A Novel Approach for Rapid Preparation of Monophasic

Jan 27, 2016 - To realize an efficient penetration effect in wood blocks by microemulsions, the SOW systems that exhibit relatively low surface tensio...
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Research Article pubs.acs.org/journal/ascecg

A Novel Approach for Rapid Preparation of Monophasic Microemulsions That Facilitates Penetration of Woody Biomass Xueyu Du,†,‡,§ Lucian A. Lucia,*,†,‡,§ and Reza A. Ghiladi*,† †

Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States Department of Forest Biomaterials, North Carolina State University, Raleigh, North Carolina 27695-8005, United States § Key Laboratory of Pulp & Paper Science and Technology of the Ministry of Education, Qilu University of Technology, Shandong Province, Jinan 250353, P.R. China ‡

S Supporting Information *

ABSTRACT: Microemulsions are a straightforward, efficient, and highly useful complex media for flooding/wetting substrates as a result of their low surface tension and viscosity. Among the four broad general classes of microemulsions (Winsor-I, -II, -III, and -IV), Winsor-IV is by far considered the ideal microemulsion type within the context of woody biomass pretreatment because it is a single phase. In the present study, a never-before reported titration method was developed with the intent of providing a rapid online determination of Winsor-IV type microemulsion formulations under fixed surfactant concentrations for expressly treating woody biomass. A total of 108 surfactant-oil−water formulations based on a sodium dodecylsulfate/ pentanol/water/sodium chloride/dodecane system were investigated for their phase behavior, 54 of which yielded Winsor-IV type microemulsions. The ability of the selected microemulsions to affect the crystallinity of cellulose was studied by X-ray diffraction as was the synergetic effect of microemulsion surface tension and kinematic viscosity on wood penetration from liquid uptake experiments. The general method described here enables rapid preparation of Winsor-IV type microemulsions that exhibit rapid wood penetration at room temperature and atmospheric pressure and potential utility as a general means for screening surfactant-oil−water formulations for effective wood biomass pretreatment or other materials applications. KEYWORDS: Surfactant-oil−water (SOW) system, Microemulsion, Surface tension, Kinetic viscosity, Wood penetration

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INTRODUCTION Lignocellulosic biomass is a significant renewable resource that takes up nearly half of the entire biomass reserves in the world.1 The versatile utilizations of lignocellulosics are not only limited to the applications based on its original state (e.g., construction and civil materials), but also upgraded to a large-scale substitution of traditional fossil resources for production of environmentally friendly energy and mainstream chemicals. However, even though lignocellulosic materials are processed for direct usage in the pulp & paper industry or used as a feedstock in the nascent bioenergy industry, both of these industries are still suffering from longstanding burdens such as high production costs, severe environmental pollution, low efficiency, etc. One of the biggest concerns regarding direct use of lignocellulosic materials is to suppress its biodegradation by preservation. However, the traditional preservation technique was commonly operated by applying vacuum in the initial stage that is followed by imposing high pressure for penetration of biocide liquids into wood structures. This whole procedure involves high operation cost, and has inherent difficulties to fully overcome internal resistance (capillary forces in the porous wood structures), which results in low efficiency of biocide diffusion throughout the treated samples and environ© XXXX American Chemical Society

mental hazards because many biocides (e.g., pyrethroids) are water insoluble and thus require organic solvents (e.g., xylene, kerosene, naphtha, etc.) for dissolution.2 Therefore, from a point of view of sustainable development, a water−organic biocide-based complex liquid is highly favored for fast wetting/ flooding porous wood structures and even distribution of preservatives. With respect to the paradigm of lignocellulosic feedstockbased “biorefineries”, we need to overcome two recalcitrant obstacles to their feasibility: (1) how to effectively open up the original compact structural elements in the biomass and thus increase accessibility for subsequent enzymatic or chemical treatments and (2) how to efficiently get rid of the natural barriers, for example, lignin−carbohydrate complexes3,4 in which the biopolymers are physically intermingled and chemically linked, so as to isolate high purity biopolymer for production of specific biomaterials and fine chemicals. Within the context of these obstacles, pretreatment is regarded as the key bottleneck because currently it consumes a large portion of total production costs due to its high pressures, high Received: December 2, 2015 Revised: January 6, 2016

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DOI: 10.1021/acssuschemeng.5b01601 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering temperatures (e.g., steam explosion,5 ammonia fiber explosion6,7), or expensive chemicals (e.g., ozone8). On the basis of the above-mentioned challenges associated with lignocellulosics applications, a more sustainable, costeffective, and environmentally friendly technique is highly needed for improvements in the wood preservation industry and pretreatment of lignocellulosics biorefinery. In comparison with the currently prevailing pretreatment/impregnation methods, imbibition of woody lignocellulosics by microemulsions has emerged as a novel and ideal approach for fast wetting/flooding of woody fibers at atmospheric pressure and room temperature9,10 because of their low surface tension and viscosity. Moreover, their remarkable affinity for oil confers to the good transportation properties for the delivery of various organic/inorganic reagents within the porous lignocellulosic network without involving harsh conditions. Microemulsions are defined as clear, thermodynamically stable, and spontaneously formed mixtures of surfactant (S), oil (O), and water (W).11 Co-surfactants and electrolytes are sometimes necessary when employing ionic surfactants. Their prevailing applications can be found in the fields of pharmaceuticals,12 oil recovery,13 cosmetics,14 etc. Among the four classic microemulsion types,15 known as Winsor-I (O/W), Winsor-II (W/O), Winsor-III (bicontinuous), and Winsor-IV (O/W or W/O or bicontinuous), the Winsor-IV type is highly favored in this study, given its single phase without a surplus oil phase or water phase. Thus, a fast phase behavior scanning method specific for monophasic microemulsion preparation is a prerequisite. Traditionally, the phase behavior scanning of the SOW systems is realized by building up the so-called pseudo-ternary phase diagram16 or estimated by the semi-empirical HLD equation.17,18 In the former case, the surfactant and cosurfactant (alcohol) are combined into one single pseudocomponent, while the electrolyte and water (brine) form another pseudocomponent. The third component of the pseudo-ternary system is the oil itself. There are three aspects featured in the pseudo-ternary phase diagram (Note: A titration process refers to a period starting from the first addition of titrant into the mother liquid to the last addition of titrant into the mother liquid; a titration procedure is a combination of many titration processes): (1) Constant ratio of surfactant and co-surfactant: This ratio is preset in the whole titration procedure and needs to be optimized by the preparation of various ratios (time consuming). (2) Complete phase scanning: During each titration procedure, all possible phase behaviors including Winsor-I, -II, -III and IV are systematically delineated from low surfactant concentration to high surfactant concentration (time consuming and oversubscribed). (3) Single component titrated: During each titration process, the mother liquid is titrated by only one component (e.g., dispersed phase) (low titration efficiency). The HLD equation is a well-known practical formulation tool for predicting the optimal formulation of a microemulsion by inputting the formulation parameters such as the characteristic values for the hydrophobic/-philic nature of the surfactant, effective alkane carbon number (EACN), temperature, salinity, and %-age of co-surfactant. However, all these parameters still need to be either predetermined or empirically derived and can still have variation even when the same surfactant is employed

with different purities (e.g., GR, AR, CP, technical, etc.). The optimal formulation acquired from the HLD equation is still limited to theoretical estimation, which needs to be further verified by experimental work. Thus, there is a critical need for a more rapid, simplified, and specific approach for scanning Winsor-IV type microemulsion formulations using economically viable constituents. In this study, a surfactant-oil−water (SOW) system comprised of economic constituents, specifically sodium dodecylsulfate (SDS), pentanol, water, sodium chloride, and dodecane, was investigated for its ability to form a Winsor-IV type microemulsion that could subsequently be used for wood impregnation studies. Instead of traditional comprehensive scanning of various phase behaviors for one SOW system, a simplified titration approach was designed for this study that focused on a quick determination of all the possible formulations for the preparation of Winsor-IV type microemulsions under a fixed concentration of surfactant. The initial screening studies demonstrated that the Winsor-IV type microemulsions exhibited high potential for wood penetration because of low surface tension and kinetic viscosity that also led to changes in short-range cellulose crystallinity.

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RESULTS AND DISCUSSION Selection of SOW System. In general, a surfactant-oil− water (SOW) system is comprised of surfactant, oil, and water of different ratios; however, in the case of SOWs made from ionic surfactants, the addition of a co-surfactant and electrolyte are typically necessary to ensure the proper charge balance of the system. For instance, sodium dodecylsulfate requires a short chain alcohol as a co-surfactant to facilitate its ability to engage in the formation of a microemulsion, while nonionic surfactants do not need any additional co-surfactants. Simple, available, economic, and saturated chemical materials (e.g., components without aromatic rings or CC double bonds to allow for follow-up determination/characterization of aromatized lignin structures) were guiding criteria in the selection of surfactant and oil phases. Thus, sodium dodecylsulfate (SDS) was a prime candidate because of its low cost, high water solubility, and wide applicability.19−23 Similarly, dodecane was selected for the SOW system because of its low price, simple structure, low toxicity, and high stability (e.g., high boiling point) relative to short chain hydrocarbons. Another reason for selection of dedocane is to match the chemical structure of SDS (both of them have 12-carbon alkyl chain). Pentanol (co-surfactant) and sodium chloride (electrolyte) were added to facilitate the spontaneous formation of the microemulsion and adjust the affinity of the surfactant between the water and oil phases, respectively. Therefore, the final SOW system was designed by combination of five different components, viz., SDS, dodecane, water, pentanol, and sodium chloride. Salinity and Co-Surfactant Effects on Microemulsion Phase Behaviors. The phase behavior of SOW systems can vary differently as its formulation or composition changes. There are four common microemulsion types or phase behaviors15 known as Winsor-I, Winsor-II, Winsor-III, and Winsor-IV, whose phase equilibria are shown in Figure 1. Under a fixed concentration of surfactant, salinity and cosurfactant amount are two major determinants for the final microemulsion types, which were investigated individually, and the detailed formulations for each single factor experiment are listed in Table 1. B

DOI: 10.1021/acssuschemeng.5b01601 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 2. Influence of salinity on microemulsion types. Figure 1. Four common microemulsion types (Winsor-I, II, III, and IV).

The function of the electrolyte, for example, NaCl, in the SOW systems is to change the affinity of the surfactant between the water and oil phases. Under lower salinity conditions (Figure 2, I and II), SDS is readily soluble in water, which facilitates formation of Winsor-I type (O/W) microemulsions. When the salinity increases, the affinity of SDS toward the water and oil phases is well balanced, and Winsor-III type microemulsions are formed (Figure 2, III and IV). If the salinity is further increased, the solubility of SDS in brine was found to sharply drop, and most of the SDS was dissolved into the oil phase to form a Winsor-II type microemulsion (Figure 2, V). In order to minimize the oil phase consumption and reach a high water-to-oil ratio (WOR), low salinity is undoubtedly recommended. The addition of pentanol (co-surfactant) is designed to significantly change the curvature of the interfacial layer (composed by surfactant and co-surfactant) between the oil and water phases.24 At low concentrations of pentanol, the highly water-soluble SDS orients its hydrophilic head toward the water phase, leaving the hydrophobic tails curving to an inner core to dissolve oil. Under these conditions, the formation of Winsor-I type (O/W) microemulsions are favored (Figure 3, I and II). As the concentration of pentanol increases, more pentanol molecules can be packed in parallel with SDS

Figure 3. Influence of pentanol concentration on microemulsion types.

molecules, gradually changing the curvature of the interfacial layer and forming a Winsor-III type microemulsion (Figure 3, III and IV). At the highest pentanol concentrations, more hydrophobic tails are exposed to the oil phase, leaving the water-soluble heads to retain the water phase in the core, and thus Winsor-II type microemulsions are observed (Figure 3, V). Therefore, during the phase transition from Winsor-I (O/W) type to Winsor-III (bicontinuous) type, the volume of dispersed oil phase in the O/W system can be continuously

Table 1. Microemulsion Compositions for Single Factor Scannings Effects of salinity (sodium chloride) on microemulsion phase behaviors SOW series dodecane (mL) H2O (mL) SDS (g) NaCl (g) pentanol (mL) microemulsion type

I

II

III

5.0 5.0 5.0 5.0 5.0 5.0 0.20 0.20 0.20 0.13 0.15 0.20 0.50 0.50 0.50 Winsor-I Winsor-I Winsor-III Effects of co-surfactant (pentanol) on microemulsion phase behaviors

IV

V

5.0 5.0 0.20 0.25 0.50 Winsor-III

5.0 5.0 0.20 0.30 0.50 Winsor-II

SOW series

I

II

III

IV

V

dodecane (mL) H2O (mL) SDS (g) NaCl (g) pentanol (mL) microemulsion type

5.0 5.0 0.20 0.15 0.40 Winsor-I

5.0 5.0 0.20 0.15 0.50 Winsor-I

5.0 5.0 0.20 0.15 0.60 Winsor-III

5.0 5.0 0.20 0.15 0.70 Winsor-III

5.0 5.0 0.20 0.15 0.80 Winsor-II

C

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microemulsions at a given surfactant concentration, salinity, and temperature (time saving). In simple terms, under a fixed amount of surfactant, all Winsor-IV type microemulsions cannot be fully determined in a single titration process by the traditional method because there is only one titrant (e.g., dispersed phase or continuous phase) introduced into the system without optimizing surfactant-toalcohol ratio at the same time. However, under a fixed amount of surfactant, all monophasic microemulsions can be completely identified in a single titration process by this new approach because there are two titrants (dispersed phase and cosurfactant) dynamically charged into the system, which achieves an online optimization of surfactant-to-alcohol ratio and thus saves the entire titration time to a great extent. Characterization of Winsor-IV Microemulsions. Representative microemulsions from different Winsor-IV type ranges, as noted by their series number (SOW-A1; SOW-B1 to B6; SOW-C1 to C9; SOW-D1 to D9, whose compositions are listed in Table 2), were selected and characterized in terms of

enriched by gradually increasing the amount of co-surfactant until the interfacial layer is incapable of enclosing any oil phase, which results in a bicontinuous status of the SOW system. Fast and Specific Phase Behavior Scanning for Monophasic Microemulsions. Four different concentrations of SDS solution (CSDS = 20, 40, 60, and 80 mg/mL H2O) were prepared and successively titrated by batch-wise addition of dodecane and pentanol to define all the Winsor-IV type possibilities (Tables S1−S4). Here, the SOW systems were formed under low salinity, featured as high water-to-oil ratios (WOR),9 and highly desirable from the standpoint of economy. During phase scanning, the appearance of the Winsor-III type was considered a termination point. Generally, the probability of forming Winsor-IV type microemulsions was higher when more concentrated SDS solution was applied (Tables S1−S4). For example, when the CSDS was 20 mg/mL H2O (initial water volume = 5 mL), only one range was detected for Winsor-IV, formulated using 0.1− 0.3 mL of dodecane with the addition of 0.2 mL of 1-pentanol (Table S1). The highest dodecane aliquot for forming a Winsor-IV microemulsion was only 0.3 mL. However, when the CSDS was raised to 80 mg/mL H2O (initial water volume = 5 mL), seven ranges of Winsor-IV type microemulsions were found, while the highest amount of dodecane added to realize a Winsor-IV microemulsion was as high as 2.9 mL when applying 0.8 mL of pentanol (Table S4). While the concentration of SDS could have been further increased from 80 mg/mL of H2O to its saturation point of 100 mg/mL of H2O, when considering the cost of SDS, this extremely high SDS concentration should be avoided. It is interesting to point out that under a given high SDS concentration, the Winsor-IV range can be reactivated by addition of fresh pentanol due to the variation of the curvature of the interfacial layer (constructed by SDS and pentanol) in favor of involving more oil into Winsor-IV (O/W type) microemulsion, until the volume of the oil phase was beyond the maximum capacity of the adopted quantity of surfactant for forming Winsor-IV type microemulsion. From the observed results in Tables S1−S4, the minimum surfactant-to-alcohol ratio (SAR) for forming a Winsor-IV type microemulsion was limited to 1:2 (g:mL), that is, Winsor-IV type microemulsions were never formed if the SAR value was lower than 1:2 (g:mL), which was confirmed during the phase scanning under four different SDS concentrations. Compared with the traditional phase scanning method by building up pseudo-ternary phase diagram, our new phase behavior scanning work also featured the following three positive aspects: (1) Dynamical optimization of surfactant-to-alcohol ratio (SAR): During each titration process, the ratio between surfactant and co-surfactant was dynamically altered and optimized to reach the maximum oil dissolution capacity by changing the curvature of the interfacial layer that is composed by surfactant and co-surfactant (time saving). (2) Exclusive phase scanning: During each titration process, all possible Winsor-IV type microemulsions under fixed surfactant concentration were exclusively delineated instead of mapping out the whole map of various phase behaviors (time saving and more specific). (3) Two components alternatively titrated: Co-surfactant and dispersed phase were alternatively titrated into the surfactant brine solution, while the aim was to determine the maximum ability of surfactant to form monophasic

Table 2. Compositions of Representative Monophasic Microemulsions compositions series no.

SDS (mg)

dodecane (mL)

H2O (mL)

NaCl (mg)

pentanol (mL)

SOW-A1 SOW-B1 SOW-B2 SOW-B3 SOW-B4 SOW-B5 SOW-B6 SOW-C1 SOW-C2 SOW-C3 SOW-C4 SOW-C5 SOW-C6 SOW-C7 SOW-C8 SOW-C9 SOW-D1 SOW-D2 SOW-D3 SOW-D4 SOW-D5 SOW-D6 SOW-D7 SOW-D8 SOW-D9

100 200 200 200 200 200 200 300 300 300 300 300 300 300 300 300 400 400 400 400 400 400 400 400 400

0.2 0.1 0.2 0.5 0.6 0.7 1.1 0.1 0.4 0.5 0.6 0.7 1.1 1.3 1.5 1.9 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.7 2.2

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

0.2 0.1 0.2 0.3 0.3 0.3 0.4 0.2 0.3 0.3 0.3 0.4 0.5 0.5 0.5 0.6 0.4 0.4 0.4 0.4 0.4 0.5 0.5 0.6 0.7

their respective surface tension (liquid−solid interfacial tension between microemulsion and platinum Wilhelmy plate, see Experimental Section) and kinematic viscosity because these physical properties are critical evaluation criteria for wood flooding and penetrating the void spaces of wood micropores. Generally, the surface tension of the Winsor-IV type microemulsions (ranging from 22.1 to 28.9 mN m−1) (Figure 4) was found to be much lower than that of water (72.5 mN m−1) and was mainly affected by the amount of dodecane. The surface tension of the microemulsions decreased as the amount of the oil phase increased, but if the quantity of dodecane exceeded a D

DOI: 10.1021/acssuschemeng.5b01601 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. Surface tensions of various Winsor-IV type microemulsions.

specific level (e.g., 1.0 mL dodecane in the case of CSDS = 80 mg/mL H2O with 5 mL initial water volume), no further decreases in surface tension were observed. Altogether, it was noted that the surface tension of microemulsions at lower amounts of dodecane was found to be in the range of 27.0− 28.9 mN m−1, while that of microemulsions with higher compositions of dodecane were in the range of 22.1−22.7 mN m−1. With regards to kinematic viscosities, lower values were detected for microemulsions with lower oil and co-surfactant contents, and the lowest value (1.24 cSt) was observed for SOW-B1 (Figure 5). Further addition of dodecane and

different types of surfactant (ionic, nonionic, or a combination of both) is proposed: Step-I: Establish a gradient range of different surfactant concentrations (extremely high concentrations can be excluded based on the view of cost-competitiveness). Utilize an appropriate salinity corresponding to a high water-to-oil ratio (WOR). Step-II: Gradually add the oil phase to the surfactant brine solution. A pre-existed small amount of co-surfactant in the SOW system will facilitate the formation of a microemulsion. The volume of each aliquot of the oil phase should be strictly controlled relative to the starting water phase volume (e.g., each aliquot of oil volume ≤2% of total starting water volume). Step-III: Once equilibrium is reached for a Winsor-I type, cease addition of the oil phase and add aliquots of the cosurfactant, again keeping the volume added to ≤2% of total starting water volume. The SOW system will then return to Winsor-IV or Winsor-II types when reaching equilibrium. If so, add the oil phase in batches until the Winsor-I type reappears. Repeat the above procedure until Winsor-III type is formed, which is regarded as a termination point. Notably, the WinsorII type may transform directly into a Winsor-III type instead of the Winsor-I after an addition of oil phase. Step-IV: Map out the Winsor-IV type ranges under the fixed concentration of surfactant based on the collected experimental data. It is necessary to highlight that the reason for employing sodium dodecylsulfate and dodecane as surfactant and oil phase, respectively, in this study is primarily based on their simple structures and low costs. According to different application purposes, the species of surfactants and oil phase can be rather flexible. Crystallinity Changes in Wood Samples After Different Microemulsion Treatments. Cellulose crystallinity of loblolly pine wood particles before and after different pretreatments was calculated from the height ratio between the intensity of the crystalline peak (I002−Iamorphous) and the intensity of (002) plane peak after subtraction of the background signals (Figure 6). In order to imitate an in situ measurement of treated wood particles by XRD, all five treated samples were kept in their original wet states. The crystallinity differences of four SOW treated samples (SOW-A1 treated, 61.4%; SOW-B1 treated, 60.9%; SOW-B2 treated, 61.9%; SOW-B4 treated, 57.9%) and the water treated sample (60.3%) were found to be similar to each other and higher than the raw

Figure 5. Kinematic viscosity of various Winsor-IV type microemulsions.

pentanol into the SOW system led to an increase in the final value of kinematic viscosity. This observation may be explained by two hypotheses: (i) Kinematic viscosity of pentanol (3.74 cSt) is considerably high, and thus the increasing amount of pentanol will lead to a corresponding increase in the system viscosity. (ii) Although the kinematic viscosity of dodecane (1.72 cSt) is not as high as pentanol, the real contribution of the oil phase to the system viscosity comes from the increased friction among the dispersed oil droplets in O/W microemulsion, whose droplet size is enlarged by the incorporation of more oil into the SOW system. For example, the size of the dispersed oil phase in SOW-B1 is only 7.2 nm by dynamic light scattering measurement, while this value is elevated to 11.2 nm in SOW-B2 and to 35.8 nm in SOW-B3. Universal Scanning Approach for Winsor-IV Type Microemulsions. On the basis of the present results, a universal Winsor-IV type microemulsion scanning approach for E

DOI: 10.1021/acssuschemeng.5b01601 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 6. XRD spectra of wood samples before and after treatment with water and SOW.

Table 3. Compositions, Surface Tensions, and Kinematic Viscosities of Five Selected Microemulsions for Wood Penetration composition series no.

SDS (mg)

dodecane (mL)

H2O (mL)

NaCl (mg)

pentanol (mL)

surface tension (mN m−1)

kinematic viscosity (cSt)

SOW-A1 SOW-B1 SOW-B2 SOW-B4 SOW-C9

100 200 200 200 300

0.2 0.1 0.2 0.6 1.9

5 5 5 5 5

100 100 100 100 100

0.2 0.1 0.2 0.3 0.6

27.2 28.9 27.8 22.6 22.2

2.93 1.24 1.93 3.23 4.93

Figure 7. Fluid uptake of Winsor-IV type microemulsions and water-treated wood samples.

Iamorphous can lead to an increase in the overall crystallinity. (iii) There is the possibility that the total crystallinity of the samples is enriched by removal of non-crystalline lignin and other carbohydrate-based contaminants. Wood Penetration by Winsor-IV Type Microemulsions. To realize an efficient penetration effect in wood blocks by microemulsions, the SOW systems that exhibit relatively low surface tension and kinematic viscosities are highly preferred. However, their penetration ability may not simply be determined by single factors (e.g., surface tension or kinematic viscosity), but by the synergetic effects of multiple factors.

material (55.4%) (Figure 6). It is suggested that this increase is probably due to the following three hypotheses: (i) There is a new formation of inter- or intra-hydrogen bonds among cellulose within wetted wood samples (e.g., H2O treated sample, 60.3%, vs raw material, 55.4%). (ii) Short crystalline ranges present in the amorphous area of cellulose were more easily disrupted by SOW treatments than the long-range crystallinity of cellulose, which was reflected by the decreased Iamorphous/I002 values of the four SOW treated samples compared to their raw material. Because the final crystallinity was calculated by (I002−Iamorphous)/I002, a larger reduction of F

DOI: 10.1021/acssuschemeng.5b01601 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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microemulsions under a fixed concentration of surfactant by dynamically optimizing the quantitative ratio between surfactant (SDS) and co-surfactant (pentanol) and may also be further considered as a universal approach for any type of SOW system. The selected microemulsions only disrupted the short crystalline ranges existing in the amorphous regions of cellulose in treated loblolly pine wood particles, but did not show any obvious reduction of crystallinity in the long-range crystallinity of cellulose, and this latter goal will be the subject of future studies. Notably, the overall ability of these microemulsions for wood penetration was governed by the synergetic effects between surface tension and kinematic viscosity rather than single factors. The Winsor-IV type microemulsions prepared using economic and environmentally friendly components not only have applications as efficient complex fluids for the rapid penetration of lignocellulosics, but may also serve as transport carriers of various reagents for potential applications in wood preservation, enzymatic hydrolysis, or delignification.

Therefore, five SOW systems (SOW-A1, B1, B2, B4, and C9) with different compositions, surface tensions, and kinematic viscosities (Table 3) were selected for liquid uptake experiments, and their respective wood penetration abilities were compared with each other and against water (Figure 7). Liquid uptake experiments were examined over 20 min. Longer impregnation times were avoided to prevent the microemulsion from evaporating, resulting in surfactant precipitation on the surface or in the inner capillaries of the wood blocks, which could therefore lead to an overestimation of the liquid uptake by weight gain. The microemulsions exhibited a more rapid initial uptake of liquid during the first 3 min when compared to the water-only system (Figure 7). At later times (>10 min), the weight gain tended to be linear and gradually stable. SOW-A1 (γ = 27.2 mN m−1; ν = 2.93 cSt) exhibited the highest penetration ability with a 26.8% weight gain after 20 min, followed by SOW-B2 (γ = 27.8 mN m−1; ν = 1.93 cSt), SOW-B1 (γ = 28.9 mN m−1; ν = 1.24 cSt), SOW-B4 (γ = 22.6 mN m−1; ν = 3.23 cSt), and SOW-C9 (γ = 22.2 mN m−1; ν = 4.93 cSt). As mentioned above, a low value for a single factor does not necessarily correlate to the strongest penetration ability due to potential synergistic effects. This is observed for the SOW-A1 system, as this microemulsion possesses midrange values of surface tension (27.2 mN m−1) and kinematic viscosity (2.93 cSt), yet was the best formulation with respect to wood penetration. It is interesting to point out that the liquid uptake amount by water was lower than those of the other five microemulsions during the first 10 min of the study, but was ultimately greater than the levels reached by SOW-B4 and SOW-C9 and nearly equivalent to that of SOW-B2 after 20 min. This may be understood from the process of liquid penetration itself in that the initial penetration rate depends on the surface tension of the liquid. For instance, a lower surface tension liquid could effectively wet fibers in a wood block to a larger extent. For water, because its surface tension is as high as 72.5 mN m−1, it demonstrated the worst initial wetting rate. However, once the majority of the wood block was wetted, the dominating factor for further liquid uptake was shifted to kinematic viscosity. During this stage, the fresh liquid was continuously siphoned up from the liquid source into the prewetted micropores of the fibers until all the fibers reached a saturated status. Although water exhibited the weakest initial wetting ability, its kinematic viscosity (0.89 cSt), the lowest of all of the solutions studied, was a more dominant driving force in liquid uptake than SOWB4 and SOW-C9. If the total contact time was prolonged, the uptake of water may likely overtake those for SOW-B1 and SOW-A1. On the basis of the above observations, microemulsions may be considered a better choice for the efficient wetting of wood fibers under short contact times, whereas for longer treatment times, water is preferred. However, the key superiority of microemulsion for woody biomass treatment is not only limited to its rapid wood penetration ability, but also contributed to by its remarkable oil solubility, which make it a good carrier for transportation of target organic reagents (e.g., biocides and catalysts) for various potential applications.

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EXPERIMENTAL SECTION

Materials. sodium dodecylsulfate (SDS) as surfactant and 1pentanol as co-surfactant were purchased from Sigma-Aldrich. Dodecane and sodium chloride, used in the oil phase and as the electrolyte in SOW system, respectively, were supplied by Fisher Scientific. Milli-Q water was used as the water phase in the preparation of the SOW system and for other experiments. Phase Behavior Scanning of SOW System. Four SDS aqueous solutions at 20, 40, 60, and 80 mg SDS/mL H2O (initial water volume = 5 mL) were prepared at constant NaCl concentration (20 mg NaCl/ mL H2O) and scanned for phase behavior. Dodecane, the oil phase, was added gradually (in 100 μL aliquots) to each of these SDS solutions until phase equilibrium reached Winsor-I. At this point, a fresh aliquot of pentanol (100 μL) was added. Usually, the phase equilibrium returned to either Winsor-IV or Winsor-II when reaching equilibrium; if so, dodecane was again added in aliquots (100 μL each) until Winsor-I was reachieved. The previous procedure of pentanol/ dodecane additions was repeated until a Winsor-III phase was achieved, which indicated that the amount of SDS was incapable of restoring the SOW system to a Winsor-IV type no matter how much 1-pentanol was added. Characterization of Microemulsions. Surface tension (liquid− solid interface) was measured by a Sigma tensiometer system (Sigma 703D) equipped with a platinum Wilhelmy plate. Kinematic viscosity was determined by a Cannon Fenske viscometer at 25 °C. The size of the dispersed phase in the microemulsion was obtained by dynamic light scattering using a Nanosizer (Malvern Instruments Ltd., Malvern, U.K.). The water-to-oil ratio (WOR) (v/v) and surfactant-to-alcohol ratio (SAR) (w/v) were calculated from each phase behavior scanning. Microemulsion Treatment of Wood Samples. Wiley milled loblolly pine wood particles (20−40 mesh) and soft pine wood blocks (1.5 cm × 1.5 cm × 2.0 cm) were prepared for this study to characterize the effects of Winsor-IV type microemulsions on cellulose crystallinity and wood penetration, respectively. Approximately 1.0 g of Wiley milled loblolly pine wood particles (20−40 mesh) was fully immersed into 10 mL microemulsion or D.I. water. After a total immersion time of 24 h, the excess liquid was removed by vacuum filtration. The residue particles were thoroughly rinsed by D.I. water under vacuum, and the sample was collected and measured by XRD analysis. For wood penetration (liquid uptake) experiments, the soft pine wood blocks (with uniform wood ultrastructure features) having dimensions of 1.5 cm (radially) × 1.5 cm (tangentially) × 2.0 cm (axially) were cut from a soft pine wood board that was precut axially along the grain and air-dried (moisture content: 7.9 wt %). The procedure of wood penetration was first that the wood block was dipped axially into the testing microemulsion to a depth of 2 mm

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CONCLUSIONS A simplified and timesaving preparation method of monophasic microemulsions was developed for wood pretreatment by utilizing sodium dodecylsulfate (SDS)/pentanol/water/sodium chloride/dodecane. This approach preferentially focused on screening all possible formulations of the Winsor-IV type G

DOI: 10.1021/acssuschemeng.5b01601 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering below the fluid surface. After contact time (5 s per contact time during the range from 0 to 60 s; 30 s per contact time during the range from 1 to 3 min; 1 min per contact time during the range from 3 to 10 min; 2 min per contact time during the range from 10 to 20 min), the wood block was removed from the microemulsion. The bottom surface of the block was quickly dried by blotting with a clean Kimtech tissue, and the overall weight was recorded. The entire accumulated contact (penetration) time for each microemulsion was up to 20 min, and the penetration measurements for each sample were performed in duplicate. XRD Analysis of Microemulsion Treated Wood Samples. XRD experiments were performed on a Rigaku Smartlab X-ray diffractometer to characterize the crystallinity change of the Wiley milled loblolly wood particles before and after microemulsion exposure. Scans were obtained from 10° to 40° 2θ in 0.1° steps for 15 s per step.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01601.

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Information as mentioned in the text. (PDF) .

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L. Lucia). *E-mail: [email protected] (R. Ghiladi). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the North Carolina Biotechnology Center (2013-MRG-1113). C.A. Carrillo is warmly thanked for his valuable advice and discussions. We sincerely acknowledge O.J. Rojas of Aalto University for the helpful discussions, input, and support that led to the development of the current manuscript. Finally, we thank the members of Rojas research group at NC State University for their time and generous use of equipment.

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REFERENCES

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DOI: 10.1021/acssuschemeng.5b01601 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX