to design surfactants that will optimize these parameters. Measurements of phase behavior, interfacial tensions, surfactant adsorption, wettability, and related parameters will be needed to fit the various requirements of different reservoirs, each of which has a unique combination of mineralogy, pore structure, temperature, pressure, oil and brine composition, etc. 

Values of adsorption capacity of granular carbons (Tab. 3) were determined in static conditions by means of FIBDM — indicator [11] (F — phenol, molecule size - 5 A, I - iodine - 10 A, B - methylene blue - 15 X, D - sodium laurylosulphate - 19 A, and M - molasses — 28 A). These values may be used for evaluation of powdered carbons and for determination of portions of adsorbents for phenol and surfactant adsorption for practical application. 

Adsorption of surfactant on reservoir rock can be determined by static tests (batch equilibrium tests on crushed core grains) and dynamic tests (core flood) in the laboratory. The units of surfactant adsorption in the laboratory can be mass of surfactant adsorbed per unit mass of rock (mg/g rock), mass per unit pore volume (mg/mL PV), moles per unit surface area (peq/m ), and moles per unit mass of rock (peq/g rock). The units used in field applications could be volume of surfactant adsorbed per unit pore volume (mL/mL PV) or mass per unit pore volume (mg/mL PV). Some unit conversions follow ... 

Surfactant Adsorption. Surfactant propagation is crucial to foam propagation. The data compiled in later sections of this chapter show that surfactants that are similarly effective as gas mobility reducing agents may have significant differences in adsorption levels. The level of surfactant adsorption and its dependence on parameters such as brine salinity and hardness may then be the deciding factors in surfactant selection for a specific application. 

The evaluation of surfactant adsorption is particularly important when foams for high salinity reservoirs, such as many Canadian reservoirs subjected to hydrocarbon-miscible flooding, are considered. This puts stringent requirements on the solubility, foaming, and adsorption properties of surfactants that may be considered for foam applications, and severely limits the types of surfactant that may be used. 

An increase in electrolyte concentration reduces the solubility of anionic surfactants in the aqueous phase and increases their tendency to accumulate at the solid—liquid interface. An increase in temperature offsets the loss in solubility to some degree For the DPES—AOS on Berea sandstone, the slopes of the lines in Figure 13a decrease as the temperature increases, and this finding lends support to the hypothesis that surfactant adsorption is related to surfactant solubility. Adsorption of surfactants that are less salt-tolerant than the DPES—AOS, such as the AOS and the IOS, increases much more steeply with salinity. Both surfactants adsorb negligibly at salinities of 0.5 mass % NaCl, but adsorb similarly to the DPES—AOS at a salinity of 2.3 mass %. At moderate salinities (on the order of 3 mass %), these surfactants precipitate, which severely limits their applicability to foam-flooding in many reservoirs that are currently being flooded with hydrocarbon solvents. 

Dependence of Adsorption, on Rock Type. Table I shows that gas injection EOR projects are being conducted in sandstone and carbonate pools. Hydrocarbon- and C02-misdble projects are run largely in carbonate reservoirs. With the exception of several studies that report adsorption levels of EOR surfactants on carbonates (4,11,12, 24, 33, 62—64, 86), the petroleum literature has dealt almost exclusively with anionic, and sometimes nonionic, surfactant adsorption on sandstones, because most studies have been carried out with surfactants used in low-tension flooding. These surfactants are not considered suitable for application in carbonate reservoirs because of their low salinity and hardness tolerance. Foam-forming surfactants suitable for high-salinity environments include amphoteric surfactants (2). The adsorption behavior of this surfactant type has also rarely been studied (10—12, 87, 88). 

This section discusses three approaches that may be used to minimize surfactant adsorption matching surfactant type to specific reservoir rock type based on surfactant ionic character and solid surface charge, application of surfactant mixtures, and sacrificial adsorbates (128). 

Adsorption layers of the same kind as at fluid interfaces are also formed at low-energy solid -water surfaces, as it was established on PE, polystyrene, paraffin, carbon black, and other related materials. The classical Langmuir or Frumkin adsorption isotherm is often applicable to describe this behaviour. Studies on surfactant adsorption at various solid surfaces have been summarised in a great number of reviews [2, 7, 8, 54, 98, 101, 111, 121, 126, 141, 144, 145, 177, 186, 190, 194-198]. The adsorption at the solid/liquid interfaces is governed by a number of factors ... 

The studies of adsorption layers at the water/alkane interface give excess to the distribution coefficient of a surfactant, which is a parameter of particular relevance for many applications. Theoretical models and experimental measurements of surfactant adsorption kinetics at and transfer across the water/oil interface will be presented. The chapter will be concluded by investigations on mixed surfactant systems comprising experiments on competitive adsorption of two surfactants as well as penetration processes of a soluble surfactant into the monolayer of a second insoluble compound. In particular these penetration kinetics experiment can be used to visualise separation processes of the components in an interfacial layer. 

Primarily, this approach was based on the formal analogy between a first order phase transition and the micellisation. When a new phase of a pure substance is formed the chemical potential of this substance and its concentration in the initial phase do not change with the total content of this substance in the system. A similar situation is observed above the CMC, where the adsorption and the surface tension become approximately constant. In reality variations of these properties are relatively small to be observed by conventional experimental methods. The application of the Gibbs adsorption equation shows that the constancy of the surfactant activity above the CMC follows from the constancy of the surfactant adsorption T2 [13]... 

In a spraying process, a liquid is forced through an orifice (the spray nozzle) to form droplets by the application of hydrostatic pressure. The effect of surfactants and/or polymers on the droplet size spectrum of a spray can be described in terms of their effects on the surface tension. Since surfactants lower the surface tension of the liquid, one would expect that their presence in the spray solution would result in the formation of smaller droplets. However, when considering the role of surfactants in droplet formation, one should consider the dynamics of surfactant adsorption at the air/liquid interface. In a spraying process, a fresh liquid surface is continuously being formed. The surface tension of this... 

This so-called Sperline relationship has been verified in a series of in situ and ex situ studies of surfactant adsorption phenomena (see Refs. [169, 170] for examples and references). The corrected formula (1.114) is also applicable to ex situ ATR spectra [171] and adsorption onto a film-coated IRE [172]. The extension of Eq. (1.114) to anisotropic films is discussed by Eringelli [173]. Pitt and Cooper [174] represented Eq. (1.114) as... 

Surfactants are used in a wide variety of applications such as ore flotation, cleaning, polymerization processes, and pharmaceuticals and agriculture. The usual role for the surfactant is to modify interfacial properties, whether they be liquid/liquid, solid/liquid, or gas/liquid interfaces. To be effective in any of these applications, however, the surfactant must adsorb strongly at the interface. In addition to its concentration at the interface, the conformation of the surfactant at the interface is also an important factor. While the influence of solution properties such as concentration, ionic strength, and pH on surfactant adsorption are well known, the properties of the other phase also exert a significant influence. 

Surfactants are generally attracted to solid-liquid and liquid-air interfaces, and this interfacial enrichment is vital to a large number of industrial applications. (Even the term surfactant—short for surface-active agent—betrays the central importance of interfaces.) One such application is foam flotation in the mining industry, in which surfactant adsorption to ore microparticles causes them to flocculate at the surfaces of air bubbles and rise to the foam layer, where they are skimmed from the remaining matrix. 

An overview of some of the significant findings of surfactant adsorption research is presented. Subjects include the importance of surfactant adsorption in petroleum applications, some history of surfactant adsorption research, the mechanisms which have been proposed to explain observed adsorption behavior, and a review of several significant surfactant adsorption studies. The emphasis of this review is understanding the mechanisms of surfactant adsorption as they relate to applications of surfactants in petroleum processes. 

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