Interfacial Properties of Sugar-based Surfactantsojrojas/PDF/2009_2.pdf · Interfacial Properties...

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Interfacial Properties of Sugar-based Surfactants

Orlando J. Rojas1, Cosima Stubenrauch2, Lucian A. Lucia1, and Youssef Habibi1

1 Forest Biomaterials, North Carolina State University, Campus Box 8005, Raleigh, NC 27695 and 2 University College Dublin, School of Chemical and Bioprocess Engineering, Belfield, Dublin 4, Ireland

IntroductionMotivationInterest is growing in the development of material technologies that are based on renewable resources. The use of biomass as feedstock is of expanding industrial significance, particularly in the energy and commodities sectors. Relative to these arenas, mono- and polysaccharides, very flexible and tunable chemical starting materials, are estimated to make up three-quarters of the world’s biomass. The efficient use of this resource is now recognized as a major future objective in a wide range of technology applications. The need exists to identify secondary streams of saccharide by-products and to use these materials as the basis for higher, value-added surfactant chemistries. Several research groups in the world are creating a platform for advancing the knowledge on the structure–performance properties and for facilitating the penetration of sugar-based surfactants in traditional markets dominated by nonrenewable nonionic surfactants. The general goals of the research work performed in these groups are to propose and test new structures for targeted applications as well as to provide the basis for future strategies aimed at enhancing cost-effectiveness. More specific aims involve both identifying molecular factors that govern the surface activity and facilitating the design and application of sugar-based surfactants as substitutes for conventional poly(ethylene oxide) (EO) surfactants and others. Significant groundwork was already done in this area (Balzer & Luders, 2000; Kiraly & Findenegg, 2000; Kocherbitov et al., 2002; Kumpulainen et al., 2004a,b, 2005; Liljekvist et al., 2001; Matsson et al., 2004; Muruganathan et al., 2003, 2004; Nickel et al., 1996; Persson et al., 2002, 2003a,b, 2004; Ruiz, 2009; Stubenrauch, 2001). Three classes of surfactants with sugar or a polyol derived from sugar as polar head group were widely researched: alkyl polyglucosides (APGs) (also known as alkylpolyglycosides), alkyl glucamides, and sugar esters (Holmberg, 2001). Both these surfactants and biosurfactants produced by microorganisms and other surfactants derived from renewable raw materials are coming progressively onto the market.

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Deleu and Michel Paquot (2004) provided an interesting summary of trends, and reported for APGs a share of ca. 3% of the total production. The reader is referred to the book edited by Hill et al. (1997) for more detailed information on this subject. Overall, APGs have established themselves as natural surfactants of choice for diverse applications—from emulsifiers for skin care to foaming agents. The possibility of future development in the area of biofuels from cellulose may open new opportunities, especially for secondary streams in related processes. We believe that four critical work packages need pursuing to successfully expand the potential of sugar-based surfactants from such sources:

Understanding and rationalizing the state-of-the-art in the area of saccharide-• based surfactants. The existing, highly fragmented activities need to be systemized and analyzed to adequately respond to current changes in economic and environmental aspects of surfactant production and use.

Identifying molecular and structural factors that govern the surface activity of • these materials so that we can facilitate their design and application in areas that are currently utilizing petroleum-based surfactants. One can support this by studies with model systems which will expand our understanding and allow us to target specific applications.

Designing and characterizing new sugar surfactants based on natural products and • on new motifs (Blunk et al., 2006) to demonstrate the economic and technical viabilities in different applications.

Exploring new synthetic approaches to utilize saccharides for the synthesis of new • surfactants. One could harvest saccharides from biomass processing (and related secondary streams) or from agricultural/food by-products. One can then combine this with an understanding of the current and projected economics of saccharide- and petroleum-based surfactant technologies to facilitate the identification of key future markets for these materials.

Recent research results show that glucose-based and sucrose-based surfactants have a range of beneficial physical and performance properties, including high levels of surfactancy (surface and interfacial activity), very rapid biodegradability, low human and animal toxicity, effective emulsification properties, and surface interactions. Therefore, we anticipate that we can rationally design saccharide-based surfactant structures, centered on the combination of potentially low-cost (based on natural starting materials), renewable saccharide components and appropriate renewable (natural fatty acids) hydrophobic pendant groups that will allow the understanding of the fundamental interactions governing their functionalities. Based on this understanding as well as on the synthetic approaches to generating sugar-based surfactants, we hope to promote interest in, and the prospects for, their utilization in a number of surfactant markets, which would ultimately lead to a significant reduction in the use of petroleum-based materials in these sectors.


Interfacial Properties of Sugar-Based Surfactants l 459

Nonionic SurfactantsNonionic surfactants represent a major component material for applications ranging from personal care to a wide range of industrial uses. Structurally based on the molecular combination of hydrophilic and hydrophobic substructures, nonionic surfactants are effective as wetting and spreading agents, as emulsifiers, and as foaming agents (while having minimal skin and eye irritation effects). Furthermore, they are characterized by a wide range of critical secondary-performance properties. The hydrophilic component of these materials is currently based largely on EO, which is petroleum-derived. In addition, a significant portion of the hydrophobic components of these materials is also petroleum-derived. The production costs of these commodity chemicals are closely linked to the highly volatile petroleum prices, which one can regard as an additional motivation to search for substitutes. In view of the large volume in the consumption of EO-based nonionic surfactants, further replacement of the established market would be very attractive for a high-value utilization of sugars from biomass. However, extensive replacement of EO-based surfactants can be challenging, and a market barrier may exist that prevents more widespread penetration of a new generation of surfactants. Nevertheless, in our opinion, the time is right to further advance efforts by starting with high-end applications such as drug emulsification, pharmaceutical formulations, etcetera, which are industries that require these technologies and can temporarily tolerate higher prices. In summary, a need is evident to broaden the application of surfactants to replace petroleum-based products toward new classes of highly biodegradable, low-irritating, low-toxic, completely naturally-derived nonionic surfactants. As superior-performance properties of current and novel sugar-based surfactants are demonstrated, they will become further established in the future “green” market.

Environmental and Product-performance ConcernsEnvironmental and toxicity issues are prompting an increased consideration of a wider use of sugar-based surfactants. For example, two of the most common types of surfactants used in large-scale applications are EO alkyl ethers and fatty-acid soaps. In particular, the use of nonylphenol (NP) ethoxylates is problematic. NP is a mixture of isomeric monoalkyl phenols, predominantly para-substituted, found in the environment as a result of the biodegradation of NP ethoxylates. NPs that are used as nonionic surfactants are released to the environment through various waste streams in industrial processes. The National Library of Health reports these surfactants as being severely irritating to skin and eyes. Vapors cause a slight irritation of the eyes or respiratory system if present in high concentrations (ILO, 1998; Lewis, 1997; NIOSH, 1983). Furthermore, NPs are suspected of being endocrine disruptors, that is, they have adverse effects on the workings of the endocrine system in humans and animals (Ren et al., 1997). Many European countries have banned the use of NPs. Recent



EU legislation on the use of NPs (Directive 2003/53/EC) states that NP and NP ethoxylates “may not be placed on the market or used as a substance or constituent of preparations in concentrations equal to or higher than 0.1% by mass” (EPHA, 2005). Therefore, an important need exists for replacing this and other toxic surfactants with environmentally-friendly alternatives. Our objective in this chapter is to introduce the interfacial characteristics of surfactants based on renewable materials (natural surfactants). In this way, one can evaluate the fundamental properties of sugar surfactants and consider them for specific applications. Overall, understanding the adsorption of biobased surfactants will not only solve an important industry and environmental problem, but will also open an avenue for a number of products with unique properties that one can generate from biomass.

Significance of Biobased SurfactantsA significant demand is apparent for new, less- expensive, nontoxic, and biodegradable surface-active materials. Their use, as conventional surfactants, is aimed at stabilizing liquid–liquid (emulsions), solid–liquid (dispersions), and gas–liquid (foams) interfaces. The economic impact that the penetration of sugar-based surfactants can have is mirrored by the substantial consumption of emulsions worldwide in which the surfactant composition amounts to 0.1–5% of the total mass of the emulsion. However, the use of surfactants is not limited to emulsions; vast numbers of other applications involve large quantities of (amphiphilic) stabilizers, including household, cleaning, food, cosmetic, pharmaceutical, and other specialty products. Numerous studies recently werecarried out to examine the viscoelastic properties, surface tension, and surface properties of sugar surfactants. Most of those studies point to the overall feasibility of employing various sugar surfactants to replace their EO-based counterparts because of their superior-surface physical responses and application performance, environmental compatibility, and chemical tunability (functionalization and modification). Thus, the sections that follow provide a summary of results that one can use as a basis for the further development of sugar-based surfactant systems. The final goal is to design biobased surfactants with properties superior to those of the traditionally-used nonionic surfactants and thus to replace the latter whenever possible and feasible.

Interfacial Properties of Sugar-based SurfactantsNumerous fundamental studies were conducted in the area of sugar-surfactant properties. Four milestones that serve as recent evidence for the superior characteristics of sugar-based surfactants compared with EO-based surfactants are discussed below in terms of “Air/Water Interfaces,” “Solid/Liquid Interfaces,” “Comparison between EO- and Sugar-Based Surfactants: Structural Aspects, Interfacial Aspects, Packing at the Interface,” and “Viscoelasticity Properties of Isomeric Sugar-based Surfactants”.



Before discussing these subjects, let us briefly outline the tools that were utilized in these experiments since most of these are not widely available in typical laboratories. Results from techniques used to measure surface tension, solution rheology, contact angle, detergency, and emulsion properties are not covered in this chapter. Instead, we describe the interfacial properties of adsorbed layers of sugar surfactants by using the methods described below.

TechniquesMeasurements and Analysis of Surface Interactions and Forces (MASIF) Force measurements were conducted using the noninterferometric surface force apparatus (Parker et al., 1989, 1994). This device, commonly known as MASIF (Measurements and Analysis of Surface Interactions and Forces), employs a bimorph force/deflection sensor that, after calibration of the spring (deflection) constant, yields the interaction force. One of the surfaces (bottom surface) is mounted on the edge of the bimorph, and the other (the top one) at the end of a piezoelectric tube. The assembly is enclosed in a stainless-steel cell of ca. a 10-mL volume, and mounted on a translation stage that is isolated from electrical and sound noise. During a typical force measurement, the surfaces are driven closer until contact, and then they are separated further apart. This is done by applying a triangular voltage wave to the piezo crystal. Simultaneously, the charge produced upon any deflection of the bimorph, due to repulsive or attractive forces, is recorded. Once the surfaces come into hard-wall contact, the linear movement of the piezo deflects the bimorph, thus enabling the force sensor to be calibrated against the known piezo-crystal expansion and contraction as measured by a linear variable differential transformer (LVDT) sensor. Provided the deflection and the spring constant of the bimorph are known, the data are used to calculate the force–distance curves from Hooke’s law. The noninterferometric surface force apparatus does not allow an absolute determination of the zero surface separation; however, one can obtain the adsorbed surfactant-layer thickness from the magnitude of inward “jumps” that typically occurs when a surfactant layer is pushed out from the contact area upon compression. Ederth et al. (1998) show that flame-polished glass surfaces are smooth enough to enable accurate measurements of surface forces down to molecular separations. The surfaces used in each experiment were prepared by melting one end of a borosilicate glass rod (diameter 2 mm, length ca. 25 mm) in a butane–oxygen burner until the tip formed into a sphere with a diameter of about 4 mm. The normal radii of curvature (r1 and r2) for each surface were determined more accurately at the end of the experiment by using a micrometer, and the local harmonic mean radius of the interaction, R, was then calculated as R = 2r1r2/(r1 + r2). The spring constant of the bimorph was measured at the end of each experiment by placing known weights on the bimorph spring and measuring the resulting deflection (usually about 100 N/m). The force was then normalized by the local harmonic mean radius of interaction (F/R).



The hydrophobic surfaces on which the adsorption of the surfactants was studied were obtained by silanization and thiolization, respectively, of the hydrophilic glass surfaces. Despite being hydrophobic, the silanated glass carries a significant net negative charge. This charge results from the dissociation of unreacted silanol groups in the glass substrate that is not completely screened by the self-assembled silane layer. The thiolated surfaces, however, are completely uncharged. One can find details about the surface preparation in previous reports (Ederth et al., 1998; Stubenrauch et al., 2004a). All procedures for assembling the measuring chamber and preparing the solutions were carried out in a laminar-flow cabinet. At the beginning of each set of experiments, the interaction profiles were first determined in air to ensure that the system showed no signs of contamination. Then a background electrolyte solution was introduced into the measuring chamber, and the interaction profiles were again determined. A stock surfactant solution was then introduced through a 0.2-µm poly (tetrafluoroethylene) (PTFE) filter until the desired concentration inside the chamber was attained. All measurements were carried out at 22 ± 1°C.

Thin-Film Pressure Balance (TFPB)The most prominent method for investigating the interactions between two surfactant films at the air/water surface (i.e., the interactions acting in foam films) is the thin-film pressure balance (TFPB) (Claesson et al., 1996; Exerowa & Kruglyakov, 1998; Exerowa & Scheludko, 1971; Mysels & Jones, 1966; Stubenrauch et al., 2003) and its modified versions. In brief, a film is formed in a film holder consisting of a porous glass disc that is connected to a glass tube. A hole is drilled in the disc in which the film is formed. This film holder is fixed in a gas-tight cell, a pressure is applied to the cell via a syringe, and the film thickness h at this particular pressure is determined interferometrically. By calculating the disjoining pressure Π from the applied pressure, one obtains the characteristic Π-h curves.

Surface Light Scattering (SLS)The surface light scattering (SLS) set up referred to in this study was reported by Rojas et al. (2005b), which is based on the original work of Hård and Neuman (1987). The respective surfactant solution was placed inside a closed 316-stainless-steel double-walled thermostated cabinet sitting on an optical table. The temperature was monitored by temperature probes located inside the chamber, and maintained by an external thermostated circulation bath (to ±0.1°C). The humidity was set close to saturation [ca. 90% of relative humidity (RH)] by placing filter papers moistened with water. In a typical experiment, the experimental autocorrelation function of the surface waves was recorded and fitted to an exponentially dampened cosine function (Bouchiat & Meunier, 1971). The correlograms were Fourier-transformed and then fitted to a four-parameter Lorentzian function (Bellman & Pennington, 1954; Hård



& Neuman, 1987). From these operations, we obtained the parameters of the power spectrum. The measured power spectrum was compared with the theoretical power spectrum (Kramer, 1971) for capillary waves in the presence of air. This theoretical power spectrum is described, among others, in terms of the surface tension γ, the transverse viscosity µ, the sum of interfacial shear elasticity and interfacial dilatational elasticity ε, and the sum of interfacial shear viscosity and interfacial dilatational viscosity κ. The experimental data [i.e., the central frequency and the dampening coefficient (after correction for instrumental broadening) together with the bulk properties of the fluids] were used to calculate the viscoelasticity coefficients from the power- spectrum equation. Hence, the sum of the real and imaginary parts of the complex modulus (elasticity and viscosity) was obtained. We used polar diagrams for direct interpretation of the rheological parameters ε and κ. These plots were constructed from the dispersion equation for a given temperature, wave number, and surface tension (or surface pressure) by using as parameters the normalized complex frequency (ωo/ωw, α/αw), where ωo is the experimental central frequency, and α is the dampening coefficient. Here the subscript “w” is used to denote the experimental values for a film-free surface (ε = 0, κ = 0) (i.e., water in our case).

Fundamental Studies on Interfacial Properties of Sugar-based SurfactantsA key aspect of our studies lies in a systematic comparison between petroleum-based surfactants and sugar-based surfactants. This is intended to highlight the benefits of sugar-based surfactant platforms. We compared an EO-based surfactant (hexaoxyethylene dodecyl ether, C12E6) with a sugar-based surfactant with the same hydrophobic group (n-dodecyl-β-d-maltoside, β-C12G2) (see Fig. 16.1) (Claesson et al., 2006; Rojas et al., 2005a). We consider here the interactions between nonpolar surfaces coated with either C12E6 or β-C12G2. As nonpolar surfaces, we chose the air/water surface, silanated

n-dodecyl--D-maltoside,-C12G2

Hexaoxyethylene dodecyl ether, C12E6

O OOOO

OH

HO

OOHOH

OOH OH

OH

HOO

O

Fig. 16.1. The n-dodecyl-β-d-maltoside (β-C12G2, top) and hexaoxyethylene dodecyl ether (C12E6, bottom) are two nonionic surfactants with the same hydrophobic group that were compared regarding their interfacial properties.



glass, and thiolated gold surfaces. The most important results with respect to the comparison of different surfaces are summarized in this section.

Air/Water InterfacesThe adsorption of nonionic surfactants at the air/water surface leads to a decrease of the surface charge of the interface (reviewed in Stubenrauch et al., 2003). This decrease eventually results in a transition from an electrostatically stabilized common black film (CBF) to a Newton black film (NBF) that is stabilized by short-range repulsive forces. This phenomenon is illustrated in Fig. 16.2 with data obtained for the nonionic surfactant hexaoxyethylene dodecyl ether (C12E6). The formed NBF consists of two densely packed monolayers, and creates a “force barrier” that prevents the film from rupturing, thus stabilizing a foam. Apart from densely-packed monolayers, a sufficient monolayer cohesion is required for the formation of a stable NBF (Stubenrauch et al., 2004a,b). From the thickness of the NBF, one can estimate the thickness of one monolayer. Comparing Fig. 16.2 with the respective results obtained for n-dodecyl-β-d-maltoside (β-C12G2) shown in Fig. 16.3, apparently, the same general trend is observed. In both cases, film thicknesses ranged from >80 nm to <5 nm, depending on the surfactant concentration and the applied pressure, which ranges from 200–9000 Pa. As was the case for C12E6 (Fig. 16.2), two different kinds of films were observed: thick CBFs stabilized by electrostatic repulsion, and thin NBFs stabilized by short-range repulsion. The thicknesses of the CBFs decrease monotonically as Π increases. While the slope d(log Π )/dh is independent of the surfactant concentration, a significant shift of the curves toward lower disjoining pressures is observed when increasing the β-C12G2 concentration from 0.034–0.137 mM. Moreover, at the highest concentration, no CBF is formed at all, but the foam film drains directly down to the NBF. Experimentally (reviewed in Stubenrauch et al., 2003) and only recently also theoretically (Kudin & Car, 2008), the air/water surface is negatively charged. This charge is responsible for the long-range electrostatic repulsive forces observed in foam films stabilized by nonionic surfactants. An increase in the nonionic surfactant concentration leads to a decrease of the surface-charge density as more uncharged molecules (i.e., nonionic C12E6 and β-C12G2 surfactants) adsorb at an originally charged surface. The electrostatic forces acting in foam films stabilized by β-C12G2 were quantified by means of the DLVO theory by using constant-charge boundary conditions and the theoretical Debye length of κ–1 = 30 nm. These calculations led to surface-charge densities of q0 = 1.55 mC m–2 for the 0.035 mM solution and q0 = 0.95 mC m–2 for the 0.137 mM solution, respectively. The decrease in surface-charge density destabilizes the CBF until no CBF is observed for c > CMC under the given experimental conditions. At these concentrations, the immediate formation of an NBF is observed. The NBFs are very thin (ca. 5 nm) with an aqueous core of 1–2 nm assuming a length of ≈2 nm for the surfactant. In other words, these films consist of



10 nm < h < 100 nm h < 10 nm

Newton Black Film, NBFCommon Black Film, CBF

10 nm < h < 100 nm h < 10 nm

Newton Black Film, NBFCommon Black Film, CBF

h / nm20 40 60 80 1000

100

1000

10000

/

Pa

0.01 mM

0.05 mM

0.1 mM

Fig. 16.2. (top) Pictures and schematic drawings of a common black film and a Newton black film, respectively. (bottom) Disjoining pressure Π as a function of film thickness h measured for three concentrations of C12E6 in 10–4 M of a NaCl solution. The two lower concentrations are below; the highest is above the critical micelle concentration (CMC) (= 0.073 mM). The solid lines are calculated according to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory assuming interactions at constant charge



two surfactant monolayers with only small amounts of water separating the headgroups (mainly hydration water). As is the case for the force barrier between nonpolar solid surfaces (to be presented later), densely-packed monolayers are required to stabilize an NBF and thus to prevent contact between the two bare surfaces (i.e., film rupture). Hence, the presence of an NBF signifies the existence of a force barrier between two air/water surfaces analogous to the force barrier between two solid surfaces (see Figs. 16.4 and 16.5). The remarkable similarities with regard to the Π-h curves of β-C12G2 and C12E6 are evident from the fact that the curves nearly lie on top of each other, which means that the surface- charge densities q0 are nearly equal. Indeed, surface-charge densities of q0 = 1.70 mC m–2 at c = 0.01 mM (0.12 CMC) for C12E6 and q0 = 1.55 mC m–2 at c = 0.035 mM (0.25 CMC) for β-C12G2 were calculated from the experimental data.

0.137 mM0.170 mM

○●

(b)

h / nm0 10 20 30 40 50 60 70 80 90 100

/

Pa

100

1000

10000

0.137 mM0.034 mM

0.170 mM

Fig. 16.3. Disjoining pressure Π as a function of film thickness h measured for three concentrations of β-C12G2 in 10–4 M of a NaCl solution. The two lower concentrations are below; the highest is above the critical micelle concentration (CMC) (= 0.14 mM). The solid lines are calculated according to the DLVO theory assuming interactions at constant charge (Stubenrauch et al., 2002).



As the surface-charge density is a property of the bare air/water surface, these values mean that a surface concentration of 3.0 × 10–6 mol m–2 C12E6 (0.12 cmc) reduces the surface-charge density to the same amount as a surface concentration of 4.0 × 10–6 mol m–2 β-C12G2 (0.12 cmc). Moreover, in both cases, the surface concentration is enough to stabilize a foam film up to 10,000 Pa. We suggest that the effective surface cross-section area covered per molecule determines the reduction in surface-charge density. Since the headgroup of C12E6 is larger than that of β-C12G2, a smaller number of C12E6 molecules would be needed to achieve a given reduction in the repulsive double-layer force. We conclude that, for each nonionic surfactant, the surface- charge density is reduced with increasing adsorption. A more thorough discussion regarding the influence of the headgroups and the chain length of the surfactant on

Fig. 16.4. Force F normalized by the radius R as a function of relative surface separation D between two thiolated solid surfaces. The forces were measured across aqueous solutions containing 0.1 mM NaCl in the absence (filled circle) and in the presence of 0.01 mM non-ionic surfactant C12E6 (open squares) (redrawn from Stubenrauch et al., 2004 and Rojas et al., 2005a). Similar behavior was observed for β-C12G2 (see Fig. 16.5).

F R

-1 /

mN

m-1

solid

liquid

surfactant

D / nm0 10 20 30 40 50

0

1

2

without C12E6

0.010 mM C12E6



film stability is found in Stubenrauch et al. (2002) and Schlarmann & Stubenrauch (2003).

Solid/Liquid Interfaces Silanated glass and thiolated gold surfaces were used as hydrophobic solid surfaces. Upon adsorption, at the lowest surfactant concentration, we noted that the interaction forces were dominated by a steric barrier at a short-surface separation. By increasing the surfactant concentration, a more robust steric barrier was formed. Interestingly, the NBF formation observed for foam films (see Figs. 16.2 and 16.3) corresponded to the appearance of this force barrier between two solid surfaces (see Fig. 16.4 in the case of thiolated gold). The barrier shown in Fig. 16.4 is a measure of the force that is needed to remove surfactant from between the two surfaces. Because this barrier is located at a distance corresponding to the thickness of a surfactant bilayer, the analogy to the NBF formation is obvious. Note that a “removal” of this bilayer results in film rupturing in the case of foam films, whereas in the case of solid/liquid interfaces it leads to a direct contact of the solid surfaces (Rojas et al., 2005a; Stubenrauch et al., 2004a). Also concluded (data not shown) is that the nature of the surface at which the surfactant adsorption takes place mainly influences the interaction forces at a low-surface coverage. Once a densely-packed surfactant layer is formed, the surfactant itself determines the interaction forces.

Comparison between EO- and Sugar-based Surfactants: Structural Aspects, Interfacial Aspects, Packing at the Interface An important conclusion drawn in the summary of results presented before is the fact that the nature of the surface at which the surfactant adsorption takes place mainly influences the interaction forces at a low-surface coverage. Once a densely-packed surfactant layer is formed, the surfactant itself determines the interaction forces. Therefore, we were able to detect structural effects (i.e., the influence of the polar headgroup) on the interaction forces at the molecular level. Comparing data obtained for β-C12G2 and C12E6, we demonstrated that for similar bulk concentrations the sugar surfactant forms a denser and more robust monolayer with higher cohesiveness (see Fig. 16.5). On the other hand, compared with the shorter tail homolog β-C10G2, β-C12G2 is anchored more strongly to hydrophobic surfaces. This is explained by a stronger interaction between the nonpolar tails within the monolayer. Our measurements indicated that the thickness of the β-C12G2 monolayer under compressive loads was marginally lower than 2 nm. This thickness value is in agreement with the surfactant molecular dimensions but smaller than the (compressed) monolayer thickness measured for C12E6. For both surfactants, the most distinctive feature in the force curve between surfactant-coated solid surfaces is the buildup of a



steep and short-range repulsion as the surfactant adsorption approaches saturation. Shrestha at al. measured the surface rheology of C12E6 and β-C12G2 under similar conditions, and found that β-C12G2 has a higher surface elasticity, which, in turn, explains the observation that β-C12G2 forms much more stable foams than C12E6 (submitted for publication). This indicates that the headgroup of the sugar-based surfactants is stiffer and less compressible, which, in turn, seems to be favorable for the stabilization of foams (Shrestha, 2008, submitted for publication) and dispersions (Lu et al., 2007; Lu & Somasundaran, 2008). In summary, we have so far exclusively considered short-range repulsive interactions between surfactant layers. They are divided roughly into repulsive forces due to hydration and steric forces (Claesson et al., 2006). Of course, the net short-range interaction—that is the only measurable quantity—is due to a balance of attractive and repulsive-force contributions. In the next sections, we recapitulate some findings about the hydration of polar groups, and relate those findings to the short-range interactions between surfactant layers.

0 5 10 15 20

0

1

2

D / nm

F R-1

/ m

Nm

-1 -C12G2

C12E6

Fig. 16.5. Steric force barriers for 0.035 mM of β-C12G2 and 0.01 mM of C12E6, respectively, between solid thiolated surfaces. A stronger force barrier is observed for the sugar-based surfactant. This steric force is responsible for the stabilization of dispersions, where the solid particles are separated by liquid films. The stronger barrier seen in the case of the sugar surfactant is explained by a larger stiffness of its polar group as compared to the EO units (Rojas, et al., 2005a).



Viscoelasticity Properties of Isomeric Sugar-based Surfactants A wealth of information about the surface tension of sugar-based surfactant is avail-able in the literature, some of which are cited in this chapter. As expected, differences in surface activity, packing density, minimal surface tension, and CMC were observed when comparing sugar surfactants of different types or different structures. Less obvi-ous is perhaps the fact that subtle changes in the stereochemistry of a given surfactant can lead to noticeable differences in the same properties mentioned above. Instead of discussing tensiometry data for sugar surfactants here, we only point out the case of an interrelated property, namely, the viscoelasticity of adsorbed monolayers. Surface light scattering by capillary waves was used to investigate the adsorption behavior of nonionic sugar surfactants at the air/water surface. The viscoelastic properties (surface elasticity and surface viscosity) of monolayers formed by the isomeric surfactants octyl β-glucoside, octyl α-glucoside, and 2-ethylhexyl α-glucoside (see Fig. 16.6) were quantified at submicellar concentrations (Rojas et al., 2005b). The same surfactant types were used in an earlier study by Waltermo who presented surface-tension data and its relationship with the foaming behaviors (Waltermo et al., 1996). Unfortunately, SLS data for C12E6 and β-C12G2 that were discussed in previous sections are not available; however, we used the surfactants illustrated in Fig. 16.6 to document the conclusion that differences in the configuration of the molecule affect the surface packing and thus the rheological behavior of the formed monolayers. Likewise, macroscopic properties such as foam ability, emulsion stability, and others are also influenced. The surfactants used (Fig. 16.6) were anomerically pure alkyl glucosides. The focus of this study was to uncover the difference between the β and α anomers of octyl glucoside (Figs. 16.6a and 16.6b, respectively). In addition, the effect of the structure of the surfactant’s hydrophobic part was addressed by comparing a linear (Fig. 16.6b) and a branched octyl chain (Fig. 16.6c). Readers interested in the processing of the SLS data to calculate the surface rheological properties are referred to (Rojas et al., 2005b). In short, we used a so-called “helmet” or polar plots in which radial and semicircular lines represent constant interfacial elasticitiy (ε) and constant interfacial viscositiy (κ), or, in other words, isoelasticity and isoviscosity curves, respectively (see Fig. 16.7). The distance of an experimental data point from the origin (ε = 0, κ = 0) in a polar plot is a measure of the surface elasticity and/or viscosity. Therefore, the surface elasticity and viscosity can simply be estimated by the inspection of the location of the experimental data within the polar plot. Figure 16.7 shows the experimental polar loci or the viscoelastic behavior of the three surfactants described earlier (at a SLS wave number of 78508 m–1). The polar plots show first the limiting case of a surfactant-free liquid dynamics (no monolayer present) [i.e., pure water (ε = 0, κ = 0)]. As the surfactant concentration increases, the respective viscoelastic behavior can be identified by the direct inspection of the curve, starting from the value corresponding to zero surfactant concentration (ε = 0,



Fig. 16.6. Isomeric sugar surfactants used to investigate the effects of the molecular structure on surface viscoelasticty: n-octyl-β-glucoside (a), n-octyl-α-glucoside (b), and 2-ethylhexyl-α-glucoside (c).

OH

O

OHHO

H H

HHO

HO H

OOH

HOH

H H

HOHO

H

OH

OOH

HOH

H H

HOHO

H

OH

(a) (b) (c)

OH

O

OHHO

H H

HHO

HO H

O

OHHO

H H

HHO

HO H

OOH

HOH

H H

HOHO

H

OH

OOH

HOH

H H

HOHO

H

OH

(a) (b) (c)

κ = 0). At low-surfactant concentrations, the tendencies indicate a deviation from the perfectly elastic film behavior (κ = 0, contour line). In these cases, a maximal propagation velocity and dampening limit is attained. More importantly, in terms of the present discussion, is the fact that the polar plots indicate small, yet distinctive, differences in the viscoelasticity of the three isomeric (sugar) surfactants. Therefore, in agreement with other studies (Neimert-Andersson et al., 2004; Niraula et al., 2004 ), the stereochemistry of the surfactant plays an important role in molecular packing, lateral interactions, and overall intermolecular forces, all of which affect the respective macroscopic surface behaviors. We note that the area per molecule for octyl α-glucoside and octyl β-glucoside surfactant was 41.5 and 43.5 Å2, respectively (Rojas et al., 2005b). Furthermore, the data presented in Fig. 16.7 demonstrate further important changes in molecular packing at the air/water surface for these isomeric surfactants. This is by virtue of very subtle configuration differences of the headgroup as well as the tail structure. One can therefore anticipate that by fine-tuning the molecular architecture of sugar surfactants, one can develop applications involving different performance requirements. For



example, one can vary foamability and emulsion stabilization by changing the polar group (position and stereochemistry), as well as the structure of the hydrophobic tail (e.g., via branching). Changes in the stereochemistry of the headgroup units are also expected to influence the interaction with the ions present in solution. Such effects were made clearer in the discussion presented in “Air/Water Interfaces” on thin-film pressure-balance experiments for other surfactants. One should stress, however, that dynamics’ effects (diffusion, relaxation, etcetera) play important roles as well. At this point, to speculate further about the relationship between surfactant structure and properties is meaningless until the nature of surface-charge density, surfactant packing, and dynamic effects is better understood. An attempt to shed some light regarding the effects of surfactant structure is made in the next sections.

Molecular Prospects To obtain experimental information on the structure of water adjacent to solutes is very difficult. The main reason for this is that in most cases the spectroscopic signal from the hydration shell is swamped by that of bulk water. However, recently some progress was made where an inherently surface-sensitive spectroscopic technique, vibrational sum frequency spectroscopy (VSFS), was employed to investigate the hydration of sugar groups and oligo(ethylene oxide) chains anchored to the air/water surface. Some very strong hydrogen bonds are formed between sugar and water, whereas the hydration of the oligo(ethylene oxide) chain is dominated by a clathrate-like structure (Claesson et al., 2006; Tyrode et al., 2005). One can apply related arguments when considering short-range interactions between surfactant layers. The fact that hydrogen bonds can form between polar

Fig. 16.7. Helmet or polar plots showing the experimental surface viscosity and elasticity obtained from the propagation (central frequency and dampening) of capillary waves probed by surface light scattering for n-octyl-β-glucoside (left), n-octyl-α-glucoside (center), and 2-ethylhexyl-α-glucoside (right) surfactants. The loci of the different curves show differences in the behavior of these isomeric surfactants (Rojas et al., 2005b).

/gs-1

(0.5, 13)

(0, 0)

(1, 14)

(3, 24)(5, 30)

(25, 42)(21, 42)

(0, 0)

(1, 14)

(3, 25)

(10, 35)

(0, 0)

(1.3, 17)

(13, 42)

(0.7, 11)

(4, 29)

Normalized Central Frequency 0-1

Nor

mal

ized

Dam

ping

0-1

Normalized Central Frequency 0-1Normalized Central Frequency 0

-1

Nor

mal

ized

Dam

ping

0-1

Nor

mal

ized

Dam

ping

0-1

/gs-1

/mNm-1

/gs-1/gs-1

(0.5, 13)

(0, 0)

(1, 14)

(3, 24)(5, 30)

(25, 42)(21, 42)

(0, 0)

(1, 14)

(3, 25)

(10, 35)

(0, 0)

(1.3, 17)

(13, 42)

(0.7, 11)

(4, 29)

Normalized Central Frequency 0-1

Nor

mal

ized

Dam

ping

0-1

Normalized Central Frequency 0-1Normalized Central Frequency 0

-1

Nor

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ized

Dam

ping

0-1

Nor

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ized

Dam

ping

0-1

-1

/gs-1

/mNm-1

/gs-1

/mNm-1/mNm



head-groups of, for example, sugar-based surfactants does not directly lead to the conclusion that the short-range interaction across water should be attractive due to a hydrogen-bond formation. To compile available data on the depth of the interaction-force minimum is thus of some interest. In this case, the pull-off force needed to separate surfaces that are in contact (for example, after bringing the surfaces together, as shown in Fig. 16.4 and Fig. 16.5) is measured. This force is related to the adhesion energy that, in our case, was measured between adsorbed nonionic surfactant layers. This will allow us to learn if the possibility of forming hydrogen bonds between two headgroups has a bearing on the short-range attraction observed in these systems (see Table 16.1). We note that one can not form direct hydrogen bonds between DDAO molecules in uncharged form, DDPO, or surfactants with EO headgroups (see Table 16.1). However, such interactions are possible for surfactants with polyhydroxyl headgroups and also for amine headgroups. By comparing the data compiled above, we note that the trend is that headgroups that are able to form interlayer hydrogen bonds display a somewhat larger attractive short-range force component than layers that are unable to form such bonds. This could indicate that a contribution from interlayer hydrogen-bond formation exists, but the extension of this effect is difficult to determine since the nature of the head-group also influences other short-range interactions, for example, van der Waals forces. Due to the lack of systematic data, we cannot compare quantitatively the hydration numbers obtained for EO and glucose headgroups. A qualitative evaluation, however, might shed some light on the differences between these surfactants. From extensive SANS measurements in bicontinuous microemulsions, we know that the area per headgroup at the water/oil interface is 0.56 nm2 for a glucose unit (Kluge 2000), which is comparable to that of a surfactant with four EO units which is 0.54 nm2 (Sottmann et al., 1997). The hydration number of the glucose headgroup, as determined by nuclear magnetic resonance (NMR), is 6 at a surfactant concentration of 3 wt%, whereas NMR estimates the hydration number to be 6 per an EO unit at a surfactant concentration of 10 wt% (i.e., 24 water molecules per tetraoxyethylene headgroup). Keeping in mind that the hydration number per EO unit is expected to be even larger at 3 wt%, one can conclude that under similar conditions and for similar headgroup sizes the hydration of EO-based surfactants is significantly higher than that of sugar-based surfactants (Claesson et al., 2006). The short-range interaction between surfactant layers exposing the polar part toward solution is due to a complex interplay between van der Waals forces, hydration, and steric effects. One can only measure the total force, and this force displays both attractive and repulsive regimes. The van der Waals force is the main attractive force, while hydrogen bonding within adsorbed layers, again directly or mediated by water molecules, increases the packing density within adsorbed surfactant layers; this increases the van der Waals force. We note that both hydration and the mobility of surfactants are important for short-range interactions. The steric interaction between surfactants with oligomeric



Table 16.1. Depth of Force Minimum between Various NonIonic Surfactant Layers (Claesson et al., 2006 and references therein)

Compound Technique

Depth of force minimum F/R (Mn/M)

Substrate surface and remarks

n-dodecyl amine oxide (DDAO)Uncharged form.

SFA 0.25 Bilayer on mica.

n-dodecyl phosphine oxide (DDPO)

SFA 0.25 Hydrophobied mica. Force minimum increases with increasing temperature.

tetraoxyethylene dodecyl ether (C12E4)

MASIF 0 Silanated glass surfaces.

pentaoxyethylene dodecyl ether (C12E5)

SFA 0.1–0.2 Hydrophobized mica. At 20°C. Force minimum increases with increasing temperature.

hexaoxyethylene dodecyl ether (C12E6)

MASIF 1–1.4 Silanated glass surfaces.

hexaoxyethylene dodecyl ether (C12E6)

MASIF 0.2–0.4 Thiolated gold surfaces.

tetraoxyethylene dodecyl amide(C12ONHE4)

SFA 0.3–0.4 Hydrophobized mica and silanated glass (Herder, 1991)

tetraoxyethylene dodecyl amine (C12NHE4)

SFA 0.1–0.2 Bilayer on mica, pH 9.4–9.9.

n-octyl-β-glucoside (β-C8G1) SFA 1.2–1.5 Hydrophobized mica.

n-decyl-β-d-glucoside (β-C10G1) MASIF 1 Thiolated gold surfaces. Estimated. (Waltermo, 1993)

n-decyl-β-d-maltoside (β-C10G2) MASIF 0.9 Silanated glass surfaces.

n-dodecyl-β-d-maltoside (β-C12G2)

MASIF 1-2 Thiolated gold surfaces.

technical sugar–surfactant mixture

SFA 2 Hydrophobized mica. Located on the repulsive side.

n-dodecyl lactobion amide (LABA), inner sugar ring is open

SFA 1.2 Hydrophobized mica

n-dodecyl lactobion amide (LABA), inner sugar ring is open

MASIF 1.5 Silanated glass surfaces.

maltose 6´-O-dodecanoate (C12OG2)

MASIF 1.0 Silanated glass surfaces

n-dodecyl amine (C12NH2)Most molecules in the outer layer are uncharged.

SFA 0.5–1 Bilayer on mica, pH 9.5–10.1

Data correspond to aqueous surfactant solutions at or above the respective CMC (i.e., to a saturated monolayer). SFA = surface-force apparatus, MASIF = measurement and analysis of surface-interaction forces.



headgroups also deserves further attention. As the headgroup size increases, the short-range repulsion goes from a hard-wall repulsion, via compressible excluded volume repulsion, to a steric repulsion described by polymer-brush theories (de Gennes, 1987). The experimental results discussed in this chapter help to demonstrate fundamental features of nonionic surfactants based on sugar polar groups (mainly glucoside and maltoside). These polar groups make them structurally more attractive in terms of their capacity to stabilize interfaces (liquid–liquid, solid–liquid, and gas–liquid) as compared to EO-based nonionic surfactants. Note that one can use the OH-groups in the sugar units for chemical reactions, and thus one has the possibility of engineering the surfactant structure and its stereochemistry (which, as was demonstrated before, play a leading role for the properties). Key molecular components (headgroup configuration, identity, number, tail length, and saturation) lead to subsequent packing aspects for “green” polysaccharide streams that one can ultimately use to manufacture nonionic surfactants. As mentioned in previous sections, nonionic, saccharide-based surfactants are a veritable plethora of tangible benefits for state-of-the-art surface, colloid, and condensed state research. Their solubility and phase behavior are typically insensitive to temperature changes in contrast to EO-based surfactants. This allows much greater amplitude in their synthetic design without compromising desirable physical and chemical features such as small molecule (e.g., blood glucose) biosensor applications, biophysical recognition (such as antigen–antibody mechanisms), host–guest chemistry, and a number of other smart functions. One needs to use separations chemistries, and chemically fine-tune the respective sugar surfactants with the appropriate physical characteristics (functional groups and/or ligands) to achieve the “smart” functionality we have described. The performance of sugar surfactants (versus EO surfactants) in dispersed systems (such as foams, emulsions, and dispersions) and their potential for the formation of new structures in these systems (such as unilamellar vesicles, ribbons, spirals, tubes, etcetera) depend on the detailed molecular structure. One may synthesize new surfactants which may offer unique templates for more sophisticated electronic, biomedical, and optical devices. Some strategies in the design of sugar surfactants to provide greater functionalities include the introduction of both mono- (glucoside) and di-saccharide (maltoside) moieties with various modes of attachments to the headgroup of synthesized surfactants and also the introduction of different degrees of hydrophobicity and crystallinity to the sugar surfactants by manipulating their aliphatic tails.

Structure–Property RelationshipsEfforts need to be devoted to the development of new surfactants based on the structure–property heuristics. In the simplest form, a new sugar surfactant that could be synthesized would consist of three discrete parts: (a) polar headgroup(s), for example, one or two glucosides or one maltoside, with or without a (b) spacer



(that could display the versatility of allowing the attachment of a second headgroup and two tails as well as two different tail placements for one group), and the (c) aliphatic tail (for example, a fatty acid). How the changes in these components will affect their performance in model systems (foams, emulsions, dispersions) is then the key question. Some leads to answering such questions were presented in previous sections. What is exciting about all of the possible configurations is the inherent potential for providing a number of different surface interactions based on the HLBG (hydrophilic and lipophilic balance) parameter (Griffin, 1954), the variation in the packing based on steric interactions (Piispanen et al., 2004), and the packing variation based on other interactions (John & Vemula, 2006). The HLBG is useful to characterize the balance between the hydrophilic and hydrophobic parts of the surfactant molecule. One can vary the HLBG by changing the length of the tail of the surfactant. This can lead to different packing behaviors. The molecules will have a pronounced ability to engage in discrete regular packing motifs, as was shown in recent work (John et al., 2003). Typically, surfactant packing is dominated by the hydrophobic interactions between the hydrophic chains, which, in turn, influence the flexibility, the orientation, and the surface energy of molecular assemblies. A serious need is noted for a systematic study of the structure–property relations that define sugar-based surfactants as the alternative of choice. Major issues in this area are:

Development of new products, production practices, and business in the biomass • industry. This requires rapid changes in consumer demand for renewable products and alternative market solutions.

Replacement (even small fractions) of petroleum-based nonionic surfactants with • sugar-based surfactants, which is important from both an environmental and energetic point of view.

Synthetic strategies to reduce the costs of sugar-based surfactants since current • barriers for the acceptance of sugar-based surfactants are their costs. Linking fundamental studies with economics and market analysis may promote this goal.

Systematic studies of the properties of sugar-based surfactants as a lack of • information about their properties and performance benefits may be an obstacle for their wide use.

The use of nonbiodegradable, petroleum-based surfactants is a major drawback, and therefore the development of biodegradable surfactants derived from agricultural resources is justified. Sugar-based surfactants represent a proven candidate, but any effort aimed at formulating dispersed systems by using the saccharide platform entails that specific technical requirements in terms of their applications must be met. The



performance of (new) surfactants in any given application is expected to be closely related to molecular factors, with the interfacial behaviors being some of the most important ones.

AcknowledgmentsO.J.R. greatly appreciates the financial support of the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant number #USDA 2008-01519. C.S. is grateful for financial support of the European Commission under the 6th Framework Program, contract No. MRTN-CT-2004-512331-Project SOCON.

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