Positional Isomers of Linear Sodium Dodecyl Benzene Sulfonate

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Articles Positional Isomers of Linear Sodium Dodecyl Benzene Sulfonate: Solubility, Self-Assembly, and Air/Water Interfacial Activity Jian-Guo Ma,†,‡ Ben J. Boyd,§ and Calum J. Drummond*,† CSIRO Molecular and Health Technologies, Bag 10, Clayton, Victoria, 3169, Australia, China National Lanxing Chemical Cleaning Company Xigu, Lanzhou, P. R. China, and Department of Pharmaceutics, Victorian College of Pharmacy, Monash UniVersity, Victoria 3052, Australia ReceiVed January 29, 2006. In Final Form: June 18, 2006 Commercial linear alkyl benzene sulfonates (ABS) are a very important class of anionic surfactants that are employed in a wide variety of applications, especially those involving wetting and detergency. Linear ABS surfactants generally consist of a complex mixture of different chain lengths and positional isomers. This diversity and level of complexity makes it difficult to develop fundamental structure-property correlations for the commercial surfactants. In this work, six monodisperse headgroup positional isomers of sodium para-dodecyl benzene sulfonate (Na-x-DBS, x ) 1-6) have been studied. The influence of headgroup position and added electrolyte (NaCl) on the solubility and self-assembly (micellar and vesicular aggregation and lyotropic liquid crystalline phase behavior) in the temperature range from 10 to 90 °C have been investigated. Additionally, the air-aqueous solution interfacial adsorption at 25 (no added NaCl) and 50 °C (from 0 to 1.0 M added NaCl) has been examined. The observed physicochemical behavior is interpreted in terms of local molecular packing constraints, and in the case of the lyotropic liquid crystalline behavior global aggregate packing constraints as well.

Introduction Industrially produced surfactants are often complex mixtures of many individual surface active components. Commercial surfactant polydispersity may arise due to the nature of the chemical reactions employed to produce the surfactant and/or the utilization of polydisperse raw materials. Synergistic or antagonistic behavior may result from mixtures of surfactants, and tailoring the polydispersity to contain certain components may provide access to properties not attainable with a monodisperse surfactant product. Developing an improved understanding of the properties of the individual components of commercial surfactant mixtures is a step toward being able to better manipulate application-related properties by controlling the relative amounts of the individual components through employing rational design principles. Linear alkyl benzene sulfonates (ABS) are arguably the most important class of commercial surfactant, used in many industrial applications. Commercial ABS surfactants are generally produced by using a process that results in a mixture of alkyl chain homologues with a range of headgroup positional isomers. The positional isomers have a molecular structure where the benzene sulfonate headgroup is attached at different positions along the alkyl chain.1 For example, the polydispersity of one commercial linear ABS surfactant is illustrated by the numerous peaks in its HPLC chromatogram in Figure 1a, highlighting the alkyl chain length distribution and the different isomeric components * To whom correspondence should be addressed. E-mail: [email protected]. † CSIRO Molecular and Health Technologies. ‡ China National Lanxing Chemical Cleaning Company Xigu. § Monash University. (1) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley & Sons: New York, 1978.

produced for each chain length. Not only are such complex mixtures difficult to characterize, and hence predict functional performance, the situation is even more complex as nominally the same surfactant, manufactured by different suppliers, may contain varying proportions of the individual isomeric components, as illustrated in Figure 1b. Dodecyl benzene sulfonates are known to form micelles in dilute solution and display an array of liquid crystalline phases at higher concentrations.2 In particular, it is already known that a micelle to vesicle aggregate transition can be induced by a change in ionic strength, resulting in a reduction of the airaqueous solution interfacial tension.3,4 This behavior is important from a practical perspective, as the ability of surfactants to reduce interfacial tension is often key to their performance. The ability to control the self-assembly aggregate structure and related solution rheology can also be desirable from a formulation and consumer acceptability perspective. However, despite the inherent complexity of linear ABS surfactant mixtures, and the potential for these components to dictate different performance-related physicochemical properties, the relative contribution of the positional isomers to this behavior is not well understood (at least by the academic scientific community). To our knowledge, there is yet to be a literature report of a full systematic study of the solubility, self-assembly, and interfacial properties of this important series of anionic surfactants and the influence of added electrolyte on the behavior of the individual components. In the present study, we describe an investigation of the solubility and self-assembly, including aggregation and lyotropic liquid crystalline phase behavior, of (2) Tezˇak, D.; Hertel, G.; Hoffmann, H. Liq. Cryst. 1991, 10, 15. (3) Farquhar, K. D.; Misran, M.; Robinson, B. H.; Steytler, D. C.; Morini, P.; Garrett, P. R.; Holzwarth, J. F. J. Phys.: Condens. Matter 1996, 8, 9397-9404. (4) Brinkmann, U.; Neumann, E.; Robinson, B. H. J. Chem. Soc. 1998, 94, 1281-1285.

10.1021/la0602822 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/19/2006

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Figure 2. Structures and abbreviations for the six positional isomers of para-dodecyl benzene sulfonate surfactants.

Figure 1. HPLC chromatograms of linear alkyl benzene sulfonates. Panel A: HPLC chromatogram of NANSA SSAS commercial ABS surfactant (Albright & Wilson). Panel B: HPLC chromatograms of the dodecyl benzene sulfonate region of three commercial surfactants, NANSA SSAS (Albright & Wilson), soft alkyl benzenesulfonate (Tokyo Kasei), and dodecene-1-LAS (Tokyo Kasei), illustrating the differences in positional isomer distribution between the commercial materials. For each chain length, the retention time sequence for the individual isomer peaks is 6 ) 5 < 4 < 3 < 2. The assignment of peaks was based on our own work with the individual Na-x-DBS isomers. Note that the relative amount (peak areas) of the individual isomers varies between products. The 1 isomer is not produced in the commercial synthetic routes.

the six positional isomers of sodium para-dodecyl benzene sulfonate. The structures of the primary n-dodecyl isomer (Na1-DBS) and the five secondary positional isomers (Na-x-DBS, x ) 2-6) are illustrated in Figure 2. The influence of headgroup position on air-water interfacial activity has also been investigated, with an emphasis on the effect of added electrolyte, by surface tension measurements. The 1-isomer is not produced in the conventional commercial synthetic routes but is included in this study so as to cover the complete series of possible paradodecyl benzene sulfonate isomers. Materials and Methods Materials. Sodium chloride (BDH, AR) was used as received. Water was obtained by passing de-ionized tap water through a Milli-Q Plus Ultrapure Water System (Millipore, Australia). Acetonitrile

was HPLC grade from Mallinckrodt. Na-x-DBS isomers were synthesized according to the method reported by Gray et al.5 The purity of the surfactants was determined by high performance liquid chromatography and by air - water interfacial tension measurements. Multiple recrystallization and drying stages were performed to remove any excess electrolyte produced in the synthetic routes. An isomer was considered to be free from excess electrolyte when the surface tension versus concentration curve did not change between two separate sequential recrystallization and drying stages. The HPLC system consisted of an Altex (San Ramon, CA) model 110A pump, a Rheodyne model 7120 syringe loading injector (Rohnert Park, CA) with a 50 µL loop attached, fitted with a Zorbax C18 column (250 × 4.6 mm, Alltech, Melbourne, Australia) and was eluted in isocratic mode (50:50 acetonitrile/ 0.15M NaClO4 in water at 1 mL/min) with UV detection at 225 nm collected by a Beckman System Gold Chromatographic system. In all cases the isomer purity was >97%. Solubility and Lyotropic Liquid Crystalline Phase Behavior. Lyotropic liquid crystalline phase behavior was investigated using polarized optical light microscopy with the water penetration scan technique6 and by viewing accurately prepared sealed samples through crossed polarized filters in a heated water bath. Polarized optical light microscopy was performed with a Mettler FP90 hot stage and viewed with an Olympus inverted IMT2 microscope fitted with crossed polarizing filters. A magnification factor of 150 was used. The microscope was connected to a Sony DXP-101C CCD camera for video capture and printout of images. Birefringent textures from the optical microscopy allowed the assignment of the anisotropic lyotropic liquid crystalline phases that were present.7,8 Isotropic phases were identified by their location next to the anisotropic phases and by their rheology. Krafft (solubility) boundaries were accurately determined by close inspection of dissolution temperatures of sealed, stirred samples under a magnifying lamp. Combination of the information obtained using all of the above techniques allowed construction of the approximate partial binary surfactant-water phase diagrams. Air-Aqueous Solution Interfacial Behavior. The method of du Nou¨y ring tensiometry9 was used to investigate the interfacial activity of the positional isomers, thermostated to either 25 or 50 ( 1 °C. Surface tension values were obtained in triplicate at each concentration studied. The critical micelle concentration (CMC), or (5) Gray, F. W.; Gerecht, J. F.; Krems, I. J. J. Org. Chem. 1955, 20, 511. (6) Rendall, K.; Tiddy, G. J. T.; Trevethan, M. J. Chem. Soc., Faraday Trans. 1983, 79, 637. (7) Rosevear, F. J. Am. Oil Chem. Soc. 1954, 31, 628. (8) Rosevear, F. J. Soc. Cosmetic Chem. 1968, 19, 581. (9) du Nou¨y, P. J. Gen. Physiol. 1919, 1, 521.

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Figure 3. Approximate partial binary surfactant-water phase diagrams for the six positional isomers of para-dodecyl benzene sulfonate. critical aggregation concentration (CAC), was determined as the average concentration at which there was a breakpoint in the surface tension versus concentration curve. From the adsorption isotherms at concentrations approaching the inflection point, the minimum area per molecule at the air/aqueous solution interface, Aa/w min, was calculated from the surface excess concentration, Γ (mol m-2), using the modified Gibbs adsorption equation10 Γ)

-1 dγ RT d ln c

(

)

(1)

1016 2 Å NAΓ

(2)

and Aa/w min )

where γ is the surface tension (N m-1), R is the gas constant, T is the temperature (K), c is the surfactant concentration (M), and NA is Avogadro’s number.

Results Lyotropic Phase Behavior. The approximate partial binary surfactant-water phase diagrams for each of the six positional isomers of Na-x-DBS obtained through the birefringence studies and Krafft boundary determinations are presented in Figure 3. At concentrations well above the Krafft point, the phase behavior of Na-1-DBS, Na-2-DBS, and Na-3-DBS was qualitatively very similar. All three surfactants displayed a micellar (L1) f hexagonal (H1) f lamellar (LR) progression with customary regions of phase coexistence being identified. The concentration boundary for the L1 f H1 regions occurred at progressively lower surfactant compositions, as the headgroup was located closer to the center of the alkyl chain, and only one type of lamellar phase was identified in the phase behavior of the first three isomers. None of these three isomers were found to display inverse phase structures at low water content between 10 and 90 °C. (10) Gibbs, J. In The Collected Works of J. W. Gibbs; Longmans: New York, 1931; p 219.

Figure 4. Krafft (solubility) boundary as a function surfactant concentration for the positional isomers of para-dodecyl benzene sulfonate. Squares correspond to Na-1-DBS, circles correspond to Na-2-DBS, and triangles correspond to Na-3-DBS.

The 0.1 wt % Krafft point, at which the crystals plus water transition to form the L1 micellar phase, decreased from approximately 52 °C for Na-1-DBS to 25 °C for Na-2-DBS, and 11 °C for the Na-3-DBS (Figure 4). The Krafft boundaries for the 4, 5, and 6 isomers were all below 0 °C. In contrast to the behavior of the 1, 2, and 3 isomers, the phase behavior of the 4, 5, and 6 isomers (Figure 3) was dominated by the presence of lamellar (LR and LR′) and L1 + LR coexistence regions. The composition at which there is an observed transition to an LR phase occurred at progressively lower surfactant content as the headgroup was positioned closer to the center of the alkyl chain. This indicated a gradual change in local molecular packing constraints, whereby higher curvature L1 micellar structures were less favored for the more bulky 5 and 6 isomers, relative to the 4 isomer (see below). The phase behavior of Na-4-DBS consisted of only three regions; the L1 phase up to 30% Na-4-DBS (w/w) and the LR

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Figure 5. Optical micrograph image (magnification ×40; using crossed polarizers) of a concentration gradient for the binary Na-6-DBSwater system at 25 °C. The phase progression with increasing water content (from left to right in the image) is assigned as inverse hexagonal (H2), bicontinuous cubic (V2), followed by two lamellar phases (LR′ and LR).

Figure 6. Krafft temperature at varying salt concentrations (0.1% (w/w) surfactant), for the positional isomers of para-dodecyl benzene sulfonate. Squares correspond to Na-1-DBS, circles correspond to Na-2-DBS, and triangles correspond to Na-3-DBS.

phase above 60% Na-4-DBS (w/w). A wide range of coexisting L1 + LR was observed between 30 and 60% surfactant (w/w). The phase diagrams of Na-5-DBS and Na-6-DBS were more complex, and although at low surfactant concentration they also displayed the L1 to L1 + LR progression, there were two major features differentiating them from the other four surfactants of the series. First, at high surfactant concentrations (>80% w/w), it appeared that inverse cubic (V2) and inverse hexagonal (H2) phases were present. At this stage, this is a tentative assignment as we did not comprehensively examine the phase boundaries in this compositional region of the phase diagram. Perhaps more interestingly, there is a region of coexisting lamellar phases (LR + LR′). For the 5 isomer there is an upper consolute temperature at approximately 35 °C, whereas for the 6 isomer there is no upper consolute temperature at least up to 90 °C. Figure 5 shows a concentration gradient image that illustrates the liquid crystalline phases present in the Na-6-DBS-water system at 25 °C. The concentration gradient image of the phase progression in the Na-5-DBS-water system at 25 °C is very similar. Influence of Added NaCl on Lyotropic Phase Behavior. The effect of added NaCl on the 0.1 wt % Krafft point was investigated for the 1, 2, and 3 isomers of Na-x-DBS (Figure 6). The addition of NaCl increased the Krafft point. For example,

Figure 7. Transition temperatures for the L1 + LR f L1 phase transition at varying salt concentration, for Na-x-DBS at 0.1% (w/ w) surfactant. Triangles correspond to Na-4-DBS, squares correspond to Na-5-DBS, and circles correspond to Na-6-DBS. Above the lines denoting the phase transition boundary for the three isomers, an isotropic L1 (micellar phase) exists, whereas below the boundary lines, an L1 + LR (vesicular dispersion) phase exists. The inset is an image of a vesicular dispersion formed by Na-6-DBS at 10wt % in 0.05 M NaCl viewed under crossed polarizing filters at 150× magnification at room temperature. The Maltese crosses disappear on heating when the system passes through the anisotropic lamellar (vesicular dispersion) to isotropic liquid (micellar solution) phase transition.

the addition of 1 M NaCl to 0.1% surfactant (w/w) resulted in an increase in the Krafft point of 33 °C for Na-1-DBS, 30 °C for the 2 isomer, and 19 °C for the 3 isomer (Figure 6). Once past the knee in the Krafft boundary, the solubility behavior of the three isomers, with increasing NaCl concentration, approximately track one another, with a temperature displacement. At 0.1% w/w surfactant, in the absence of added NaCl, the 4, 5, and 6 isomers displayed a L1 micellar phase. However, as seen in Figure 7, the addition of a small amount of electrolyte to these solutions induces an L1 (micellar solution) f L1 + LR (vesicular dispersion) phase transition. The vesicular dispersion induced by the addition of salt to a solution of Na-6-DBS is illustrated in the inset showing the characteristic Maltese cross appearance. Added electrolyte markedly increased the temperature required for the vesicular dispersion to transition to an isotropic

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Table 1. Critical Aggregation Concentration (CAC), Minimum Area Per Molecule at the Air/Aqueous Solution Interface (Aa/w min), and Surface Tension at a Concentration Just above the CAC (γCAC) for Na-x-DBS at 25 °Cg CAC (mmol dm-3) surfactant

this work

Na-1-DBSa Na-2-DBS Na-3-DBS

1.04 1.38

Na-4-DBS Na-5-DBS Na-6-DBS

1.65 1.94 2.53

literature 1.08,b 1.19c 1.50,b 1.46,c 1.40,d 1.48,e 2.22e 1.85,b 1.59c 2.30b 2.56,b 1.80f

Aa/w min (nm2)

γCAC (mN‚m-1)

0.43 0.54

40.2 38.5

0.55 0.61 0.65

36.5 34.5 33.6

a No values could be determined for Na-1-DBS due to its very high Krafft temperature rendering it insoluble at 25 °C even at very low surfactant concentrations. b Conductivity measurement.12 c Conductivity measurement.13 d Interfacial tension measurement (aqueous solution/ decane).15 e Conductivity measurement.14 f Conductivity measurement.11 g CAC is used generically in this table as an indication of a transition to either micelles or other aggregates.

solution, i.e., an L1 + LR (vesicular dispersion) f L1 (micellar solution) transition. A lower electrolyte concentration is required to stabilize the vesicular dispersion for the 6 isomer; 0.2 M NaCl is sufficient to increase the thermal stability to >70 °C, whereas for the 5 and 4 isomer, 0.3 and >0.5 M NaCl were required to achieve comparable thermal stability, respectively. Air-Aqueous Solution Interfacial Behavior. The interfacial adsorption behavior of five of the six positional isomers was studied at 25 and 50 °C. The terminal positional isomer, Na1-DBS, was not investigated, as its solubility, even in very dilute solutions, was the limiting factor at these experimental temperatures (Figure 4). The adsorption isotherms for the five isomers at the air-aqueous solution interface, at 25 °C, displayed conventional behavior, with a decrease in surface tension with increasing surfactant concentration being observed before a plateau is reached. No minima were observed in the surface tension versus concentration plots,z reflecting the purity of the surfactants synthesized for this study. The individual adsorption isotherms can be accessed in the Supporting Information for this article. The critical aggregation concentration (CAC), minimum area per molecule at the air/aqueous solution interface (Aa/w min) and surface tension at a concentration just above the CAC (γCAC) derived from the adsorption isotherms are listed in Table 1. The term CAC is used in a generic sense in this discussion to indicate the formation of surfactant aggregates in general, as indicated already, either micelles or vesicles can form depending on solution electrolyte conditions and temperature. The data for the CACs (Table 1) generally agree well with previously determined values obtained from the literature.11-15 The CAC shows a trend to higher concentrations as the headgroup is positioned closer to the center of the alkyl chain. A plot of isomer number versus log [CAC] in Figure 8 reveals an approximately linear relationship between log [CAC] and headgroup position along the dodecyl chain. The CAC trend is consistent with the HPLC retention time trend (Figure 1) and shows that the hydrocarbon part of the free energy of transfer from dilute solution to an aggregate is qualitatively similar to the individual isomer hydrocarbon contributions to the partitioning phenomena in the HPLC process. From the CAC values the order of hydrophobicity of the isomers is 2 > 3 > 4 > 5 > 6. Similarly, Figure 8 also illustrates the trend toward increasing minimum area per molecule with increasing isomer number, 2 where the value for Aa/w min for the 2 isomer is 0.43 nm , increasing to 0.65 nm2 for the 6 isomer (Table 1). Also, on this table the

Figure 8. Dependence of CAC values (filled squares) and minimum area per molecule at the air/aqueous solution interface (Aa/w min) (unfilled circles) on the position of attachment of the headgroup for Na-x-DBS at 25 ( 1 °C. The individual adsorption isotherms from which these data were derived can be obtained in the Supporting Information for this article.

value of γCAC was observed to monotonically decrease, reflecting an increase in effective hydrocarbon per unit surface area, as the headgroup was positioned closer to the center of the alkyl chain. Influence of Added NaCl on Air-Aqueous Solution Interfacial Behavior. The influence of salt on the interfacial behavior was investigated at 50 °C, and not at 25 °C, due to the increase in Krafft boundary temperature observed for the 2 and 3 isomer with increasing electrolyte, as illustrated in Figure 6. The effect of addition of 0.01, 0.1, and 1.0 M NaCl on the CAC, minimum headgroup area, and surface tension at the CAC, at 50 °C, are detailed in Table 2. There is a clear trend in the CAC data in Table 2, whereby the addition of salt results in progressively lower values for CAC and is of a greater magnitude than the effect of moving the headgroup along the hydrocarbon tail. The shift toward lower CAC values on addition of electrolyte is consistent with the data trend provided by Zhu et al., who have studied the surface-active properties of the same surfactants in hard river water.16 The interfacial tension at the CAC also dropped substantially as the salt concentration was increased for all five isomers. Figure 9 shows the influence of salt addition on the adsorption isotherms for Na-2-DBS, and is representative of similar plots obtained for the other isomers (see the Supporting Information). The CAC data for each isomer obtained from the individual adsorption isotherms with added salt are plotted in Figure 10, showing the decrease in CAC value with increasing salt concentration in each case. It should be noted that the magnitude of change in CACs in Figure 10 are comparable to those shown in Figure 8 for CAC; the two plots being scaled differently on the log [CAC] axis. All five isomers also displayed a reduction in minimum area per molecule at the air/aqueous solution interface with increasing salt concentration. The Aa/w min values for each isomer across the four salt concentrations are illustrated in Figure (11) Sein, A.; Engberts, J. B. F. N. Langmuir 1995, 11, 455-465. (12) Dick, S. G. M.Sc. Thesis; The University of Melbourne: Melbourne, Australia, 1972. (13) Ludlum, D. B. J. Phys. Chem. 1956, 60, 1250-1244. (14) van Os, N. M.; Daane, G. J.; Bolsman, T. A. B. M. J. Colloid Interface Sci. 1988, 123, 267-274. (15) van Os, N. M.; Rupert, L. A. M.; Smit, B.; Hilbers, P. A. J.; Esselink, K.; Bohmer, M. R.; Koopal, L. K. Colloids Surf. A 1993, 81, 217-229. (16) Zhu, Y.-P.; Rosen, M. J.; Morrall, S. W.; Tolls, J. J. Surfactants Deterg. 1998, 1, 187-193.

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Table 2. Critical Aggregation Concentration (CAC),a Minimum Area per Molecule at the Air/Aqueous Solution Interface (Aa/w min), and Surface Tension at a Concentration Just above the CAC (γCAC) for Na-x-DBS at 50 °C, at Various Concentrations of Added NaCl CACa (µmol dm-3) [added NaCl] (M) surfactant

γCAC (mN‚m-1)

2 Aa/w min (nm )

[added NaCl] (M)

[added NaCl] (M)

0

0.01

0.1

1

0

0.01

0.1

1

0

0.01

0.1

1

832 1490 1580 1818 2530

246 442 572 743 934

58.9 115 161 202 247

7.65 14.5 16.8 20.0 23.7

0.43 0.53 0.56 0.59 0.67

0.40 0.46 0.50 0.53 0.54

0.40 0.47 0.48 0.50 0.51

0.39 0.45 0.43 0.47 0.49

36.1 38.6 36.0 34.6 32.1

34.6 34.8 31.9 30.2 29.6

30.9 30.2 27.9 25.9 24.8

27.4 27.1 25.9 25.0 24.7

Na-1-DBSb Na-2-DBS Na-3-DBS Na-4-DBS Na-5-DBS Na-6-DBS

a The term CAC is used in this table to indicate the formation of surfactant aggregates in general, as micelles themselves may not explicitly be formed. b No values could be determined for Na-1-DBS due to its very high Krafft boundary temperatures rendering it insoluble at 50 °C, even at 0 M added NaCl.

Figure 9. Adsorption isotherms for Na-2-DBS at the air-aqueous solution interface at varying added salt concentration at 50 ( 1 °C. Filled squares correspond to no added salt, unfilled circles correspond to 0.01 M NaCl, unfilled squares correspond to 0.1 M NaCl, and filled circles correspond to 1 M NaCl. Isotherms were also obtained for the other four isomers at the same concentrations of NaCl (see the Supporting Information).

Figure 11. Dependence of minimum area per molecule at the air/ aqueous solution interface (Aa/w min) on the position of attachment of the headgroup for Na-x-DBS at 50 ( 1 °C. Filled squares correspond to no added salt, unfilled circles correspond to 0.01 M NaCl, unfilled squares correspond to 0.1 M NaCl, and filled circles correspond to 1 M NaCl.

of the isomers when 0.01 M or greater salt is added to the aqueous medium.

Discussion

Figure 10. Dependence of CAC values on the position of attachment of the headgroup for Na-x-DBS at varying added salt concentration at 50 ( 1 °C. Filled squares correspond to no added salt, unfilled circles correspond to 0.01 M NaCl, unfilled squares correspond to 0.1 M NaCl, and filled circles correspond to 1 M NaCl.

11. Interestingly, the almost linear trend in Aa/w min with isomer number when no salt is present is not reflected in the behavior

The polydispersity of commercially produced surfactants is often a result of financial considerations: the cost of producing highly pure, well characterized surfactants from monodisperse raw materials is frequently prohibitive. As a consequence, the behavior of these complex mixtures is often difficult to interpret. Specific work is generally required to understand which components provide the different behavioral attributes to the mixture in a given application and how best to optimize the distribution of components in the mixture during synthesis to maximize performance. In previous work, we have taken the approach of understanding the behavior of the individual components from mixtures such as alkyl polyglucosides17-19 and then attempt to relate these to the behavior of the commercial mixtures. In the current study, we apply this strategy to linear alkyl benzene sulfonates, arguably the most important of these complex commercial surfactant mixtures. The components of the com(17) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Weerawardena, A.; Furlong, D. N.; Grieser, F. Langmuir 2001, 17, 6100-6107. (18) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Grieser, F. Langmuir 2000, 16, 7359-7367. (19) Weerawardena, A.; Boyd, B. J.; Drummond, C. J.; Furlong, D. N. Colloids Surf. A 2000, 169, 317-328.

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mercial mixtures differ primarily in alkyl chain length and position of the benzene sulfonate headgroup along the chain (Figure 1a). As a first step toward this task, we have synthesized pure dodecyl benzene sulfonate isomers, where the headgroup is attached sequentially from positions 1 to 6 along the alkyl chain. Structural Effects. The lyotropic phase behavior can be interpreted in terms of local molecular and global aggregate packing constraints. The solubility is governed by how amenable the molecular shape is to forming strong intermolecular interactions in the crystalline solid-state. The Krafft boundary temperatures decreased as the headgroup was positioned closer to the center of the alkyl chain. Clearly the strength of the intermolecular interaction, presumably dictated by van der Waals interchain interaction, follows the isomer sequence of 1 > 2 > 3 > 4, 5, and 6. This finding is in line with previous observations for the positional isomers of dodecyl β-D-glucoside.17 The role of the effective molecular shape in governing selfassembly behavior, in low to mid concentration surfactant solutions, can be discussed in terms of the critical packing parameter (CPP) popularized by Israelachvili et al.,20,21 where

CPP )

V a0lc

(3)

with lc defined as the effective length of the surfactant chain in the molten state, a0 as the effective surfactant headgroup area (determined by the balance of interchain attractive and headgroup repulsive interactions, and taken as the area at the headgoupchain interface), and V as the average volume occupied by the surfactant molecule. In the absence of strong interactions between aggregates the preferred aggregate morphologies for different values of CPP are spheres for CPP ) 1/3, cylinders for 1/3 < CPP < 1/2, bilayers for 1/2 < CPP < 1, and inverted structures for CPP > 1. In concentrated surfactant solutions, global aggregate packing constraints become important. For example once the physical limit is reached for packing a particular aggregate morphology into a given volume, in order for more surfactant to be added to the lyotropic liquid crystalline system, an aggregate morphology change must occur, e.g., cubic close packing of spheres may transition to hexagonal close packing of cylinders which in turn may transition to bicontinuous cubic structures which may transition to lamellae which may transition to inverse structures. This model sequence of phase transitions is not necessarily observed in all surfactant-water systems. Some phase transitions will be precluded by factors relating to the molecular structure and solvating environment. The lyotropic liquid crystalline phase behavior was dominated by the formation of a micellar L1 phase for Na-1-DBS, Na-2DBS, and Na-3-DBS at lower concentration and a hexagonal phase followed by a lamellar phase as the concentration was increased. The effective molecular shape of the 1, 2, and 3 isomers is a normal truncated cone shape. Consequently, at relatively low surfactant concentrations, the micelles will be cylindrical, and as the surfactant concentration (and dissociated counterion concentration) increases, the isotropic solution of micelles transitions to a two-dimensional ordered hexagonal phase. As the surfactant concentration is increased further, dissociated counterions will screen the headgroup charge and will reduce a0, with the effective molecular shape transforming to a more (20) Israelachvili, J.; Mitchell, D.; Ninham, B. J. Chem. Soc., Faraday Trans. 2 1975, 72, 1525. (21) Israelachvili, J.; Mitchell, D.; Ninham, B. Biochim. Biophys. Acta 1977, 470, 185.

cylinder-like structure. Additionally, once the hexagonal close packing limit is reached, a phase transition must occur in order to continue to accommodate more surfactant in the lyotropic liquid crystalline system. Both of these factors promote a hexagonal (2D ordered state) to lamellar (1D ordered state) phase transition. The 4, 5, and 6 isomers did not form a normal hexagonal phase but rather formed a lamellar phase at increasingly lower concentrations, indicating the preference for packing of these surfactants in bilayers. Wide coexistence regions of L1 + LR are present. Na-5-DBS and Na-6-DBS exhibited a region of coexisting lamellar phases (LR + LR′), which has been observed previously for Na-5-DBS.22 The partial phase diagram presented in that work22 agrees well with that presented in Figure 3, although the compositional boundary for the LR f LR + LR′ transition at low temperature was at a slightly lower surfactant content. It is not possible from these studies, or the deuterium NMR studies of the previous work by Ockelford et al.,22 to suggest the structural difference between the coexisting lamellar phases. Coexisting lamellar phases have also been reported in cationic surfactant systems by McGrath and Drummond.23 The phase diagrams for Na-1-DBS, Na-4-DBS, and Na-6DBS have been reported in the literature2 but differ in some aspects from those which we present here. Although the phase diagram for the 4 isomer was very similar, there was no hexagonal phase reported for the 1 isomer. The previous report also details the same vesicular dispersion at low surfactant content for the 6 isomer; however, they detected a transition from the L1 + LR dispersion to a nematic phase at approximately 60 °C. This phase was not detected in this work. The previous authors also did not report coexisting lamellar phases at higher surfactant content. The reasons for these discrepancies are not clear at this time. At very high surfactant content greater than 80% w/w for Na-5-DBS and Na-6-DBS, a series of transitions from lamellar phase through inverse cubic and inverse hexagonal phases are observed in both cases. This has not been reported previously for these surfactants but is very similar to the phase progression reported for Aerosol OT,24 an analogous surfactant with an anionic headgroup. The effective molecular shape of the 4, 5, and 6 isomers also appears to be a normal truncated cone at low surfactant concentrations. However, the CPP value must be approaching a value of 1/2, as the micellar solution transitions to a lamellar phase with increasing concentration (Figure 3). For the 5 and 6 isomers, as the surfactant concentration becomes relatively high, dissociated counterions must screen the headgroup charge and reduce a0 to an extent where, in combination with the bulky hydrocarbon chain region, inverse structures are favorable. The influence of surfactant structure on adsorption behavior at the air-water interface is in accord with previous work on an analogous series of sodium decyl-x-benzene sulfonates.14 Moving the headgroup to the center of the alkyl chain in both cases induced a trend toward higher concentrations at the CAC (Figures 8 and 10). We also observed a trend toward a greater headgroup area at the interface, which is consistent with the findings of Das et al., who propose that for Na-6-DBS one of the arms of the alkyl chain occupies space close to the aromatic ring, essentially increasing the effective headgroup area.25 A similar trend has (22) Ockelford, J.; Timimi, B. A.; Narayan, K. S.; Tiddy, G. J. T. J. Phys. Chem. 1993, 97, 6767. (23) McGrath, K. M.; Drummond, C. J. Colloid Polym. Sci. 1996, 274, 316333. (24) Rogers, J.; Winsor, P. A. Nature 1967, 216, 477-479. (25) Das, S.; Bhirud, R. G.; Nayyar, N.; Narayan, K. S.; Kumar, V. V. J. Phys. Chem. 1992, 96, 7454-7457.

Isomers of Sodium Dodecyl Benzene Sulfonate

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Figure 12. Corrins-Harkins plot showing the dependence of CAC values on concentration of added electrolyte for Na-x-DBS at 50 ( 1 °C. Filled squares correspond to Na-2-DBS, unfilled circles correspond to Na-3-DBS, filled triangles correspond to Na-4-DBS, unfilled squares correspond to Na-5-DBS, and filled circles correspond to Na-6-DBS.

Figure 13. Dependence of minimum area per molecule at the air/ aqueous solution interface (Aa/w min) on concentration of added electrolyte for Na-x-DBS at 50 ( 1 °C. Filled squares correspond to Na-2-DBS, unfilled circles correspond to Na-3-DBS, filled triangles correspond to Na-4-DBS, unfilled squares correspond to Na-5-DBS, and filled circles correspond to Na-6-DBS.

been observed in the phase behavior of a series of positional isomers of a nonionic carbohydrate based surfactant, dodecyl β-D-glucoside.17 Electrolyte Effects and Vesicle Formation. The influence of salt on the behavior of Na-x-DBS surfactants is profound. The increase in Krafft point observed for Na-1-DBS, Na-2-DBS, and Na-3-DBS in Figure 6 has implications for the practical use of these materials in the presence of electrolyte, in light of potential solubility and precipitation concerns for materials that contain high levels of the 2 and 3 isomer. The presence of added electrolyte has already been reported to induce micellar to lamellar phase transitions for ABS surfactants.3,4 A critical salt concentration (CSC) for sodium tridecyl-6-benzene sulfonate at 25 °C has been reported to be ∼20 mM NaCl, above which this molecule forms vesicles, rather than micelles at the CAC.4 This molecule is structurally similar to those presented here. On the basis of the relationship between the previous findings for sodium tridecyl-6-benzene sulfonate and the present study (Figure 7), we propose that, for the series of positional isomers Na-4-DBS, Na-5-DBS and Na-6-DBS at 25 °C, CSCs exist at approximately 0.22, 0.10, and 0.04 M NaCl, respectively, whereas at 50 °C, the CSCs are approximately 0.38, 0.19, and 0.11 M NaCl, respectively. If we assume that as salt is added changes in the effective surfactant headgroup area (a0), due to charge screening by the sodium counterions control the micelle to vesicle transition, then we can employ the Aa/w min values to examine what a0 value ) is the phase transition trigger. From the (estimated from Aa/w min data (Table 2 and Figures 7 and 11), the 50 °C CSC and Aa/w min trigger values are estimated as 0.47, 0.50, and 0.51 nm2 for Na4-DBS, Na-5-DBS, and Na-6-DBS, respectively. In other words, a0 values greater than these trigger values provide an effective molecular shape that promotes micellar solution formation in preference to vesicular dispersion formation. To further investigate this phenomenon, the data were reviewed by utilizing a Corrins-Harkins plot, whereby log [CAC] was plotted against total electrolyte concentration (Figure 12). Departure from linearity in this type of plot is indicative of an electrolyte-induced phase change, and has been used previously to identify spherical to rodlike micelle transitions in cationic

surfactants.26,27 For all five Na-x-DBS isomers, there appears to be a break in linear-like behavior between the CAC + [NaCl] values of 0.1 and 1.0 M. For the 4, 5, and 6 isomers this may reflect a difference in the free energy of transfer of a surfactant monomer from dilute solution to a micellar state and to a vesicular state. The gradient change between CAC + [NaCl] values of 0.1 and 1.0 M for the 2 and 3 isomer, as well, may reflect that the micelles undergo an aggregate morphology change in this concentration range. The trend for minimum area per molecule at the air/aqueous solution interface with no added salt at 50 °C (Table 2 and Figure 11) is an approximately linear progression with isomer number and of similar magnitude to that at 25 °C (Table 1 and Figure 8), with the 6 isomer having the highest Aa/w min value and the 2 isomer the lowest. The trends observed for the Aa/w min values, in the no added salt condition, are presumably reflecting the combined influence of the repulsive interaction between the charged headgroups and the attractive interaction between the hydrocarbon portions of the isomers, with this last effect also influenced by steric considerations. Figure 13 displays the minimum area per molecule at the air/aqueous solution interface as a function of the total electrolyte concentration. The results of Figures 11 and 13 suggest that as salt is added to the aqueous solution the component due to the headgroup repulsion becomes less influential and the packing requirements of the hydrophobe begin to assume more importance in dictating the Aa/w min values.

Summary and Conclusions Six headgroup positional isomers of sodium para-dodecyl benzene sulfonate (Na-x-DBS, x ) 1-6) have been investigated. The 1, 2, and 3 isomers exhibit a crystal plus water to isotropic liquid phase transition in dilute surfactant solution, with the Krafft (solubility) boundary sequence for the isomers being 3 < 2 < 1. Above the solubility boundary, with increasing surfactant concentration, these three isomers have phase transitions from (26) Johnson, S. B.; Drummond, C. J.; Scales, P. J.; Nishimura, S. Langmuir 1995, 11, 2367-2375. (27) Johnson, S. B.; Drummond, C. J.; Scales, P. J.; Nishimura, S. Colloids Surf. A 1995, 103, 195-206.

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micellar to normal hexagonal to lamellar. The 4, 5, and 6 isomers do not exhibit a solubility boundary in the temperature region that was studied. In the absence of NaCl, with increasing surfactant concentration, the 4, 5, and 6 isomers have phase transitions from micellar to lamellar. The 5 and 6 isomers display two different lamellar phases and also appear to exhibit both a cubic and an inverse hexagonal phase at relatively high surfactant concentrations. For the five individual isomers, where the solubility boundary was not problematic, the sequence for the critical aggregation concentration (CAC) and the minimum molecular packing area at the air-water interface (Aa/w min) is 2 < 3 < 4 < 5 < 6. The air-water interfacial tension (γCAC) at a surfactant concentration just above the CAC is generally in the sequence 6 < 5 < 4 < 3 < 2. The addition of NaCl increases the Krafft temperature for the 1, 2, and 3 isomers. The addition of NaCl to relatively dilute solutions of the 4, 5, and 6 isomers can induce an isotropic liquid (micellar solution) to an anisotropic lamellar (vesicular dispersion) phase transition. For the 4, 5, and 6 isomers, at certain NaCl concentrations, heating can lead to a phase transition from a vesicular dispersion to a micellar solution. The addition of NaCl decreases the CAC, Aa/w min, and γCAC, but the relative isomer trends remain the same. The observations are consistent with the physicochemical behavior being predominantly governed by local molecular packing constraints in the low surfactant concentration regions. In line with expectations, global aggregate packing constraints also become important in the mid to high surfactant concentration regions. Industrial surfactant scientists have known for many years that varying the ratio of the isomers through modifications in the synthetic procedures can dramatically change the properties of the final ABS formulations. These scientists also know that the structure of ABS solutions can be markedly changed by adding electrolyte. The work reported herein develops structure-property correlations for some of the individual isomeric components of linear ABS surfactants. We demonstrate that the individual

Ma et al.

isomers have distinctly different physicochemical properties in aqueous solutions. We also show that the addition of electrolyte can induce significant phase and adsorption changes. This is consistent with the experience of the industrial scientists on the more complex commercial mixtures. It is anticipated that a better understanding of the role of the individual surfactant components of linear ABS surfactants should assist formulation scientists. The formation of vesicular structures, and the accompanying decrease in surface tension at the CAC, occurs at well defined electrolyte concentrations, as evidenced in the current study. This reduction in surface tension is of particular importance for the use of these materials in the removal of soils. It is wellknown that a reduction in surface tension can often be correlated with enhanced detergency, due to enhanced roll-up or displacement of oily soils from surfaces. Phase transitions in the binary surfactant-water system may also correspond to the formation of liquid crystalline structures at the surface of hard soils, facilitating removal. Thus, the formation of vesicular structures, the concurrent influence of electrolyte on the surface tension at the CAC, and their impact on detergency performance for the Na-x-DBS isomers are the subject of a forthcoming publication.28 Acknowledgment. We thank Dr. Irena Krodkiewska for assistance with some of the synthesis. J.-G.M. thanks CSIRO for hosting his visit, and particularly thanks C.J.D. for support and supervision. C.J.D. acknowledges the receipt of an Australian Research Council Federation Fellowship. Supporting Information Available: For Na-x-DBS isomers where x ) 2, 3, 4, 5, and 6, air-water interfacial tension (γLV) versus surfactant concentration curves at 25 °C (Figure S1) and γLV versus surfactant concentration curves at 50 °C with 0, 0.01, 0.1, and 1.0 M NaCl added for x ) 3-6 (Figures S2-S5). This material is available free of charge via the Internet at http://pubs.acs.org. LA0602822 (28) Weerawardena, A.; Boyd, B. J.; Drummond, C. J. Manuscript in preparation.