Physicochemical Properties of Noncovalently Constructed Sugar

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Physicochemical Properties of Noncovalently Constructed SugarBased Pseudogemini Surfactants: Evaluation of Linker Length Influence Xue Min Liu, Xiong Liao, Shi Hui Zhang, Shuo Chang, Lin Cheng, Meng Ge, and Xin Ge* School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China

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S Supporting Information *

ABSTRACT: Natural renewable glucose or lactose upon reaction with dodecylamine has been converted into N-dodecylglucosylamine or N-dodecyllactosylamine, which upon acid−alkali reaction with dicarboxylic acid (HOOC(CH2)n‑2COOH, n = 3,4,5,6,8) gives a series of sugar-based pseudogemini surfactants (G-n, L-n, respectively). Some physicochemical properties such as the critical micelle concentration (CMC), equilibrium surface tension at the CMC (γCMC), effectiveness (πCMC), efficiency (pC20), maximum surface excess (Γmax), minimum surface area (Amin), counterion binding of micelles (β), and the changes of standard Gibbs free energy (ΔG0m), enthalpy (ΔH0m) and entropy (ΔS0m) for processes of micellization in the range 298.15 K to 328.15 K have been evaluated by surface tension and electroconductometry methods in aqueous solutions of these pseudogemini surfactants. The results revealed that most of the above properties depend on dicarboxylic acid linker length and headgroup saccharide size. These findings help with understanding the structure-properties relationships of surfactants so as to construct new pseudogemini surfactants.

1. INTRODUCTION Pseudogemini surfactants (also named as Gemini-like surfactants or counterion-coupled gemini surfactants), fabricated by noncovalent interactions, such as hydrogen bonding, electrostatic attraction, host−guest recognition, charge transfer interaction, metal coordination, etc., have attracted increasing attention. Compared with the conventional gemini surfactants through covalent bonds, pseudogemini surfactants can avoid complicated synthetic and purification procedures, which are widely applied in the household and industry fields. In recent years, there have been some reports in the literature on the fabrication of pseudogemini surfactants. One way to construct pseudogemini surfactants is mixing traditional single-chained surfactants with gemini-type molecules. Pahi et al.1 synthesized a novel counterion-coupled gemini surfactant via the 2:1 coupling reaction between 4-(2-dodecyl)benzenesulfonic acid and polypropylene glycol-bis(2aminopropyl)ether. They found that both critical micelle concentrations and surface saturation are one order lower than those of conventional surfactants. Yu et al.2 constructed a new pseudogemini surfactant with a traditional single-chained anionic surfactant sodium dodecyl sulfate (SDS) and a dicationic “Gemini-type” organic salt 1,2-bis(2-benzylammoniumethoxy) ethane dichloride (BEO) through intermolecular electrostatic binding between the two-charged “Gemini type” organic salt and oppositely charged SDS, assisted by the hydrophobic interaction between the hydrocarbon chains of SDS and the π−π interaction between the benzene rings of BEO. For the SDS/BEO solution with a molar ratio of 5:1, they observed a series of aggregate transitions upon increasing © XXXX American Chemical Society

the total SDS and BEO concentration, for example, large loose irregular aggregates, vesicles, and long thread-like micelles. Zhu et al.3 have fabricated the pseudo-oligomeric surfactants through noncovalent interactions with a traditional singlechained anionic surfactant sodium dodecyl sulfate and oligomeric “gemini-type” organic salt. In the aqueous solution of which can form vesicles, spherical micelles, and threadlike micelles with the increase of concentration at pH 3.0. Zhang et al.4 also constructed a pseudogemini surfactant from the commercially available anionic surfactant sodium dodecyl sulfate and the protonated N,N,N′,N′-tetramethyl-1,3-propanediamine molecule with a precise stoichiometric ratio of 2:1, this mixture can be reversibly changed between viscoelastic and low-viscosity fluids through several cycles of alternate bubbling and removing CO2. Sun et al.5,6 reported a new kind of pseudogemini and linear pseudo-oligomeric surfactants, formed by sodium dodecyl benzenesulfonate (SDBS), butane1,4-bis(methylimidazolium bromide), and butane-1,4-bis(methylpyrrolidinium bromide), a linear tricationic imidazolium bromide salt by noncovalent interactions, and found that this kind of pseudogemini surfactant can form vesicles, and their phase behaviors also show intense concentration and molar ratio dependence. Tang et al.7 found that mixing anionic single chain surfactant sodium lauryl ether sulfate containing three ether groups (SLE3S) with positively bicharged organic salt 1,2-bis(2-benzylammoniumethoxy)ethane dichloride Received: June 3, 2018 Accepted: November 22, 2018

A

DOI: 10.1021/acs.jced.8b00459 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Specification of Chemical Samples chemical name glucose lactose monohydrate 1-dodecyl amine malonic acid succinic acid glutaric acid adipic acid suberic acid ethanol methanol cyclohexane ultrapure water N-dodecyl glucosylamine

source Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Energy Chemical Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Jiangnan University synthesis

N-dodecyl lactosylamine

synthesis

G-n and L-nc

construction

initial mole fraction purity

CAS no.

purification method

final mole fraction purity

>0.99

50-99-7

none

>0.99

14641-93-1

none

>0.995

124-22-1

none

>0.98

141-82-2

none

>0.995

110-15-6

none

>0.99

110-94-1

none

>0.995

124-04-9

none

>0.99 >0.998

505-48-6 64-17-5

none none

>0.995

67-56-1

none

>0.97

110-82-7

none

18.2 MΩ·cm, 298.15K

7732-18-5

none recrystallization

0.994

recrystallization

0.997

>0.99

analysis method

amine value measurement, 1 H NMRa,IRb amine value measurement, 1 H NMR, IR 1H NMR

a

Nuclear magnetic resonance. bInfrared spectroscopy. cn = 3,4,5,6,8.

switchable pseudogemini wormlike micelle system composed of N-erucamidopropyl-N,N-dimethylamine and maleic acid in a molar ratio of 2:1 by electrostatic attraction. Noori et al.16 investigated the surface active properties of counterion coupled gemini surfactant from N,N-dimethylalkylamine and adipoyl chloride. Zhang et al.17 developed novel pH-reversible wormlike micelles with characteristics of a facile, convenient, cost-effective switchable process by simply mixing N-(3(dimethylamino)propyl)palmitamide (PMA) and citric acid (HCA) at a molar ratio of 3:1. PMA and HCA groups exhibit excellent thickening ability and high pH sensitivity. Lu et al.18 constructed a pseudogemini surfactant with two types of effective pH-responsive groups by mixing the N,N-dimethyl oleoaminde-propylamine and disodium oxalate in water with a molar ratio of 2:1. The system exhibits three appearance states of transparent solution just like water, fluid with high viscoelasticity, and solution with white precipitate and low viscosity with increasing pH. Recently, our research group has also reported aggregate morphology transformation in salt-free pseudogemini surfactant systems.19 As is well-known, sugar possesses a polyhydroxyl-hydrophilic structure. Sugar-based surfactants from renewable resources sugar compounds are gaining increased attention due to the advantages with regard to performance, health of consumer, and environmental compatibility compared to some the conventional surfactants. The sugar-based gemini surfactants are made from monosaccharides or disaccharides or their derivatives.20−30 In addition, sugar-based gemini surfactants with a disaccharide spacer have been synthesized by Menger and Mbadugha.31 Despite all these reports, as far as we are aware, the studies about noncovalently constructed sugarbased pseudogemini surfactants have been comparatively few and unsystematic, and thereby the physicochemical properties

(BEO) could form large, loosely arranged spherical aggregates, closely packed spherical aggregates, and long threadlike micelles with an increase in the BEO/SLE3S concentration while fixing the BEO/SLE3S mixing molar ratio at 0.25. Li et al.8 constructed a novel pseudogemini surfactant with bolatype dicarboxylic acid (sebacic acid, SA) and zwitterionic surfactant tetradecyldimethylamine oxide, driven by electrostatic interaction and hydrogen bonding, and observed the formation of a variety of bilayer structures including unilamellar vesicles, onions, and open and hyperbranched bilayers. Rose et al.9 found a pH-switchable pseudogemini surfactant in mixing cationic single chain surfactant cetylpyridinium chloride and phthalic acid system. Tang et al.10 constructed a series of gemini-like surfactants by using dicarboxylic acid sodium salts (NaOOC(CH2)n‑2COONa, CnNa2, n = 4,6,8,10,12) and cationic ammonium single-chain surfactant cetyltrimethylammonium bromide (CTAB). These surfactants exhibit lower critical micelle concentrations (CMC) than that of CTAB, and the CMC values of the CTAB/CnNa2 mixtures decreases gradually with increasing spacer chain length. Another way to construct salt-free pseudogemini surfactant is choosing two proper nonsurface-active compounds. Zhou et al.11 fabricated an anionic pseudogemini surfactant from nonsurface-active compounds oleic acid and diethylenetriamine, its aggregate morphologies transform from the micrometer-sized giant vesicles via the coexistence of small vesicles and bilayers to a network formed by the aggregated vesicles. Antonietti et al. 12,13 synthesized a series of pseudogemini surfactants with oxalate, fumarate, terephthalate, tartrate and citrate as counterions. These surfactants are found to form very stable microemulsions with very large specific interface areas. Feng et al.14,15 have developed a novel pHB



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observed at the air/aqueous solution interface and/or in aqueous solution are worth reporting. Herein we report a systematic surface active properties study of a salt-free pseudogemini surfactant constructed with nonsurface-active sugar-based compounds, N-dodecyl glucosylamine or Ndodecyl lactosylamine, and a series of dicarboxylic acids (HOOC(CH2)n‑2COOH, n = 3,4,5,6,8). This work compares these pseudogemini surfactants to the conventional gemini surfactants,32−34 thus revealing the relationship of the solution behavior and spacer length of salt-free sugar-based pseudogemini surfactants constructed by noncovalent bonds.

99.73% according to the amine value measurement. The NMR and IR spectra are shown in Figures S8 and S9. To 2.0 mmol of N-dodecyl lactosylamine in 50 mL of methanol was added 1.0 mmol of dicarboxylic acid (malonic acid, succinic acid, glutaric acid, adipic acid, and suberic acid). The mixture was first heated to 323.15 K until dicarboxylic acid and N-dodecyl lactosylamine were completely solved, then stirred at room temperature for 48 h to give a homogeneous solution. After the solvent methanol was evaporated at 313.15 K under vacuum, the N-dodecyl lactosylamine pseudogemini surfactants were obtained. They were named L-3, L-4, L-5, L-6, and L-8, respectively. The NMR spectra are shown in Figures S10−S14. 2.4. Surface Tension Measurements. The surface tension (γ) was measured at 298.15 ± 0.1 K using the pendant drop method on an optical contact angle measuring instrument (OCA40, Dataphysics, Germany), which was calibrated by ultrapure water before measurement. The resolution of this instrument was ±0.01 mN·m−1 and the surface tension measurement of each solution was repeated three times. 2.5. Electroconductometric Measurement. The electroconductivity (κ) was measured with a conductivity meter (FE30, Mettler Toledo instruments (Shanghai) Co. Ltd.) at the range of 298.15−328.15 K using DC-2006 refrigerated/ heated water circulating to control the temperature with an accuracy of ±0.1 K, which was calibrated by 0.01 mol·kg−1 of KCl solution before measurement. The resolution of this electroconductivity meter was ±0.5%, and the electroconductivity measurement of each solution was repeated three times.

2. EXPERIMENTAL SECTION 2.1. Apparatus and Reagents. Spectra of 1H NMR were acquired using Bruker AVANCE III HD spectrometer (400 MHz) with CD3OD as solvent. Total reflection IR spectra were recorded by Nicolet 6700 spectrometer (Thermo Fisher Scientific Co. Ltd.). The surface tension was measured by the pendant method using an OCA 40 optical contact angle instrument (Dataphysics, Germany). The electroconductivity was measured using a FE30 conductivity meter (Mettler Toledo Instruments (Shanghai) Co. Ltd.). The chemicals as well as their purities and suppliers are listed in Table 1. 2.2. Construction of N-dodecyl Glucosylamine Pseudogemini Surfactants. N-Dodecyl glucosylamine was prepared based on previous works.35−37 In detail, glucose (50 mmol, 9.98 g), 1-dodecyl amine (55 mmol, 10.194 g), and methanol (100 mL) were added into a three-neck flask with a round-bottom, and stirred at room temperature for 24 h. The final mixture was suction filtered to remove methanol, then washed three times with cyclohexane, once with water, recrystallized twice with ethanol, and dried in vacuum to give a white solid powder (42.5 mmol, 15.1938 g, mp 379.05− 379.65 K) at the yield of 85%. The synthesized compound at a purity of 99.37% was obtained according to the amine value measurement. The NMR and IR spectra are shown in Figures S1−S2 (Supporting Information). To 2.0 mmol of N-dodecyl glucosylamine in 40 mL of methanol was added 1.0 mmol of dicarboxylic acid (malonic acid, succinic acid, glutaric acid, adipic acid, and suberic acid). The mixture was first heated to 323.15 K until dicarboxylic acid and N-dodecyl glucosylamine were completely solved, then stirred at room temperature for 48 h to give a homogeneous solution. After the solvent methanol was evaporated at 313.15 K under vacuum, the N-dodecyl glucosylamine pseudogemini surfactants were obtained. They were named G-3, G-4, G-5, G-6, and G-8, respectively. The NMR spectra are shown in Figures S3−S7 (Supporting Information). 2.3. Construction of N-dodecyl Lactosylamine Pseudogemini Surfactants. On the basis of previous works,38 Ndodecyl lactosylamine was prepared. In detail, lactose monohydrate (3 mmol, 1.081 g) was dissolved in 6 mL of ultrapure water, and 1-dodecylamine (5 mmol, 0.9268 g) was dissolved in 10 mL of isopropyl alcohol. After the two solutions were mixed and stirred at room temperature for 24 h, the mixture was heated at 333.15 K for 30 min. Then the solvents were evaporated at 318.15 K under vacuum. After recrystallization from absolute ethanol and washing with dry ether, recrystallized twice with ethanol, a white powder of Ndodecyl lactosylamine (2.9 mmol, 1.4601 g, mp 404.75− 404.95 K) was obtained at the yield of 98% with the purity of

3. RESULTS AND DISCUSSION 3.1. Effect of Linker Length on Surface Properties of Pseudogemini Surfactants G-n and L-n. As we know, surfactants have the property of reducing the surface tension of water, and self-assembling to form aggregates. First, we investigate the effect of the linker length on the critical micelle concentration (CMC) of G-n and L-n at 298.15 ± 0.1 K. CMC values of the obtained pseudogemini surfactants (G-n and L-n) have been found graphically from dependence of the surface tension (γ) of their aqueous solutions from their bulk molalities (in mol/kg solvent) at 298.15 K as shown in Figure 1 and Figure 2 and Tables S1−S2. The results show that the surface tension decreases sharply with increasing molality in

Figure 1. Surface tension (γ) of G-n aqueous solution as a function of surfactant molality (b) at 298.15 K: ▲, G-3; orange ◆, G-4; green ◆, G-5; purple ■, G-6; red □, G-8. C


Journal of Chemical & Engineering Data Γmax =

−1 n × 2.303RT

Article

ij dγ yz jj z jj d log b zzz k {T

(1)

1 N × Γmax

A min =

(2)

where γ is the equilibrium surface tension measured at the surfactant molality of b, γ0 is the surface tension of pure water, γ0 is 71.8 mN·m−1 at 298.15 K, T is the absolute temperature, N is the Avogadro’s number, and R is the gas constant. The C20 value is defined to be the surfactant molality where a decrease in the surface tension of 20 mN·m−1 from pure water is recorded (pC20 = −log C20), this value is indicative of an efficiency in lowering the surface tension.39 The value of “n” in the Gibbs equation characterizes the number of ions formed by the surfactant. For the sugar-based pseudogemini surfactants (G-n, L-n), the value of “n” is taken as 3, because they dissociate strongly in distinction to other sugar-based pseudogemini surfactants.40 For the G-n and L-n surfactants consisting of two dodecyl chains linked with a different dicarboxylic acid linker chain length spacer, except G-3 and L-3, the increase in the dicarboxylic acid linker chain length results in (i) a decrease in the CMC, and (ii) an increase in the pC20. The combination of these results suggests that the adsorption of the G-n and L-n surfactants to the air/aqueous solution interface occurs more effectively for the shorter dicarboxylic acid linker chain than for the longer one. This necessarily leads to the formation of a closely packed monolayer film of the shorter linker chain sugarbased pseudogemini surfactant at the air/aqueous solution interface, which is supported by the greater Γmax value (and thereby, the smaller Amin value) of the shorter linker chain sugar-based pseudogemini surfactant. This greater effectiveness of the sugar-based pseudogemini surfactants in the adsorption is also supported by (i) the slightly lower γCMC and (ii) the larger π CMC of the shorter linker chain sugar-based pseudogemini surfactants. Similarly, the results of Zana32 also showed that the covalently bonded gemini surfactant (Cm-sCm) has a strong spacer group effect. The short carbon chain linker (s ≤ 4) makes the two hydrophobic chains more tightly connected; however, when s ≥ 10, the long spacer bends into the micelle hydrophobic core due to a conformational change in the surfactant molecule, acting like additional hydrocarbon

Figure 2. Surface tension (γ) of L-n aqueous solution as a function of surfactant molality (b) at 298.15 K: ▲, L-3; orange ◆, L-4; green ◆, L-5; purple ■, L-6; red □, L-8.

the region of low molality of pseudogemini surfactants (G-n and L-n) aqueous solutions; apparently, the values of the surface tension measured above a certain molality are almost constant. This surfactant molality is assumed to be the critical micelle concentration (CMC) of each surfactant. Compared to that of the other reported pseudogemini surfactants (Table S13), such a typical surfactant aqueous solution behavior in the γ-log b curve has been observed for these pseudogemini surfactants (G-n and L-n), which proves that G-n and L-n with surface activity have been successfully constructed by the in situ neutralization reaction of N-dodecyl glucosylamine or Ndodecyl lactosylamine with a series of dicarboxylic acids in water, and no lowest point of γ-log b curve in Figure 1 and Figure 2 is found. This also indicates that the purities of these surfactants G-n and L-n reach the requirements for the study of the surface chemical properties of the aqueous solutions. The surface tension data shown in Figure 1 and Figure 2 allow us to calculate some physicochemical parameters (see Table 2): critical micelle concentration (CMC); equilibrium surface tension at the CMC (γCMC); effectiveness (πCMC = γ0 − γCMC); efficiency (pC20); maximum surface excess (Γmax); and minimum surface area (Amin), where Γmax and Amin are calculated from eqs 1 and 2, respectively.

Table 2. Surface Activity Parameters of G-n and L-n aqueous solution at 298.15 Ka γCMC

CMC surfactant G-3 G-4 G-5 G-6 G-8 L-3 L-4 L-5 L-6 L-8

−1

mmol·kg 2.27 2.46 2.12 1.65 1.31 2.43 2.55 2.30 2.25 1.57

−1

mN·m

26.33 30.17 31.65 31.53 30.42 26.71 27.84 29.37 30.06 30.39

Γmax −10

10

πcmc

Amin

mol·cm

−2

1.96 1.76 1.68 1.70 1.73 1.90 1.83 1.72 1.62 1.61

2

pC20

CMC/C20

mN·m−1

0.85 0.94 0.99 0.98 0.96 0.87 0.91 0.96 1.02 1.03

3.41 3.33 3.37 3.48 3.60 3.39 3.36 3.40 3.43 3.59

5.77 5.21 5.01 4.97 5.27 5.91 5.82 5.76 6.04 6.14

45.47 41.63 40.15 40.27 41.38 45.09 43.96 42.43 41.74 41.41

nm

a Notation: critical micelle concentration, CMC; equilibrium surface tension at the CMC, γCMC; maximum surface excess, Γmax; minimum surface area, Amin; efficiency, pC20; effectiveness, πCMC; CMC/C20. The standard uncertainties u are u(t) = 0.1 K and u(p) = 10 kPa. The combined expanded uncertainties Uc are Uc(CMC) = 10−5 mol·kg−1, Uc(γ) = 0.1 mN·m−1, Uc(Γmax) = 10−12 mol·cm−2, Uc(Amin) = 0.01 nm2, Uc(πCMC) = 0.1 mN·m−1, and Uc(pC20) = 0.02 (0.95 level of confidence).

D



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Figure 3. Dependence of the electroconductivity (κ) of G-n and L-n aqueous solution on surfactant molality (b) at different temperatures. Temperature, K: ●, 298.15; blue ○, 308.15; green ■, 318.15; red □, 328.15. Left, from top to bottom: G-3, G-4, G-5, G-6, G-8. Right, from top to bottom: L-3, L-4, L-5, L-6, L-8.

chains. The two alkyl chains would be in a gauche or trans position at low s values and in a cis position at higher s, so the CMC of Cm-s-Cm shows a maximum at s = 5−6. From Table 2, it can be seen that the CMC of G-n (or L-n) reaches a

maximum at n = 4. This may occur because the noncovalent bond is more flexible or when the number of carbon atoms in spacer group is the same, the noncovalently bonded pseudogemini surfactant has a longer linker. Such a E



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chain length increases, the CMC value increases first and then decreases, with the maximum at n = 4. The dissociation degree of micelles (α)47 and the degree of bonding counterions by micelle (β) are obtained from the slope of the fitted line of the monomer region (S1) and the micelle region (S2) in Figure 3:

predominance of effect of spacer nature on surface properties of a pseudogemini surfactant has similarly been reported in the recent literature.10,41−43 It is worth noting that in the more recent literature,44 the spacer chain length of gemini surfactant has not only a great effect on micelle formation, but also plays an important role in antibacterial activity and influences the cytotoxicity. Notably, N-dodecyl glucosylamine is insoluble in water;45 however, sugar-based pseudogemini surfactant G-n aqueous solution is clear and transparent. Compared with N-dodecyl glucosylamine, G-n has two hydrophilic groups which can greatly improve the water-solubility of the molecule. At the same time, G-n contains two hydrophobic tails closely linked through the linker. The increase of the number of hydrophobic tails leads to the increase of hydrophobicity, resulting in the increase of hydrophobicity, G-n monomer is easier to aggregate in aqueous solution to form micelles with lower CMC. In G-n aqueous solution from Table 2, G-3 has the smallest γcmc value (the largest πCMC) in series of G-n pseudogemini surfactants, indicating that the shorter linker malonic acid gives the surfactant the greatest effectiveness to reduce the surface tension of water; G-8 has the largest pC20 value, indicating that the longer linker suberic acid gives surfactants the greatest efficiency in reducing surface tension. G-6 has the lowest CMC/C20 value, while G-3 has the highest CMC/C20 value, indicating that N-dodecyl glucosylamine molecules are more prone to micelle formation in the presence of adipic acid, while are more prone to adsorb on the surface in the presence of malonic acid. From the data of L-n aqueous solution in Table 2, with the increase of n, the CMC of L-n solution increases first and then decreases, and the maximum value is obtained when n = 4, but the CMC of the L-n solution is slightly larger than that of G-n with the same dicarboxylic acid linker chain length spacer. 3.2. Effect of Linker Length on Micellization of Pseudogemini Surfactants G-n and L-n. The measurement of electroconductivity is one of the most effective methods for studying the aggregation behavior of aqueous solutions of ionic surfactants. The results of electroconductivity measurements of the surfactants G-n and L-n aqueous solutions at different temperatures (298.15 K, 308.15 K, 318.15 K, and 328.15 K) are shown in Figure 3 (or Supporting Information, Figures S15−S24 and Tables S3−S12). It may be seen in Figure 3 that there are two linear parts in the typical curves and the steep change of the slope is assigned to the micellization of the surfactants when the molality increases; the corresponding molality value is the value of CMC. The CMC values of surfactants L-3 and L-6 aqueous solutions measured at 298.15 K by the conductivity method are smaller than that in the reference,46 and this may be caused by the different purification methods. N-Dodecyllactosylamine reacted with dicarboxylic acids for 2 days at room temperature to obtain di(N-dodecyllactosylammonium) dicarboxylates. Zhang46 reported that the separation was carried out by removing methanol and washing with acetone and n-hexane to get the final product. However, as N-dodecyllactosylammoniums were noncovalently constructed stoichiometrically on the acid-alkali reaction, we directly removed methanol to obtain the surfactant. The CMC value of surfactants G-n and L-n aqueous solutions measured at 298.15 K by the conductivity method changes with the increase in dicarboxylic acid linker length, and the presented CMC values are consistent with the results measured by the surface tension method. As the linker

α=

S2 S1

(3)

β=1−α

(4)

The respective results presented in Table 3 show that the values of β decrease a little with an increase of dicarboxylic acid linker length and a rise in temperature. Also, the electroconductivity of the solution increases when the temperature increases. There will be some surfactant ions with oppositely charged ions (counterions) binding to the micelle surface when the ionic surfactant forms a micelle due to the effect of the Coulomb force. The increase in temperature will enhance the thermal motion of the surfactant micelles, which will reduce the number of counterions that are immobilized on the micelles and the degree of counterion binding, while the number of counterions in the solution will increase. Therefore, the electroconductivity of the solution will increase. The micellar thermodynamic parameters of the aqueous surfactant solution are calculated with the procedure proposed by Zana48 designed for gemini surfactants with monovalent counterions. The ΔG0m, ΔH0m, and ΔS0m for the pseudogemini surfactants G-n and L-n in aqueous solution at 298.15−328.15 K are calculated from eqs 5 and 6, respectively, and are listed in Table 3. ΔGm0 = (0.5 + β)RT ln Xcmc − 0.5RT ln 2

(5)

where Xcmc is the molar fraction of a surfactant at CMC, R is the universal gas constant (8.314 J·(mol·K)−1) and T is the absolute temperature. i d ln Xcmc zy zzz ΔHm0 = −(0.5 + β)RT 2jjjj k dT {

(6)

ΔSm0 = (ΔHm0 − ΔGm0)/T

(7)

The respective results presented in Table 3 show that the ΔG0m values of the systems investigated are negative within the whole temperature range, taking G-3 as an example, the Gibbs free energy of micellization changes from −34.74 kJ·mol−1 to −36.75 kJ·mol−1 in the range 298.15−328.15 K. With the increase of temperature, the absolute value of ΔG0m gradually increases, and their small changes result from the enthalpic− entropic compensation effect. From the point of view of thermodynamics, the formation of micelles described in this study is spontaneous. At the same temperature, it can be found that the absolute value of Gibbs free energy ΔG0m decreases with an increase in the dicarboxylic acid spacer length for both G-n and L-n, except for the case of G-8 and L-8. This means that a decrease in the chain length and hydrophobicity of dicarboxylic acid linker strongly promote the binding of dicarboxylic acid with the sugar-based pseudogemini surfactants (G-n, L-n) micelles and make their binding stronger and more spontaneous. The absolute value of ΔG0m of L-n is smaller than the value of G-n, because the L-n monomer is more soluble in water than the G-n monomer, micelle formation is inhibited, obviously giving the influence of the headgroup on F



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between the hydrophobic groups, and the thermodynamic changes in the conformation of the linking groups are the main factors affecting the thermodynamic function. At the same time, the interaction between van der Waals force and the hydrophobic tail leads to a negative value of ΔH0m. The value of TΔS0m is larger than the absolute value of ΔH0m, indicating that the micellization process is an entropy-driven process. As the dicarboxylic acid linker length increases, the value of TΔS0m decreases first and then increases. The value of TΔS0m reaches the minimum value when the dicarboxylic acid linker is succinic acid in the series of sugar-based pseudogemini surfactants G-n. 3.3. Effects of the Head Group of Sugar-Based Pseudogemini Surfactants. It is well-known that the nature of the headgroup is one of the most important structural factors that control the properties of surfactants, and the structure of headgroup could influence the aggregate forms and surface adsorption through the packing parameter.50 For different carbohydrate headgroup sugar-based pseudogemini surfactants G-n and L-n, this effect is also obvious from Figure 4, the critical micelle concentration (CMC) increased with the

Table 3. Thermodynamic Parameters of Micellization of Gn and L-n Aqueous Solution at 298.15 K, 308.15 K, 318.15 K, and 328.15 Ka CMC

ΔG0m

ΔH0m

TΔS0m

surfactant

T/°C

mmol·kg−1

β

KJ·mol−1

KJ·mol−1

KJ·mol−1

G-3

25 35 45 55 25 35 45 55 25 35 45 55 25 35 45 55 25 35 45 55 25 35 45 55 25 35 45 55 25 35 45 55 25 35 45 55 25 35 45 55

2.333 2.372 2.513 2.534 2.772 2.876 2.958 2.992 2.275 2.292 2.335 2.425 1.848 1.915 1.920 1.956 1.403 1.443 1.505 1.600 2.506 2.630 2.760 2.766 2.825 2.866 2.941 2.970 2.348 2.431 2.516 2.541 2.047 2.092 2.100 2.161 1.522 1.523 1.542 1.578

0.856 0.841 0.844 0.813 0.815 0.787 0.767 0.762 0.722 0.752 0.763 0.732 0.703 0.699 0.704 0.688 0.897 0.871 0.848 0.844 0.755 0.724 0.690 0.675 0.739 0.718 0.723 0.712 0.623 0.682 0.686 0.707 0.719 0.784 0.784 0.743 0.815 0.872 0.826 0.823

−34.74 −35.46 −36.48 −36.75 −33.15 −33.43 −33.90 −34.79 −31.47 −33.27 −34.58 −34.69 −31.61 −32.45 −33.64 −34.18 −37.52 −37.98 −38.42 −39.28 −31.99 −32.12 −32.11 −32.71 −31.23 −31.70 −32.77 −33.47 −28.90 −31.28 −32.30 −33.85 −31.71 −34.40 −35.50 −35.38 −35.10 −37.81 −37.72 −38.74

−3.063 −3.236 −3.457 −3.593 −2.500 −2.614 −2.743 −2.906 −1.898 −2.077 −2.234 −2.318 −1.538 −1.638 −1.753 −1.840 −4.504 −4.722 −4.949 −5.249 −3.194 −3.328 −3.449 −3.623 −1.612 −1.692 −1.811 −1.910 −2.252 −2.532 −2.708 −2.932 −1.499 −1.687 −1.798 −1.852 −1.174 −1.308 −1.348 −1.431

31.67 32.22 33.03 33.16 30.65 30.82 31.16 31.88 29.57 31.20 32.35 32.35 30.07 30.82 31.88 32.34 33.02 33.25 33.47 34.04 28.80 28.79 28.66 29.09 29.61 30.00 30.96 31.56 26.65 28.75 29.59 30.92 30.21 32.71 33.71 33.53 33.93 36.51 36.37 37.31

G-4

G-5

G-6

G-8

L-3

L-4

L-5

L-6

L-8

Figure 4. Dependence of the minimum surface area (Amin) and the critical micelle concentration (CMC) of sugar-based gemini surfactants (G-n, L-n) on the carbon number of linker dicarboxylic acid at 298.15 K: ●, G-n; ★, L-n; red ●, G-n; red ★, L-n.

increase of headgroup size, while the minimum surface area (Amin) depends not only on headgroup size, but also on the dicarboxylic acid spacer length. Nonlinearity was observed both in CMC and Amin versus the dicarboxylic acid spacer length; the CMC value of L-n with two more sugar rings headgroup is larger than the CMC value of G-n at the same temperature and the dicarboxylic acid spacer length, but it is still the same order of magnitude. The CMC of sugar-based psedogemini surfactants (G-n, L-n) increases first and then decreases with increasing the dicarboxylic acid spacer length, and the maximum value is obtained at n = 4. In Figure 4, the Amin values of G-n and L-n gradually increase as the dicarboxylic acid spacer length is increased from 3 to 8, which suggests that sugar-based psedogemini surfactants with a dicarboxylic acid spacer length of three carbon atoms have higher packing densities at the air/aqueous solution interface than do those with a dicarboxylic acid spacer length of eight carbon atoms. A possible explanation is that the longer dicarboxylic acid spacers are more prone to curl and thus make the Amin value larger. This is not only consistent with previous

a Temperature, T; critical micelle concentration, CMC; degree of counterion binding to micelles, β; Gibbs free energy of micellization, ΔG0m; enthalpy of micellization, ΔH0m; entropy of micellization, ΔS0m. The standard uncertainties u are u(t) = 0.1 K and u(p) = 10 kPa. The combined expanded uncertainties Uc are Uc(ΔG0m) = 0.02 kJ·mol−1, Uc(ΔH0m) = 0.001 kJ·mol−1, and Uc(TΔS0m) = 0.02 J·(mol·K)−1 (0.95 level of confidence).

micelle formation. The negative ΔH0m value of the solution indicates that the micellization process is an exothermic process, and the absolute value of ΔH0m increases with increasing temperature. At the same temperature, the absolute value of ΔH0m decreases with an increase in the dicarboxylic acid spacer length for both G-n and L-n, except for the case of G-8 and L-8. Diamant and Andelman49 pointed out that the van der Waals forces between the carbon chains, the repulsive forces between the hydrophilic head groups, the interactions G



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findings,51 but further indicates that saccharide segment size is an important determining factor in physicochemical properties of noncovalently constructed sugar-based pseudogemini surfactants.

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4. CONCLUSIONS In summary, we have investigated the role of the dicarboxylic acid linker length and headgroup saccharide size on physicochemical properties of noncovalently constructed sugar-based pseudogemini surfactants. We have shown that the CMC of G-n and L-n aqueous solution has dependence of the dicarboxylic acid linker, which increases first and then decreases with increasing n and is the largest at n = 4. At the same linker length, the CMC values of L-n are always larger than that of G-n, but it is still the same order of magnitude. Thermodynamic data show that the micellization in aqueous solution of these pseudogemini surfactants (G-n, L-n) is a thermodynamically spontaneous entropy-driven exothermic process.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00459.

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IR, and 1H NMR spectra of N-dodecyl glucosylamine and N-dodecyl lactocosylamine; 1H NMR spectra of G-n and L-n; dependence of conductivity from molality of Gn and L-n at different temperatures; dependence of surface tension from logarithm of molality for G-n and L-n at 298.15 K; dependence of conductivity from molality of G-n or L-n at different temperatures; CMCs of other reported pseudogemini surfactants constructed from component 1 and component 2 (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.(Fax): +86-051-85917713. E-mail: [email protected]. cn. ORCID

Xin Ge: 0000-0001-9058-0544 Funding

The authors are grateful for the financial support from the Natural Science Foundation of China (21606104), the National Key Research and Development Program of China (2016YFB0301800), and the Opening Foundation from Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology (ACEMT-17-03). Notes

The authors declare no competing financial interest.

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REFERENCES

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Physicochemical Properties of Noncovalently Constructed Sugar

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