Surfactant‐Based Materials*
4.4 Surfactant‐Modified Sorbents
4.4.1 Surfactant‐Modified Mineral Oxides
To date, SPE has been the most popular sample pretreatment technique in common use due to its remarkable advantages including high enrichment performance, low solvent‐
consumption, short extraction time, and ease of automation. The core of SPE is the
OH (a)
(b)
(c)
OH O O
O O O
O O
S H2
C H2
C H2
C CH2 C
H2
H2 C CH2
H2
C H2
C CH2 C
H2 CH3
H2
H2 C H2 C
C H2
C H2
C CH2
CH2 CH2 H2C
H3C
CH3 CH3
OH Si
O
O O O O O O O
Si Si
OH N
CH2
H2 C CH2
H2
C CH2 C
H2 CH3
H2 H2 C H2 C
C H2
C H2
C CH2
CH2 CH2 H2C
H3C
CH3 CH3 + O
– –
+–
+–
OM+ O Si
N
Al Al Si
CH2
H2 C CH2
H2 C CH2 C
H2 CH3
O O
O
Al Al Al
Figure 4.6 Adsorption models of surfactants on mineral oxide surfaces: (a) SDS–alumina, (b) CTAB–silica, and (c) CTAB–zeolite.
Surfactant-Based Materials 145
adsorbent that determines the selectivity and sensitivity of the method. However, the widely used commercial SPE adsorbents, such as bonded silicas (C8, C18), graphene, alumina, and nanomaterials are often limited by their shortcomings, such as the high costs, potential health and environmental risks, and the narrow applicable ranges.
Thus, developing new SPE adsorbents is of high value. Recently, it has been reported that surface modification by ionic surfactants, such as the anionic surfactants sodium deoxycholate, SDS, and sodium dodecanoate, can significantly improve the interfacial properties. The anionic surfactant molecules are supposed to electrostatically adsorb onto the oppositely charged surface and form hemimicelles under certain conditions.
In this state, their charged head groups are bonded to the solid surface with the carbon‐
chain tails toward the solution phase, leading to great enhancement of the surface hydrophobicity [101].
Anionic surfactant‐modified alumina has been widely studied since alumina has high a surface area with positive charge at the solution pH below its point of zero charge, and anionic surfactants such as SDS can adsorb onto an alumina surface. Unmodified alu- mina has low sorption ability for organic compounds due to its low hydrophobicity. The affinity for organic compounds is increased when it is coated with SDS. SDS–γ‐alumina admicelles have been investigated for concentrating traces of chlorophenols in water. It is pointed out that chlorophenols were concentrated onto the admicelles. The sorption capability for chlorophenols was also increased with increasing hydrophobicity [102].
Application of SDS‐treated alumina for removal of herbicides from water has also been reported; the enhancement in sorption of herbicides on surfactant treated alumina was observed [103].
The cationic surfactant is effectively adsorbed onto the silica surface because of nega- tive charges on the silica surface. The properties of silica nanoparticles modified with cationic surfactant have been studied. It has been demonstrated that a cationic sur- factant formed bilayers on the silica surface. Surfactant‐coated silica has been applied for phenanthrene partitioning [104]. It was investigated for protein purification and found to be efficient media for protein separation as well [105].
Zeolites have became an impressive support material for surfactant modification as they present high surface areas and negatively surface, permitting the adsorption of cationic surfactants. Zeolites are hydrated aluminosilicate materials with high cation exchange capacities. Sorption of surfactant molecules on zeolites is limited to sites of external exchange only. This is of course due to the zeolite channel diameter, which is expected to be sufficiently large for exchangeable cations but too small for surfactant cations. Surfactant molecules form a monolayer or hemimicelle at the solid–aqueous interface via strong Coulombic interaction at a surfactant concentration at or below its cmc. Just as surfactant molecules in solution form a micelle above the cmc, surfactant exposed to a negatively charged zeolite surface will form a bilayer or admicelle and the charge on the zeolite surface is reversed from negative to positive. The positively charge head groups are balanced by anionic counterions, which make surfactant‐modified zeo- lites a potential sorptive media to sorb anionic contaminants such as arsenate oxyanions via an ion exchange mechanism. The study shows that surfactant‐modified zeolites are effective sorbent for the removal of As(V) from aqueous solution [106]. Natural zeolites are inexpensive and readily available in nature. Furthermore, surfactant‐modified natu- ral zeolites are much less expensive than granular activated carbon or synthetic ion exchange resins due to very low specific gravity of high‐porosity zeolites.
CTAB is regularly used for modification of zeolite surfaces since the CTAB‐modified zeolites can have both hydrophilic and hydrophobic phases; thus, they can be applied widely to groups of target compounds. For example, CTAB‐modified zeolite NaY has been employed as sorbent for preconcentration of carbamate pesticides in vortex‐
assisted dispersive micro‐solid phase extraction (VA‐D‐μ‐SPE) [107]. The method was based on the application of CTAB‐modified zeolite NaY sorbent following its dispersion in sample solution by vortex agitation to enhance the extraction efficiency and facilitate a fast extraction process. The presented method achieves low LODs, which are below the maximum residue limits (MRLs) of the carbamate residues in agricultural products.
Surfactant‐modified zeolite has been studied extensively for the removal of inor- ganic anions and other ionizable organic solvents. The study focused on the sorption of diclofinac by surfactant‐modified zeolite under different physico‐chemical conditions in order to elucidate the mechanism of diclofinac sorption by surfactant‐
modified zeolite and to expand its application further. The results showed that the diclofinac was retained on the external surfaces of surfactant‐modified zeolite with an extremely fast removal rate. Both anion exchange and partitioning of diclofinac into the adsorbed surfactant micelles (admicelles) were responsible for the extended diclofinac sorption [108].
The automated modification of zeolite NaY with CTAB and subsequent investigation for extraction/preconcentration of carbamate pesticides has been developed for appli- cation in various water matrices [109]. The full on‐line steps, including modification, extraction/preconcentration, determination, and re‐modification, were pointed out.
The SPE column was coupled with a HPLC system for simultaneous preconcentration/
determination of carbamate pesticides. The quantitative retention of target pesticides on the admicellar sorbent was based on hydrophobic interaction with the hydrocarbon core of CTAB aggregates and π–cation interaction, between the polar head of the sur- factant and the aromatic rings of pesticides. The high surface area and reusability of zeolite NaY were utilized sufficiently for the proposed method. The created sorbent established high sorption capacity resulting from the high surface area of the material.
The developed system offers cost‐effectiveness due to the reuse of sorbent material, a high enrichment factor, time‐saving, and use a small volume of the eluent, with the less organic waste, too.
By way of SPE, a mixed‐mode sorbent based on surfactant‐modified mineral oxide has been developed for multi‐class pesticide residues. A multifunctional sorbent made up of admicelles of SDS and tetrabutylammonium (TBA) on alumina surfaces was pro- posed for extraction of pesticides belonging to different structural groups. Moral and his colleagues studied the performance of pure SDS aggregate and mixed TBA‐SDS aggregate on alumina surface for extraction/preconcentration of 17 pesticides, repre- sentative of all common groups (triazines, phenylureas, carbamates, azols, anilides, chloroacetanilides, organophosphorius, aryloxy acids, and phenols) [110]. TBA‐SDS mixed hemimicelles/admicelles on alumina showed high performance for determi- nation of pesticide multiresidues. The suitability of a TBA‐SDS mixed hemimicelles/
admicelles sorbent to extract and preserve multi‐class pesticides was assessed in a separate work [111]. Most of the studied pesticides were stable for one month at room temperature and three months at 4 °C in darkness. That was long enough to permit easy shipping and storage of samples for monitoring of pesticides.
Surfactant-Based Materials 147
Different surfactant‐modified solid sorbents have been investigated comparatively for the retention of carbamate pesticides in aqueous solution [112]. Three modified‐
sorbents, including SDS treated alumina, CTAB coated silica, and CTAB coated zeolite, were created using different surfactant concentrations. Among various initial concen- trations of surfactant, SDS‐modified alumina and CTAB‐modified silica treated at the cmc showed the highest sorption percentages for most carbamate pesticides. At the cmc, surfactant molecules began to form micelles and sorbed on some area of sorbent surfaces to form admicelles; accordingly, mixed hemimicelles/admicelles were created on sorbent surfaces. The mixed hemimicelles/admicelles provided both hydrophilic and hydrophobic phases. Therefore, interaction of carbamate molecules and modified‐
sorbents occurred via both hydrophobic and hydrophilic interaction, resulting in a high sorption capacity. However, the configuration of surfactant aggregate on solid surfaces was found to be related to the amount of surfactant, nature of surfactant, and the char- acteristics of the solid surface. Zeolites, which have a high sorption capacity for CTAB, were reported as showing the greatest pesticide adsorption when using a sorbent treated with surfactant at a concentration higher than the cmc.
In addition, it has also been shown that mixtures of anionic–non‐ionic surfactants can exhibit much higher surface activities than their single components in the solid–
solution interfaces. An anionic–non‐ionic surfactant pair of sodium dodecyl benzene sulfonate (SDBS) with Triton X‐100 (TX100) was modified on the surface of an eggshell membrane (ESM) to prepare the mixed SDBS‐TX100 surfactants modified eggshell membrane (SDBS‐TX100‐ESM), and subsequently applied for the simultaneous deter- mination of trace Sudan I–IV [101]. The adsorption mechanism of SDBS‐TX100‐ESM adsorbent for target analytes was discussed based on π–π interaction, hydrophobic effect, and π–π electron donor–acceptor. Finally, this SDBS‐TX100‐ESM/SPE‐HPLC‐
UV method was applied practically for analysis of trace Sudan I–IV in real chili powder, chili sauce, and ketchup samples.
In recent years, nano‐sized metal oxides, including nano‐sized Fe2O3, Fe3O4, TiO2, SiO2, Al2O3, MgO, and CeO2, have delivered some promising applications as adsorbents due to their large surface areas and high activity levels, caused by the size quantization effect.
Treatment of this kind of nano‐sized metal oxide with surfactants can enhance their adsorptive tendency towards organic pollutants. Surfactants that have been used for this purpose have either anionic or cationic head groups, with long chain hydrocarbon mole- cules forming the surfactant tail. The pH point of a zero charge of nano‐sized sorbent characterizes the type of behavior of modifiers. Due to the hydrophobic interactions of micelles formed on the surface of nano‐sized metal oxides, such as alumina, the organic contaminants escape from the aqueous phase and become concentrated in the microscopic hydrophobic phase. One study deals with the simultaneous removal of Brilliant Green and Crystal Violet by surfactant‐modified alumina. The utilization of alumina nanoparticles with SDS anionic surfactant as a novel and efficient adsorbent has been successfully car- ried out to remove two cationic dyes from aqueous solutions in binary batch systems [113].