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Self-Assembly and Functions of Star-Shaped Oligomeric Surfactants Yaxun Fan, and Yilin Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00290 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Self-Assembly and Functions of Star-Shaped Oligomeric Surfactants Yaxun Fan,† and Yilin Wang*,†,‡ †

Key Laboratory of Colloid, Interface and Chemical Thermodynamics, CAS Research/Education

Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

ABSTRACT: Oligomeric surfactant consists of three or more amphiphilic moieties which are connected by spacer groups covalently at the level of headgroups. It provides a possible route to bridge the gap from conventional single-chain surfactants to polymeric surfactants and leads to many profound improvements in the properties of surfactants in aqueous solution and at the air/water and water/solid interfaces. Generally oligomeric surfactants are categorized into linear, ring-like and star-shaped base on the topological structures of their spacer groups, and their aggregation behavior strongly depends on the resultant topological structures. In recent years, we studied trimeric, tetrameric and hexameric surfactants with a star-shaped spacer which spreads from a central site of nitrogen or carbon element and their charged headgroups connect with each other through the spacers. It has been found that both the nature of spacer groups and the oligomerization degree show important influences on the self-assembly of oligomeric surfactants and provide great possibilities in fabricating various surfactant aggregate morphologies by adjusting the molecule conformations. The unique self-assembly behavior endows them with superior physicochemical properties and potential applications. This feature article summarizes the development of star-shaped oligomeric surfactants, including self-assembly at the air/water and water/solid interface, 1

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self-assembly in aqueous solution and their functions. We expect that this review could provide a comprehensive understanding of the structure-property relationship and various potential applications of star-shaped oligomeric surfactants and offer additional motivation for their future research.

INTRODUCTION Surfactants show strong self-assembling ability to form different aggregate structures from nanoto microscales and exhibit interfacial activity and versatile phase behaviors, and thus display widespread applications in various industrial processes ranging from conventional fields (e.g., cosmetics, foods and pharmaceuticals) to modern technologies (e.g., synthesis of advanced materials, biological techniques and environmental protection). The wide applications stimulate continuous exploration for more efficient and environmentally friendly surfactants, catalyzing more and more novel and optimized surfactant structures. The vast literature1-5 has revealed that gemini surfactant, which is comprised of two amphiphilic moieties connected by a spacer convalently at the level of the headgroups, possesses unique properties and aggregation behaviors compared to the corresponding single-chain surfactant. This fact demonstrates that the increment of oligomerization degree provides extensive possibilities to fabricate versatile aggregate structures and improves the performance of surfactants, consequently giving birth to oligomeric surfactants. Generally, oligomeric surfactants are considered to be constructed by more than two amphiphilic moieties covalently linked by spacer groups at or in close vicinity to the headgroups, and the spacer group connecting the headgroups can be linear, ring-like or star-shaped. The architectures of well-defined oligomeric surfactants can be mainly controlled by the degree of oligomerization and the structure 2


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of the spacers (Figure 1). The combination of these two structural parameters with the normal structure controlling factors of surfactants (alkyl chains, headgroups and counterions) enlarges the pool of surfactant molecules and creates novel surfactant molecular structures.4, 6

Figure 1. Different architectures of well-defined oligomeric surfactants with two molecular variables: degree of oligomerization and topological structures. Most of the pioneering works in investigating oligomeric surfactants are about linear oligomeric surfactant by Zana et al.,7-11 who have laid the basis for the understanding of the self-assembly of oligomeric surfactants with more complicated spatial conformations. Up to now, the most investigated linear oligomeric surfactants are made up of amphiphile moieties - dodecylmethyl and dodecyldimethyl ammonium bromide - connected by short polymethylene spacers. They show unique self-assembly and rheological properties and are easier to be synthesized. Their molecular structures can be denoted as m-s-(m-s)x-m, where m and s are respectively the number of carbon atoms in alkyl chain and spacer group and x = j-2 (j is the degree of oligomerization, 2 ≤ j ≤ 4) (Figure 2). The hydroxy groups or rigid groups were also introduced into the spacers of the linear oligomeric surfactants.12-14 Zana et al.,7, 8, 10, 11 Ikeda et al.,12 Laschewsky et al.,14, 15 and Esumi et al.,16-18 have devoted their great efforts to explore diversified and functional structures of linear 3


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oligomeric surfactants, and have systematically studied the effects of oligomerization degree, spacer nature, alkyl chain length and counterions on the self-assembling structures and the related properties. These linear oligomeric surfactants can be considered as the natural extension of gemini surfactants in one-dimension. The synergistic effect between multiple alkyl chains can be enhanced more strongly while the electrostatic repulsion among charged headgroups can be minimized more efficiently due to more spacer groups in each molecule. Therefore, the higher degree of oligomerization gives rise to stronger adsorption ability of the surfactants at the air/water interface or on solid surface, and more abundant aggregate structures in aqueous solution in comparison with single-chain and gemini surfactants. For the analogs with a spacer of two methylene groups, the surface tension values at critical micellar concentration (CMC) decreases with the increase of oligomerization degree, such as in the order of DTAB (38.6 mN m-1) > 12-2-12 (31.4 mN m-1) > 12-2-12-2-12

(25.2

mN

m-1).

DTAB

is

the

single-chain

surfactant

N,N,N-trimethyl-1-dodecanaminium bromide. By calculation, the values of surface excess (Γmax) at the air/water interface decrease as the oligomerization degree increases, suggesting that the molecules of trimeric surfactants are packed much more compactly than those of gemini and single-chain surfactants.16 The various aggregate morphologies are also generated by adjusting the oligomerization degree and spacer length, from spherical micelles (DTAB)19 to linear wormlike micelles (12-3-12),20 branched wormlike micelles (12-3-12-3-12),7 and closed-loop (ring) micelles (12-3-12-4-12-3-12).10 In addition, the influences of alkyl chain length and counterion are broadly in agreement with those for conventional single-chain and gemini surfactants.

4


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N

CH2

(CH2)m-1 CH3

s

N

CH2

(CH2)m-1 CH3

s

N

jBr

(CH2)m-1 CH3 x = j-2

Oligomerization Degree (j = 2, 3, 4)

Figure 2. The molecular structure and denotations of linear oligomeric surfactants. By contrast, star-shaped oligomeric surfactants have been much less reported. They possess dendritic configuration, where the spacer groups radiate from a central moiety such as nitrogen or carbon, and the headgroups connect with each other by alkyl chains or planar aromatic groups.3, 6 A natural conformation of such a surfactant is symmetric or dissymmetric around the center and often displays a branched three-dimensional structure, which can enhance the intermolecular hydrophobic association. Meanwhile, the multiple alkyl chains of star-shaped oligomeric surfactants can produce strong hydrophobic interaction and overcome the rigidity of spacer groups and electrostatic repulsion of charged headgroups under a certain condition, leading to the change of molecular conformation. This kind of characteristics generates unique self-assembly behavior, specific self-assembly structures and superior performances. However, so far there is not a comprehensive review about star-shaped oligomeric surfactants yet. This feature article summarizes the self-assembly of star-shaped oligomeric surfactants at the air/water and water/solid interface, the self-assembly of star-shaped oligomeric surfactants in aqueous solution, and their possible applications. A brief future perspective is also presented at the end based on our understanding. The molecular formulas of typical star-shaped oligomeric surfactants included in this review are shown in Table 1. Both the series numbers in the table and the abbreviations named in original references will be used in the following text so that the 5


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surfactant structures can be easily found.

Table 1. Molecular Structures of Star-shaped Oligomeric Surfactants. The Abbreviations Used in Original References are also Provided in Red. NO.

Molecular Structures

Ref. 21

SS1

SS2a

22

SS2b

SS2c

23

SS3

SS4 24, 25 SS5

SS6

26

SS7

27

6


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SS8

28

SS9

29

SS10

30, 42

SS11

30

SS12

31

SS13

32

SS14

33

(CH2)N(CH3)2CnH2n+1 N

SS15

(CH2)N(CH3)2CnH2n+1 3Br-

34-36

(CH2)N(CH3)2CnH2n+1

3CntrisQ

n = 8, 10, 12, 14

SS16

37

SS17

38

7


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SS18

39

SS19

40

SS20a

(CH2)2CONH(CH2)2N(CH3)2C12H25 (CH2)2CONH(CH2)2N (CH2)2CONH(CH2)2N(CH3)2C12H25

CH2N


8Br-

SS20b


CH2N


G2QPAMC12

(CH2)2CONH(CH2)2N(CH3)2C12H25

48

(CH2)2CONH(CH2)2N (CH2)2CONH(CH2)2N(CH3)2C12H25

(CH2)2CONH(CH2)2N


CH2N


(CH2)2CONH(CH2)2N


SS20c

16Br-


(CH2)2CONH(CH2)2N


CH2N


(CH2)2CONH(CH2)2N


G3QPAMC12

51

SS21a

8


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SS21b

SS21c

53

SS22

O(CH2)3SO3Na CH2CHC10H21 CH2N O(CH2)3SO3Na

SS23

72

CH2CHC10H21 CH2N CH2CHC10H21 O(CH2)3SO3Na

TED-(C10SO3Na)3

SS24a

74 SS24b

SELF-ASSEMBLY AT THE AIR/WATER AND WATER/SOLID INTERFACE Self-Assembly at the Air/Water Interface. Surface activity is the most basic information about the performance and efficacy of surfactants. The variation of surface activity is a consequence of the difference in the self-assembly of surfactants at the air/water interface. Surfactant structures affect 9


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the orientation and density of molecular packing at the air/water interface and in turn reduce the surface tension of water. The surface tension values at CMC (γCMC), surfactant surface excess concentration (Γmax) and surface area occupied by a surfactant molecule (Amin) reflect the basic information of surfactant self-assembly at the air/water interface. In the following text, we will summarize the self-assembly at the air/water interface in the order of anionic, cationic, nonionic and zwitterionic. A series of double- or triple-chain atypical star-shaped oligomeric surfactants with two sulfonate (SS1 and SS2a), two sulfate (SS2b) or two carboxylate (SS2c) groups and a series of typical star-shaped trimeric surfactant bearing three sulfonate groups (SS3) were studied by Okahara, Masuyama and Nakatsuji et al.21-23 The surface tension values at CMC (γCMC) for the triple-chain surfactants (SS1) with two sulfonate headgroups are much smaller than those of the corresponding gemini surfactants, and are reduced at least an order of magnitude by changing the alkyl chain length from 1 to 10 carbon number.21 Replacing the headgroups into sulfate (SS2b) or carboxylate (SS2c) groups makes the surfactant solubility in water higher than the corresponding sulfonate surfactants (SS2a) bearing the same alkyl chain, but the γCMC values are almost the same for the three surfactants.22 However, the γCMC value of the star-shaped trimeric surfactant bearing three sulfonate groups (SS3) slightly increases with elongating alkyl chain length, and is even larger than those of the corresponding double-chain bis(sulfonate) and triple-chain bis(sulfonate) surfactants. Normally the surface activity increases with elongating the alkyl chain length for single-chain and gemini surfactants, and with the increment of oligomerization degree from single-chain surfactants to gemini surfactants and linear oligomeric surfactants.4 The abnormal phenomenon of the surface activity for the star-shaped oligomeric surfactants may be attributed to the stretched and star-shaped 10


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spacer, which may lead to less ordered molecular packing at the air/water interface. Yoshimura and Esumi et al. synthesized several anionic star-shaped trimeric surfactants with different number of carboxylate headgroups and different alkyl chain length, 2R12dtda (SS4),24, 25 3R12tata (SS5)24,

25

and 3RntaAm (SS6).26 The trimeric surfactants 3R12tata provides greater

efficiency in lowering the surface tension (γCMC = 33.2 mN/m) than the corresponding single-chain surfactant C10H21CH(NH2)COONa (46.0 mN/m).25 However, the triple-chain surfactant with two charged headgroups (2R12dtda) shows much lower γCMC (29.3 mN/m) than the triple-chain surfactant with three charged headgroups (3R12tata). Clearly, the triple chains promote the surfactant molecules to closely self-assemble at the air/water interface, but the orientation of 3R12tata may be less effective at the air/water interface than 2R12dtda due to stronger electrostatic repulsion among more charged headgroups and more bulky structure from 3-aza-1,5-pentanediamine.25 Moreover, the molecular orientation is strongly influenced by the alkyl chain length. As a result, the trimeric surfactants with 10-carbon chains in the series of 3RntaAm,26 with 14-carbon chains in the series of SS727 and with 10-carbon chains in the series of SS828 are the optimal molecular structures in lowering the surface tension of aqueous solutions in the corresponding surfactant series. Furthermore, Yoshimura and Esumi29 synthesized cationic star-shaped oligomeric surfactants by the reaction of zero-generation poly(amidoamine) dendrimers and dimethylalkylammonium bromide. They found that the star-shaped tetrameric surfactant with 8-carbon chain C8qbG0 (SS9) is more efficient in lowering the surface tension than cationic ammonium single-chain surfactant DTAB and gemini surfactant 12-2-12.11 But the occupied area of C8qbG0 is much larger than that of DTAB. The area per octyl chain of C8qbG0 is almost the same as the dodecyl chain of DTAB.29 Our group developed the above synthesis route, and systematically synthesized star-shaped 11


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trimeric, tetrameric and hexameric ammonium surfactants by using tris(2-aminoethyl)amine and ethylenediamine as the core moiety of spacers linked to three, four or six quaternary ammonium headgroups and 12-carbon chains. The trimeric, tetrameric and hexameric ammonium surfactants are denoted as DTAD and DDAD (SS10 and SS11),30 PATC (SS12)31 and PAHB (SS13),32 respectively. This series shows two special phenomena at the air/water interface, different from most of reported surfactants and even anionic star-shaped oligomeric surfactants. One is the weaker ability of lowering the surface tension. The obtained γCMC values of these four surfactants are all above 42 mN/m, and even up to 45 mN/m for PAHB. The oligomerization degree is higher, γCMC is larger. The reason is that the multiple positive charged headgroups and the rigid branched spacer result in the looser molecular packing of surfactants at the air/water interface. Another phenomenon is that the surface tension continues to significantly decrease beyond CMC and this decrease lasts in a large concentration range, which demonstrates that the self-assembly situation of these surfactants still keeps changing beyond CMC at the air/water interface. This is attributed to the molecular conformation changes and the concurrent change in the molecular packing at the air/water interface beyond CMC. This phenomenon is accompanied with the continuous transitions of aggregate structures in aqueous solutions beyond CMC. Thus, they will be discussed together in detail in the latter section of “Self-Assembly in Aqueous Solution”. That the surface tension decreases beyond CMC also appears in the cationic trimeric surfactant series of Tn (SS14),33 but these surfactants show lower surface tension above CMC due to their more flexible spacer. In particular, Yoshimura et al.34-36 found that the diffusion coefficients of the cationic trimeric series of 3CntrisQ (SS15) at the air/water interface are located in the order of CTAB > 3C8trisQ > 3C10trisQ > 12-2-12 > 3C12trisQ while comparing with those of single-chain 12


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surfactant CTAB and gemini surfactant 12-2-12. This indicates that either increasing the oligomerization degree or elongating the alkyl chain length decreases the self-assembly kinetics at the air/water interface due to the increased molecular bulkiness. In addition, if cationic star-shaped trimeric surfactants use a carbon as the central atom (SS16 and SS17),37, 38 the occupied area per molecule at the air/water interface becomes smaller and continues to become smaller with increasing alkyl chain length because of more effective self-assembling ability at the interface. Compared with anionic and cationic ones, the reports on nonionic and zwitterionic star-shaped oligomeric surfactants are even scarcer. A series of nonionic star-shaped trimeric surfactants (SS18) were prepared from tris(2-amino-ethyl)amine with altering the length of alkyl chain and poly(ethylene glycol) chain by Mohamed et al.39 It was found that the surfactant with octyl chains has the strongest ability to lower surface tension. The surface activity gradually reduces with increasing the alkyl chains from octyl, decyl to dodecyl, because the surfactant molecules with shorter alkyl chain adsorbs at the air/water interface to orient themselves efficiently, while the longer one is directed away from water. But the increase in the poly(ethylene glycol) chain length leads to a decrease in the γCMC and Amin instead, because the longer polyethylene glycol chain may lead to the chain coiling in order to minimize any probable interactions between them. In comparison, zwitterionic trimeric betaine surfactants SS19 possess lower surface tension at a smaller concentration with the increase of alkyl chain length.40 The γCMC value is lowered to 24 mN/m for SS19 with dodecyl chain. It indicates that the alkyl chain of trimeric betaine surfactants is longer, the tendency of the surfactant to self-assemble at the air/water interface is greater. These two series of trimeric surfactants do not show the anomalous increase in surface tension with increasing the alkyl chain length and oligomerization degree, probably because nonionic and zwitterionic 13


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oligomeric surfactants pack more closely at the air/water interface owing to weaker electrostatic repulsion among the nonionic and zwitterionic headgroups. In brief, star-shaped oligomeric surfactants show very special features in the self-assembly at the air/water interface. The increment of oligomerization degree is not always the favorable factor for the improvement of surface activity, depending on the charge number and charge property of headgroups and the rigidity of spacers. Moreover, because the special structure of star-shaped spacers, longer alkyl chains often do not lead to lower surface tension. In particular, continuous decreasing in surface tension above CMC is a normal phenomenon for star-shaped oligomeric surfactants, suggesting that the self-assembly remain changing. These properties are quite distinguished from traditional single-chain surfactants, gemini surfactants and even linear oligomeric surfactants.

Self-Assembly at the Water/Solid Interface. Many applications of surfactants are based on their self-assembly ability at the various water/solid interfaces. So far most of the studies about the self-assembly at the water/solid interface are limited to single-chain and gemini surfactants. There are only few works about the self-assembly at the water/solid interface for oligomeric surfactants. Esumi et al.17, 41 comparatively studied the self-assemblies of single-chain, gemini, and linear trimeric cationic quaternary ammonium surfactants on silica. The physicochemical properties of the self-assembled layer on silica are significantly impacted by the alkyl-chain number of the surfactants. The packing density of the surfactants decreases in the order of single-chain, gemini and trimeric surfactants, and the adsorption on silica of gemini and trimeric surfactants is much stronger than that of single-chain surfactant. 14


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Our group42 reported the self-assembly of a cationic star-shaped trimeric surfactant DTAD (SS10) on mica and silica (Figure 3). There are two crucial factors prompting the formation of firm and well-ordered surface patterns through molecular self-assembly. One is the matching between the crystalline structure of the substrate and the geometry of surfactant molecule, which could adjust the oriented growth of self-assembled aggregates.43 The other is the intermolecular interactions including hydrophobic interaction, hydrogen bonding and π-π stacking, which may improve the robustness and stabilization of aggregate structures.44, 45 As to the self-assembly of DTAD on mica, the images of atomic force microscope (AFM) on mica substrates prepared with different DTAD concentrations indicate that the DTAD aggregate morphology shows a concentration-dependent variation on a mica surface. With the DTAD concentration increases from 0.08 to 20 mM (above CMC), the aggregate morphology varies in the order of continuous and semicontinuous islands, ordered parallel stripes coexisting with many circular islands, highly stretched and parallel stripes and highly ordered bilayer patterns (Figure 3A1 ~ 3A4). Meanwhile, the thickness of the aggregates becomes more and more regular, and maintains at a constant value of 3.1 ± 0.2 nm. Different from the self-assembled aggregates on mica, only randomly distributed bilayer patches can be observed on silica and a consistent thickness at 3.4 ± 0.4 nm is also kept (Figure 3B). According to the CPK model, one chain length plus one headgroup for DTAD is about 1.7 nm, so both the thickness on mica and on silica suggests a bilayer structure of DTAD. Although both silica and mica are hydrophilic surfaces and the values of the DTAD aggregate thickness on them are the same, there is an obvious difference between the aggregate morphologies, which could be ascribed to the crystalline structures of the surfaces. The mica surface has alumino-silicate six-ring sites compensated by potassium ion, which can be exchanged with the cationic headgroup of DTAD via 15


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electrostatic interaction.31,46 Meanwhile, the simulation calculation displays that three spacer groups of DTAD are approximately 0.38, 0.75 and 0.75 nm from the central nitrogen atom of tertiary amide to the three neighboring nitrogen atoms of headgroups, respectively (Figure 3D), whereas the distance between each mica lattice site is about 0.50 nm.47 As a result, three mica lattice sites can be occupied by three cationic headgroups of DTAD as a triangle shape, and the special distance matching leads to the highly ordered morphology of DTAD. The strong hydrophobic association among the alkyl chains of DTAD results in the compressed bilayer structure, and the hydrogen bonding between the secondary amide groups leads to array as a zigzag shape for DTAD molecules assisting the directional growth of such bilayer structures (Figure 3D). We studied the self-assembled structures of tetrameric and hexameric surfactants on mica but did not obtained clear ordered bilayer structures. Therefore, the matching of the molecular structure of the surfactants with the crystalline structure of mica is a key factor to form the unique ordered bilayer structure.

Figure 3. Tapping-mode AFM images (2.5 µm × 2.5 µm) of the DTAD aggregate (A) on mica substrates prepared with the DTAD concentrations of 0.08, 2, 10, and 20 mM, and (B) on a silica substrate with 20 mM DTAD. (C) Chemical structure of DTAD. (D) Possible model of DTAD molecular arrangement on mica as a zigzag shape and possible bilayer model of DTAD molecules.42 Reproduced from Ref. [42]. Copyright [2008] American Chemical Society. 16


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In another report, Zhang et al.48 studied the adsorption kinetics and equilibrium of dendrimer surfactants on silica surface and the self-assembled layer as a function of generation number and concentration. The studied dendrimer surfactants GnQPAMC12 are based on poly(amidoamine) modified with a 12-carbon chain, where n represents the generation number and the surfactants correspond to SS20a, SS20b and SS20c when n is 1, 2 and 3, respectively. The oligomerization degree of GnQPAMC12 increases exponentially with the increase of the generation number, making these star-shaped surfactants bearing 4, 8 and even 16 amphiphile moieties. The adsorption behaviors of GnQPAMC12 exhibit multiple stages depending on the concentration and generation number of the surfactants. Below CMC, the adsorption mass increases rapidly at the initial stage, and then the adsorption increase slows down, gradually reaching equilibrium. Above CMC, the adsorption is fast, and an equilibrium is reached very quickly. The one- and two-step models were employed to further depict the adsorption kinetics. As a result, the adsorption kinetics includes two steps: the adsorption of individual molecules occurs below CMC, whereas the adsorption of individual molecules and aggregates simultaneously takes place above CMC. Moreover, the flattened films containing numerous pores of sizes and thickness at 3-4 nm are formed at the silica/water surface for GnQPAMC12 with different oligomerization degrees. Among all of them, G3QPAMC12 forms a more compact quasi-fibrous film, while the dendrimer surfactants of lower-generation form loosely connected island-like structure. The limited studies on the self-assembly of star-shaped oligomeric surfactants on the water/solid interface suggest that their self-assembly on a solid surface are controlled by their oligomerization degree and concentration as well as surface structures. If the appropriate oligomerization degree and spacer structure of the surfactants can match the crystalline structure of the surface, the 17


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self-assembly can generate highly ordered and stable structures on the surface.

SELF-ASSEMBLY IN AQUEOUS SOLUTION Self-Assembly without Additives. The self-assembly of surfactants in aqueous solution is crucially important for the application of surfactants and is characterized by the aggregation ability and aggregate morphology. Both experimental research7,

10, 19, 20

and molecular dynamic

simulation49, 50 have indicated that with the increment of oligomerization degree, linear oligomeric surfactants enhance the aggregation ability, form richer aggregate morphology and optimize colloidal property. Compared with linear oligomeric surfactants, star-shaped oligomeric surfactants display more unique self-assembly behavior in aqueous solution. Menger et al.51 synthesized three families of multi-armed and multi-cationic surfactants, having pentaerythritol (PE, SS21a), adamantane cores (AD, SS21b), or dipentaerythritol (DPE, SS21c), respectively (Figure 4A). The surfactants with 4-carbon or 6-carbon chains are highly water-soluble but have no ability to self-assemble. The surfactants with 8-carbon chains form small micelles (< 3 nm diameter) at the concentration of 3-6 mM, and the CMC is much lower than that of the corresponding single-chain surfactant.52 Then the surfactants with 12-carbon chains are too water-insoluble to form micelles. Based on these phenomena, the authors put forward the geometry and connectivity as the key influencing factors of aggregate structures and presumed that the multi-armed surfactants can immerse itself in a hydrophobic environment to form a “dendritic-like” aggregate as illustrated in Figure 4B. But eventually the dendritic growth of the surfactants was not observed possibly due to chain pairing, chain looping, and associative ring formation (Figure 4C). Even so, they proposed that the surfactants must be imparted to a more water-solubilizing functional 18


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group than quaternary ammonium nitrogen to achieve larger micelles and even a network structure.

Figure 4. (A) The molecular structures of multi-armed and multi-cationic surfactants having pentaerythritol (PE, SS21a), adamantane cores (AD, SS21b), or dipentaerythritol (DPE, SS21c). (B) Schematic of the first steps in a possible “dendritic growth” of a multi-armed surfactant. (C) Highly schematic representation of a multi-armed surfactant micelle showing ring formation, chain pairing, and chain coiling, all of which serve to diminish dendritic growth.51 Reproduced from Ref. [51]. Copyright [1999] American Chemical Society. Inspired by the Menger’s hypothesis above, our group synthesized cationic ammonium star-shaped trimeric surfactants DTAD (SS10) and DDAD (SS11).30 In this case, the solubility of star-shaped surfactants are improved by replacing very rigid spacer with a more flexible amide-type spacer groups, while the long alkyl chain and quaternary ammonium group are kept. The two 19


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surfactants have comparable molecular structures, but the former is an entirely symmetric structure whereas the latter is asymmetric due to a slightly different spacer. Both form large vesicles in aqueous solution just beyond their critical aggregation concentration (CAC), following which the vesicles transform into small spherical micelles gradually with increasing surfactant concentration (Figure 5A). This result is different from the normal aggregation behavior of traditional surfactants and even from linear oligomeric surfactants. Normally surfactants form smaller aggregates at lower concentration but larger aggregates at higher concentration. We proposed that the formation of vesicles just above CAC is attributed to the loose packing of the alkyl chains triggered by rigidity of spacer and the electrostatic repulsion of headgroups. As the hydrophobic interaction of the alkyl chains becomes sufficiently strong to compel the molecular conformation to change into pyramid-like, the vesicles transfer into micelles correspondingly. From then on, the variation of aggregate morphologies along with the transition of molecular conformation has been considered as one of the specific features of star-shaped oligomeric surfactants. But there was still no evidence of network-like aggregates over the concentration range we studied. Furthermore, our group made great efforts to increase the oligomerization degree of star-shaped oligomeric surfactants up to six and investigated their self-assembly in aqueous solution,31, 32 in spite of the complications of their synthesis and purification. The increasing number of amphiphile moieties in the tetrameric and hexameric surfactant molecules PATC (SS12) and PAHB (SS13) results in a remarkable decrease in CMC values, which is at least an order of magnitude smaller than CMC values of cationic gemini surfactants. By comparing with some surfactants consisting of very similar cationic ammonium amphiphile moiety, it was clearly found that from single-chain to gemini, trimeric, tetrameric and hexameric surfactant, the enthalpy change of each amphiphilic 20


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moiety becomes more and more exothermic, which are respectively -1.01, -1.84, -6.3, -7.6 to -17.4 kJ/mol. Undoubtedly, the enhanced ability to aggregate with higher oligomerization degree is primarily ascribed to the larger contribution of inter- and intramolecular hydrophobic interactions from each alkyl chain.32 In addition, higher temperature, smaller counterion and longer alkyl chain were also used to enhance the aggregation ability of the star-shaped oligomeric surfactants (SS18 and SS22).39, 53 More interestingly, the large network-like premicellar aggregates in tetrameric and hexameric surfactant solutions (PATC and PAHB) were found and verified below CMC by the 1H NMR results of spin-spin relaxation time, 2D NOESY and cryogenic transmission electron microscopy (Cryo-TEM) eventually. The molecules of star-shaped oligomeric surfactants present a branched molecular conformation at very low concentrations as a consequence of the rigidity of their spacer groups and strong electrostatic repulsion between their cationic headgroups. As the oligomerization degree becomes large enough, the large network-like premicellar aggregates are formed in the PATC and PAHB aqueous solutions with this type of molecular conformation through intermolecular hydrophobic interaction among the alkyl chains. At higher concentration of surfactant, the stronger hydrophobic interaction makes the alkyl chains approach to each other closer, resulting in the formation of pyramid-like molecular conformation for PATC and transition from claw-like to pyramid-like conformation for PAHB. This kind of conformation transition induces the PATC premicelles to convert into small spherical micelles (Figure 5B) and leads the PAHB premicelles to transfer into larger spherical aggregates and then small spherical micelles (Figure 5C). Additionally, the elongation of alkyl chain is an alternative approach to induce the formation of premicellar aggregates. Within a series of trimeric star-shaped surfactants SS14 synthesized by Gao et al.,33 the 21


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premicellar aggregation can also be observed in the surfactant with 16-carbon chain33 and elongated vesicles transfer to smaller spherical micelles gradually at higher surfactant concentration. This unique aggregation behavior is also attributed to the transformation of surfactant conformations from stretched to compacted pyramid-like shape with increasing concentration driven by the strong hydrophobic interactions among the alkyl chains. The above works reveal that sufficiently high oligomerization degree and long alkyl chain are required to form network-like premicelles for star-shaped oligomeric surfactants with good water-solubility.

Figure 5. The possible variation of the molecular configuration and the related aggregate transitions for (A) trimeric DTAD and DDAD (SS10 and SS11), (B) tetrameric PATC (SS12) and (C) hexameric surfactants PAHB (SS13).30-32 Reproduced from Ref. [30-32]. Copyright [2010, 2011] 22


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American Chemical Society. Another interesting phenomenon for the self-assembly of star-shaped oligomeric surfactants in aqueous solution is that the effect of the spacer bulkiness on the rheological properties of the solution.34,

35

Kurata et al. synthesized star-shaped trimeric surfactants 3CntrisQ (SS15), which

consists of three cationic amphiphile moieties linked by tris(2-aminoethyl) amine as a core. The results of rheology, small angle neutron scattering (SANS) and Cryo-TEM indicate that 3C10trisQ and 3C12trisQ form ellipsoidal micelles at low concentration, and for 3C12trisQ the structures transform into thread-like micelles of very few branches with increasing the concentration. As for 3C14trisQ, the threadlike micelles form at much lower concentrations and no more transitions take place due to the limitation of the solubility at high concentration.34 For the charged wormlike micelles, the micellar growth is mainly controlled by the micellar surface charge and the end-cap energy of the micelles.54 The length of the micelle becomes longer with the high enough scission energy of a wormlike micelle, whereas the molecular structure affects the end-cap energy for wormlike micelles. The end-cap energy value of the 3C12trisQ wormlike micelles is 50.2 ± 1.14 (kBT), much higher than that for DTAB micelles (11.7 ± 1.24 (kBT)) and slightly higher than that for 12-2-12 micelles (41.4 ± 1.3 (kBT)). Thus, it can be concluded that the higher oligomerization degree enhances the end-cap energy and promotes the micellar growth. The probable reason is that the intramolecular motion is limited by more spacer chains. The end-cap energy of 3C12trisQ is also much smaller than that of linear oligomeric surfactant 12-3-12-3-12, because star-shaped surfactant can form a rounder structure than linear one and in turn result in a lower end-cap energy.35 In conclusion, the unique aggregation behavior of star-shaped oligomeric surfactants is dominated by branched topological structure, long alkyl chain length and high oligomerization degree, which 23


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determine the surfactant molecular conformation. Any factors that influence the molecular conformation can affect the self-assembly of these surfactants. Moreover, the micellar growth and improvement of rheological properties can be realized by increasing the degree of oligomerization or reducing the steric effect of spacer.

Self-Assembly with Additives. To further optimize the properties of star-shaped oligomeric surfactants, adding opposite charged organic salts, other surfactants, and polymers is one of the most effective methods. The additives can strongly affect the electrostatic interaction between the multi-charged headgroups and the hydrophobic interaction among the alkyl chains for star-shaped oligomeric surfactants, so that the transitions of molecular conformation are promoted.

Self-Assembly with Organic Salt. Kusano et al.36 studied the self-assembly of the mixture of trimeric star-shaped surfactants 3C12trisQ (SS15) and sodium salicylate (NaSal) in aqueous solution at various surfactant and salt concentrations. The 3C12trisQ solution undergoes a series of aggregate transitions from ellipsoidal micelles to rod-like micelles, wormlike micelles and multi-lamellar vesicles with the increase in the NaSal concentration (Figure 6). This aggregate growth is ascribed to not only the electrostatic screening of NaSal molecules to the charged headgroups of 3C12trisQ, but also the dehydration from the inner micelles by the penetration of NaSal into the micelles. Compared with gemini surfactant 12-2-12, the sphere-to-rod transition occurs at lower salt and surfactant concentrations, which is attributed to the increase in the end-cap energy introduced by trimeric surfactants.34,

35

These results demonstrate that the interactions between the oppositely

charges species are strongly enhanced in the star-shaped oligomeric surfactant-containing mixed systems, and the aggregate transition to large aggregates, especially to wormlike micelles, is 24


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remarkably promoted.

Figure 6. The aggregate transitions of the mixture of trimeric surfactant and sodium salicylate with increasing salt concentration.36 Reproduced with permission from Ref. [36]. Copyright [2016] Elsevier.

Self-Assembly with Single-Chain Surfactant. Adding an oppositely charged single-chain surfactant is another approach to further optimize the performance of oligomeric surfactants and has been shown to be effective by several works on linear oligomeric surfactant mixed systems.55, 56 So we mixed anionic single-chain surfactant sodium dodecyl sulfate (SDS) with hexameric surfactant PAHB (SS13, Figure 7B), expecting to adjust aggregation behavior of PAHB through affecting its molecular conformation. As mentioned above,32 increasing the PAHB concentration induces the transition of its molecular conformation from stretched star-shaped to claw-like and pyramid-like sequentially, thereby inducing the alteration from network-like premicelles into large spherical aggregates and small spherical aggregates correspondingly. The effects of SDS on the PAHB molecular conformation and the aggregates were studied, in which three typical PAHB concentrations were selected to initiate from the three different molecular conformations.57 Combining with isothermal titration microcalorimetry (ITC), turbidity, ζ-potential and conductivity measurements, the results indicate that the electrostatic repulsion between multiple charged 25


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headgroups of PAHB is greatly weakened while the hydrophobic interaction between the alkyl chains of PAHB and SDS is significantly enhanced due to the binding of SDS with PAHB. The variation in the PAHB molecular conformation is strongly promoted, so that either the stretched star-shaped or claw-like conformations converts into the pyramid-like conformation, producing small spherical aggregates firstly. Upon increasing the SDS concentration, the binding of PAHB with more SDS molecules cause the multi-charged surfactant molecules to be further neutralized and induce the hydrophobic interaction to become stronger. Being assisted by hydrogen bonding between the amide bonds on the PAHB spacer, large spherical fingerprint-like aggregates are formed (Figure 7A) and the molecules are closely packed. According to the peak position in small angle X-ray scattering (SAXS, q ≈ 0.15 Å-1) and Chem3D simulation, the layer thickness is slightly longer than length of one PAHB molecule but shorter than that of two PAHB molecules, indicating that the surfactant molecules adopt a model of interdigitated chain-packing. Moreover, the well-ordered and steady large aggregates with multilayer structures can be fabricated over a wide range of mixing ratios regardless of PAHB concentration and even with high positive or negative net charges (Figure 7C). Hence, this work demonstrates that a single-chain surfactant with oppositely charge is able to drive the variations in the molecular conformation and aggregate morphologies of star-shaped oligomeric surfactants, while strong synergistic interactions allow the further optimization in the performances of star-shaped oligomeric surfactants. In addition, Yoshimura et al.29 investigated the interaction between SDS and star-shaped cationic tetrameric surfactant C8qbG0 (SS9), which has four octyl chains and four ammonium groups. The multiple alkyl chains and multiple hydrophilic groups lead to strong interactions between SDS and C8qbG0, thus lead to a dramatic reduction of CMC and generate large aggregates. 26


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Figure 7. (A) Cryo-TEM images of the PAHB aggregates at PAHB concentrations cPAHB = 0.002 (a1), 0.500 (b1), and 5.00 mM (c1), and SDS/PAHB mixed aggregates that were formed in the presence of 0.04 mM PAHB at SDS/PAHB molar ratios R = 1.00 (a2), 6.00 (a3), and 10.00 (a4); 0.20 mM PAHB at R = 0.10 (b2), 3.00 (b3), and 12.00 (b4); and 2.00 mM PAHB at R = 0.30 (c2), 2.00 (c3), and 12.00 (c4). (B) Molecular structures of PAHB (SS13) and SDS. (C) Phase boundaries of SDS/PAHB mixtures and simplified models of the transitions of the PAHB molecular configurations and the SDS/PAHB aggregate at different SDS/PAHB molar ratios and PAHB concentrations.57 Reproduced with permission from Ref. [57]. Copyright [2016] Wiley. 27


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Self-Assembly with Polymers. The polymers which have been studied with oligomeric surfactants are limited to hydrophobically modified polymers (HM polymers) until now. HM polymers have amphiphilic properties in aqueous solutions and often display superior properties compared to their unmodified relatives. Particularly, mixing surfactants with HM polymers induce the formation of associated structures, and can improve solution and interfacial properties. Zana’s group58 studied the interaction of linear trimeric cationic surfactants with different spacer lengths (12-3-12-3-12 and 12-6-12-6-12) with hydroxypropyl guar (HPG) and its modified derivative (HMHPG), respectively. The effects of the corresponding single-chain surfactant DTAB and gemini surfactant 12-n-12 on the mixtures of HM polymers and surfactants were also comparatively studied. It was found that the apparent viscosity of the dilute mixtures increases with higher degree of oligomerization. Our group studied the effects of the star-shaped trimeric and hexameric cationic surfactants DTAD (SS10) and PAHB (SS13) on the self-assembly of anionic hydrophobically modified polyacrylamide (C12PAM).59 Figure 8 shows the plots of the observed enthalpy changes, turbidity and ζ-potential values of the surfactant/polymer mixtures as a function of the DTAD or PAHB concentration at a fixed C12PAM concentration of 0.3 g/L, and the scanning electron microscopy (SEM) images of C12PAM with DTAD or PAHB in different concentration regions. DTAD/C12PAM and PAHB/C12PAM exhibit similar changing profiles in the ITC, turbidity and ζ-potential curves, and both the mixtures form soluble network-like aggregates, denser crosslinked precipitated aggregates, and soluble spherical aggregates with the increase in the surfactant concentration. However, a smaller amount of PAHB molecules participate in the formation of smaller spherical aggregates compared with the DTAD/C12PAM mixture. The aggregate transitions are principally induced by the interactions between C12PAM and the oligomeric surfactants, and by the resultant 28


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change in the molecular conformation of DTAD and PAHB. Electrostatic binding between the quaternary ammonium headgroups of the oligomeric surfactants and the oppositely charged units of C12PAM, and hydrophobic association among the alkyl chains of the surfactants and polymers strengthen the formation of network-like aggregates of the polymer-bound oligomeric surfactants at very low concentrations. The resultant aggregate transitions are caused by the variation in the surfactant molecular conformation from stretched star-shaped to a pyramid-like. Although the studies on the interactions between star-shaped oligomeric surfactants and other oppositely charged species are still rare to date, the current reported works demonstrate that the self-assembly of individual star-shaped oligomeric surfactants have strong effects on the aggregation behaviors of the mixed systems due to the multiple headgroups and alkyl chains. Their own features, such as strong ability for micellar growth and the variation of molecular conformation during the aggregate transitions, are still reflected in the mixed systems. Meanwhile, oppositely charged additives further promote the self-assembly and the transition of molecular conformation of the star-shaped oligomeric surfactants and strengthen the characteristics of the self-assembling structures of the star-shaped oligomeric surfactants. These understandings provide helpful guidance to apply an oppositely charged component to adjust the self-assembly of star-shaped oligomeric surfactants and to take advantage of unique properties of star-shaped oligomeric surfactants to develop their applications.

29


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Figure 8. (A1 and B1) ITC curves for titrating 5 mM DTAD (SS10) and PAHB (SS13) into 0.3 g/L C12PAM solution (■) and water (□). Turbidity and ζ-potential as a function of DTAD (A2-A3) and PAHB (B2-B3) concentrations in the presence of 0.3 g/L C12PAM. Insets are molecular structures of DTAD, PAHB and C12PAM, as well as SEM images of the DTAD/C12PAM and PAHB/C12PAM aggregates.59 Reproduced from Ref. [59]. Copyright [2014] American Chemical Society.

FUNCTIONS As mentioned above, the oppositely charged additives can greatly weaken the electrostatic repulsion between multi-charged headgroups of star-shaped oligomeric surfactants or enhance the hydrophobic interactions between the alkyl chains. Thus, the characteristics of individual star-shape oligomeric surfactants are strongly promoted, leading to the various transitions of aggregate structures. Thereby, the strong self-assembly ability and unique aggregation behaviors of star-shaped oligomeric surfactants can influence the self-assembly of additives, especially for 30


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functional molecules, endowing them with enhanced functionalities. The following text will present the function explorations of star-shaped oligomeric surfactants. Stable Encapsulation. Encapsulation is one of the most important functions for surfactant aggregates, which contains both hydrophobic and hydrophilic domains simultaneously. Rosen et al.22 designed and prepared a series of atypical star-shaped trimeric surfactants bearing two hydrophilic head groups and three 16-carbon or 18-carbon chains (SS2c-C16, SS2c-C18). These surfactants have greater surface activity and stronger micellization ability than the corresponding gemini surfactants. The multiple alkyl chains in each molecule provide a positive contribution to the higher surface active and stronger aggregation behaviors attributed to the simultaneously strengthened inter- and intramolecular hydrophobic interactions. Afterwards, Sumida et al.60-62 comparatively studied the ability in suppressing the leakage of substances from vesicles made of SS2c and conventional phosphatidylcholines. By comparing the released percentage of 5(6)-carboxyfluorescein (CF) for the SS2c vesicles with the vesicles of the corresponding phospholipids bearing the same alkyl chain length (SS2c-C16 vs. DPPC and SS2c-C18 vs. DSPC, respectively), it was found that the SS2c vesicles are much more stable than the lipid vesicles.61 In particular, less than 10% of CF is released from the vesicles of SS2c with 18-carbon chains (SS2c-C18) after 12 months under the experimental conditions used. Three octadecyl chains in the vesicle bilayer with much tighter packing facilitate the long-term stability of the SS2c-C18 vesicles toward the leakage of trapped CF compared to other lipids. The high barrier effect of SS2c is attributed to the promoted surface charge of vesicles and enhanced hydrophobic interaction of the bilayer membrane. Furthermore, the SS2c vesicles are more stable against increasing temperature than the lipid vesicles. This is consistent with the fact that the microfluidity 31


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of bilayer membrane for SS2c is less sensitive to temperature change than that for phosphatidylcholines.60 Meanwhile, the SS2c-C18 vesicles show a pH-dependent release of trapped compounds owing to the carboxylate groups, while the DSPC vesicles do not show pH sensitivity.62 The SS2c-C18 vesicles only release less than 10% CF at pH 7.5 after 1 year, but almost fully release the trapped CF into the bulk phase below pH 3.5-4.0. The latter result is relevant to the complete protonation of the carboxylate groups and alteration in the aggregate structures of SS2c-C18. Disassembly of Amyloid Fibrils. β-Amyloid (Aβ) deposits were proposed as the fundamental cause of Alzheimer's disease (AD), so decreasing or eliminating neurotic plaques composed of fibrillar β-amyloid might be one of the potential strategies to prevent or cure AD. During the exploration of various effective and safe approaches to disassemble fibrils, we have found that cationic ammonium gemini surfactant can regulate the fibrillogenesis63 and effectively disassemble mature Aβ(1-40) fibrils in vitro, but the surfactant concentration above CMC is required.64 Positive charges and strong self-aggregation ability are two key factors for the surfactants to disassemble Aβ fibrils. Based on the above understanding, star-shaped tetrameric quaternary ammonium surfactant PATC (SS12), which has a very low CMC (0.08 mM) and can form large network-like aggregates even below its CMC, was applied to induce Aβ fibril disaggregation.65 The effect of PATC on the disaggregation of Aβ(1-40) fibrils was studied either below or above the CMC at pH 7.4. The thioflavin T (ThT) fluorescence was utilized to monitor the variation of the Aβ(1-40) fibril contents in the presence of 12-6-12 monomers, 12-6-12 micelles, PATC premicelles and PATC micelles. Fourier transform-infrared (FT-IR) spectroscopic technique and AFM were used to test the secondary structure and morphology change of the Aβ(1-40) fibrils in the disaggregation process, 32


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respectively. Cationic gemini surfactant 12-6-12 was selected to be the reference compound. Parts of the results are shown in Figure 9. The results show that mature Aβ(1-40) fibrils can be disassembled effectively and thoroughly by either PATC premicelles below CMC or PATC micelles above CMC. The Aβ(1-40) fibrils is disrupted by PATC and smaller mixed aggregates are formed eventually. Moreover, the disassembling ability of PATC keeps strong even at the PATC concentration of 0.02 mM, while 12-6-12 has already lost disassembly efficacy at this concentration and even just below its CMC (1 mM). The strong self-assembling ability of PATC below CMC is considered as the key factor in the fibril disassembly. This finding reveals an effective approach to disassemble the Aβ(1-40) fibrils. Although cationic surfactants cannot pass through the blood brain barrier, this work provides helpful guidance to molecular design in constructing efficient disassembly agents to Aβ fibrils and other stable aggregates.

Figure 9. (A) Molecular structures of 12-6-12 and PATC (SS12). (B) ThT fluorescence results showing the dependence of the Aβ(1-40) fibril content on time in the presence of 0.02 mM 12-6-12, 1.16 mM 12-6-12, 0.02 mM PATC and 1.16 mM PATC. (C) AFM images (5 × 5 µm2) for the mixtures of Aβ(1-40) fibrils with 12-6-12 and PATC at the different time intervals: (A1, A2) 12-6-12 (0.02 mM); (B1, B2) 12-6-12 (1.16 mM); (C1, C2) PATC (0.02 mM); and (D1, D2) PATC (1.16 33


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mM). (D) The proposed mechanism of the PATC premicellar aggregates disassembling the Aβ(1-40) fibrils.65 Reproduced from Ref. [65]. Copyright [2011] American Chemical Society. Condensation of DNA. Cationic gemini surfactants are widely considered as one kind of potential alternatives as nonviral condensing agents for gene delivery.66 The unique structures of gemini surfactants enhance their binding ability to DNA at very low concentration, endowing low toxicity and stronger transfection ability. Based on the performances of gemini surfactants for DNA delivery, it can be expected that the efficiency of surfactants in DNA condensation may be enhanced by increasing the number of amphiphilic moieties in surfactant molecules. Hence, star-shaped oligomeric surfactant (PAHB, SS13) bearing six cationic amphiphile moieties was chosen to study the effect of surfactants with higher oligomerization degree on the conformation of calf thymus DNA and the properties of PAHB/DNA complex.67 The various techniques were used to study the cooperative behaviors and their mechanism, including dynamic light scattering (DLS), ζ-potential, ITC, AFM, circular dichroism (CD), and ethidium bromide exclusion assay. The cytotoxicity of the PAHB/DNA complexes were also evaluated with normal and cancer cells. The DNA molecules display numerous successive transitions during condensation by the electrostatic and hydrophobic interactions with PAHB, i.e., from coil state to partially condensed states (including loose cluster-like aggregates, globules-on-a-string structure and spherical aggregates), and completely condensed state finally (Figure 10A and 10B). More importantly, the secondary structure of DNA does not show notable change and the PAHB/DNA complexes do not display cytotoxicity for condensed DNA, although the DNA conformation is changed by PAHB significantly (Figure 10C and 10D). Obviously, the increase in the degree of oligomerization of surfactants is positive to improve the efficiency of surfactants in DNA condensation while weakening the cytotoxicity and 34


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damage of the DNA secondary structure during the process. The unique manner of DNA condensation and excellent property of PAHB/DNA complexes are also ascribed to the variations in molecular conformation and aggregate morphology of the star-shaped oligomeric surfactant motivated by the highly cooperative association between the special surfactant and DNA. Along with the binding, the less-charged DNA chains are prone to bind with each other, while the alkyl chains of the DNA-bound PAHB molecules tend to interact with each other. As the PAHB molecules bound onto the DNA chains become more and more, electrostatic repulsion between each DNA chain is strongly weakened and hydrophobic interaction among the PAHB alkyl chains adhesive on the DNA chains is significantly enhanced. Then the association of DNA chains occurs and PAHB conformation varies from stretched star-shaped to claw-like and compact pyramid-like. Consequently, the PAHB/DNA complexes transfer from loose cluster-like aggregates to globules-on-a-string structure and then spherical aggregates sequentially.

Figure 10. (A) DNA morphologies observed by AFM for the PAHB/DNA mixed systems of 0.15 mM DNA with PAHB at different concentrations and (B) illustration of the mechanism of PAHB-induced DNA condensation. (C) Circular dichroism spectra for the PAHB/DNA mixed 35


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solution. (D) Cell viability of MCF-7 cells after 24 h and 48 h incubation, and HaCaT cells after 48 h incubation with the PAHB/DNA mixed solutions at 0.03 mM DNA and different PAHB/DNA charge ratios.67 Reproduced from Ref. [67]. Copyright [2017] American Chemical Society. Reconstitution of Lipid Vesicle. Surfactants can also be used to realize the reconstitution and purification of membrane proteins and lipids in a native environment,68-70 so knowledge of lipid−surfactant interaction is a key for this. Surfactant molecular structures are the crucial factor for the interaction between lipid and surfactant and to regulate the effective molar ratio of surfactant to lipid (Re) in the interaction. At very low Re, lipid−surfactant interaction is a partition process of surfactant monomers between lipid bilayers and aqueous phase; While at larger Re, the lipid−surfactant interaction becomes a solubilization process of lipid vesicles by surfactants. Our previous reports3, 71 have illustrated that gemini surfactants are more effective in the solubilization of lipid vesicles than the corresponding single-chain surfactant. Compared with single-chain and gemini surfactants, surfactants with higher oligomerization degree have more alkyl chains and charged headgroups, and thereby exhibit more various molecular conformations and unique morphologies of aggregate. Thus, studying the interactions between star-shaped oligomeric surfactants and lipid vesicles is significant to reconstitute lipid vesicles and promote the related biological applications. Our

group72

have

systematically

investigated

the

interactions

of

the

1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicles with cationic single-chain surfactant (DTAB), gemini surfactant (12-3-12), trimeric surfactant (DDAD)30 and anionic single-chain surfactant (SDS), gemini surfactant (C12C3C12(SO3)2)73, trimeric surfactant (TED-(C10SO3Na)3, SS23) in aqueous solution (Figure 11A). Meanwhile, the resultant effects of the topological 36


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structure of star-shaped oligomeric surfactants on the interactions have explored as well. The partition coefficients of the surfactants between the lipid bilayers and aqueous phase, and the solubilization process of lipid were studied by two separate ITC experiments from the thermodynamic aspect, and the turbidity and DLS were used to obtain the corresponding aggregate transitions during the solubilization process. It was found that the increment of oligomerization degree significantly increases the partition coefficient of the surfactants between the lipid bilayers and aqueous phase, and enhances the permeability of the lipid membrane, which is proved by more negative enthalpy change and Gibbs free energy for the surfactant transition from water to bilayer and the promotion of dye released from the DOPC vesicle (Table 2). Meanwhile, the lipid solubilizing power of the surfactants is evaluated by the slopes of the phase boundaries of the critical concentrations for the solubilization against the DOPC concentration from onset (Dtsat) to end (Dtsol), i.e., the effective molar ratios of surfactant to lipid corresponding to the onset (Resat) and end (Resol) of solubilization. Lower Re means that the ability to solubilize lipid vesicles is stronger. The intercepts are the equilibrium concentrations of surfactant monomer at the boundaries, designated as Dwsat and Dwsol. Clearly, the parameters (Table 2) indicate the increment of oligomerization degree is beneficial in the solubilization of DOPC vesicles for either anionic or cationic oligomeric surfactants. As illustrated in Figure 11B, these phenomena can be interpreted by the structure and shape of the surfactant molecules. More charged headgroups and alkyl chains of oligomeric surfactants produce stronger hydrophobic and electrostatic interactions with the lipid vesicles, and stronger ability to incorporate into the lipid bilayer. The multiple alkyl chains of the oligomeric surfactants can form a more efficiently packed shape with smaller packing parameter like a cone, which has more free volume and more obvious differences from the situation of lipids. 37


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In this case, a more remarkable disturbance is exerted to the lipid bilayers, i.e., the ability in disrupting and solubilizing lipid vesicles becomes stronger. Therefore, the higher degree of oligomerization endows surfactants with stronger ability in improving permeability of lipid membrane and solubilization of lipid vesicles.

Figure 11. (A) Chemical Structures of DTAB, 12-3-12, DDAD (SS11), SDS, C12C3C12(SO3)2 and TED-(C10SO3Na)3 (SS23). (B) Illustration of the molecular packing of oligomeric surfactants and their interactions with lipid bilayers.72 Reproduced from Ref. [72]. Copyright [2016] American Chemical Society.

Table 2. The Partition Coefficient (P) and Thermodynamic Parameters for the Partitioning of the Surfactants between DOPC Bilayer and Water (∆Hb/w, ∆Gb/w and T∆Sb/w), the Effective Surfactant to DOPC Molar Ratio for the Onset (Resat) and End (Resol) of the DOPC Solubilization, and the Surfactant Monomer Concentration (Dwsat and Dwsol) at the Phase Boundaries for the Interaction of Surfactants with DOPC Vesicles.72 Reproduced from Ref. [72]. Copyright [2017] American Chemical Society. Surfactant

P×104

∆Hb/w (kJ/mol)

∆Gb/w (kJ/mol)

T∆Sb/w (kJ/mol)

Resat

Resol

Dwsat

Dwsol

CMC (mM)

DTAB

0.40

-2.10

-21.5

19.4

2.33

6.41

10.0

13.9

14.1

38


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12-3-12 DDAD

1.76 2.30

-23.8 -90.2

-24.2 -24.9

0.50 -65.3

0.35 0.05

1.30 0.21

0.68 0.03

6.40 0.21

1.10 0.21

SDS C12C3C12(SO3)2 TED-(C10SO3Na)3

0.58 2.70 15.0

-22.6 -23.5 -25.4

-21.5 -25.5 -29.5

-1.10 2.00 4.10

1.84 0.07 0.04

9.20 0.22 0.16

5.88 0.06 0.03

7.61 0.72 0.24

8.40 0.08 0.16

Antibacterial Activity. Cationic surfactants have been widely used as antimicrobial agents in detergents, cosmetics, disinfectants, etc. Many surfactant structures continue to be designed to target the increase in their effectiveness and specificity. Star-shaped oligomieric surfactants have shown very high antimicrobial activity as described below. Caran et al.74 synthesized two series of triple-headed, double-tailed atypical trimeric surfactants connected by a mesitylene (M) core and studied the effects of alkyl chain lengths on their antimicrobial activity. One of the three charged headgroups is either trimethylammonium (SS24a) or pyridinium (SS24b). The minimum inhibitory concentration (MIC) values of the two series of surfactants were measured for four Gram-positive strains, i.e., Staphylococcus aureus, Streptococcus agalactiae, Enterococcus faecalis, and Bacillus cereus, and two Gram-negative strains, i.e., Pseudomonas aeruginosa and Escherichia coli. For both cases, the MIC value decreases with elongating the alkyl chain length until 12-carbon chain as an optimal value, above which the MIC increases. More especially, the compounds with 12-carbons chain can kill P. aeruginosa at relatively low concentrations (8 µM SS24a-C12 and 16 µM for SS24b-C12). This bacteria is very difficult to be killed for many antibacterial agents.75 Notably, the MIC values of the two surfactants against this organism are comparable to those of tobramycin and cefepime, which are normally utilized to treat infection in cystic fibrosis patients and an antipseudomonal cephalosporin, respectively. The enhancement of antibacterial activity should be related to the increase of cationic headgroup numbers.76 It was also found that the replacement of the trimethylammonium with a pyridinium as 39


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headgroup does not have obvious effect on the bioactivity, and the surfactant aggregation is not an necessary condition to kill these bacteria.77 Recent studies demonstrated that the incorporation of amide linkages is a positive element to improve antibacterial activity and biocompatibility of surfactants.78,

79

Our group80 studied the

effects of the oligomerization degree on the antibacterial activities and antibacterial mechanism to Gram-negative E. coli for star-shaped cationic ammonium oligomeric surfactants. Cationic trimeric, tetrameric and hexameric surfactants bearing amide linkages, i.e., DTAD (SS10), PATC (SS12), and PAHB (SS13), were applied, and their cytotoxicity was evaluated with mammalian cells as well. The results show that the micelles made up of the cationic star-shaped oligomeric surfactants are highly effective against E. coli but nontoxic to mammalian cells under the same conditions (Figure 12A). The MIC value of the three oligomeric surfactants against Gram-negative E. coli is in the range of 1.70-0.93 µM, much lower than the corresponding single-chain surfactant and gemini surfactant, and their antibacterial activity becomes stronger with increment of oligomerization degree from trimeric to hexameric. The surfactants with higher oligomerization degree bear more amphiphile moieties and have stronger ability to form micelles, resulting in the stronger interaction with bacterial cell from its outer to inner membranes. The ITC curves, ζ-potential values and SEM images reveal that these cationic micelles killing E. coli experiences two steps: (1) Disrupt the integrity of outer membrane due to the electrostatic interaction between the cationic micelles and the oppositely charged surface of E. coli, resulting in the damage of the barrier function; (2) Disintegrate the inner membrane of E. coli because of the hydrophobic interaction among the alkyl chains of surfactants and the lipids in E. coli membranes, leading to the cytoplast leakage and the death of bacteria eventually. These results are broadly consistent with the conclusions on the 40


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interactions of the star-shaped oligomeric surfactants with lipid membrane discussed above.72

Figure 12. (A) Antibacterial activity of DTAD (SS10), PATC (SS12), and PAHB (SS13) toward E.

coli, cell viability of Hela cells after incubation with the aqueous solutions of these surfactants at different concentrations, and schematic graph of antibacterial mechanism of the surfactant micelles to E. coli.80 (B) Schemes of the CD/DTAD complexes, antibacterial activity toward E. coli of DTAD and CD/DTAD mixtures, and binding constants (Kb) of the interaction of DTAD or CD/DTAD complexes with zein.81 Reproduced from Ref. [80, 81]. Copyright [2016] American Chemical Society. Furthermore, α-CD, β-CD and γ-CD were applied to form inclusion complexes with the above cationic star-shaped trimeric surfactant DTAD (SS10) to optimize the mildness of the surfactant and keep its high antibacterial activity.81 The multiple alkyl chains of DTAD facilitate the inclusion by 41


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the CDs, and the hydrogen bonds between the hydroxyl groups on the rim of the CD cavity and the amide groups of DTAD are also favorable for the binding. Moreover, the mode of the interaction between CDs and DTAD is significantly relevant to the sizes of CD cavity and the asymmetry of DTAD spacers. As a result, four types of complexes, including α-CD@DTAD, 2α-CD@DTAD,

β-CD@DTAD and γ-CD@DTAD, are formed by the CD/DTAD mixtures (Figure 12B). Although the formation of these inclusion complexes results in the decrease in the charge density and the alkyl chain number, the high activity against Gram-negative E. coli are still exhibited by these CD/DTAD complexes. The reason is that the ability in the formation of the CD/DTAD complexes is improved significantly by the incorporation of CDs, while the electrostatic interaction between DTAD headgroups and the negatively charged surface of E. coli could synergistically act with the hydrogen bonds between the hydroxyl groups of CDs with E. coli. Their MIC values are in the range of 2.22-2.48 µM, and the activity of these complexes are comparable to that of DTAD itself (1.70 µM). Furthermore, these CD/DTAD complexes display much weaker ability in interacting with and solubilizing zein (a skin model protein) than DTAD, because lower CAC of these CD/DTAD complexes reduces the concentration of surfactant monomers and the formation of large aggregates just above CAC enhances the steric repulsion of the complexes to zein backbone. The mildness of these different CD/DTAD complexes follows the order of 2α-CD@DTAD > β-CD@DTAD >

γ-CD@DTAD > α-CD@DTAD. Interfacial Lubrication. As introduced above, star-shaped cationic trimeric surfactant DTAD (SS10) forms highly ordered self-assemblies on mica surface.42 Interestingly, we found that this property can be utilized to reduce the interfacial friction. We studied the relationship between the surface structures of DTAD on mica and the interactions of two DTAD-coated surfaces by AFM 42


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and surface force balance (SFB) (Figure 13). When the DTAD concentration is five times CAC, i.e., 3 mM, the mica surfaces are coated with the larger bilayer vesicles and wormlike micelles/hemimicelles (∼ 80 nm) formed by DTAD molecules. Repulsive normal interactions between the surfaces demonstrate the presence of a net charge and the ion concentration is close to CAC as expected. Moreover, this coated surface is strongly lubricating up to some tens of atmospheres, ascribed to the hydration−lubrication mechanism acting at the charged, exposed and highly hydrated headgroups of DTAD. The friction force is extremely low (0.16 ± 0.3 µN) and the average friction coefficient µ is reduced to (13 ± 9) × 10−5 under ∼ 25 atm. With replacing DTAD with water, the self-assembling structures of surface are substituted with a hydrophobic DTAD monolayer both in water and 0.1 M salt solution, which jump into adhesive contact on approach. These monolayers are rather unusual since they are charged and hydrophobic simultaneously and can stay stable over a few days even across the salt solution, which is ascribed to the several available stabilization approaches for DTAD on the mica surface. This work indicates that the star-shaped trimeric DTAD is a very good candidate as an interfacial lubricant.

43


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Figure 13. (A) Chemical structure of DTAD (SS10) and schematic illustration of the surface configuration (not to scale) equivalent to the actual lens/spring assembly used in the SFB. (B) AFM scan of mica under a 3 mM DTAD solution. (C) Force Fn(D)/R vs distance (D) profiles between two mica surfaces across pure conductivity water or a 3 mM DTAD solution, and schematic of the mica and the adsorbed surfactant as wormlike micelles and vesicles. (D) Summary of the shear forces measured between two mica surfaces across a 3 mM DTAD solution at different loads and pressures upon compression or decompression.82 Reproduced from Ref. [82]. Copyright [2016] American Chemical Society.

CONCLUSIONS AND PERSPECTIVES This feature article has attempted to review recent works on a novel class of surfactants, i.e., the star-shaped oligomeric surfactants, from basic research of their self-assembly to various potential chemical or biological functions. In principle, the star-shaped oligomeric surfactants adopt a stretched and branched symmetric or asymmetric three-dimensional molecular conformation. The multiple alkyl chains produce strong intermolecular hydrophobic association. Moreover the strong intermolecular hydrophobic interaction can overcome the rigidity of spacer groups and electrostatic repulsion of charged headgroups by increasing concentration or by the assistance of other additives and thus lead to the change of molecular conformation. These structure characteristics bring about unique self-assembly behaviors at interface and in bulk solutions. Different from one-dimensional structural extension from single-chain to gemini and linear oligomeric surfactants, the increment of oligomerization degree does not always facilitate the improvement of the surface activity at the air/water interface. In contrast, the self-assembly of star-shaped oligomeric surfactants on solid 44


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surface and in solution becomes very special due to the higher oligomerization degree. More interestingly, star-shaped oligomeric surfactants can form large network-like aggregates far below the CMC when the alkyl chain length and the oligomerization degree are large enough. As the molecules change from a stretched conformation to a closed pyramid-like conformation, the large aggregates transfer into smaller aggregates. Based on these unique structures and self-assembly behaviors, as explored preliminarily in the recent few years, the star-shaped oligomeric surfactants have displayed fantastic performances in the fields of stable encapsulation, disassembly of amyloid fibrils, condensation of DNA, reconstitution of lipid vesicles, antibacterial activity and interfacial lubrication. Overall, star-shaped oligomeric surfactants represent an attractive and powerful class of amphiphilic compounds by their enhanced properties compared with conventional single-chain surfactants and gemini surfactants. However, so far both the basic research and functional exploration remain very limited as we described. Systematic and extensive investigations are highly desired, such as developing the structures by adding various functional groups, setting up more convenient and less-cost building methods, understanding the self-assembly at different conditions (pH, temperature, various organic, inorganic additives, etc.), and exploring functions in more extensive fields. It is expected that breakthroughs will arise from such an extensive research and some practical applications will be realized in the near future.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] 45


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ORCID Yaxun Fan: 0000-0003-0057-0444 Yilin Wang: 0000-0002-8455-390X Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT We are grateful for financial support from National Natural Science Foundation of China (Grants 21603239, 21633002, 21761142007).

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Biographies

Yaxun Fan has been an associate professor in Institute of Chemistry, Chinese Academy of Sciences (ICCAS) since 2018. She received her B.Sc (2006) and M.Sc (2009) in physical chemistry from Yangzhou University in China, and her Ph.D. (2012) in physical chemistry from ICCAS. She worked as a postdoc in ICCAS in 2012-2014 and University of Massachusetts, Amherst in 2014-2015. Her main research interests include self-assembly behavior of oligomeric surfactants with and without additives, construction and application of simple coacervation, and phase behavior of surfactant-containing systems in applied products.

Yilin Wang has been a full professor in Institute of Chemistry, Chinese Academy of Sciences (ICCAS) since 2002 and a professor in University of Chinese Academy of Sciences since 2014. She received her B.Sc (1988) and M.Sc (1991) from Lanzhou University, and Ph.D (1997) from ICCAS. Then she undertook postdoctoral research in University of Florida and Indiana University-Purdue University at Indianapolis (1997-2001). She was granted “Hundred Distinguished Young Scholars” 57


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by CAS (2002) and “Distinguished Young Scientists” by National Natural Science Foundation of China (2010). Her research focuses on development and applications of novel surfactants, and the interactions and phase behaviors of surfactants with polymers and biomacromolecules. She has published more than 160 papers and obtained 16 patents.

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