Materials, Chemicals, and Energy from Forest Biomass - American

Chapter 12

Lignins as Emulsion Stabilizers 1

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Orlando J. Rojas , Johnny Bullón2, Fredy Ysambertt , A n a Forgiarini4, Jean-L. Salager , and Dimitris S. Argyropoulos

Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: April 16, 2007 | doi: 10.1021/bk-2007-0954.ch012

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Department of Forest Biomaterials Science and Engineering, North Carolina State University, Raleigh,NC27695-8005 Lab. LMSSI, Universidad de Los Andes, Mérida, Venezuela Lab. Petroquimica y Surfactantes, LUZ, Maracaibo, Venezuela Lab. FIRP, Universidad de Los Andes, Mérida 5101, Venezuela 2

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This is a report on the use of (kraft and soda) lignins as polymeric amphiphiles for the stabilization of emulsions. The lignins' phase behavior in oil/water systems is presented and explained in terms of their molecular affinities. Emulsions with various oils (including crude oils) were formulated and their properties were rationalized in terms of lignin surface activity as a function of p H and salinity. It is concluded that lignins are an effective emulsion stabilizer; the resulting emulsions behave according to the balance of affinities between oil and water phases and lignin-based emulsions can be tailored to meet specific demands.

A by-product of chemical digestion of lignocellulosics, "black liquor," is highly concentrated in organic materials, with lignin being the main component of the total dry mass. Only about 1.5 % of the ca. 70 million tons of lignin yearly available in the pulp and paper industry is commercially used (about 1 million tons per year) (1-4). Furthermore, these 70 million tons represent only a very small fraction of the lignin produced by nature in biomass every year. New opportunities which appear in industrial production appear in other contexts such as bioethanol production for the replacement of transportation fuel 182

© 2007 American Chemical Society In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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of fossil origin. In all cases, there is an intensive use of the carbohydrate fraction of the lignocellulosic resources while lignin is a non-negligible part of the raw material that cannot be transformed into e.g., sugars. Therefore, these bio-energy efforts and other forest-based industrial initiatives lead to the generation of even larger amounts of lignin side streams. For achieving economic viability in such current and future processes, lignin's highest possible co-product value becomes one of the key success factors.

Lignins as Polymeric Surfactant Since the pioneering work of Landoll (5), studies on naturally-occurring polymeric surfactants have been centered on derivatives of polysaccharides. The various polymer types and the characteristic property profiles distinguishing polymeric surfactants have been discussed recently (6). Such polymers seem attractive in many fields because of their good performance in lowering surface tension. Despite the fact that lignins from black liquors don't have the chemical structure of conventional amphophilic molecules, they exhibit surface activity. It is hypothesized that lignin adsorbs at the air/water and oil/water interfaces forming a condensed, viscoelastic surface or interfacial film. In the literature, lignin molecules have been categorized as polymeric surfactant when separated from spent black liquors (e.g., lignin sulphonates). Surface activity has been demonstrated in the case of lignins obtained directly from black liquors, with little or no modification (7,8), after chemical derivatization (9-16) and also in combination with other species (17-19). A l l in all, as is the case of alkyl polyglucosides and other biosurfactants, lignin polyelectrolytes derived from black liquors can be classified as a natural, renewable polymeric amphiphile.

Emulsion Formulation Only very few studies have been reported on the use of lignin in emulsion technology (8,20-24). The preparation of emulsions allows many degrees of freedom with respect to the formulation and the emulsification protocol. These degrees of freedom could be classified as variables that influence the properties of the final emulsion. Furthermore, the actual effect of these variables is very complex (25). Three different factors or variables are important in emulsion manufacture: (1) "formulation variables" that depend on the nature of the components; (2) "composition variables" that quantitatively describe the composition of the system in terms of, e.g., volume fractions; and (3) the ones that describe the way the emulsion is prepared, that is, the stirring and "emulsification" conditions.

In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.


184 The physicochemical variables that depend on the nature of the different ingredients are those that are able to alter the prevailing physicochemical condition at the liquid-liquid interfaces. There are at least four such variables, i.e., (1) the alkane carbon number (ACN) when the oil is an alkane or the equivalent A C N (EACN) when it is not (26-29); (2) the ionic strength or salinity of the aqueous phase, which in turn depends on the nature and content of the electrolyte; (3) the surfactant and, (4) the temperature. The implications of this multiplicity in the number of variables are overwhelming. A systematic study with a few values of each variable would require an astounding number of tests. Obviously, a systematic approach is needed to rationalize the involved phenomena. Winsor's pioneering theoretical work (30) showed that the formulation concept could be described through a single parameter that gathered all effects. This parameter, Λ, is the ratio of the interaction energy of the surfactant with the oil phase, to the interaction energy of the surfactant with the aqueous phase, R^fAço-O.SAooyfAcw-O.SAw), where A is the interaction energy between the surfactant (c) and the oil phase (o) molecules; A the interaction energy between the surfactant and water; A the interaction energy between two oil molecules and so forth. The 0.5 coefficients are introduced because cross interactions (A and A^) involve one surfactant molecule and both an oil molecule and a water molecule, while the self interactions (A a n d ^ ) involve two molecules of oil, and two of water. Winsor's research showed that the phase behavior at low surfactant concentration, say from 0.1% to 5%, as in most applications, is directly linked with the R value (30,31). For R\ the opposite occurs, i.e., the surfactant-rich phase is the oil phase, which is in equilibrium with an aqueous phase that contains essentially no surfactant, and the dominant interaction of the surfactant is for the oil phase. When R=l, the surfactant interactions for the oil and water phases are equal and the system splits into three phases in equilibrium: a microemulsion that contains most of the surfactant and that co-solubilizes large amounts of oil and water, and two excess phases that contain essentially pure oil and pure water (30,31), which is a complex but not uncommon phase behavior situation. Since R varies with one or more changes in interaction energy, those factors that affect the A's are likely to change R, and consequently, the phase behavior. For instance, an increase in surfactant lipophilic group length tends to increase the A and thus increases R. If the starting R value is smaller than unity, and the final value is R>\, or vice versa, then a complete phase behavior transition is co

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185 exhibited. Similar transitions may be attained by changing any of the formulation variable that can affect any of the A's (salinity, pH, T, etc.). A more quantitative description of the physical-chemical formulation is available through the so-called surfactant affinity difference (SAD) and the Hydrophilic-Lipophilic Deviation (HLD) (see Refs. 32-36) which translate Winsor's conceptual approach into numerical values. At present, there is an accumulated knowledge for emulsion formulation mainly for conventional, monomeric surfactants. In this collection of information it is evident that correlations for lignin-based surfactants are lacking. Therefore, the objective in this effort is to specifically address this lack of scientific data in anticipation that such information will eventually permit the development of new, lignin-based emulsion formulations that may effectively compete with their synthetic counterparts.

Lignins and Characterization Methods In order to illustrate the abilities of lignins to act as an emulsion stabilizer, different lignin sources are cited in this chapter. Industrial black liquor was obtained from Smurfit-Mocarpel pulp mill (Yaracuy, Venezuela), where a lowsulfidity Kraft process is used in the digestion of Pinus caribaea (KBL-P). In this case, the black liquor samples were collected in the recovery boiler (pH 13.5, 31% solids content and 18% Klason lignin) and suspended solids were separated by filtration with a Whatman filter No. 1. Industrial black liquor was also obtained from the same pulp mill after soda digestion of sugar cane bagasse (SBL-B). Finally, there is reference to two commercially-available lignin samples, both from Westvaco Corp. (Charleston, SC, USA), namely, Indulin C, a sodium salt of Kraft pine lignin and Indulin AT, a purified form of pine Kraft lignin, free of hemicellulosic materials. The details about the chemical and physical characteristics of the industrial and laboratory black liquors can be found elsewhere (19,37,38). The surface tension (f) of aqueous solutions of lignins were measured by employing the Wilhelmy plate technique in a Dataphysics tensiometer, model D CA T 11 (Germany). A thermostated bath with circulating water at 25 °C was used to ensure constant temperature conditions throughout the measurements. Phase Behavior and lignin partitioning between oil and aqueous phases was studied by using formulation scans in equilibrated systems (31,34,39). Lignin concentration in both organic and aqueous phases was obtained by U V spectroscopy (280 nm). Oil-in-water emulsions with different internal phase content and different pHs of the aqueous phase were prepared by using standard emulsification protocols. The aqueous phase consisted of distilled water with adjusted salinity



186 and pH to which lignin (as 1% solids or as otherwise specified) was added as an emulsifier. Different oleic phases were employed. Depending on the experiments ,kerosene (Aldrich) was used with a measured Equivalent Alkane Carbon Number of 9 or an extra heavy crude oil (Cerro Negro) from the Orinoco Belt (Venezuela) of 8.5 °API at 60 °C (Viscosity 500-2500 mPa.s at down hole pressure of 700-1300 psi and 50-70 °C) (40). Additional phases consisted of mixtures as specified in the respective sections. In most cases, a Water-to-Oil Ratio (WOR) of 30/70 was used or as specified otherwise. In the case of the extra heavy crude oil, the preparation of the emulsion involved heating of both phases to 60 °C, and the emulsification was carried out in a turbine blender (Taurus) at 12,000 rpm for 30 seconds. For all other oil phases emulsification was conducted at 25 °C. In all cases emulsion properties were measured at 25 °C. The resulting emulsions were analyzed in terms of drop size distribution by laser light diffraction (Malvern) and stability. The stability was estimated both by drop size evolution with time and by a phase separation technique (41). The rheological behavior of the emulsions was studied with a Rheometric Scientific SR-5000 using parallel plates geometry (40 mm diameter, 1 mm gap), and the flow curves for less viscous emulsions were obtained in a parallel cylinder geometry using a Rheomat 30 (Contraves A G , Zurich).

Lignin Surface Activity When a surface active substance is added to water (or oil), it spontaneously adsorbs at the surface, and decreases the surface tension γ. In the case of small surfactant molecules, a monolayer is formed with the polar parts of the surfaceactive molecules in contact with water, and the non-polar parts in contact with air (or oil). The surface activity of lignin solids separated after the evaporation of black liquors at low temperature is illustrated in Fig. 1. For comparison purposes, the surface tension curves for a commercial lignin (Indulin C) and a typical anionic surfactant (petroleum sulphonate) are also included. Minimum surface tensions of ca. 30 mN/m were observed for the lignin solutions. Despite the fact that the reduction in surface tension is not as large as for conventional surfactants, it is low enough to unequivocally suggest the adsorption and packing of lignin molecules at the interface. The reduction in surface tension is not enough for lignin to act as typical emulsifiers work, by facilitating the creation of surface area or droplets due to the lowering in interfacial energy. However, as will be demonstrated in later sections, lignin derivatives perform effectively as an emulsion stabilizer since it prevents droplet coalescence by means of electrostatic and steric repulsion as well as by the formation of a viscoelastic skin or interfacial film around the droplets of the dispersed phase (22). A break in the surface tension tendency is observed at solids concentrations between 1 and 10%. In the case of monomeric surfactants this break indicates the



187 critical micelle concentration (one). In the case of lignin molecules it is unlikely the association of the molecules in the form of micelles and therefore this concentration is rather described as a pseudo-c/wc. These figures are similar to those reported by Rojas and Salager for SBL-B (7). It can be concluded that given the right conditions, the lignins considered here behave as polymeric surfactants in aqueous media. Adsorption of conventional surfactants is generally very fast, but for larger molecules, such as lignins, the adsorption process is slower (see evidence in (7)) and adsorption is generally irreversible. Furthermore, it is hypothesized that lignin adsorbs in the form of aggregates, the surface layers become thick, and a solid-like "skin" is formed.

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Figure 1. Left: Surface tension γ (25 °C) of solutions of commercial lignin (Indulin C) compared to a typical anionic surfactant (petroleum sulphonate). Right: Surface tension for solids separatedfrom kraft black liquor, KBL-P.

In our earlier efforts it has been demonstrated that lignin can be separated from spent pulping liquors in high yields and purity by acid precipitation and ultrafiltration, U F (7,42). These methods of lignin separation offer the significant advantage of simplicity over other alternative routes (43-45). Furthermore, one can take advantage of the fact that separation by molecular size is coupled with the selective concentration of functional groups, O/C ratios and H/C ratios in the separated fractions (46). These results indicate that the concentration of phenolic groups in the lignin fractions obtained by U F of industrial kraft lignins decreased by increasing mass-weighted hydrodynamic radii of the diffusants. Interestingly, this leads to differences in the surface activity of the separated molecules, as can be observed in Fig. 2 (left). The lignin samples used in Fig. 2 were obtained by ultrafiltration (UF) using a Minitan™ (Millipore) unit with polysulfone semi



188 permeable membranes in a plate-and-frame set up. The initial feed consisted of black liquor solution diluted to 15 % total solids content. This solution was fed to the membrane of M W C O 100 kDa. The retentate in each U F step was recycled to the feed stream. The permeate from the membrane of M W C O 100 kDa was filtered using the membrane of M W C O 30 kDa and the same was done successively for the membranes of smaller M W C O s . It is seen that the permeate solutions for the membranes with M W C O 100 and 30 kDa show high surface activity. However, this is not the case for the permeate of the membrane of 1 kDa M W C O . From these results it can be concluded that lignin fractions of higher molecular weight are more surface active than the lower molecular weight counterparts. The amphiphilic nature of these anionic polyelelectrolytes is explained by the hydrophobicity of the molecule balanced by a high number of anionically-charged groups. A s the lignin size is reduced, the hydrophilic/hydrophobic balance is changed making the molecule less hydrophobic and less surface-active.

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Figure 2. Left: Surface tension ofpermeates from ultrafiltration with polysulfone membranes of KBL-P with membranes of nominal MWCO 1, 30 and 100 kDa. Right: Surface tension of SBL-B lignin dissolved in aqueous media of pH 7 (squares), 8(triangles), 9 (filled circles) and 10 (open circles).

The way lignin adsorbs at the interface is still an issue under scrutiny. In the bulk solution a randomly branched model for lignin derivatives has been proposed with a hydrophobic backbone and hydrophilic side chains as well as flat, fractal structures (47-49). It is envisioned that the conformation at the interface involves molecular orientation according to the affinities of the involved structures. Evidently, more research is needed to confirm or elucidate these issues.



189 It is worth mentioning that the knowledge of surface tension alone is not enough to understand emulsion properties (see later sections), and the surface viscoelasticity plays an important role in a number of dynamic processes. There is research in laboratories underway to explore this aspect. Nevertheless, there is evidence provided here of the interesting properties obtained when lignin is used as emulsion stabilizer. Finally, the surface activity of lignin solids when dissolved in aqueous media of different p H is addressed as follows. As can be seen in Fig. 2, the surface activity of lignin is increased when dissolved in high pH aqueous solution. This effect is explained by the ionization of functional groups, mainly carboxylic and phenolic hydroxyl groups. A right molecular size is predicted to give better surface activity i f the molecule contains enough hydrophilic groups, depending on the p H of the aqueous system. A fully dissociated group such as sulphonate would be required to provide surface activity less dependant on the p H of the solution. Furthermore, chemical modification of lignin can be used for optimizing its surface activity.

Phase Behavior in Oil-Water Systems The investigation of the phase behavior of lignin in oil/water systems is required in order to understand the affinity of the molecule towards these phases as a function of the pH, salinity and oil type (oil's Equivalent Alkane Carbon Number, E A C N ) . Figure 3 shows a typical phase behavior for a kraft lignin in a kerosene/water system. In this case the lignin was first dissolved in the aqueous phase (1.5% lignin concentration) and then an equal amount of oil (kerosene) was placed in contact with the aqueous solution. After gentle mixing and equilibration for at least 24 h at constant temperature (25 °C), the partitioning of the surface active molecule is followed as a function of a formulation variable (pH of the aqueous phase in the case of Fig. 3). As expected, it is observed that at high pHs, the system can be described as a Winsor I (Λ

Materials, Chemicals, and Energy from Forest Biomass - American

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