m-s-m Cationic Gemini and Zwitterionic Surfactants â Spacer

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B: Fluid Interfaces, Colloids, Polymers, Soft Matter, Surfactants, and Glassy Materials

m-s-m Cationic Gemini and Zwitterionic Surfactants – Spacer Dependent Synergistic Interactions Aleisha Arlene McLachlan, Kulbir Singh, Emily Piggott, Michael James McAlduff, Shannon MacLennan, Victoria Sandre, Taryn Reid, and D. Gerrard Marangoni J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b09771 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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The Journal of Physical Chemistry

m-s-m Cationic Gemini and Zwitterionic Surfactants – Spacer Dependent Synergistic Interactions Aleisha McLachlan, Kulbir Singh, Emily Piggott, Michael McAlduff, Shannon MacLennan, Victoria Sandre, Taryn Reid, and D. Gerrard Marangoni* Dept. of Chemistry, St. F.X. University, Antigonish NS, B2G 2W5 Phone – (902) 867-2324; Fax – (902) 867-2414; E-mail - [email protected]

Running title – Synergistic interactions in cationic gemini/zwitterionic surfactant mixtures.

*

To Whom Correspondence should be addressed.

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

Abstract Critical micelle concentration (cmc) values have been determined for the mixed zwitterionic/cationic gemini systems of N-dodecyl-N,N-dimethyl-3-ammonio-1propanesulfonate (ZW3-12)/N,N’bis(dimethyldodecyl)-,-alkanediammonium dibromide (12s-12) systems. The cmcs for the mixed systems were determined through conductivity measurements. The degree of nonideality of the interaction in the mixed micelle (m), for each system, was determined according to Rubingh’s nonideal solution theory. In most cases, the systems exhibited negative deviations (-m values) at high surfactant mole fractions of zwittergent (ZW3-12). Specifically, the ZW3-12/12-4-12 system displayed -m values at ZW3-12  0.5, while both the ZW3-12/12-5-12 and the ZW3-12/12-6-12 system displayed –m values

over the entire mole fraction range. Except for the low mole fraction range in the 12-4-12 system, these mixed surfactant systems demonstrated almost identical behaviour to the DTAB/12-2-12 system studied by Bakshi et al. providing further evidence that ZW3-12 tends to behave as a cationic surfactant in mixed surfactant systems. The manner in which the cosurfactants aggregate in the micelles was determined via 2D-NOESY spectroscopy. In the case of both the ZW3-12/12-5-12 and the ZW3-12/12-6-12 systems, the 2-D NOESY spectra exhibited strong cross peaks between the gemini and zwitterionic surfactants over the entire micellar composition range in the case of the ZW3-12/12-5-12 system. In the case of the ZW312/12-4-12 system, little cross peak intensity was observed between the gemini and the zwitterionic surfactant at low micellar compositions of the zwittergent. The results suggest some micelle demixing is occurring between the gemini and the zwittergent certain micellar composition ranges, a phenomenon rarely associated with hydrocarbon surfactants.


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3

Introduction Zwitterionic and gemini surfactants have many unique and interesting properties, and as a result, mixtures of these two surfactants could prove beneficial in many commercial and industrial applications 1,2. Surprisingly, there have been few studies, presented in the literature involving mixtures of zwitterionic and gemini surfactants. There have, however, been numerous reports on mixtures involving gemini surfactants with ionic and non-ionic conventional surfactants3-16. Overall, binary mixtures of gemini surfactants with conventional surfactants have a greater probability of displaying synergism than mixed systems involving conventional surfactants with other conventional surfactants16-18. For example, Bakshi et al. have reported the existence of synergistic interactions in some mixtures of cationic geminis with conventional cationic surfactants despite the fact that structurally similar surfactants typically mix in an ideal fashion13,19. In light of these findings, the authors determined that studies focusing on mixed systems of gemini surfactants with zwitterionic surfactants would provide a clearer understanding of the role that head group and tail moieties play in synergistic interactions. The mixed zwitterionic/gemini systems which Bakshi and Singh studied involved an m-sm type gemini and alkyldimethylammoniopropanesulfonate (zwitterionic) surfactants20. The authors were primarily interested in studying the effect of increasing hydrophobicity, of both gemini and zwitterionic surfactants, on the mixed micellar properties. Appropriately, gemini surfactants of varying chain length were studied in combination with zwitterionic surfactants of varying chain length while the gemini spacer was maintained at two methylene groups (s=2). The length of the gemini spacer can have varied effects on the surfactant properties and the nature of the micelles formed. However, studies involving mixed gemini and zwitterionic surfactants in which emphasis is placed on gemini spacer length have yet to be presented in the



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4 literature. There are primarily two reasons for our interest in these mixed zwitterionic/gemini systems. Firstly, studying the 12-s-12/ZW3-12 systems will enable us to see the effects that slight changes in spacer length have on the nature of the interactions in these systems. Also, interactions in these systems can be compared with those of the 12-2-12/ZW3-12 system studied by Bakshi and Singh 20. Secondly, these studies will give us the opportunity to compare and contrast the behaviours of the 12-s-12 geminis with their monomeric equivalent, DTAB, in mixed systems involving zwitterionic surfactants. Interactions between DTAB and ZW3-12 have been the subject of previous work in our laboratory21. In this paper, we will examine the self-assembly process in ZW3-12/12-s-12 systems for s = 4, 5, and 6. The cmc values for the mixed systems have been determined through conductance measurements. Via the application of Rubingh’s regular solution theory22 to the conductance derived cmc values, the interaction parameters between the zwittergent and the gemini surfactant was determined at various mole fractions of zwittergent to gemini surfactant, at a constant total surfactant concentration. Additionally, two-dimensional Nuclear Overhauser Effect Spectroscopy (NOESY) was used to determine the proximity of the protons of the constituent surfactants to one another in the mixed aggregates. All these results are used to investigate the subtle differences that exist in the self-assembly process for these systems as a function of the spacer length of the gemini surfactant.

Experimental Materials N,N-Dimethyldodecylamine, 1,4-dibromobutane, and 1,5-dibromopentane, having purities of 97%, 99%, and 97% respectively were received from Sigma-Aldrich. N-dodecyl-


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5 N,N-dimethyl-3-ammonio-1-propanesulfonate (ZW3-12) with purity of 99% was received from Sigma-Aldrich and used without further purification.

Methods Gemini Surfactant Synthesis The N,N’-bis (dimethyldodecyl)-,-alkanediammonium dibromide (12-s-12) surfactants were prepared, according to the method described by Wettig and Verall 23,24. The appropriate ,dibromoalkane was reacted with a 2 molar eq (plus a 10% excess) of N,Ndimethyldodecylamine by refluxing in HPLC-grade acetonitrile for 48 hours. After the solution was cooled, the solid material was collected by vacuum filtration and recrystallized several times from acetonitrile. The yields obtained for the gemini surfactants were above 80%. Critical micelle concentration values obtained through conductivity measurements were in excellent agreement with the literature values25. CMC Determination by Conductivity Measurements For the 12-s-12 / ZW3-12 systems, ~ 10 mM stock solutions of each surfactant were prepared, and these stock solutions were mixed in the appropriate quantities to prepare the different mole fractions in each of the mixed systems. These systems were studied in the zero to 0.90 mole fraction range at 0.10 unit intervals. Conductivity measurements were made on a YSI Conductivity meter with a conductasnce cell having a nominal cell constant of 1.0 cm-1. The readings were taken in a jacketed beaker, which was maintained at a temperature of 25.0C by a Julabo C water bath. Dilutions of the stock solutions into the solvent (triply deionized water) were made using Eppendorf pipettes that were routinely calibrated with the deionized water. Conductivity experiments were performed in



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6 triplicate. The break point in the plot of conductivity versus the total surfactant concentration was taken to be the cmc at that mole fraction. NMR Experiments 1D and 2D nmr experiments (NOESY and COSY) were performed on the AVANCE-500 spectrometer at the Atlantic Region Magnetic Resource Center (ARMRC) and on the AVANCEII 400 MHZ spectrometer at StFX University. The mixing times and the delay times for the NOESY experiments were estimated from the spin-lattice relaxation times (T1 values) of the surfactant in micellar form, determined in separate experiments. In all cases, an acquisition delay of ~ 5-10 x T1 and a mixing time of ~1 x T1 were used to obtain the NOESY spectra. Stock solutions of each surfactant were prepared in D2O (Sigma-Aldrich), and the appropriate volumes of the stock solutions were mixed to prepare the surfactant solutions at each mole fraction. Each system was studied at mole fractions (in terms of zwittergent, ZW3-12) of 0.20, 0.50, and 0.80. Eppendorf pipettes, which were routinely calibrated with water, were used for solution preparation.

Results and Discussion The conductivity data for the three 12-s-12 surfactants and the mixed ZW3-12/12-s-12 systems are presented in Figures 1 (ZW3-12/12-4-12), 2 (ZW3-12/12-5-12), and 3 (ZW3-12/126-12), respectively. In all cases, break points are ascribed to the cmc values of the pure surfactant or the mixed surfactant system; plots of molar conductivity versus the square root of the total surfactant concentration (not shown) were also used to verify the cmc values, particularly in the case of the high mole fraction zwittergent solutions. The cmc value of the pure ZW3-12 system was taken from our previous work26. These figures clearly demonstrate


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7

400 350

Conductance / mS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 250 200 150 100 50 0 0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

Csurf,t(mM) Figure 1 Conductivity data for the ZW3-12/12-4-12 system as a function of the mole fraction of zwitterionic surfactant in the surfactant mixture (αZW3-12): • 0.00; • 0.10;  0.20; ▲ 0.30; ○ 0.40; ◊ 0.50; □ 0.60; △ 0.70; ⋇ 0.80; + 0.90.



8

400 350

Conductance / mS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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300 250 200 150 100 50 0 0.00

1.00

2.00

Csurf,t(mM)

3.00

4.00

Figure 2 Conductivity data for the ZW3-12/12-5-12 system as a function of the mole fraction of zwitterionic surfactant in the surfactant mixture (αZW3-12):  0.00;  0.10;  0.20; ▲ 0.30; ○ 0.40; ◊ 0.50; □ 0.60; △ 0.70;

⋇ 0.80; + 0.90.


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9

400 350 300

Conductance / mS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


250 200 150 100 50 0 0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

CSurf,t / mM

Figure 3 Conductivity data for the ZW3-12/12-6-12 system as a function of the mole fraction of zwitterionic surfactant in the surfactant mixture (αZW3-12):  0.00;  0.10;  0.20; ▲ 0.30; ○ 0.40; ◊ 0.50; □ 0.60; △ 0.70;

⋇ 0.80; + 0.90;



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10

that as the mole fraction of zwitterionic surfactant was increased in each system, the break points in the plots, taken to be the cmc, were still easily distinguished. Several theories have been developed to predict and analyze cmc values of binary surfactant mixtures. Clints’ equation (1) can be used to predict the cmc for a system in which mixing of surfactants is ideal. 1

α𝑖 ∑ = 𝑖 𝐶𝑀𝐶𝑚𝑖𝑥 𝐶𝑀𝐶𝑖

(1)

In this equation CMCmix is the cmc of the mixture; i is the mole fraction of each component i in the solution; and CMCi is the cmc of each component i 27,28 . Therefore, if the mixed micellization process for a mixed binary surfactant system behaves ideally i.e., mixed micelles are in equilibrium with both monomers in the system, the cmc values should fall on the line predicted by the following relationship: 𝟏

𝑪𝑴𝑪𝒎𝒊𝒙 =

𝛂

𝑪𝑴𝑪𝒂 +

(𝟏 ― 𝛂) 𝑪𝑴𝑪𝒃

(2)

where CMCmix, CMCa and CMCb are the critical micelle concentrations of the mixed surfactant, surfactant a, and surfactant b, respectively, and  is the mole fraction of surfactant a in the mixed system29. This theory works well for binary mixtures of homologous surfactants in which head groups are similar. For non-ideal surfactant mixtures, i.e., non-homologous surfactant mixtures, the strength and nature of the interaction between two surfactants can be measured by values termed the m parameter. These values show the degree of non-ideality of the interaction in the mixed micelle30. The following relationships, arising from Rubingh’s non-ideal solution theory, can be used for determination of the m parameter for mixed micelle formation in an aqueous medium.


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11 α 𝐶𝑀𝐶𝑚𝑖𝑥 (χ1)2ln( 1 χ1𝐶𝑀𝐶1)

=1 (1 ― χ1)ln[(1 ― α1)𝐶𝑀𝐶𝑚𝑖𝑥 (1 ― χ1)𝐶𝑀𝐶2]

(3)

Here 1 is the mole fraction of surfactant 1 in the mixed micelle, and CMC1, CMC2, and CMCmix are the critical micelle concentrations for surfactant 1, surfactant 2, and their mixture respectively, at mole fraction  1 27,28. Knowing 1 , the m parameter can then be calculated using the following equation

(𝛂𝟏𝑪𝑴𝑪𝒎𝒊𝒙 𝛘𝟏𝑪𝑴𝑪𝟏)

𝐥𝐧

𝛃𝒎 =

(𝟏 ― 𝛘 𝟏 )𝟐

(4)

Interactions between the surfactant head groups lead to deviations of m from zero. Positive values of m indicate repulsive interactions (i.e., an antagonistic micelle formation process) while negative values indicate attractive interactions31. Within the regular solution approximation, the m value should be constant with respect to the change in composition for a given surfactant mixture12. However, in some cases, the m value has been found to change with changing surfactant composition. In a study of mixed non-ionic/ionic systems and zwitterionic/ionic systems, Misselyn-Bauduin et al.12 found absolute values of m to be roughly constant over the entire range of composition for the C10E5/SDS system while greater deviations in m values over the range of composition were found for the LAPB/SDS system. The m parameter can also be indicative of whether or not synergism exists in a given mixed binary system. The following two conditions must be met in order for synergism to exist in mixed micelle formation: (1) m must be negative, and (2) |β𝑚| > |ln(𝐶𝑀𝐶1 𝐶𝑀𝐶2)|30.



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12 In Figures 4a-c, we show a comparison between ideal cmcs (obtained from Clint’s equation, Eq. 1) and experimental cmcs for the ZW3-12/12-s-12 gemini systems. These plots show that the experimental cmcs correspond well with the ideal cmc values particularly at low mole fractions of zwittergent. At high mole fractions of zwittergent, the negative deviations from ideal behavior become quite apparent, most notably in the ZW3-12/12-5-12 system and the ZW3-12/12-6-12 systems. This trend is very similar to the plot comparing the ideal and experimental cmc values for ZW3-12/12-2-12 determined by Bakshi and Singh 18. This is in stark contrast to the behaviour displayed by the ZW3-12/DTAB system, studied in previous work 21,

which displayed positive deviations from ideal behaviour over the entire mole fraction range.

Tables 1-3 present the micellar mole fractions of ZW3-12 (ZW3-12), and the interaction parameter values (m) at solution mole fractions of ZW3-12 (ZW3-12) for the zwitterionic/gemini systems studied. The mixture cmc values obtained from conductivity measurements were substituted into Equation 3 to determine the ZW3-12 values. These values along with the experimental mixture cmc values were then substituted into Equation 4 to determine the m values at the various solution mole fractions of ZW3-12. Equation 3 was solved using the solver function in Microsoft Excel. Figures 5a-c shows a comparison between experimental ZW3-12 values and ideal ZW3-12 values, at ZW3-12, for all the ZW3-12/12-s-12 systems studied. From Figures 5a-c, it is evident that the exp values are close to ideal values over most of the mixing range for both ZW3-12/12-s-12 systems. This was also demonstrated in the ZW312/12-2-12 system studied by Bakshi and Singh 18. The determined ZW3-12 values show that the mole fraction of ZW3-12 in the micelle is always less than the mole fraction in solution. This is not surprising seeing as how the 12-s-12 surfactants have lower cmcs than ZW3-12. Interesting comparisons can also be made between the m values for the ZW3-12/12-s-12 systems and those


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13 for the ZW3-12/DTAB system determined from previous work 21. The m values for the ZW312/DTAB system are positive over the entire mole fraction range. In comparison, the ZW312/12-4-12 system displays negative m values (mavg = -0.178) at ZW3-12

4a



14 3.00

CMCmix

2.50 2.00 1.50 1.00 0.50 0.00 0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.60

0.70

0.80

0.90

1.00

ZW3-12 3.00

4b

2.50

CMCmix

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 48

2.00 1.50 1.00 0.50 0.00 0.00

0.10

0.20

0.30

0.40

0.50

ZW3-12


Page 15 of 48

15

3.00

2.50

4c

2.00

CMCmix

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.50

1.00

0.50

0.00 0.00

0.10

0.20

0.30

0.40

0.50

ZW3-12

0.60

0.70

0.80

0.90

1.00

Figure 4 Comparison between ideal () and experimental cmc's () for the ZW3-12/12-s-12 system. 4a) 12-4-12; 4b) 12-5-12; and 4c) 12-6-12

Table 1 Mixture cmc values, calculated ZW3-12 values, and m values for ZW3-12/12-4-12 system as a function of the mole fraction of zwittergent (ZW3-12).

ZW3-12

CMCmix

ZW3-12

m

0.00

0.00120

0.000

----

0.10

0.00130

0.0175

1.04

0.20

0.00141

0.0565

0.68

0.30

0.00147

0.141

0.18

0.40

0.00158

0.213

0.13

0.50

0.00162

0.317

-0.16

0.60

0.00177

0.402

-0.10



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16 0.70

0.00186

0.513

-0.32

0.80

0.00204

0.634

-0.48

0.90

0.00222

0.778

-1.34

1.00

0.00274

1.000

----

Table 2 Mixture cmc values, calculated ZW3-12 values, and m values for ZW3-12/12-5-12 system as a function of the mole fraction of zwittergent (ZW3-12).

ZW3-12

CMCmix

ZW3-12

m

0.00

0.00112

0.000

----

0.10

0.00118

0.045

-0.04

0.20

0.00126

0.096

-0.05

0.30

0.00134

0.159

-0.11

0.40

0.00142

0.229

-0.16

0.50

0.00158

0.293

-0.04

0.60

0.00167

0.392

-0.19

0.70

0.00177

0.500

-0.41

0.80

0.00189

0.619

-0.80

0.90

0.00210

0.761

-1.73

1.00

0.00274

1.000

----


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17 Table 3 Mixture cmc values, calculated ZW3-12 values, and m values for ZW3-12/12-6-12 system as a function of the mole fraction of zwittergent (ZW3-12).

ZW3-12

CMCmix

ZW3-12

m

0.00

0.00106

0.000

----

0.10

0.00113

0.0503

-0.23

0.20

0.00118

0.112

-0.33

0.30

0.00122

0.183

-0.47

0.40

0.00126

0.258

-0.62

0.50

0.00129

0.335

-0.80

0.60

0.00132

0.410

-1.01

0.70

0.00146

0.484

-1.05

0.80

0.00155

0.568

-1.21

0.90

0.00180

0.681

-1.37

1.00

0.00274

1.000

----

5a



18  

ZW3-12

         





















0.60

0.70

0.80

0.90

1.00

ZW3-12 1.00 0.90 0.80

ZW3-12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 48

5b

0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 0.00

0.10

0.20

0.30

0.40

0.50

ZW3-12


Page 19 of 48

19

1.00 0.90

5c

0.80

ZW3-12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

ZW3-12 Figure 5 - Micellar mole fractions, exp () and ideal (---), of ZW3-12 versus solution mole fraction () of ZW3-12 for ZW3-12/12-s-12 systems. 5a) 12-4-12; 5b) 12-5-12; and 5c) 12-6-12

 0.5, while m values for the both the ZW3-12/12-5-12 and the ZW3-12/12-6-12 system are

negative over the entire mole fraction range (mavg = -0.436 and -0.670, respectively). Bakshi and Singh had also determined an average negative m value of approximately –0.5 for the ZW312/12-2-12 system over the entire mole fraction range20. Also interesting is the fact that the m values become increasingly positive with increasing mole fraction of ZW3-12 in the ZW312/DTAB system, while m values become increasingly negative with an increase in the mole fraction of ZW3-12 for the ZW3-12/12-s-12 systems. The increased favourable interactions in the zwitterionic/gemini systems in comparison with the zwitterionic /DTAB systems, studied previously 11, can likely be partly attributed to the increased hydrophobic interactions due to the presence of the dual tails of the gemini surfactants. The study by Bakshi and Singh has demonstrated that increasing hydrophobicity plays a role in increasing the attractive interactions,



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20 which is demonstrated by greater negative m values. Decreased repulsions between gemini head groups upon incorporation of zwitterionic head groups can also play a role in the presence of the synergistic interactions as suggested by Bakshi and Singh20. While negative m values indicate attractive interactions in mixed systems, interactions in the systems studied here cannot be said to be truly synergistic as the 2nd condition for synergism,  m  lnCMC1 CMC 2  , is not satisfied. The zwittergent/12-4-12 and 12-5-12 systems only satisfy this condition at ZW3-12 = 0.9, whereas the ZW3-12/12-6-12 systems satisfies this above ZW3-12 = 0.6. Previous studies of mixed zwitterionic/ionic systems by McLachlan and Marangoni 21, and Li et al.30, and of mixed ethoxylated alcohol/zwitterionic systems by Mullally and Marangoni 10, have concluded that the zwitterionic surfactant is behaving more as a cationic surfactant in these mixed systems. These findings indicate that the ammonium functionality is primarily responsible for the behaviour of the zwitterionic surfactant. It is interesting to compare the results obtained for the ZW3-12/12-s-12 geminis studied in this paper with those of the DTAB/12-2-12 system studied by Bakshi et al.19,20. The authors actually reported slight synergistic interactions in the DTAB/12-2-12 system. In fact, the plot showing a comparison between the ideal cmcs and experimental cmcs for this system demonstrates an almost identical trend to that of the ZW3-12/12-5-12 and the ZW3-12/12-6-12 systems- presented here (Figures 4b and 4c). Therefore, these results suggest that ZW3-12 is also likely behaving as a cationic surfactant in mixed systems with ionic gemini surfactants as well. It is, however, important to point out the notable difference between the DTAB/12-2-12 system with the ZW3-12/12-s-12 systems. m values for the DTAB/12-2-12 system remain relatively constant over the entire mole fraction range, while in the ZW3-12/12-s-12 system, m values decrease steadily as the mole fraction of ZW3-12 increases. This may be indicative of a possible change in the


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21 conformation of the ZW3-12 as its mole fraction in the mixed micelle increases. Further insights as to the conformation of the head groups and tail moieties of the zwitterionic surfactant alone and in mixed systems with 12-4-12 and 12-5-12 gemini surfactants will be provided by NOESY experiments, to be discussed shortly. In comparing m values for the ZW3-12/12-4-12 versus the ZW3-12/12-5-12 and ZW312/12-6-12 systems, we see that greater attractive interactions are apparent in the mixed micelles with the 5 and 6 carbon tethers. The difference in m values between the two systems is significant! In the ZW3-12/12-5-12 and the ZW3-12/12-6-12 systems, m values are negative over the entire mole fraction range while for the ZW3-12/12-4-12 system, m values are negative at ZW3-12  0 .5. This difference is apparent in the plots comparing ideal vs experimental cmcmix values for the three systems (Figure 4). These values seem to suggest that increasing spacer length increases the attractive interactions between the cationic gemini surfactants and zwitterionic surfactants. It is likely that the addition of another methylene group in the spacer allows for a greater distance between the repulsing head groups of the gemini, and thus allows more room to accommodate the head groups of the zwittergent. However, if we take into account the ZW3-12/12-2-12 system studied by Bakshi and Singh 18 it becomes difficult to accurately predict a trend for the effect of increasing spacer length on the interaction parameter. The authors only present an average m value, mavg ~ = -0.5, which falls in between the values obtained for the ZW3-12/12-4-12 and the other two ZW3-12/gemini surfactant systems presented here.

2D-NOESY Spectroscopy 2D NOESY spectroscopy has been shown to be valuable tool for examining the local orientations of the molecules that make up self-assembled system like micelles 32-39. The



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22 presence of cross peaks in the NOESY spectra is indicative of protons that are in close enough proximity to one another (less than 5 angstroms) to interact with one another via through-space dipolar H-H interaction40-43. Therefore, 2D NOE spectroscopy can be a valuable tool in the study of surfactants and mixed surfactant systems, as it can provide a clearer understanding as to how the surfactants are orientating themselves in the micelles. In the case of mixed micelles with typical carbon chain lengths, the motions of the respective protons in the micelles undergoing cross-relaxation are in the extreme narrowing region (i.e., the reorientation of the chains in the micelle is in the picosecond range44-47). In this case, the cross peaks of the 2D NOESY spectrum will be of opposite sign to those of the diagonal and can be interpreted in terms of the manner in which the surfactant are co-assembling in the mixed micelles. Figures 6 presents the 2D NOESY spectrum (a) along with the chemical structure (b) for the zwitterionic surfactant. The strong cross peaks between the N-methyl groups and the CH2 bonded to the sulfonate indicate that the two groups must be in close proximity to one another, which means that the sulfonate group is likely bending back towards the ammonium functionality. This provides further support for the existence of a six-member, ring-like structure due to electrostatic interactions, which was proposed by Mullally and Marangoni26. From the NOESY spectrum it is also apparent that the end of the alkyl chain is bending back to come in contact with the methyl groups on the cationic nitrogen. Perhaps more interesting is the cross peak between the end of the alkyl chain and the CH2 bonded to the sulfonate group. Yet, there is no apparent evidence of interaction between the end of the alkyl chain with the other CH2 groups of the intercharge arm. This provides further evidence that the sulfonate group is bending towards the ammonium group.


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23 Figures 7-9 show the 2D NOESY spectra (a) along with the chemical structures (b) for the 12-s-12 cationic gemini surfactants. Peak assignments for the 1D proton spectra were made in accordance with the 2D COSY spectra obtained for each of these surfactants. The appearance of cross peaks in a COSY spectrum is indicative of two nuclei that are scalar coupled to each other40. The NOESY spectra show evidence that the alkyl chains are folding back to interact with the head group regions for both of the gemini surfactants. For the 12-4-12 gemini (Figure 7a) this is demonstrated by the cross peaks between the nitrogen methyl groups and the middle and end portions of the alkyl chain. Fairly intense cross peaks between the spacer protons and all parts of the alkyl chain are also clearly visible. In the 12-5-12 and the 12-6-12 NOESY spectra, (Figures 8a and 9a, respectively) there are no visible cross peaks between the end of the alkyl chain and the spacer protons. However, cross peaks between the nitrogen methyl groups and both the middle and end portions of the alkyl chain as well as the cross peak between the spacer protons and the middle portion of the alkyl chain do indicate that the 12-5-12 (Figure 8) and the 12-6-12 (Figure 9) alkyl chains are also bending back to interact with the head groups.



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24

6a

6b

Figure 6 (a) 2D NOESY spectrum, and corresponding 1D proton spectrum of 0.010M ZW3-12 , peak labels correspond to the chemical structure of ZW3-12. (b) Chemical structure of ZW3-12.


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25

7a

7b

Figure 7 – (a) 2D NOESY spectrum, and corresponding 1D proton spectrum of 0.010M 12-4-12 gemini. Peak labels correspond to the chemical structure of 12-4-12. (b) Chemical structure of 12-412.



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26

8a

8b

Figure 8 – (a) 2D NOESY spectrum and corresponding 1D spectrum of 0.010M 12-5-12 gemini. Peak labels correspond to the chemical structure of 12-5-12. (b) Chemical structure of 12-5-12.


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27

(C) (H) ((F) (A)&(B)

(G)

(I)

(D) & (E)

9a a

9b

(F) (B) (H) (E)H3C BrCH2 CH2 CH2 CH2 CH2 (I) CH2 CH2 CH2 + CH2 CH2 CH2 CH2 N CH2 CH2 CH2 CH2 + CH2 CH2 CH2 CH2 CH3 CH2 CH2 CH2 CH2 CH2 (D) N CH2 CH2 CH2 CH3 Br CH3 (A) (G) (C) CH3

H3C



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28 Figure 9 –(a) 2D NOESY spectrum and corresponding 1D spectrum of 0.010M 12-6-12 gemini. Peak labels correspond to the chemical structure of 12-6-12. (b) Chemical structure of 12-6-12.

The 2D NOESY and corresponding 1D proton spectra for the mixed ZW3-12/12-4-12 system at ZW3-12 = 0.8, 0.5, and 0.2 are shown in Figures 10 (a-c), respectively. The intense cross peaks, which exist amongst the CH2’s bonded to the nitrogen atoms in the head group region of the gemini and the zwittergent, i.e., the CH2 bonded to N+ in the gemini spacer, and the N+ methyls, indicate that there are strong interactions occurring between the head group regions of the two surfactants. This is most evident at ZW3-12 = 0.5 in which the zwittergent and gemini in solution are present in equivalent amounts. There is no apparent interaction between the protons of the zwittergent intercharge arm with those of the gemini spacer, as indicated by the lack of cross peaks between the middle CH2 of the intercharge arm with the spacer methylene groups. This may indicate that the zwittergent and gemini are oriented in such a fashion that the intercharge arm of the zwittergent and the gemini spacer are not in close proximity. What is most surprising about these spectra is that the ZW3-12 = 0.8 possesses number of intermolecular cross peaks, indicating significant mixing between both surfactants in the micellar phase (i.e., there is a reasonable distribution of both surfactants in the mixed aggregates). A similar result is found for the all three spectra for the other ZW3-12/12-s-12 gemini surfactants, except for the ZW3-12/12-4-12. Overall, in the ZW3-12/12-4-12 system, there seems to be less interaction between the end of the alkyl chains and the head group regions than what was observed in both the ZW3-12/12-5-12 and the ZW3-12/12-6-12 systems. In those individual NOESY spectra, there was clear evidence that the alkyl chain was bending back to interact with the head group regions in both the zwitterionic and gemini surfactants. However, for all mole fractions studied in the ZW3-12/12-4-12 system, there are no longer cross peaks evident between the alkyl chain


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29 and the CH2 bonded to the sulfonate of the zwittergent, nor between the end of the alkyl chain and the gemini spacer protons. Also, there are no visible cross peaks between the end of the



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30

10a


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31

10b



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32

10c

Figure 10 – 2D NOESY spectrum and corresponding 1D spectrum of ZW3-12/12-4-12 mixed system with total surfactant concentration = 0.010M, at a) ZW3-12 = 0.8; b) ZW3-12 = 0.5; and c) ZW3-12 = 0.2


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33 alkyl chain and the nitrogen methyl groups at ZW3-12 = 0.8 and 0.5. It is likely that the interaction between the alkyl chains of the two surfactants is prohibiting their ability to fold back. This corresponds with observations made by Bakshi et al. that the increased hydrophobic interactions, when the dual chains of a gemini are packed into the mixed micelle with the single tail of a conventional surfactant, play a predominant role in synergistic interactions (-m values)13,19,20. At ZW3-12 = 0.2, when the system is rich in gemini component, the cross peak between the end of the alkyl chain and the nitrogen methyl groups is now apparent. This is likely because with less zwittergent in the system the tails of the gemini are not as prohibited from folding back. The fact that there are weak cross peaks are present between the zwittergent and the gemini surfactant at that composition is consistent with the fact that the ZW3-12 values are significantly smaller than the solution mole fractions (ZW3-12). In fact, the 2D-NOESY at ZW3-12 = 0.2 has characteristics of the 2D-NOESY spectra of the individual surfactants, which may indicate a preference for the individual surfactants to form separate micelles in this composition range. At the lower amounts of added zwittergent, there is a decrease in the intermolecular hydrophobic interactions, which leads to less intermolecular mixing in the micelles, consistent with the fact that the m values positive have been obtained at lower ZW3-12 values for the ZW3-12/12-4-12 system. NOESY spectra for the mixed ZW3-12/12-5-12 and ZW3-12/12-6-12 systems at ZW3-12 = 0.2, 0.5 and 0.8 (Figures 11 (a-c) and Figures 12(a-c) ) show strong similarities in cross-peak patterns and intensities with each other, and some differences versus those of the ZW3-12/12-412 system. There are intense cross peaks in the head group regions indicating strong interactions between the head groups of the two surfactants, not unlike those interactions observed in the NOESY spectra of the ZW3-12/12-4-12 systems; these interactions appear to be more favourable



Page 34 of 48

34 in both the ZW3-12/12-5-12 and the ZW3-12/12-6-12 systems as demonstrated by the existence of more numerous, and stronger cross peaks. This corresponds with the calculated m values for these systems, which show that more favourable interactions exist between the ZW3-12 surfactant and the 5 and 6 carbon spacer length gemini surfactants. As mentioned previously, this may be attributed to the additional CH2 groups in the spacer, which allows the repulsing ammonium groups of the gemini to achieve greater separation which in turn provides more room to accommodate the zwittergent head groups. Interaction between the end of the alkyl chain and the nitrogen methyl groups is visible at all three mole fractions studied, unlike in the ZW3-12/124-12 system, although these interactions are greatly minimized in comparison with the NOESYs for the individual surfactants. Once again, there is no evidence of interaction between the protons of the zwittergent intercharge arm and those of the gemini spacer. Also notable is the fairly intense cross peak that exists between the CH2 bonded to the sulfonate of the zwittergent and the nitrogen methyl groups at all mole fractions in both zwittergent/gemini systems studied. From the NOESY of the zwittergent alone, it has been determined that this CH2 group is in close proximity to the N+ methyls. But, because peaks for the N+ methyl protons of the gemini and zwittergent overlap, it is difficult to determine if interactions between the CH2 bonded to the sulfonate of the zwittergent and the N+ methyl groups of the gemini are partly contributing to this cross peak. However, the fact that the cross peak is more intense at ZW3-12 = 0.5 than at ZW3-12 = 0.8, and remains fairly intense at ZW3-12 = 0.2, when the system is rich in gemini component, suggests that the sulfonate region of the zwittergent is indeed interacting with at least one of the ammonium functionalities of the gemini.


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35

11a



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36

11b


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37

11c




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38

12a


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12b



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40

12c



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41

Conclusions The mixed zwitterionic/cationic gemini systems have been analyzed through application of Rubingh’s Regular Solution Theory as well as 2D NOE Spectroscopy. The NOESY spectrum of ZW3-12, alone, indicates that the sulfonate group is bending back to interact with the ammonium group. This provides further support that the intercharge arm of the zwittergent is arranged in a ring-like structure, as proposed by Mullally and Marangoni26. This, perhaps, accounts for the cationic behaviour of the ZW3-12 in mixed systems with ionic surfactants, which have been studied previously21,30. The ZW3-12 also appears to be behaving as a cationic surfactant in the mixed systems with cationic geminis presented in this paper. The similarity in behavior between the DTAB/12-2-12 system studied by Bakshi et al.19 and the ZW3-12/12-s-12 systems, studied in this work, is evidence of this. NOESY spectra for ZW3-12, 12-4-12 and 125-12, alone, also show that the alkyl chains bend back to interact with the head group regions. The zwittergent/gemini systems studied display favourable interactions. The ZW312/12-4-12 system displays m values at ZW3-12  0.5, while –m values exist at all mole fractions in the ZW3-12/12-5-12 and the ZW3-12/12-6-12 systems. Interactions in the two systems are not truly synergistic as the second condition of synergism, |β𝑚| > |ln(𝐶𝑀𝐶1 𝐶𝑀𝐶2)| , is not fully met over all concentrations studied. Our results are in good agreement with results obtained for the ZW3-12/12-2-12 system studied by Bakshi and Singh20. The favourable interactions in these systems, in comparison with slight repulsive interactions observed for the ZW3-12/DTAB system, can be partly attributed to increased hydrophobic interactions due to the dual tails of the gemini surfactant. NOESY spectra of some of the mixed systems, which show minimal, or no, interaction between the alkyl chains and head groups, seem to support this theory.



Page 42 of 48

42 Greater attractive interactions have been observed in the ZW3-12/12-5-12 and the ZW312/12-6-12 systems than in the ZW3-12/12-4-12 system. This is in accordance with NOESY spectra, which show more numerous and more intense cross peaks in the mixed micelles of the zwittergent and the longer tether cationic gemini surfactants. This can possibly be attributed to the additional CH2 groups in the spacer, which may allow the repulsing ammonium groups of the gemini to achieve greater separation than in the 12-4-12 gemini hence allowing more room to accommodate the zwitterionic head groups.

Acknowledgements The financial support of NSERC (research grant, D.G.M., S.M., and A.M.) and St. F.X. University (T.R.) are greatly appreciated. D.G. and E.P. acknowledges the grant of NSERC Undergraduate Summer Research Awards. V.S. acknowledges the support of the Irving Foundation. We are grateful to Mike Lumsden, Pratap Bahadur, and Soumen Ghosh for stimulating discussions.


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43

References 1. Lomax, E. G. Amphoteric Surfactants.; M. Dekker: New York, 1996. 2. Friedli, F. E. Detergency of Specialty Surfactants; Marcel Dekker: New York, 2001. 3. Yang, Q.; Zhou, Q.; Somasundaran, P. 1H NMR Study of Micelles Formed by Mixture of Nonionic N-Dodecyl-Β-D-Maltoside and Cationic Gemini Surfactants. J. Mol. Liq. 2009, 146. 105-111. 4. Sugihara, G.; Nagadome, S.; Oh, S.; Ko, J. A Review of Recent Studies on Aqueous Binary Mixed Surfactant Systems. J. Oleo Sci. 2008, 57, 61-92. 5. Kumar, A.; Alami, E.; Holmberg, K.; Seredyuk, V.; Menger, F. M. Branched Zwitterionic Gemini Surfactants Micellization and Interaction with Ionic Surfactants. Colloids Surf. Physicochem. Eng. Aspects 2003, 228, 197-207. 6. Kabir-ud-Din; Sheikh, M. S.; Mir, M. A.; Dar, A. A. Effect of Spacer Length on the Micellization and Interfacial Behavior of Mixed Alkanediyl-,ω-Bis(Dimethylcetylammonium Bromide) Gemini Homologues. J. Colloid Interface Sci. 2010, 344, 75-80. 7. Bakshi, M. S.; Singh, J.; Kaur, G. Fluorescence Study of Solubilization of L-[Alpha]Dilauroylphosphatidylethanolamine in the Mixed Micelles with Monomeric and Dimeric Cationic Surfactants. J. Photochem. Photobiol. A. 2005, 173, 202-210. 8. Bakshi, M. S.; Sachar, S. Influence of Temperature on the Mixed Micelles of Pluronic F127 and P103 with Dim Ethylene-Bis-(Dodecyldimethylammonium Bromide). J. Colloid Interface Sci. 2006, 296, 309-315. 9. Yoshimura, T.; Ohno, A.; Esumi, K. Mixed Micellar Properties of Cationic Trimeric-Type Quaternary Ammonium Salts and Anionic Sodium N-Octyl Sulfate Surfactants. J. Colloid Interface Sci. 2004, 272, 191-196. 10. Sierra, M. L.; Svensson, M. Mixed Micelles Containing Alklyglycosides: Effect of the Chain Length and the Polar Head Group. Langmuir 1999, 15, 2301-2306. 11. Lainez, A.; Burgo, P. d.; Junquera, E.; Aicart, E. Mixed Micelles Formed by N-Octyl--DGlucopyranoside and Tetradecyltrimethylammonium Bromide in Aqueous Media. Langmuir 2004, 20, 5745-5752. 12. Misselyn-Bauduin, A.; Thibaut, A.; Grandjean, J.; Broze, G.; Jérôme, R. Mixed Micelles of Anionic-Nonionic and Anionic-Zwitterionic Surfactants Analyzed by Pulsed Field Gradient NMR. Langmuir 2000, 16, 4430-4435.



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44 13. Bakshi, M. S.; Singh, J.; Singh, K.; Kaur, G. Mixed Micelles of Cationic 12-2-12 Gemini with Conventional Surfactants: The Head Group and Counterion Effects. Colloids Surf. Physicochem. Eng. Aspects 2004, 237, 61-71. 14. Fang, X.; Zhao, S.; Mao, S.; Yu, J.; Du, Y. Mixed Micelles of Cationic-Nonionic Surfactants: NMR Self-Diffusion Studies of Triton X-100 and Cetyltrimethylammonium Bromide in Aqueous Solution. Colloid Polymer Sci. 2003, 281, 455-460. 15. Yang, Q.; Zhou, Q.; Somasundaran, P. Mixed Micelles of Octane-1,8 Bis(Dodecyl Dimethyl Ammonium Chloride) and N-Dodecyl-Β-D-Maltoside by 1H NMR Study. Colloids Surf. Physicochem. Eng. Aspects 2007, 305, 22-28. 16. Bakshi, M. S.; Kaur, G. Mixed Micelles of Series of Monomeric and Dimeric Cationic, Zwitterionic, and Unequal Twin-Tail Cationic Surfactants with Sugar Surfactants: A Fluorescence Study. J. Colloid Interface Sci. 2005, 289, 551-559. 17. Bakshi, M. S.; Kaura, A.; Mahajan, R. K. Effect of Temperature on the Micellar Properties of Polyoxyethylene Glycol Ethers and Twin Tail Alkylammonium Surfactants. Colloids Surf. Physicochem. Eng. Aspects 2005, 262, 167-174. 18. Bakshi, M. S.; Singh, J.; Kaur, J. Estimation of Degree of Counterion Binding and Thermodynamic Parameters of Ionic Surfactants from Cloud Point Measurements by using Triblock Polymer as Probe. J. Colloid Interface Sci. 2005, 287, 704-711. 19. Bakshi, M. S.; Singh, J.; Singh, K.; Kaur, G. Mixed Micelles of Cationic Gemini with Tetraalkyl Ammonium and Phosphonium Surfactants: The Head Group and Hydrophobic Tail Contributions. Colloids Surf. Physicochem. Eng. Aspects 2004, 234, 77-84. 20. Bakshi, M. S.; Singh, K. Synergistic Interactions in the Mixed Micelles of Cationic Gemini with Zwitterionic Surfactants: Fluorescence and Krafft Temperature Studies. J. Colloid Interface Sci. 2005, 287, 288-297. 21. McLachlan, A. A.; Marangoni, D. G. Interactions between Zwitterionic and Conventional Anionic and Cationic Surfactants. J. Colloid Interface Sci. 2006, 295, 243-248. 22. Rubingh, D. N. Mixed Micelle Solutions. In Solution Chemistry of Surfactants: Volume 1; Mittal, K. L., Ed.; Springer New York: Boston, MA, 1979, pp 337-354. 23. Wettig, S.; Verrall, R. E. Studies of the Interaction of Cationic Gemini Surfactants with Polymers and Triblock Copolymers in Aqueous Solution. J. Colloid Interface Sci. 2001, 244, 377-385. 24. Li, X.; Wettig, S. D.; Verrall, R. E. Isothermal Titration Calorimetry and Dynamic Light Scattering Studies of Interactions between Gemini Surfactants of Different Structure and Pluronic Block Copolymers. J. Colloid Interface Sci. 2005, 282, 466-477.


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47 CH3

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