Hindawi Publishing Corporation
`Journal of Chemistry
`Volume 2014, Article ID 908476, 14 pages
`http://dx.doi.org/10.1155/2014/908476
`
`Review Article
`Effects of Surfactants on the Rate of
`Chemical Reactions
`
`B. Samiey,1 C.-H. Cheng,2 and J. Wu2
`1 Department of Chemistry, Faculty of Science, Lorestan University, 68137-17133 Khoramabad, Iran
`2 Department of Chemical Engineering, Ryerson University, Toronto, ON, Canada M5B 2K3
`
`Correspondence should be addressed to B. Samiey; babsamiey@yahoo.com
`
`Received 13 August 2014; Accepted 16 October 2014; Published 30 December 2014
`
`Academic Editor: Tomokazu Yoshimura
`
`Copyright © 2014 B. Samiey et al. This is an open access article distributed under the Creative Commons Attribution License, which
`permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
`
`Surfactants are self-assembled compounds that depend on their structure and electric charge can interact as monomer or
`micelle with other compounds (substrates). These interactions which may catalyze or inhibit the reaction rates are studied with
`pseudophase, cooperativity, and stoichiometric (classical) models. In this review, we discuss applying these models to study
`surfactant-substrate interactions and their effects on Diels-Alder, redox, photochemical, decomposition, enzymatic, isomerization,
`ligand exchange, radical, and nucleophilic reactions.
`
`1. Introduction
`
`Self-organized assemblies such as micelles can change the
`rates of chemical and enzymatic reactions. Effects of micelles
`of surfactants on these reactions can be attributed to their
`electrostatic and hydrophobic interactions with reactants.
`Surfactants are amphiphilic organic compounds, containing
`both hydrophobic groups (their tails) and hydrophilic groups
`(their heads). Thus, a surfactant molecule contains both a
`water insoluble component and a water soluble component.
`The tail of most surfactants consists of a hydrocarbon
`chain. Surfactants are classified into four types. (1) Anionic
`surfactants, such as sodium dodecyl sulfate (SDS), contain
`anionic functional groups at their head, that is, sulfate, sul-
`fonate and phosphate. (2) Cationic surfactants, for example,
`cetyltrimethylammonium bromide (CTAB), have cationic
`functional groups such as quaternary ammonium cation.
`(3) Zwitterionic surfactants have one cationic center and
`one anionic center both attached to the same molecule. The
`cationic part is based on primary, secondary, or tertiary
`amines or quaternary ammonium cations and the anionic
`part can be, for example, sulfonate and carboxylate [1]. (4)
`Nonionic surfactants (such as Triton X-100) do not ionize
`in an aqueous solution because their hydrophilic groups are
`
`nondissociable. Gemini surfactants (such as gemini 16-2-16)
`are a relatively new class of amphiphilic molecules containing
`two head groups and two aliphatic chains, linked by a rigid
`or flexible spacer [2]. They show greatly enhanced surfactant
`properties relative to the corresponding monovalent surfac-
`tants, Figure 1.
`A micelle is an aggregate of surfactant molecules dis-
`persed in a liquid colloid. Micelles form only when the
`concentration of surfactant is greater than the critical micelle
`concentration (CMC). This type of micelle is known as a
`normal-phase micelle (oil-in-water micelle). In a nonpolar
`solvent, a reverse micelle (water-in-oil micelle) forms in
`which the hydrophilic groups of surfactant are sequestered
`in the micelle core and the hydrophobic groups extend away
`from the center [3], Figure 2.
`
`2. Classification of Kinetic Models
`In this study, three models used to study kinetics of reactions
`in the presence of surfactants are discussed.
`
`2.1. Pseudophase Model. The pseudophase (or pseudophase
`ion-exchange (PPIE)) model was first introduced by Menger
`and Portnoy [4] in 1967 to study effects of surfactant micelles
`
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`2
`
`Journal of Chemistry
`
`O
`O Na
`
`S
`
`OO
`
`SDS
`
`H3C
`
`+N
`
`Br−
`
`w(CH2CH2O)OH
`
`(OCH2CH2)xOH
`
`O
`
`H
`
`n
`
`O
`
`O
`
`N+
`
`Br−
`
`(OCH2CH2)yOH
`
`O
`
`O
`
`(OCH2CH2)z
`
`O
`
`C17H33
`
`Where sum of w, x, y, and z = 20
`
`Tween-80
`
`C16H33–N+(CH3)2–N+(CH3)2–C16H33
`
`Gemini 16-2-16
`
`CH3(CH2)6CH2CH=CHCH2(CH2)6CH2(OCH2CH2)10OH
`
`CTAB
`
`O
`
`Triton X-100 (TX-100), n = 9-10
`Triton X-305 (TX-305), n = 30 (avg)
`Triton X-405 (TX-405), n = 40
`
`Septonex
`
`Brij 97
`
`Figure 1: Structures of several surfactants.
`
`Hydrophilic head
`
`Aqueous
`solution
`
`Hydrophobic tail
`
`(a)
`
`Oil
`
`H2O
`
`(b)
`
`Figure 2: Typical structures of (a) micelle and (b) reverse micelle.
`
`on the chemical reaction rates. They considered surfactant
`micelles as a pseudophase that can interact with some or all
`of reactants (or substrates), can further dissolve substrates,
`and can alter the reaction rate of substrates. Therefore, this
`model cannot study the interaction between the substrate and
`surfactant molecules below the CMC. With respect to the
`definition of micelle as a pseudophase, there is no stoichio-
`metric ratio between the substrate and surfactant molecules
`for the presence of this interaction. The distribution constant
`of each substrate between solvent and micelle is defined as the
`binding constant of the substrate with a micelle. The substrate
`
`(𝑆) distributes between the solvent and a micelle (𝐷
`follows:
`
`𝑛
`
`) as
`
`S
`
`+
`
`Dn
`
`KS
`
`kw
`
`Products
`
`SDn
`
`km
`
`Products
`
`(1)
`
`𝑤 and 𝑘𝑚 are the observed rate constants in the solvent
`
`where 𝑘
`𝑆 is the association constant of
`and micelles, respectively. 𝐾
`
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`Journal of Chemistry
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`3
`
`the substrate with the micelles. In this model, it is assumed
`that a single equilibrium relation, thus one 𝐾
`𝑆 value, is applied
`within the whole surfactant concentration range. On the basis
`of the above model, the following relation for the observed
`rate constant (𝑘obs) has been derived:
`1
`1
`(𝑘obs − 𝑘
`
`=
`
`)
`
`𝑤
`
`(𝑘
`
`𝑚
`
`− 𝑘
`
`𝑤
`
`+
`
`)
`
`(𝑘
`
`𝑚
`
`− 𝑘obs) 𝐾
`
`𝑆
`
`,
`
`1
`([𝐷] − CMC)
`(2)
`where [𝐷] is the surfactant concentration. Depending on the
`number of substrates and other compounds (such as salts),
`relations of 𝑘obs can be written as different forms.
`
`R + n1S
`
`K1↔ RSn1
`
`sc
`
`RSn1 + n2S
`
`K2↔ RSn1+n2
`
`120
`
`110
`
`100
`
`90
`
`80
`
`70
`
`kobs(M−1min−1)
`
`60
`0.00
`
`0.01
`
`0.02
`0.01
`[surfactant] (mM)
`Figure 3: Typical equilibrium relations between reactant and
`surfactant molecules in a two-region system.
`
`0.02
`
`0.03
`
`2.2. Cooperativity Model. Piszkiewicz presented cooperativ-
`ity model [5] in 1976 analogous to the enzyme-catalyzed
`reactions. This model is used only for reactions catalyzed
`) forms a
`by surfactants. He assumed that a micelle (𝐷
`𝑆) with the substrate (𝑆) before the
`noncovalent complex (𝐷
`catalysis takes place:
`
`𝑛
`
`𝑛
`
`𝐷
`
`𝑛
`
`+ 𝑆 𝐾←→ 𝐷
`
`𝑛
`
`𝑆,
`
`𝐷
`
`𝑛
`
`𝑆
`
`𝑘
`
`𝑚󳨀→ products,
`
`𝑘
`
`𝑆
`
`(3)
`
`concentration. 𝑛 is known as the cooperativity index and is a
`measure of the association of additional surfactant molecules
`to an aggregate in the whole surfactant concentration range.
`If 𝑛 value is greater than one, the cooperativity of interaction
`is positive and if its value is less than one, the cooperativity
`of interaction is negative and if its value is equal to 1, the
`interaction is noncooperative.
`
`0
`
`𝑚
`
`,
`
`𝑛1
`
`0󳨀→ products,
`where 𝐾 is the association constant of the micelle-substrate
`𝑚 is the rate constant for micelle-catalyzed reac-
`complex, 𝑘
`0 is the rate constant for the reaction in the
`tion, and 𝑘
`absence of micelle. Similar to pseudophase model, this model
`assumed that there is only one equilibrium relation, thus one
`𝑆 value within the whole surfactant concentration range.
`𝐾
`The 𝑘obs at any concentration of surfactant is given by
`𝐾 (([𝐷] − CMC) /𝑛)
`+ 𝑘
`𝑘
`1 + 𝐾 (([𝐷] − CMC) /𝑛)
`where 𝑛 is the number of surfactant molecules per micelle.
`Thus, this model can study interactions between the substrate
`and surfactant molecules above the CMC. An alternative
`cooperativity model, analogous to the Hill model applied to
`enzyme-catalyzed reactions, was proposed that the substrate
`𝑆,
`and surfactant molecules aggregate to form micelles, 𝐷
`which may then react to yield product
`
`(4)
`
`𝑛
`
`𝑘obs =
`
`𝑛𝐷 + 𝑆
`
`𝐾
`
`𝐷←→ 𝐷
`
`𝑛
`
`𝑆,
`
`𝐷
`
`𝑛
`
`𝑆
`
`𝑘
`
`𝑚󳨀→ products,
`
`(5)
`
`0󳨀→ products.
`The model gives the following rate equation:
`
`𝑆
`
`𝑘
`
`log [
`
`𝑚
`
`0
`
`] = 𝑛 log [𝐷]
`
`𝑡
`
`− log 𝐾
`
`𝐷
`
`,
`
`(6)
`
`(𝑘obs − 𝑘
`)
`(𝑘
`− 𝑘obs)
`where 𝐾
`the dissociation constant of micellized
`𝐷 is
`𝑡 is the total surfactant
`surfactant-substrate complex and [𝐷]
`
`2.3. Stoichiometric Model. Samiey introduced the stoichio-
`metric (classical) model [6] in 2004. In the stoichiometric
`model [6], it is assumed that, in each range of surfac-
`tant concentration, the surfactant and substrate can bind
`together and an equilibrium relation exists. The surfactant
`concentration in which the equilibrium relation between the
`added surfactant (𝑆) and the species already present in the
`solution (𝑅) ends and a new equilibrium relation between the
`added surfactant and the compound resulted from previous
`) starts which is called “substrate-
`equilibrium relation (𝑅𝑆
`surfactant complex formation point” (or abbreviated as sc
`point) and is as follows:
`
`𝑅 + 𝑛
`
`1
`
`𝑆
`
`𝑅𝑆
`
`𝑛1
`
`+ 𝑛
`
`2
`
`𝑆
`
`𝐾
`
`1←→ 𝑅𝑆
`
`𝑛1
`
`𝐾
`
`2←→ 𝑅𝑆
`
`𝑛1 +𝑛2
`
`(in the first region) ,
`
`(in the second region) ,
`
`(7)
`
`...
`The CMC value of a surfactant is also a sc point and there
`may be some sc points higher and lower than CMC as
`well. The range of surfactant concentration which covers an
`equilibrium relation is named “region,” Figure 3.
`Surfactant molecules either monomeric or micellar can
`bind to the substrate molecules. Micelles can bind to the sub-
`strate by one or more number of their surfactant molecules.
`Thus, we can obtain the stoichiometric ratios and binding
`constants of interactions between surfactant molecules and
`with the substrate in various surfactant concentration ranges.
`The following equation holds for each equilibrium relation
`[6]:
`
`ln 𝑘󸀠 = 𝑐 −
`
`𝐸
`
`𝑆
`
`𝑅𝑇
`
`[𝑆]
`
`𝑡
`
`,
`
`(8)
`
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`4
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`Journal of Chemistry
`
`tot) values for each substrate, in
`total stoichiometric ratio (𝑛𝑖
`the 𝑖th region, can be obtained from following equations:
`
`(11)
`
`𝑖 ∏ 𝑗
`
`=1
`
`𝐾
`
`,
`
`𝑗
`
`tot = 𝐾
`𝐾𝑖
`
`1
`
`⋅ ⋅ ⋅ 𝐾
`
`𝑖−1
`
`𝐾
`
`𝑖
`
`=
`
`𝑛
`
`𝑗
`
`.
`
`𝑖 ∑ 𝑗
`
`=1
`
`tot = 𝑛
`𝑛𝑖
`
`1
`
`+ ⋅ ⋅ ⋅ + 𝑛
`
`𝑖−1
`
`+ 𝑛
`
`𝑖
`
`=
`
`Also, using this model we can study interactions of mixed
`micelles with substrate molecules and calculate the stoi-
`chiometric ratios and binding constants of their surfactant
`molecules with substrate molecules [6].
`
`2.4. Comparison of Stoichiometric, Cooperativity, and PPIE
`Models. (1) In the PPIE model, the colloidal particles of sur-
`factant (after cmc) are considered as an ion exchanger and the
`binding of substrate to them is considered like the partition of
`a substrate between the two phases (micelle and solvent). In
`the PPIE and cooperativity models, the stoichiometric ratio of
`surfactant (as micelle) to the substrate is 1 : 1 and there is one
`average binding constant for substrate-surfactant compound
`in the whole surfactant concentration range, while in the
`stoichiometric model the stoichiometric ratio of surfactant
`(either micellar or monomeric) to the substrate is n : 1 and in
`each region there is a new equilibrium relation and therefore
`a new binding constant, a new stoichiometric ratio, and
`negative or positive cooperativity [6].
`(2) The PPIE and cooperativity models is not applicable
`in the region before the cmc point of surfactant, but in
`the stoichiometric model the binding of substrate to the
`monomeric surfactant is considered.
`(3) In the PPIE and cooperativity models, for the cases in
`which the reaction rate increases in one range of surfactant
`concentration and decreases in another range, it is assumed
`that in average there is one type of interaction between surfac-
`tant and substrate molecules. Therefore, there is one binding
`constant for whole range of the surfactant concentrations.
`But, in these cases, in the stoichiometric model it is assumed
`that the substrate molecules have different interactions with
`surfactant molecules and the reaction is catalyzed in one or
`more regions and inhibited in another region(s). Therefore,
`the binding constants are not identical in different regions.
`(4) In the PPIE and cooperativity models, it is assumed
`) is not usually equal to
`that the rate constant in micelle (𝑘
`zero. But in the stoichiometric model, it is assumed that the
`rate constant in micelle for catalysis of reaction is more than
`the rate constant of free substrate and in the state of inhibition
`of reaction, it is equal to zero.
`(5) In the PPIE and cooperativity models, only one sc
`point is assumed which corresponds to the cmc of surfactant.
`But in the stoichiometric model, there are various sc points
`including cmc.
`(6) In the PPIE and cooperativity models, the binding
`constant and stoichiometric ratio of single type substrate-
`surfactant interaction are measured. But in the stoichiometric
`model, we can evaluate the stoichiometric ratios and binding
`constants of multiple type substrate-surfactant interactions in
`each region [6].
`
`𝑚
`
`where 𝑘󸀠, 𝑐, [𝑆]
`𝑡, 𝑅, and𝑇 are the rate constant in the presence
`of surfactant, ln 𝑘 (at first region) or ln 𝑘sc (for other regions),
`total surfactant concentration, universal gas constant, and
`𝑆 is the catalytic or inhi-
`absolute temperature, respectively. 𝐸
`bition energy of reaction at constant temperature and various
`surfactant concentrations. 𝑘sc is the 𝑘obs in the starting of
`in the
`each region except region one and 𝑘 is the 𝑘obs
`absence of surfactants. Equation (8) is introduced as “Samiey
`equation” [6] and determines the concentration range of each
`region. If the reaction rate decreases with the increase of
`𝑆 is positive and is
`surfactant concentration, the sign of 𝐸
`called “inhibition energy” and if the reaction rate increases
`with increasing the surfactant concentration, the sign of
`𝑆 is negative and is named “catalytic energy” at constant
`𝐸
`temperature and various surfactant concentrations [6]. The
`unit of 𝐸
`𝑆 is kJ (mol molar (surfactant))−1. In this model, it is
`assumed that in each region one substrate molecule, 𝑅, binds
`to 𝑛 molecules of surfactant and we have
`
`𝑅 + 𝑛𝑆 𝐾←→ 𝑅𝑆
`
`𝑛
`
`,
`
`(9)
`
`where 𝐾 is the binding constant of the substrate-surfactant
`interaction in each region. According to stoichiometric
`model, these interactions contain two types: Type I is the
`interaction of which surfactant molecules have an inhibitory
`effect on the reaction rate, yielding a decreased reaction
`rate; Type II is the interaction of which surfactant molecules
`exert a catalytic effect on the reaction rate, resulting in
`an increased reaction rate [6]. Some surfactants, show an
`increased reaction rate in a certain concentration range (type
`I) and a decreased reaction rate in the other range (type
`II). The 𝑘obs, which indicates the interaction between one
`species of substrate with one kind of the surfactant, is species
`dependent and is related to the surfactant concentration as
`follows [6]:
`
`𝑘obs =
`
`{{{{
`{{{{
`{
`
`𝑘 + 𝑘
`
`𝑆
`
`𝐾 [𝑆]𝑛
`
`𝑡
`
`1 + 𝐾 [𝑆]𝑛
`
`𝑡
`
`𝑡
`
`(region one) ,
`
`(all other regions) ,
`
`(10)
`
`𝑘sc + 𝑘
`1 + 𝐾 ([𝑆]
`
`𝑆
`
`𝑡
`
`− [sc])𝑛
`𝐾 ([𝑆]
`− [sc])𝑛
`
`in the absence of surfac-
`where 𝑘 and 𝑘sc are the 𝑘obs
`tant (beginning of the first region) and at each sc point,
`respectively. 𝑘
`𝑆 is the reaction rate constant in the substrate-
`surfactant complex and is greater than reaction rate in
`pure solvent (𝑘) but when the surfactant has an inhibitory
`= 0. Going from one region to the next one,
`effect, 𝑘
`if 𝐾1/𝑛 value (the average binding constant of interaction
`between one substrate molecule and one surfactant molecule
`in each region) increases, the cooperativity of interaction
`is positive and if 𝐾1/𝑛 value decreases, the cooperativity of
`interaction is negative. The total binding constant (𝐾𝑖
`tot) and
`
`𝑆
`
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`
`(7) In the stoichiometric model, K values calculated for
`each region obey the Van’t Hoff equation whereas the binding
`constants obtained from the PPIE and cooperativity models
`are not so in most of the cases.
`
`3. Change in the Chemical Reaction Rate in
`the Presence of Surfactants
`Interaction of surfactant molecules with substrates can result
`in decreasing or increasing the reaction rate or changing the
`yield of reaction and sometimes these surfactant molecules
`act as reactants. In this section, we discuss the role of temper-
`ature and cosolvents on the interactions between surfactants
`and substrates as well as the effects of head group, chain
`length, charge, and concentration of surfactants in a series
`of reactions, for example, Diels-Alder, redox, photochemical,
`decomposition, enzymatic, isomerization, ligand exchange,
`radical, and nucleophilic reactions. Furthermore, this section
`also discusses the potential role of surfactants as a reactant.
`
`3.1. Diels-Alder Reactions. The Diels-Alder reaction is an
`organic chemical reaction (specifically, a [4 + 2] cycloaddi-
`tion) between a conjugated diene and a substituted alkene,
`commonly termed the dienophile, to form a substituted
`cyclohexene system. Because the majority of the diene and
`dienophile intermolecular Diels-Alder reactions have a rather
`pronounced nonpolar character, an efficient binding of both
`substrates to micelles is anticipated. This would imply that
`the effective reaction volume for the Diels-Alder reaction is
`significantly reduced, leading to micellar catalysis [7].
`A study on the reaction of cyclopentadiene with a series of
`dienophiles shows the roles of charge and substituent groups
`in their interaction with surfactants [8–12].
`In the reaction of acridizinium bromide (a cationic
`dienophile) with cyclopentadiene, a 10-fold reaction rate is
`induced by anionic SDS micelles, whereas nonionic TX-100
`and cationic 1-N-dodecyl-4-methylpyridinium bromide have
`only modest effects on the reaction rate [8]. The efficient
`catalysis by SDS most likely results from electrostatically
`enhanced binding of the dienophile to the micelles [8, 9]. But
`the reaction rate of 1,2-dicyanoethylene with cyclopentadi-
`enedecreases with the increase of SDS concentrations which
`is due to weak interactions between 1,2-dicyanoethylene and
`SDS micelles [10]. It seems to point toward the Stern region
`of the micelles as the prominent site for this Diels-Alder
`reaction.
`Rispens and Engberts [11] studied the reaction rate of
`cyclopentadiene with a series of N-substituted maleimides
`in SDS micellar media. They observed that, up to 30 mM
`of SDS, the reaction rate of cyclopentadiene with N-methyl
`maleimide is constant while its rate with N-butyl and N-
`benzyl substituted maleimideincreases with the size of sub-
`stituent group. This is because the butyl and benzyl sub-
`stituent groups lead to deeper solubilization of N-substituted
`maleimide in the SDS micelle compared to the methyl-
`substituted compound. Evidence suggests that the reaction in
`the micellar phase mainly takes place in the region between
`the core and the Stern layer, thereby still experiencing a
`polar environment. In all the above-mentioned cases, the
`
`apolar cyclopentadiene might be expected to mainly reside
`in the apolar micellar core. It was observed that if the
`SDS concentration is more than 30 mM, the reaction rate
`decreases. Pseudophase model considers just one kind of
`interaction occurring between SDS and substrate molecules
`within the whole SDS concentration range and calculated that
`
`𝑚 value is less than 𝑘𝑤. However, it seems that pseudophase
`𝑘
`
`𝑚 is greater than 𝑘𝑤 when the SDS
`model fails to show that 𝑘
`concentration is less than 30 mM.
`Simonyan and Gitsov [12] studied the first Diels-Alder
`reaction performed in an aqueous medium with highly
`hydrophobic compounds, such as fullerene (C60) as the
`dienophile and anthracene or tetracene as the dienes,
`respectively. The reactions were performed in nanocon-
`tainers, constructed by self-assembly of
`linear-dendritic
`amphiphilic copolymers, Figure 4. Surfactants can also affect
`the endo/exoselectivity [13], regioselectivity [14], and enan-
`tioselectivity [15] of the Diels-Alder reactions.
`
`−
`
`3.2. Redox Reactions. The catalytic effects of SDS, NaBDS
`(anionic gemini surfactant), and mixed surfactants (SDS +
`NaBDS) on the oxidation rate of D-fructose by alkaline
`chloramine-T have been investigated [16]. The observed
`catalytic effect of mixed micelle on the oxidation rate was
`always less than the combination of the catalytic effects
`of two individual surfactants, suggesting an antagonism
`(negative synergism) in the mixed micelle. The antagonism
`has also been confirmed by determining the CMC and the
`interaction parameter (𝛽𝑚) of mixed micelle. According to
`
`𝑆 and 𝑘𝑚 values of interaction of D-
`the pseudophase model, 𝐾
`fructose with SDS were 8.2 M−1 and 16.5 × 10−4 s−1 and those
`of D-fructose with NaBDS were 400 M−1 and 17.9 × 10−4 s−1
`at 35∘C, respectively.
`The catalytic effects of zwitterionic micellar solutions of
`SB3-14 and SB3-16 on the redox reaction of Br− + BrO3
`have been studied using the pseudophase model [17]. The
`
`𝑆 and 𝑘𝑚 values of BrO3
`− with SB3-14 were 310 M−1 and
`𝐾
`1.24 × 10−3 s−1 and those of BrO3
`− with SB3-16 were 3100 M−1
`and 0.99 × 10−3 s−1, respectively. In the presence of the same
`concentrations of surfactants, the reaction rate of using SB3-
`16 is less than that of using SB3-14. It seems that deeper
`− in SB3-16 micelles decreases its
`solubilization of BrO3
`reaction rate with Br−.
`Vanadium (V) oxidation of D-glucose was studied in
`the presence of CPC, SDS, and TX-100 [18]. CPC inhibits
`the reaction, while SDS and TX-100 accelerate the reaction
`to different extents. The observed effects were studied by
`the cooperativity model and were explained by considering
`the hydrophobic and electrostatic interactions between the
`surfactants and substrates. Similarly, oxidation reactions of
`2− [20, 21] and MnO4
`Ce(IV) [19] or oxyanions such as CrO4
`[22] with organic compounds have been studied in the
`presence of surfactants.
`Surfactants can affect the nucleation and growth kinetics
`[23, 24] and the reduction [25] of nanocompounds. For
`instance, colloidal silver particles in the nanometer size range
`were synthesized in ethanol, by the reduction of AgNO3
`with nonionic surfactants Brij 97 and Tween 80 [25]. The
`main conclusion is that surfactants reduce silver ions to the
`
`−
`
`Opiant Exhibit 2308
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`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 5
`
`

`

`6
`
`Journal of Chemistry
`
`H2O
`
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`
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`
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`
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`O O
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`
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`
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`
`Diels-Alder
`products
`
`O
`
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`O
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`
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`
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`
`+
`
`Figure 4: Mass transfer in and out of the micellar nanoreactor entity [12].
`
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`
`

`

`Journal of Chemistry
`
`7
`
`neutral state through the oxidation of oxyethylene groups
`into hydroperoxides. Surfactant molecules subsequently were
`adsorbed onto the surface of particles, promoting steric
`stabilization. This adsorption permits also the transfer of the
`particles into nonpolar solvents through a dry state where no
`particle aggregation occurs.
`As reported [26–29], the electron transfer reaction was
`studied in the presence of surfactants. The electron transfer
`reaction between [Co(NH3)5(N-cyanopiperidine)]3+, pen-
`taammine(N-cyanopiperidine)-cobalt(III), and hexacyano-
`ferrate(II) has been studied in aqueous solutions [29]. The
`alteration of electron transfer rate constant in the presence
`of SDS, Brij 35, and TX-100 has been investigated at 298.2 K.
`The rationalization of the experimental data was assisted
`by the use of the Marcus theory on electron transfer. The
`conclusion is that the micellar effects on the electron transfer
`rate constant can be explained by considering the micelles as
`a special background electrolyte with a high electric charge
`and a strong power of hydration.
`Surfactants can affect the time stability of cation radicals
`[30, 31]. As reported, the time stability of diethazine cation
`radical (DE+∙) decreased in the presence of surfactants [30].
`Below the CMC, Septonex (a cationic surfactant) monomers
`cannot interact with DE+∙ and do not affect the degradation
`rate. Above the CMC, the DE+∙ degradation is catalyzed by
`Septonex micelles that are able to bind to DE+∙ particles
`through hydrophobic interactions, without associating with
`H+ ions. The DE+∙ degradation, occurring in the micellar
`pseudophase relatively poor in H+ ions, is therefore much
`faster. The effect of nonionic surfactant TX-305 is similar with
`the cationic surfactant but less significant. On the other hand,
`the associations of DE+∙ with SDS premicelle aggregates
`which do not bind to H+ ions formed below the CMC of SDS
`and the rate of DE+∙ decomposition quickly increases with
`the increase in SDS concentration. For the concentrations of
`SDS higher than the CMC, DE+∙ radicals are bound to the
`negatively charged surface of micelles, together with H+ ions.
`Therefore, the DE+∙ decomposition reaction is inhibited at
`higher surface local H+ activity and the DE+∙ degradation
`rates decrease.
`
`3.3. Photochemical Reactions. The solutions of 3-(4-chlo-
`rophenyl)-l,l-dimethylurea (monuron) were photolyzed in
`aqueous media containing nonionic surfactants [32]. The TX-
`100, TX-405, TMN-6, and TMN-10 were used to elucidate
`the influences of aryl- and alkyl-substituted polyoxyethylene
`glycol surfactants. Concentrations of all surfactant solutions
`were above each individual CMC. Samples were examined
`under oxygenated and nonoxygenated conditions. The
`presence of surfactants enhances the degradation rate of
`monuron, eliminates ring hydroxylation reactions, and
`promotes the reductive dechlorination reaction. Monuron
`is adsorbed on the lyophilic surface or into lipophilic core
`of micelles. Similar to that observed for the photolysis of
`nitroaromatic [33] and 2-chlorophenol [34] compounds, the
`results indicate that these photochemical reactions occur in
`the organic phase of the micelles rather than the aqueous
`phase of the solvent. In addition to the interaction with
`
`substrate, surfactants can sometimes act as an additional
`source of hydrogen for the reaction [35, 36].
`The TiO2 photosensitized oxidation of 4-dodecyloxyben-
`zyl alcohol, which is water insoluble, was investigated in
`aqueous solutions of anionic, cationic, and nonionic surfac-
`tants [37]. The reaction, which is practically absent in water,
`is greatly enhanced by several surfactants at concentrations
`higher than CMC and the effect is strongly dependent on
`the nature of the surfactant. The increase of surfactant
`concentration leads to more substrate molecules, solubilized
`in micelles, which are transported close to the TiO2 particle
`surface where the photooxidation reaction takes place. After
`a certain concentration which varies with the nature of the
`surfactant, the presence of competitive partition of comicel-
`lized 4-dodecyloxybenzyl alcohol (substrate) between TiO2
`surface (where the reaction occurs) and bulk solvent tends to
`diminish the beneficial kinetic effect of surfactant.
`For N,N,N 󸀠,N 󸀠-tetramethylbenzidine solubilized in mixed
`micelles of C12E6/SDS or C12E6/DTAC, the electron spin-
`echo and electron spin resonance spectra of photogenerated
`cations show that the photoionization yield depends on the
`sign of net charge of the mixed micelle and on the strength
`of the photocation-water interaction [38]. It is found that the
`photoyield is enhanced by the presence of mixed micelles
`with a net positive charge, probably due to the fact that
`electron escaping from the micelle is facilitated in cationic
`micelles.
`Photogalvanic effects were studied in photogalvanic cells
`containing SDS as a surfactant, EDTA as a reductant, and
`azur-B as a photosensitizer [39]. The used SDS solubilizes the
`dye more easily and stabilizes the system and may increase
`the probability of charge transfer between the surfactant
`and the dye in the system compared to the tunneling of
`photoelectrons from the micellar phase to the aqueous phase.
`
`𝑚
`
`3.4. Decomposition Reactions. In the presence of SDS
`2 ) is incorporated
`micelles, 1-naphthalenediazonium (ArN+
`𝑆 of
`into the micellar aggregates, given the estimated 𝐾
`290 M−1 and 𝑘
`= 9 × 10−4 s−1. It shows that a significant
`fraction of 1-naphthalenediazonium is incorporated into
`the micellar pseudophase at low-surfactant concentrations,
`where it undergoes thermal decomposition in the Stern layer
`[40].
`[41] studied the decarboxylation of
`Brinchi et al.
`anionic 6-nitrobenzisoxazole-3-carboxylate (6-NBIC) and its
`5-methyl derivative (6-NBIC-5-Me) in the presence of a
`series of several cationic cetyltrialkylammonium bromide
`surfactants including CTAB, CTEAB, CTPAB, and CTBAB,
`Figure 5.
`Cationic micelles of cetyltrialkylammonium bromide
`facilitate the decarboxylation of both 6-NBIC and 6-NBIC-5-
`Me by decreasing activation enthalpies. It was observed that
`𝑆 values increase with the head group
`the reaction rates and 𝐾
`size of surfactants. It was also observed that the reaction
`rate increases with the surfactant concentration when the
`surfactant concentration is low and then decreases with
`further increase in the surfactant concentration. It seems
`that with increase in the surfactant concentration, different
`interactions are involved between substrate and surfactants
`
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`Page 7
`
`

`

`8
`
`Journal of Chemistry
`
`O
`
`−
`
`O
`
`N
`
`O
`
`R
`
`O2N
`
`#−
`
`O
`
`O
`
`C
`
`C
`
`O
`
`N
`
`R
`
`O2N
`
`N
`
`C
`
`O−
`
`+ CO2
`
`Figure 5: Decarboxylation of substrates 6-NBIC and 6-NBIC-5-Me [41].
`
`𝐻
`
`activated in the presence of SDS, SDS/DS mixed micelles,
`SDS, and DTAB, respectively, and their activities, due
`to denaturation,
`are decreased at higher
`surfactant
`concentrations. The kinetic parameters of
`interaction
`of two samples of PPO with surfactants [46, 47] were
`calculated by the stoichiometric model [6]. Results show
`that the positive cooperativity is observed during these
`interactions. But, activity of PPO extracted from beet root
`[50] is increased in the applied SDS concentration range
`and its kinetic parameters were calculated using the Hill
`equation that is similar to (6) used in the cooperativity
`model. For soluble PPO, the values of Hill coefficient (𝑛
`)
`of tyramine, dopamine, L-tyrosine, and L-DOPA substrates
`were 2.2, 2.7, 3.9, and 4.2, indicating that the number of
`SDS molecules needed for activation is higher for more
`hydrophilic substrates. These results corroborate that the
`ability of SDS to activate the enzyme involves a limited
`conformational change due to the binding of small amounts
`of SDS [51]. The access of hydrophobic substrates to the
`active site is favored since the first molecules of SDS are
`bound to the enzyme, while hydrophilic substrates require a
`deeper change for full access (activity).
`Enzymatic synthesis using lipase in organic solvents
`has several advantages [52, 53]. The solubility of nonpolar
`substrates is increased in organic solvents, and the reaction
`direction can be shifted to favor synthesis over hydrolysis.
`However, like all other natural enzymes, organic solvents
`easily denature lipase. To avoid the deactivation of enzyme
`in organic media, modification of enzyme surface by coating
`it with surfactants has been studied [52]. For example, it
`was observed that whereas the unmodified lipase from B.
`cepacia was insoluble in tert-butyl alcohol, the propylene
`glycol monostearate-coated lipase exhibited an enhanced
`solubility in tert-butyl alcohol at the reaction temperature
`[52]. The formation of reverse micelles stabilized the enzyme
`in the organic solvent; otherwise, the enzyme would have
`been denatured by removing the surrounding microaqueous
`layer, Figure 7.
`
`3.6. Isomerization, Ligand Exchange, and Radical Reactions.
`Gille et al. [54] studied the thermal cis-tra