Effect of Surfactant Tail Structure on Phase Behavior of Branched and Linear Alkylbenzene Sulfonate in Water and Oil Ternary Systems

Transcription

1 Effect of Surfactant Tail Structure on Phase Behavior of Branched and Linear Alkylbenzene Sulfonate in Water and Oil Ternary Systems Abeer Al Bawab 1, 2, Ayat Bozeya 2, Fadwa Odeh /s Hamdi Mango Center for Scientific Research, The University of Jordan, Amman, 11942, Jordan 2 Chemistry Department, Faculty of Science, The University of Jordan, Amman, 11942, Jordan * Author for correspondence; drabeer@ju.edu.jo or abeerbawab@yahoo.com, Tel , Fax Abstract Systematic investigations into the phase behavior of a ternary system of linear alkyl benzene sulfonate (LAS) or branched alkyl benzene sulfonate (BAS) in oil (corn or olive oil) and water were carried out. The study showed that there is a relationship between the surfactants chemical structure, aggregation properties, and the formation of micelles. The aggregation properties were dramatically affected by small structural variations of the hydrophobic group at constant carbon number. Micelles formation was found to be correlated with the extent of chain branching by affecting oil-water interfacial tension, where the branched alkyl tail has significantly more ability to form micelles than the linear one. It was found possible to distinguish between LAS and BAS based on their phase behavior. Keywords-Linear alkyl benzene sulfonate; Branched alkyl benzene sulfonate; Phase diagram; Micelles; Aggregation I. INTRODUCTION Surfactants play an important role in many field of chemistry due to their unique behavior at interfaces. The opposing interaction of the hydrophilic head group and hydrophobic tail with the surrounding medium is the basis for the interfacial properties of surfactants. Alkyl benzene sulfonate (ABS) is an anionic surfactant characterized by hydrophobic (nonpolar) and hydrophilic (polar) groups. It is composed of an aliphatic alkyl chain which represents the hydrophobic part, a benzene ring and a sulfonate group which represent the hydrophilic part. The benzene ring is randomly distributed in all positional isomers, except the 1-phenyl and the sulfonate group which is in the para position [1]. According to the aliphatic alkyl chain structure, there are two types of alkyl benzene sulfonates, Linear alkyl benzene sulfonate (LAS) with linear alkyl chain, and branched alkyl benzene sulfonate (BAS) with branched alkyl chain [2]. The linear alkyl chain has typically 10 to 13 carbon atoms and the commercial LAS consists of more than 20 individual components of closely related homologues and isomers, representing different alkyl lengths and aromatic ring positions along the linear alkyl chains [2]. The best known BAS is tetrapropylene benzene sulfonate (TPBS), which is produced by reacting benzene with alkyl group (mainly tetrapropylene) derived from polymerizing propylene. However during the polymerization of propylene cracking and recombination result in the formation of a complex mixture of highly branched isomers and homologues. This mixture gives a highly branched BAS upon sulfonation [1, 2]. Historically, BAS surfactants -such as those based on tetrapropylene- were used in detergent formulation. A growing use of BAS led to the unexpected formation of strong foam in sewage treatment plants and sewage-polluted surface water. Due to the fact that BAS was found to be very DOI: / _

2 poorly biodegradable, it has become a threat to drinking water supplies [3]. Many efforts were made to improve ABS manufacturing to produce them as linear as possible [4]. The switch to better degradable LAS was not carried out by all nations, i.e. BAS are still extensively used in developing countries. Although LAS offers advantages in terms of performance characteristic, such as foam generation and detergency, BAS exhibit favorable physical properties in special formulations, like synthetic laundry bars which are more desirable for hard water and/or cold water cleaning properties. Another argument for the use of BAS arises from cheaper raw materials which can be of importance, particularly in less economically favored countries [5]. Some studies have been conducted to determine the effect of surfactant tail and head group structure on the air-water and water-oil interface [6-11], microemulsion [12] and liquid crystals [13]. These studies showed that tail structure has large effect on the packing, contact angle of water drops and efficiency of surface tension lowering of the surfactant in the studied systems [6-11]. Also, it was found that presence of the benzene ring increases the disorder degree compared with tails without a benzene ring [11]. On the other hand, no effect was found for variations in the head group structure. However, there is an information deficiency regarding the effect of surfactants structure on the phase diagram of oil/surfactant/water systems. This lack of information is due to difficulties in determining the complete phase diagram, which needs considerable work and skills. This work aimed to study the effect of LAS and BAS tail structure on the phase behavior of the surfactant in water, when solubilized with two different oils separately (corn and olive), to relate the effect of tail structure of ABS surfactant molecules in water and oil system ternary phase diagram II. MATERIALS AND METHODS without further purification: linear dodecylbenzene sulfonic acid sodium salt standard (Commercial- LAS) (Acros), and sodium dodecyl benzene sulfonate branched (BAS) (Pilot), corn oil (Afia) and olive oil (Nabilsi). Water was deionized and distilled. An isomerically pure para form of linear dodecylbenzene sulfonate (pure-las) was prepared by the direct sulfonation of 1-phenyldodecane (Acros) with chlorosulfonic acid (Acros) [13] The commercial-las was consisted of a mixture of linear and branched alkyl benzene sulfonate isomers. The Pure-LAS was consisted of only one linear isomer of alkyl benzene sulfonate (dodecyl) B. Phase Diagram Determination Two types of LAS standards were used. The isomerically pure-las which were prepared in our lab and the commercial LAS which consisted of linear isomers and different homologue in addition to branched alkyl benzene sulfonate. A stock solution 21% of BAS in water was used for BAS. Two oils were used, corn and olive oils; those oils were chosen as representative oils of what is usually used in daily. C. Sample Preparation for the Phase Diagram The solubility regions were determined by addition of oil to a combination of surfactant and water, while taking note of the point of clarity and turbidity. The extent of the solubility regions were confirmed first by centrifugation of the sample at 4000 rpm for 5 minutes; second, by preparing a series of samples with composition close to the solubility limits; and third, by storing samples at room temperature for several days. The liquid crystal phases were identified through visual observation of the light transmitted through them. A series of samples was prepared, then stored at room temperature for several days and identified by the visual observation of light transmitted through them, with the additional help of polarized microscopy. A. Materials The following chemicals and materials were used 33

3 III. RESULT AND DISCUSSION Fig. 1 shows the 1H-NMR spectra for pure-las and commercial-las with chemical shift values (δ, ppm) assigned for pure-las. The differences in the chemical shift between the two spectra suggested that the commercial-las consisted of a mixture of linear and branched alkyl benzene sulfonate. one-phase solid region (S1) was formed from the pure-las edge. A two-phase region (solid and liquid) was observed from the corn oil-pure-las axis and extended to a maximum of 21% of water at 55% of corn oil and in the region close to the waterpure-las axis. A three-phase region (solid and two liquids) was observed in the middle of the phase diagram with some difficulties in determining its boundaries. Fig. 2. Partial Phase Region in the System of Corn oil / Pure-LAS /water, S1: Solid area, W: water Fig H-NMR spectra A: pure-las and B: commercial-las in dimethyl sulfoxide, resonances assigned for pure-las are indicated The results are reported in the following order; first describing phase diagrams (Figs. 2 to7), then a comparison-among the phase diagrams of the commercial-las, the pure LAS and BAS in the presence of similar oil is discussed. Fig. 2 shows the phase diagram of Corn oil/ pure-las/ water system where the solubility of water in pure-las reached 43% (by mass), while the solubility of pure-las in water was insignificant; solubility of corn oil in pure-las was 36%, while the solubility of pure-las in corn oil was insignificant. Neither water nor corn oil was soluble in each other in any significant percent. A Fig. 3 shows the phase diagram of corn oil/ BAS / water system where the solubility of water in BAS reached 31%, while the solubility of BAS in water was 27%. Neither the solubility of BAS in corn oil nor the solubility of corn oil in BAS was determined due to difficulties in getting pure BAS surfactant without water. Two one-phase regions were determined from the BAS edge (S1). The second one (L1) started from the water edge, and extended to 9% BAS at 2% corn oil. The lamellar liquid crystal phase was difficult to determine as a pure region but there was a two-phase region (solid and liquid crystal) at water-bas axis in the range of 60-70% of water in BAS (S1+ D). Three two-phase regions were observed; the first one (solid and liquid) from the corn oil-bas axis and extended to a maximum of 32% of water at 68% of BAS axis. The second one (solid and liquid) was in the middle of the phase diagram. The last one (two liquids) was close to the region of corn oil-water axis. Two types of three-phase region were observed. The first one (liquid crystal and two liquids) in the range of 20-30% of BAS located in the water rich region of the phase diagram. The second one (solid and two 34

4 liquids) in the range of 3-13% of BAS was located in the corn oil rich region of the phase diagram, with some difficulties in determining this region s boundaries. Fig. 4. Partial phase region in the system of corn oil / commercial- LAS / water, S1: solid area, L1: normal micellar solution, D: lamellar liquid crystal Fig. 3. Partial phase region in the system of corn oil / BAS /water, S1: solid area, L1: normal micellar solution, D: lamellar liquid crystal. The phase diagram of corn oil/ commercial-las/ water system is presented in Fig. 4. The solubility of water in commercial-las reached 12%, while the solubility of commercial-las in water was 28%. The solubility of corn oil in commercial-las was 33%, while the solubility of commercial-las in corn oil was insignificant. Two one-phase regions were determined. The first one Solid region (S1) started from the commercial-las edge and extended to a maximum of 10% water at 14% corn oil. The second one was (L1) from the water edge and extended to 38% of LAS at 4% corn oil. The lamellar liquid crystalline phase was difficult to determine as a pure region but there was a twophase region of solid and liquid crystal at watercommercial-las axis in the range of 30-60% of water in commercial-las. Two types of two-phase region were observed. The first one (solid and liquid) from the corn oil-commercial-las axis and extended to a maximum of 15% of water at 73% of corn oil. The second one (two liquids) started from the water-corn oil axis. A three-phase (solid and two liquids) region was observed in the middle of the phase diagram with some difficulties in determining the boundaries of this region. Fig. 5 shows the phase diagram of olive oil/ pure- LAS/ water system where the solubility of water in pure-las reached 43%, while the solubility of LAS in water was insignificant. Solubility of olive oil in LAS was 31%, while the solubility of LAS in olive oil was also insignificant. Neither water nor olive oil was soluble in each other at any significant percent. One-phase solid region (S1) was determined from the LAS edge. Two-phase (solid and liquid) region was observed from the olive oil-pure-las axis, The second region was close to the water area containing S1 and water. Only one three-phase region (solid and two liquids) was observed in the middle of the phase diagram with some difficulties in determining the boundaries of this region Fig. 6 shows the phase diagram of olive oil/ BAS/ water system where the solubility of water in BAS reached 30%, while the solubility of BAS in water was 26%. Neither the solubility of BAS in olive oil, nor the solubility of olive oil in BAS was determined. Two one-phase regions were determined; the first one from the BAS edge and extended to a maximum of 16% water at 15% olive oil. The second one was from the water edge and extended to 14% of BAS at 1% olive oil. The lamellar liquid crystalline phase was difficult to determine as a pure region but there was a twophase region of solid and liquid. 35

5 Fig. 5. Partial phase region in the system of olive oil / pure-las / water, S1: solid area, W: water. Two one-phase regions were determined; the first one from the commercial-las edge that was extended to a maximum of 12% water at 5% olive oil. The second one (L1) was from the water edge and was extended to 30% of commercial-las at 5% olive oil. The lamellar liquid crystalline phase was difficult to determine as a pure region but there was a two-phase region of solid and liquid crystal at water-commercial-las axes in the range of 25-65% of water in commercial-las. Two kinds of twophase regions were observed, the first was, solid and liquid from the olive oil-commercial-las axes and was extended to a maximum of 25% of water at 2% of olive oil. The second one was of two liquids from, the water-olive oil axes and was extended to a maximum of 62% of water at 10% of olive oil. Only one three-phase (solid and two liquids) region was found in the middle of the phase diagram with some difficulties in determining the boundaries of this region. Fig. 6. Partial phase region in the system of olive oil / BAS / water, S1: solid area, L1: normal micellar solution, D: lamellar liquid crystal. Two types of two-phase region were observed; the first one was (solid and liquid) from the corn oil-bas axis and was extended to a maximum of 24% of water at 6% of olive oil and in the middle of the phase diagram. The second one (two liquids) was in the region close to the olive oil-water axis. Two kinds of three-phase region were observed. The first one (liquid crystal and two liquids) in the range of 20-30% of BAS located in the water rich region of the phase diagram. The second one (solid and two liquids) in the range of 3-13% of BAS located in the olive oil rich region of the phase diagram. The phase diagram of olive oil/ commercial- LAS/ water system is shown in Fig. 7, where the solubility of water in commercial-las reached 12% while the solubility of commercial-las in water was 29%. Solubility of olive oil in commercial-las was 25%, while the solubility of LAS in olive oil was insignificant. Fig. 7. Partial phase region in the system of olive oil /commercial-las /water., S1: solid area, L1: normal micellar solution, D: lamellar liquid crystal. The nature of the system and the conditions in which the surfactants are present as well as the surfactants' properties were responsible for the overall behavior of surfactant in the phase diagrams of Oil / Surfactant / Water system. The basic difference between the two surfactants (LAS and BAS) is the structure of the alkyl chain, which causes the distinction in the phase behavior in systems with the same oil Figs A comparison between the two surfactants phase diagrams will be discussed according to the following facts: firstly the critical packing parameter, secondly the krafft temperature and finally the degree of order upon crystallization. 36

6 Figures 2 and 3, suggest that there is a relation between the surfactant tail structure and the condition under which oil and water solubilized. The branched alkyl tail has significantly more ability to form normal micelles than the linear tail. This difference is expected because the micelles formation is related to the "critical packing parameter", (v / a l) which is known as a surfactant number (Ns), and where v is the volume of the hydrocarbon chain, (l) is the critical chain length and (a) is the head group area. Hence Ns gives relative values of the head group area (a) and tail group effective area (v / l). For normal micelles to be formed, Ns should be equal to As for LAS and BAS, they have the same head group so same value of a, but they are different in their tail group, LAS has a linear alkyl chain of 12 carbon atoms while BAS has a branched alkyl tail of 12 carbon atoms i.e. different v / l values and as a result they have different Ns values and different phase behavior [7]. Secondly, in order to form normal micelles from the ionic surfactants when their hydrocarbon chains are sufficiently fluid, temperatures should be above their chain melting temperature. Below a specific temperature for a given surfactant -the krafft temperature (TK) - the surfactant becomes insoluble rather than self-assembling [14]. For pure-las this temperature is around 52 ºC and for BAS it is around 22 ºC [13] and only above these two temperatures the micelles can be formed. The phase diagram was done at room temperature ~ 25º C. This explain why BAS (TK = 22º C) has more ability to form normal micelles than pure-las (TK = 52º C). Finally, the solid region in pure-las was larger than that for BAS. This difference shows that the linear alkyl chain can solubilize water and oil more than the branched one. This explanation is based on the well-known ability of the linear chain to have a higher degree of order (crystallinity) relative to the branched alkyl chain [14]. As shown in Figure 4 the commercial-las has an intermediate behavior of that of pure-las (Figure 2) and BAS (Figure 3). The normal micelles region which was absent in the pure-las phase diagram becomes larger than that of pure BAS surfactant in such a crystalline state that will only solubilize and form micelles if another surfactant assists it in overcoming the forces that keep it crystallized. And this is the rule in the commercial- LAS surfactant that consists of a mixture of linear and branched alkyl chain of different homologues and isomers which enhanced the micellar solution region. The solid phase region becomes smaller than both the pure-las and pure BAS. This result gives evidence that the commercial-las consisted of a mixture of linear and branched alkyl benzene sulfonate as was confirmed by 1H-NMR (Figure 1). This explanation is based on the well-known ability of the mixed surfactants system in solubilization more than the single surfactant system [15]. The variation of commercial-las chain length and structure creates a disorder of the end part of the hydrocarbon i.e. less order and hence less crystallinity. This is the reason that causes the solid region to be smaller than that of pure-las. As an average of Ns the micelles region becomes larger than that of BAS. In addition, the TK value for commercial-las is around 22 ºC [13], which is close to the BAS (TK = 22º C) and not the pure- LAS value (TK = 52 ºC). The olive oil system, Figs. 5-7 show the same results as the corn oil system (Figures 2-4) with small differences in solubility limits. This indicates that the structure of the surfactant molecules determines the conditions under which oil and water stably mixed. There were no significant differences between olive and corn oil phase diagrams with pure-las, BAS or commercial-las. IV. CONCLUSIONS The results of the ternary phase behavior showed that changes in the molecular structure of the surfactant molecules have important consequences on the solubilization of LAS or BAS with water and oil. This makes phase diagram an easy and quick approach for distinguishing between the two surfactants LAS and BAS. Differentiating between the two surfactants can be achieved with the help of their phase behaviors, for example: a sample containing up to 9% of BAS, 2% of oil and 89% of 37

7 water will form a microemulsion phase (normal micelle), whereas a sample containing the same percent of water and oil but with 9% of pure-las will form a two-phase of solid and liquid. As another example a sample containing 17% of water, 17% of oil and 66% of pure-las will form onephase of solid, whereas a sample containing the same percent of water and oil with 66% of BAS will form a two-phase region of solid and liquid. [15] M. Gradzielsk, H. Hoffman, " Influence of charges on structure and dynamics of an O / W microemulsion. Effect of admixing ionic surfactants", J. Phys. Chem. 98 (1994) REFERENCES [1] J. Blackmon, S. Li, A. Demange, T. C. Jao, "Linear Sulfonate Detergents as Pour Point Depressants", Lubric. Sci. 16 (2000) 127. [2] U.S. EPA HPV Chemical challenge program: A, April 2008, alkybenz/c14187rt.pdf). [3] D. C. Connor, J. J. Scheibel, J. C. Laurent, T. A. Cripe, P. K. Vinson, " Process for preparing a modified alkylaryl", US Patent no. US B1 (2003). [4] L. Cavalli, A. Gellera, A. Landone, " LAS removal and biodegradation in a wastewater treatment plant", Environ. Toxicol. Chem. 12 (2001) [5] P. Eichhorn, M. E. Flavier, M. L. Paje, T. P. Nipper, " Occurrence and fate of linear and branched alkylbenzenesulfonates and their metabolites in surface waters in the Philippines", The Science of the Total Environment 269 (2001) 75. [6] J. L. Conboy, M. C. Messmer, G. L. Richmond, " Effect of alkyl chain length on the conformation and order of simple ionic surfactants adsorbed at the D2O/CCl4 interface as studied by sum-frequency vibrational spectroscopy ", Langmuir 14 (1998) [7] S. Nave, J. Eastoe, J. Penfold, " What Is So Special about Aerosol-OT? 1. Aqueous systems ", Langmuir 16 (2000a) [8] S. Nave, J. Eastoe, J. Penfold, " What Is So Special about Aerosol-OT? 2. Microemulsion Systems", Langmuir 16 (2000b) [9] S. Nave, J. Eastone, R. K. Heenan, D. Steytler, "What Is So Special about Aerosol-OT? Part III-Glutaconate versus Sulfosusuccinate Headgroups and Oil-Water Interfacial Tensions", Langmuir 18 (2002) [10] S. Nave, A. Paul, J. Eastoe, A. R. Pitt, R. K. Heenan, "What Is So Special about Aerosol-OT? Part IV. Phenyl-Tipped Surfactants", Langmuir 21 (2005) [11] M. R. Warty, G. L. Richomond, "Comparison of the adsorption of liner alanesulfonate and linear alkylbenzenesulfonate surfactants at liquid interfaces ", J. Am. Chem. 122 (2000) 875. [12] Y. Barakat, L. N. Fortney, R. S. Schechter, W. H. Wade, S. H. J. Yiv, " Criteria for structuring surfactants to maximize solubilization of oil and water ", J. Colloid Interface Sci. 92 (1983) 561. [13] N. A. Lockwood, J. J. de Pablo, N. L. Abbott, "Influence of surfactant Tail Branching and Organization on the Orientation of Liquid Crystals at Aqueous-Liquid Crystal Interfaces", Langmuir 21 (2005) [14] R. Pashley, M. Araman, Applied colloid and surface chemistry, 1 st ed. England: John Wiley and Sons Ltd., Abeer F. Al Bawab Professor of Applied Physical Chemistry in The University of Jordan (JORDAN). She is also the director of research center inside the University of Jordan with the name of Hamdi Mango Center for Scientific Research (HMCSR) since 2008 till now. She obtained her Ph.D degree in Physical Chemistry from Clarkson University (USA) in Her main research interest are: chemistry of Nanoscience in the field of environment & water, application of colloid chemistry in nature by studying colloidal solution stability and associated structure of surfactant systems in different Environments such as Emulsions, Microemulsions, Liquid Crystals, Micelles and Foams, Fragrances and Flavors Systems. She was involved in some number of research projects granted institutional, governmentally, and internationally. She is author or coauthors of approximately 50 scientific papers published on international Journals. Ayat A. Bozeya Has a master degree in chemistry from the University of Jordan in She is now researcher and a PhD student in a research center inside the University of Jordan with the name of Hamdi Mango Center for scientific Research (HMCSR). Her main research areas are: Colloid and surface chemistry in Emulsions, Microemulsions, phase behavior of three component system; water remediation; superconductors. She is highly qualified in performing the following: 38

8 Spectroscopic techniques; Chromatographic techniques; Light Scattering Techniques; Particle size and zeta potential measurements. She is author or coauthors of 13 papers published on international journals. Associate Professor of applied physical chemistry in the Department of chemistry in the University of Jordan. Her main research areas are: Targeted drug-delivery systems for cancer treatment; Preparation of new materials for slow and/or controlled release of drugs from natural resources (e.g. metakaolinite); Characterization and modification of nanoparticles in solutions using non-invasive techniques especially Dynamic NMR, self-assembly behavior of sparingly soluble organic compounds in solution and its effect on the activity of such molecules as corrosion inhibitors. She is author or coauthor of approximately 25 papers published on international journals or proceedings of international conferences. Fadwa M. Odeh obtained her PhD in physical chemistry from Clarkson University (NY, USA) in She is now 39