Surface Pressure Isotherms, Dilatational Rheology, and Brewster

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Surface Pressure Isotherms, Dilatational Rheology, and Brewster Angle Microscopy of Insoluble Monolayers of Sugar Monoesters Gorgias Garofalakis and Brent S. Murray* Food Colloids Group, Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, U.K. Received December 10, 2001. In Final Form: March 18, 2002 Surface pressure isotherms of insoluble monolayers of high purity, enzymatically synthesized sucrose, lactose, glucose, and galactose stearate at the air-water and n-tetradecane-water interfaces were obtained. The dilatational moduli of these surfactant monolayers were also determined by oscillations of the interface at 0.1 Hz. Results have indicated similarities in the properties of sucrose and lactose stearate. Brewster angle microscopy has shown the transitions from gaseous to liquid expanded to liquid condensed phases upon compression of these monolayers at the air-water (A-W) interface. It was also shown that the interfacial phase separation is strongly dependent on the rate of deformation of the interface. Glucose and galactose stearate formed more aggregated monolayers at the A-W interface and (particularly galactose stearate) had higher dilatational moduli, compared to sucrose and lactose stearate. Brewster angle microscopy confirmed the extensive aggregation and the formation of a solid phase in the case of galactose stearate. The isotherms of glucose and galactose stearate at the oil-water (O-W) interface exhibited a first-order phase transition, suggesting a different aggregation pattern, in comparison to the aggregation at the A-W interface, where, possibly, no solid phase was formed. This suggested that more extended aggregation was taking place, where the interactions between the monosaccharide headgroups was the major attractive force.

Introduction Low molecular weight surfactants are among the components that can achieve colloid stabilization.1 The stability of fluid-fluid colloid systems is achieved by the existence of forces that can successfully oppose interfacial deformation, such as a change in the area or the shape of the interface. The study of interfaces with respect to stability often includes the determination of the dilatational (where an area change is imposed) and shear (where a shape change is imposed) moduli of the interface.2,3 Sugar esters, as their name indicates, are amphiphiles where the hydrophilic part is a carbohydrate. Certain sugar esters occur in nature.4,5 The synthesis of sugar esterscanbeachievedeitherchemically6,7 orenzymatically,8-13 * To whom correspondence should be addressed. Telephone: 44 (0)113 2332962. Fax. 44 (0)113 2332982. E-mail b.s.murray@ food.leeds.ac.uk. (1) Dickinson, E. An Introduction to Food Colloids; Oxford University Press: Oxford, 1992. (2) Murray, B. S.; Dickinson, E. J. Food Sci. Technol. Int. 1996, 2, 131. (3) Dickinson, E. Colloids Surf. B 1999, 15, 161. (4) Chortyk, O. T.; Kays, S. J.; Teng, Q. J. Agric. Food Chem. 1997, 45, 270. (5) Arrendale, R. F.; Severson, R. F.; Sisson, V. A.; Costello, C. E.; Leary, J. A.; Himmelsbach, D. S.; Vanhalbeek, H. J. Agric. Food Chem. 1990, 38, 75. (6) Vlahov, I. R.; Vlahova, P. I.; Linhardt, R. J. J. Carbohydr. Chem. 1997, 16, 1. (7) Goueth, P.; Gogalis, P.; Bikanga, R.; Gode, P.; Postel, D.; Ronco, G.; Villa, P. J. Carbohydr. Chem. 1994, 13, 249. (8) Chamouleau, F.; Coulon, D.; Girardin, M.; Ghoul, M. J. Mol. Catal. B: Enzymatic 2001, 11, 949. (9) Fregapane, G.; Sarney, D. B.; Vulfson, E. N. Biocatalysis 1994, 11, 9. (10) Gao, C. L.; Whitcombe, M. J.; Vulfson, E. N. Enzyme Microb. Technol. 1999, 25, 264. (11) Soultani, S.; Engasser, J. M.; Ghoul, M. J. Mol. Catal. B: Enzymatic 2001, 11, 725. (12) Tsitsimpikou, C.; Daflos, H.; Kolisis, F. N. J. Mol. Catal. B: Enzymatic 1997, 3, 189.

and their large-scale production is considered feasible.14 In the literature, sugar esters appear to have been successfully tested for a wide range of applications. Sugar esters found in various plants are part of their insect resistance mechanism and have been identified as potential insecticidal agents.15,16 In the food industry, sugar esters can be used as efficient emulsifiers, foamers, or viscosity modifiers.17-21 The cosmetic and personal care industries are interested in the mildness to the skin and the low toxicity of some sugar esters.14,22 Additionally, sugar esters are thought to be relatively benign to the environment, as they are biodegradable and the raw materials for their synthesis come from renewable resources.23,24 Currently, the number of published works on the systematic study of the surface properties of sugar esters is rather limited. Among the sugar esters, the sucrose (13) Ferrer, M.; Angeles Cruces, M.; Plou, F. J.; Bernabe, M.; Ballesteros, A. Tetrahedron 2000, 56, 4053. (14) Hill, K.; Rhode, O. Fett-Lipid 1999, 101, 25. (15) Xia, Y.; Johnson, A. W.; Chortyk, O. T. J. Econ. Entomol. 1997, 90, 1015. (16) Stansly, P. A.; Liu, T. X. Bull. Entomol. Res. 1997, 87, 525. (17) Akoh, C. C. J. Am. Oil Chem. Soc. 1992, 69, 9. (18) Partal, P.; Guerrero, A.; Berjano, M.; Mun˜oz, J.; Gallegos, C. J. Text. Stud. 1994, 25, 331. (19) Herrington, T. M.; Midmore, B. R.; Sahi, S. S. In Microemulsions and emulsions in Foods; El-Nokaly, M., Cornell, D., Eds.; ACS Symposium Series 448; American Chemical Society: Washington, DC, 1991; p 82. (20) Bee, R. Gas cells in a liquid medium. EP0521543A1, 1992. (21) Franco, J. M.; Berjano, A.; Gallegos, C.; Gallegos, C. Food Hydrocolloids 1995, 9, 111. (22) Matsumura, S.; Imai, K.; Yoshikawa, S.; Kawada, K.; Uchibori, T. J. Am. Oil Chem. Soc. 1990, 67, 996. (23) Baker, I. J. A.; Matthews, B.; Suares, H.; Krodkiewska, I.; Furlong, D. N.; Grieser, F.; Drummond, C. J. J. Surf. Detergents 2000, 3, 1. (24) Warwel, S.; Bruese, F.; Demes, C.; Kunz, M.; Rueschgen Klaas, M. Chemosphere 2001, 43, 39.

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Figure 1. (a) 6-O-Stearyl glucose, (b) 6-O-stearyl galactose, (c) 6′-O-stearyl sucrose and (d) 6′-O-stearyl lactose. R ) C17H29-. The “wavy” bond of glucose, galactose, and lactose indicates Ror β-conformation.

esters are the most studied,6,25-30 whereas data on nonsucrose esters26,30,31 is sparse, to the best of the authors’ knowledge. In the present work we have studied the equilibrium and dynamic interfacial properties of spread monolayers of four, water-insoluble stearate esters with different carbohydrate headgroups (sucrose, lactose, glucose, and galactose) at the air-water and n-tetradecane-water interfaces. The aim of this work is to study the effect of the sugar headgroup on the properties of the corresponding surfactant monolayers. Materials and Methods Materials. The surfactants used in this study were monostearate esters of sucrose,32 lactose,33 glucose, and galactose,9 enzymatically synthesized, with minimum purities of 98%, 99%, 92%, and 90%, respectively, and were kindly provided by Dr. D. B. Sarney (Institute of Food Research, Reading, UK). Sucrose and lactose were 6′-O acylated, whereas glucose and galactose were 6-O acylated. The chemical structures of these surfactants are given in Figure 1. For the sake of simplicity, sucrose, lactose, glucose, and galactose monostearate will be referred to as Suc18, Lac18, Glu18, and Gal18, respectively. Enzyme-catalyzed reactions are highly stereospecific; therefore, these sugar ester samples can be considered to be isomerically pure with respect to the point where the esterification has taken place on the sugar molecule. The sugar surfactants were dissolved in an appropriate spreading solvent consisting of dichloromethane:methanol:nheptane 2:1:1 v:v:v. The solvents were of HPLC grade, purchased from Sigma (Poole, UK), BDH (Poole, UK), and Sigma, respectively, and were used without any further purification. The subphase in all experiments was double-distilled water with a surface tension of 72 mN m-1 at 25 °C. In oil-water interface experiments n-tetradecane (Sigma, Poole, UK) was used a model nonpolar phase. Prior to use n-tetradecane was treated by trickling it through a column packed with magnesia and alumina (Sigma, Poole, UK), previously heated for a minimum of 5 h at 550 °C to ensure that they were free of organic contaminants. (25) Garofalakis, G.; Murray, B. S. Colloids Surf. B 2001, 21, 3. (26) Garofalakis, G.; Murray, B. S.; Sarney, D. B. J. Colloid Interface Sci. 2000, 229, 391. (27) Herrington, T. M.; Sahi, S. S. Colloids Surf. 1986, 17, 103. (28) Bazin, H. G.; Polat, T.; Linhardt, R. J. Carbohydr. Res. 1998, 309, 189. (29) Husband, F. A.; Sarney, D. B.; Barnard, M. J.; Wilde, P. J. Food Hydrocolloids 1998, 12, 237. (30) Soederberg, I.; Drummond, C. J.; Furlong, D. N.; Godkin, S.; Matthews, B. Colloids Surf. A 1995, 102, 91. (31) Drummond, C. J.; Wells, D. Colloids Surf. A 1998, 141, 131. (32) Sarney, D. B.; Barnard, M. J.; MacManus, D. A.; Vulfson, E. N. J. Am. Oil Chem. Soc. 1996, 73, 1481. (33) Sarney, D. B.; Kapeller, H.; Fregapane, G.; Vulfson, E. N. J. Am. Oil Chem. Soc. 1994, 71, 711.

Garofalakis and Murray Methods. All glassware used in this work was cleaned via a bath of concentrated nitric acid (BDH, Poole, UK), in which they were left overnight. They were then thoroughly rinsed with double-distilled water and dried prior to their use. The data presented in this study correspond to average values obtained from at least three experiments. All experiments were carried out at room temperature (20-24 °C). Surface Pressure Isotherms. A specialist Langmuir trough featuring a rhombic PTFE barrier, described in detail elsewhere,34 was used throughout this work. The surface pressure was measured by the Wilhelmy plate method, using a thoroughly cleaned, roughened mica plate, which was suspended in the middle of the trough area from a sensitive force transducer. The experimental setup allowed the study of both air-water (A-W) and oil-water (O-W) interfaces. The surfactant spreading was performed by leaving small amounts of the spreading solution on the water surface using a microsyringe. Prior to each experiment sufficient time (approximately 20-30 min) was allowed for the spreading solvent to evaporate. In O-W interface experiments the nonpolar phase was gently layered over the aqueous phase after the surfactant was spread in the usual way. To acquire a surface pressure versus area per molecule (π-A) isotherm, the trough area was compressed at the rate of 10.2 mm2 s-1, starting from an initial (maximum) trough area of 22 500 mm2. Decreasing the compression rate was found to have no significant impact on the π-A isotherm of the surfactants. Repeated compression-expansion cycles at this speed gave no significant hysteresis in the π-A curves obtained for each surfactant, also indicating that the surfactants were insoluble in both the aqueous and the oil phases. Dilatational Rheology. The dilatational elastic modulus, E, of each surfactant monolayer was determined at a number of different surface pressures by subjecting the interface to a sinusoidal area deformation of a frequency of 0.1 Hz and amplitude of 5%. The dilatational modulus was calculated from E ) -dπ/d(ln A), fitting this formula to the measured π of the film. Strictly, E should be termed the complex modulus, but in all of the measurements made there was no significant measurable phase difference (i.e., >0.3 s) between the π response and the motion of the barrier. Thus any loss modulus was negligible. The formula for E above was also applied to the π-A isotherms, resulting in values of the so-called static dilatational modulus, Est, as a function of π. The value of Est is expected to differ from the measured value of E at all but the very lowest frequencies of deformation, unless the film is purely elastic, in which case the dilatational modulus is frequency independent.2 Typical errors in these surface rheological measurements had a maximum value of 4 mN m-1. Brewster Angle Microscopy. The interfacial texture was observed by a “BAM2plus” Brewster angle microscope (NFT, Go¨ttingen, Germany) placed above the Langmuir trough described above. The entire setup was mounted on a vibrationisolated base (Alkyonics, Go¨ttingen, Germany). The CCD of the microscope reported the reflected light intensity in terms of gray levels (GL), with 0 corresponding to black and 255 corresponding to saturated white. CCDs do not necessarily respond linearly to the incoming radiation. In the case of our equipment the CCD response to the incoming light was found35 to be linear in the GL range from 0 to 185 for a given camera shutter speed. The software of the microscope could provide an average reflected light intensity that was calculated as the average intensity of all the points on the line connecting the top left corner with the bottom right corner of an acquired image. All the interface images were taken when the trough barrier was at rest. Most images were obtained with the angles of the polarizer (P) and the analyzer (A) set to 0°. The resolution of the BAM is approximately 1 µm. Although slightly improved resolution could sometimes be obtained by varying these settings for some monolayers, there was then the increased difficulty of comparing different systems under different settings, so that little was gained by using different analyzer and polarizer settings. With the experimental setup used in this (34) Murray, B. S.; Nelson, P. V. Langmuir 1996, 12, 5973. (35) Patino, J. M. R.; Sanchez, C. C.; Nino, M. R. R. Langmuir 1999, 15, 4777.

BAM Insoluble Monolayers of Sugar Monoesters

Figure 2. Suc18. π-A isotherms at the A-W interface (filled symbols) and O-W interface (open symbols); Est at the O-W interface (continuous line). work it was only possible to obtain monolayer images at the A-W interface. Image analysis, when appropriate, was performed with Image Tool v2.00a3, an image analysis software developed by the Health Science Centre of the University of Texas, San Antonio, TX.

Results and Discussion π-A Isotherms and Dilatational Rheology. The π-A isotherms of Suc18 at the A-W and O-W interfaces are shown in Figure 2. The π-A isotherms can be used to estimate the minimum cross-sectional area of the surfactant molecules, A0, at the interface. An example is shown in Figure 2, where the cross-sectional area of Suc18 at the A-W interface (68 ( 3 Å2) was determined as the intercept of the extrapolation of the steep part of the isotherm with the A-axis. (This assumes that the monolayer is solidlike in this region.) At the O-W interface the surface pressure curve appears to be more expanded, especially in the lower surface pressure region. This can be attributed to the solvation of the hydrocarbon chains by the oil.36 Figure 2 also shows the static dilatational modulus, Est, calculated using the data points of the O-W isotherm. (The noise in the values of Est is due to the slight noise in the π-A isotherms, which is magnified in taking the gradient of the isotherm to calculate Est.) Other points of interest in the π-A isotherms include the surface pressure where the film collapses, πc, the molecular area at that point, Ac, and the surface pressure where the Est reaches a maximum, πEm. The latter corresponds to the point where the slope of the isotherm changes, an indication of a change in the structure of the film. These characteristics of the π-A curves of Suc18 at the two interfaces are summarized in Table 1. Figure 3 shows the static and dilatational moduli of the Suc18 monolayer at the two interfaces. The values of E are lower at the O-W interface possibly because of reduced interactions between the fatty acid chains of the surfactant molecules after their solvation by molecules of the oil phase.36 The proximity of the values of E and Est for each of the interfaces studied, in the low and moderate surface pressure regions, is an indication that the film is mostly elastic at these regions.2 In the high π region (i.e., greater than ca. 25 mN m-1), E is significantly greater than Est at both the A-W and O-W interfaces. This indicates that closer to the collapse pressure relaxation phenomena (e.g., desorption and multilayer formation) become increasingly dominant, as expected, but that the time scales for these phenomena are such that they are not so evident at the frequency (0.1 Hz) at which E is measured. (36) MacRitchie, F. Chemistry at Interfaces; Academic Press: London, 1990.

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The equilibrium and dilatational behavior of Suc18 at the A-W interface was also probed on a subphase of 0.1 M NaCl, KCl, and CaCO3 (data not shown). No significant deviations from the behavior on a pure water subphase were observed. This may be taken as an indication that the surfactants used here did not contain any charged surface active impurities, such as the original fatty acids from which they had been synthesized. Figure 4 shows the π-A isotherms of Lac18 at the A-W and O-W interfaces. The cross-sectional area of Lac18 at the A-W interface derived from the isotherm is 66 ( 3 Å2. This value is quite close to the value obtained for Suc18 at the A-W interface (68 ( 3 Å2). The characteristics of the isotherms of Lac18 are summarized in Table 1. Figure 5 displays E and Est of the Lac18 monolayer. At both interfaces there is good agreement between E and Est, except near the collapse surface pressure of the A-W interface, indicating that the film behaves as an almost purely elastic body at the frequency (0.1 Hz) and amplitude employed. Figure 6 shows the π-A isotherms of Glu18 at the two interfaces. Their characteristics are summarized in Table 1. The cross-sectional area of Glu18 at the A-W interface was 44 ( 3 Å2. At the O-W interface a surface pressure plateau appeared at approximately 16.4 mN m-1 between 51 ( 3 and 37 ( 3 Å2. The plateau is also markedly displayed by the derived curve of Est, shown in Figure 6, and by the minimum of E at the O-W interface shown in Figure 7. At both interfaces E reached higher values for Glu18 than for Suc18 and Lac18. The differences between E and Est at each of the interfaces are more pronounced than in the case of Suc18 and Lac18, indicating that the film is not purely elastic. Figure 8 shows the π-A isotherms of Gal18. The behavior of this surfactant was significantly different compared to the other sugar esters. The Gal18 film at the A-W interface appeared to be much less compressible than the other films studied. The collapse pressure was 66 ( 0.5 mN m-1, compared with 43 ( 0.5 mN m-1 for Glu18 and even less for Suc18 and Lac18, and the dilatational modulus at the A-W interface (Figure 9) reached a very high value. As shown in Figure 9, E at the A-W interface was only determined up to π ) 35 mN m-1. The reason was that at higher surface pressures the 5% change in area used to measure E would have raised the surface pressure close to monolayer-collapse limits. This would produce a rather large error in the determination of E and potentially create nonreversible changes in the film structure. In contrast to its behavior at the A-W interface, Gal18 at the O-W interface exhibited behavior similar to that of the other surfactants with respect to the collapse surface pressure and the dilatational modulus, as Figures 8 and 9 indicate. Similarly to Glu18, Gal18 also exhibited a surface pressure plateau at the O-W interface at π ) 5.6 ( 0.5 mN m-1 between A ) 68 ( 3 Å2 and A ) 40 ( 3 Å2. The characteristics of the π-A isotherms of Gal18 are summarized in Table 1. In a previous study26 we have reported estimates of the limiting areas per molecule for a number of sugar esters with the same headgroups as the ones used here, but which are soluble in the aqueous phase by virtue of possessing a shorter alkyl chain. The estimates were derived by applying the Gibbs adsorption equation to plots of surface tension versus log surfactant concentration, in the usual way. A comparison between the values of A0 obtained by the two methods is given in Table 2. Excellent agreement is found in the cases of Gal18 and Lac18. Glu18 and Suc18 headgroups appeared to be bulkier than the A0 values suggested by their water-soluble equivalents. It should

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Table 1. Characteristic Values of π-A Isotherms of Sugar Esters at the Air-Water (A-W) and Oil-Water (O-W) Interfacesa πEm ( 0.5 mN m-1

A0 ( 3 Å 2

πc ( 0.5 mN m-1

Ac ( 3 Å 2

compound

A-W

O-W

A-W

O-W

A-W

O-W

A-W

O-W

Suc18 Lac18 Glu18 Gal18

68 66 44 32

74 74 43 38

20.4 27.5 29.6 32.3

20.5 23.1 29.8 27.3

37.6 40.8 43.1 66.0

39.8 36.2 39.4 36.7

31 31 22 21

28 30 22 21

a A , estimated cross-sectional area of the surfactant molecules; π , surface pressure where the film collapses; A , molecular area at that 0 c c point; πEm, surface pressure where Est reaches a maximum.

Figure 3. Suc18. Dilatational modulus, E, at 0.1 Hz and 5% area deformation (open symbols) and static dilatational modulus, Est (filled symbols). (], [) A-W interface; (O, b) O-W interface.

Figure 6. Glu18. π-A isotherms at the A-W interface (filled symbols) and O-W interface (open symbols); Est at the O-W interface (continuous line).

Figure 4. Lac18. π-A isotherms at the A-W interface (filled symbols) and O-W interface (open symbols).

Figure 7. Glu18. Dilatational modulus, E, at 0.1 Hz and 5% area deformation (open symbols) and static dilatational modulus, Est (filled symbols). (], [) A-W interface; (O, b) O-W interface.

Figure 5. Lac18. Dilatational modulus, E, at 0.1 Hz and 5% area deformation (open symbols) and static dilatational modulus, Est (filled symbols). (], [) A-W interface; (O, b) O-W interface.

Figure 8. Gal18. π-A isotherms at the A-W interface (filled symbols) and O-W interface (open symbols).

be noted that the cross-sectional area of Glu18 is compared, in Table 2, with the values of the two glucosides (i.e., in which the alkyl chain is esterified to anomeric carbon atom) as there was no water-soluble equivalent available.

Therefore, the differences between the glucose-based surfactants shown in Table 2 are most likely due to the different stereochemistry of the two types of surfactants. Viewed collectively, the π-A isotherms of Suc18, Lac18, Glu18, and Gal18 at the A-W interface show some


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Figure 9. Gal18. dilatational modulus, E, at 0.1 Hz and 5% area deformation (open symbols) and static dilatational modulus, Est (filled symbols). (], [) A-W interface; (O, b) O-W interface. Table 2. Limiting Areas per Molecule, A0, of Various Sugar Esters Derived by Two Different Methods compound C12-β-D-glucoside C12-R-D-glucoside Glu18 Gal14 Gal16 Gal18 Lac14 Lac16 Lac18 Suc12 Suc14 Suc16 Suc18

A 0 ( 3 Å2 a

A0 ( 4Å2 b 34 38

44 33 35 32 60 68 66 54 57 53 68

a

Derived by π-A data as described in this work. b Derived by adsorption data as previously presented by Garofalakis et al.26

common features. First, there are no first order phase transitions, which are typically manifested by a plateau in the isotherm. Instead all the films seemed to undergo a higher order phase transition. This is manifested as a change in the slope of the isotherm and is also indicated by the existence of a clear maximum in the static dilatational modulus, Est. At this point the compressibility of the film, which can be taken as 1/Est,36 reaches a minimum. Ruckenstein37 has suggested that inclined phase transitions from a liquid expanded (LE) to a liquid condensed (LC) phase may be nonconventional first order transitions that are a consequence of the fact that repulsive interactions between the surfactant molecules are longer range than the attractive forces. In this case, the phase change occurs by the formation of a large number of domains of the condensed phase within the expanded one, which appears to be the case here. The pattern of behaviour of the water-insoluble surfactants at the O-W interface was not coherent. Suc18 and Lac18 presented the expected behavior, where the π-A isotherms appeared to be slightly more expanded than at the A-W interface, probably due to the solvation of the fatty acid chain by molecules of n-tetradecane.36 In agreement with this, the dilatational properties of Suc18 and Lac18 were characterized by lower values of the dilatational modulus E at the O-W interface than at the A-W interface (Figures 3 and 5). Glu18 and Gal18 exhibited significantly different behavior at the O-W interface compared to the esters of the disaccharides. The isotherms of these two surfactants (37) Ruckenstein, E. Colloids Surf. A 2001, 183, 423.

at the O-W interface were also significantly different from the isotherms of these same compounds at the A-Winterface. More specifically, they underwent a first order phase transition (possibly LE/LC), as indicated by the plateau in their O-W π-A isotherms. Additionally, the collapse pressure for Gal18 was much lower at the O-W interface than at the A-W interface (similar to that of the other surfactants). The latter could be explained by the effect of oil in reducing the hydrophobic interactions between the fatty acid chains.36 It is, therefore, believed that there is no solid (S) phase formed in the Glu18 and Gal18 monolayers at the O-W interface. For any aggregation occurring at the O-W interface the main contributing factor should be the sugar-sugar interactions (which are competing with the sugar-water interactions). Considering that sugars slightly increase the surface tension of water, due to their preferential exclusion from the interface,44,45 then having oil at the interface should make the presence of hydrated sugars at the interface even less favored. Thus, there is a tendency to reduce the sugar-water contact at the interface, and that can be satisfied by the formation of large surfactant domains (since the fatty acid chain prevents the surfactant molecules from migrating to the bulk). Formation of large surfactant domains would be made via a first order phase transition, clearly manifested by a plateau in the corresponding π-A isotherm.37 In the case of Suc18 and Lac18, their (strongly hydrated) bulky structure hinders extended sugar-sugar attractive interactions and so no large domains are formed at the O-W interface. Glu18 and Gal18, on the contrary, being smaller, would be easier to interact with neighboring sugar headgroups. Brewster Angle Microscopy. Brewster angle microscopy (BAM) can usefully complement the data obtained from surface pressure measurements by adding qualitative information on the “texture” of the surfactant monolayers as a function of compression or expansion. Figure 10 shows images of the texture of the A-W interface after the spreading of Suc18. Figure 10a shows the interface immediately after the evaporation of the spreading solvent. It is clear that the interface is nonuniform and that there are domains of variable surfactant concentration. These domains are mobile. Approximately 10 min later the “foam” texture of Figure 10b was observed. The twodimensional foam structure disappeared when the film was compressed and surface pressure of the film began to rise. At approximately 10 mN m-1 bright, point-sized domains were observed, the brightness of which remained unchanged by rotating the analyzer. With increasing surface pressure the number of point-sized domains increased and the brightness of the entire interface also increased. At higher surface pressures bright “clouds” began to appear. Instances of the A-W interface during the compression of the Suc18 film are shown in Figure 10c-g. Re-expanding the film to π ) 0 mN m-1 led to the texture observed in Figure 10h. The two-dimensional foam structure was this case is much more regular. Similar (38) Patino, J. M. R.; Sanchez, C. C.; Nino, M. R. R. Langmuir 1999, 15, 2484. (39) Lipp, M. M.; Lee, K. Y. C.; Waring, A.; Zasadzinski, J. A. Biophys. J. 1997, 72, 2783. (40) Henon, S.; Meunier, J. Brewster angle microscopy and ellipsometry. In Modern Characterization Methods of Surfactant Systems; Binks, B. P., Ed.; Marcel Dekker: New York, 1999; p 109. (41) Clint, J. H. Surfactant Aggregation; Blackie: Glasgow, 1992. (42) Hossain, M. M.; Yoshida, M.; Kato, T. Langmuir 2000, 16, 3345. (43) Banipal, P. K.; Banipal, T. S.; Lark, B. S.; Ahluwalia, J. C. J. Chem. Soc., Faraday Trans. 1997, 93, 81. (44) Lee, J. C.; Timasheff, S. N. J. Biol. Chem. 1981, 256, 7193. (45) Xie, G. F.; Timasheff, S. N. Biophys. Chem. 1997, 64, 25.

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Figure 10. Suc18. A ) P ) 0°. (a) π ) 0 mN m-1, 20 min after spreading; (b) π ) 0 mN m-1, 30 min after spreading; (c) π ) 10 mN m-1; (d) π ) 20 mN m-1; (e) π ) 29 mN m-1; (f) π ) 35 mN m-1; (g) π ) 36 mN m-1; (h) π ) 0 mN m-1 after a compression-expansion cycle. The white bar represents 100 µm.

foamlike structures have been observed for several other surfactants (e.g., Patino et al.38). The effect of the rate at which the interface was compressed on the texture observed via BAM was investigated. A Suc18 film of initial interfacial area 12 194 mm2, where π = 21 mN m-1, was compressed to an interfacial area of 7946 mm2, where π = 33 mN m-1. The compression rates used were 2.0, 10.2, 20.3, 101.7, and 203.3 mm2 s-1. Immediately after the compression an image of the interface was taken. Examples of the images recorded are shown in Figure 11. It is important to stress that the exactly fixed parameters of these tests were the initial and final interfacial areas and the compression rate. The initial and final surface pressure or light intensities were not specifically regulated, because the aim was to collect images quickly after the change in area. However, for the given surfactant and the particular range of compression rates used, the initial and final surface pressures and light intensities were practically the same for all the compression rates. A general observation that can be made in Figure 11 is that the number of bright domains increases with increasing compression rate. This can be quantified by measuring the number of domains (Np) at the interface after each compression. Figure 12 shows the change in Np with increasing compression rate for Suc18. It is interesting to note that Np at first increased with increasing compression rate, but then decreased at the highest compression rates studied. Possibly the formation of the bright domains is kinetically inhibited at the highest compression rates; i.e., there is not enough time, or high enough fluidity in the interface, to allow formation of the equilibrium proportion of bright domains at high compression rates. It could be argued that the total area of the domains formed would be a better parameter to

Figure 11. Suc18. Interface after compression from 12 193.8 mm2 to 7945.8 mm2 at different compression rates. (a) 2.0 mm2 s-1; (b) 10.2 mm2 s-1; (c) 100.7 mm2 s-1. The white bar represents 100 µm.

Figure 12. Effect of compression rate on number of interfacial domains, Np, for (O) Suc18 and (b) Lac18.

monitor, but the small size of the domains meant that Np could be determined far more accurately than the domain area.


Figure 13. Lac18. A ) P ) 0°. (a) π ) 0 mN m-1; (b) π ) 5 mN m-1; (c) π ) 35 mN m-1. The white bar represents 100 µm.

The texture of a Lac18 interface was qualitatively similar to that of Suc18, as shown by Figure 13. At π ) 0 mN m-1 a two-dimensional foam structure was observed. Figure 13a corresponds to the state of the film after the first compression-expansion cycle (the brightness and contrast of this image were manipulated so as to make it appear clearer and sharper). The two-dimensional foam disappeared after a small rise in the surface pressure. Unlike the case of Suc18, a number of small, point-sized, bright domains was present even at 0 mN m-1, their number increasing with increasing surface pressure. Rotating the analyzer affected uniformly the reflectivity of the background, but the brightness of point-sized domains remained unaffected. Thus these bright domains were not optically active, and this suggested that they may have been aggregates of considerable thickness. The increase in surface pressure, as shown in Figure 13b,c, was also accompanied by an increase in the surface reflectivity. The texture of the interface as a function of the compression rate was also studied in the case of Lac18.

Langmuir, Vol. 18, No. 12, 2002 4771

The interface was compressed from an initial area of 12 149 mm2, corresponding to a surface pressure of approximately 3 mN m-1, to a final area of 7807 mm2, corresponding to a surface pressure of approximately 20 mN m-1. The compression rates used were 2.0, 6.1, 20.3, 101.7, and 203.3 mm2 s-1. As in the case of Suc18, the surface pressure at the beginning and at the end of the compression was not specifically regulated. However, for all five of the compression rates the final surface pressure and the final reflected light intensity were approximately the same. Figure 12 also shows the change of Np with increasing compression rate for Lac18, for comparison with the behavior of Suc18. The number of domains observed was far higher for Lac18 than for Suc18. Also, although for Lac18 the rate of increase in Np decreased with increasing compression rate, the rate was still positive at the highest compression rates, in contrast to the behavior of Suc18 (see above), where Np fell at the highest compression rates. Evidently the formation of these domains is more rapid and more strongly driven in Lac18 than Suc18. Figure 14 shows BAM images of the Glu18 A-W interface. At low surface pressures the images were similar to those of Suc18 and Lac18. At a low surface pressure, a two-dimensional foam structure that disappeared on compression was also observed. Small, point-sized domains were also present, even at low surface pressures, e.g., Figure 14a. However, at π > 22 mN m-1, a new type of domain became apparent. The shape of the new domains was dendritic, e.g., Figure 14b,c. The slightly elongated form of this new type of domains was possibly due to the direction of the film compression, which is indicated by the arrows in Figure 14b. The texture of the A-W interface when covered by a monolayer of Gal18 was significantly different from the texture due to Suc18, Lac18, or Glu18. Figure 15 shows two instances of the interface at π ) 0 mN m-1. The presence of a bright, condensed phase is apparent. It is also notable that the condensed phase could be contained within, or hosted by, other types of surfactant-rich domains, as can be clearly seen in Figure 15a. After a compression-expansion cycle, the interfacial texture changed to the more evenly distributed two-dimensional pattern of Figure 15b. The different light intensities within the domains of Figure 15 are a result of the optical anisotropy of the surfactant material. Compression of the Gal18 film led to the destruction of the two-dimensional patterns seen at π ) 0 mN m-1. As the film was compressed, the domains of the condensed phase started to grow (Figure 16). The domain growth was anisotropic, which led to the domains having dendritic shapes. However, when the monolayer was kept at a constant surface pressure for a prolonged period, the shape of the domains changed to circular (data not shown). This indicates that the domain shapes featured in Figure 16a,b are, in fact, nonequilibrium shapes. A low line tension between the two surfactant phases would facilitate the formation of noncircular domains.39 However, it should be noted that altering the compression rate to 5 times lower did not seem to change the isotherm of Gal18, and the dendritic domains were still apparent (data not shown). The domains came into close proximity at a surface pressure of approximately 10 mN m-1. This is in agreement with the π-A isotherm of Gal18 (Figure 8) which shows a sharp surface pressure increase after π = 9 mN m-1 is exceeded. Different, bright point-sized domains appeared (see Figure 16b,c) at π > 15 mN m-1, similar in appearance to those that were present in the monolayers of Suc18, Lac18, and Glu18.

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Figure 15. Gal18. (a) π ) 0 mN m-1, A ) P ) 0°; (b) π ) 0 mN m-1 after a compression-expansion cycle, A ) 0°, P ) 0°. The white bar represents 100 µm.

Figure 14. Glu18. A ) P ) 0°. (a) π ) 5 mN m-1; (b) π ) 25 mN m-1; (c) π ) 35 mN m-1. The arrows in (b) show the direction of the film compression. The white bar represents 100 µm.

An additional qualitative characteristic of the Gal18 monolayers that could be discerned from the BAM images was the marked lack of mobility of the interface, even at surface pressures as low as 5-10 mN m-1. In contrast, the monolayers of Suc18, Lac18, and Glu18 appeared to be much more mobile, i.e., the domains were seen to be in continuous motion against the background phase, or could be easily set in motion by the slightest mechanical disturbance of the trough, though this mobility did appear to decrease slightly at the highest surface pressures employed. It is possible to use the reflected light intensity as a measure of the behavior of the monolayers as a function of their compression. Figure 17 shows the average reflected light intensity (in gray levels) as a function of 1/A for Suc18, Lac18, and Glu18, measured using the same analyzer (A) and polarizer (P) settings, and also the same angles of incidence and reflection. The reflected light intensity from Gal18 exceeded the range of linearity of the CCD even at low surface pressures. Additionally, the monolayer of

Gal18 was anisotropic, so that obtaining an acceptably reproducible average reflectivity from successive images of a film under the same conditions proved impossible. The corresponding GL versus 1/A plot for Gal18 is not included in Figure 17. The parameter 1/A may be taken as a measure of the surface concentration, whereas the reflected light intensity depends on a number of other factors besides surface concentration,40 e.g., thickness, anisotropy, and orientation of the monolayer. It is obvious that, for the same values of 1/A, the intensity is always lower for the monosaccharide surfactant, Glu18, than for the two disaccharide surfactants. Taking into account the above complications in interpreting changes in intensity, this might be is expected, due to the less bulky headgroup of Glu18. It may also be noted that for Glu18 and Lac18 the relationship between surface concentration and intensity is approximately linear, suggesting the thickness of the surfactant layer in these cases does not vary very much over the range measured, or that the thickness increases linearly with surface concentration. This is in contrast with the behavior of Suc18, where a distinct increase in slope of the plot occurs at the higher surface concentrations, suggesting a distinct change in the orientation, or efficiency of packing, of Suc18 molecules as the monolayer is compressed. The A-W isotherms presented do not provide a detailed description of the very low surface pressure region, i.e., where π < 1 mN m-1. Gaseous (G) films typically exist at surface pressures of this order of magnitude.41 BAM provided evidence for the existence of surfactant-rich and surfactant-poor domains at π ) 0 mN m-1, or very close to π ) 0 mN m-1. It is reasonable, therefore, to assume that the surfactant films underwent a G/LE phase transition at some π. BAM also showed the existence of “point-sized” domains in all monolayers studied. Blank experiments clearly


Langmuir, Vol. 18, No. 12, 2002 4773

Figure 17. Change in intensity of reflected light (GL) with 1/A, using identical BAM settings for ([) Suc18, (2) Lac18, and (O) Glu18. The camera shutter speed was 1/50 s.

Figure 16. Gal18. A ) P ) 0°. (a) π ) 5 mN m-1; (b) π ) 15 mN m-1; (c) π ) 20 mN m-1. The white bar represents 100 µm.

illustrated that such entities did not correspond to dust particles. Additionally, Figures 14 and 16 suggest that these point domains are in dynamic equilibrium with the other structures within the film. It is possible that these were LC domains, as observed in other surfactant systems.35 However, their coexistence with surfactant material in the gaseous phase could only occur as a metastable condition. A different possible explanation of the nature of the point-sized domains is that they are crystals. The presence of very small amounts of impurities (stearic acid, for instance) could promote crystal growth, even at very low surface pressures. These microdomains could then act as nuclei for phase transitions at higher surface pressures.42 Examples of such phase transitions were apparent in the cases of Glu18 (Figure 14) and Gal18 (Figure 16), where the domain growth was directed by the presence of these microdomains. However, even in the cases of Suc18 (Figure 10) and Lac18 (Figure 13) the size of these domains increased with increasing surface pressure.

Gal18 at the A-W interface behaved in a significantly different way. It is to be expected that the formation of condensed phases is easier in the cases of Glu18 and Gal18 than in the cases of Suc18 and Lac18. The latter group of surfactants, having a (more bulky) disaccharide as a headgroup, have a decreased likelihood of interfacial aggregation due to hydrophobic interactions between the tails. On the other hand, comparing Glu18 and Gal18, the relatively minor difference in molecular volume between the glucose and galactose residues43 seems to lead to significant differences in surface aggregation. Both the BAM observations and the isotherms at the A-W interface suggest that at elevated surface pressures the film of Gal18 behaved as solid. The dilatational rheology also substantiated this, since Gal18 exhibited very high values of E, especially at the higher surface pressures. The length scale of the domains observed via BAM and the changes in them as a function of the rate of compression (Figure 11) are noteworthy. Evidently, it is possible in such systems for a macroscopic monolayer property (such as the surface pressure, averaged over the length of the Wilhelmy plate) to correspond to more than one film texture of the same surfactant. For example, a singlephase LE monolayer could be “macroscopically” the same (i.e., have the same surface tension) as a monolayer of the same surfactant consisting of a number of LC domains within a LE phase. Up to now, such behavior has not been demonstrated for this class of surfactants. Since the structure of the interface at the microscopic scale is believed to be an important factor in determining colloidal stability, where the interactions between interfaces over very short length scales are important, such variations in film structure should clearly be taken into account when trying to explain the surfactant action of sugar surfactants. It is for the same reason that the interfacial tension measured is averaged over large regions, which span all the different phase regions visible in the BAM images, that “relaxation” effects are seen in the BAM images while there is often little difference between Est and E (at 0.1 Hz). From a practical point of view, i.e., stirring, whipping, foaming, emulsification, etc., much higher frequencies are relevant, and it would be interesting to probe higher frequencies where the film molecules would not have enough time to rearrange and reach the equilibrium surface pressure for a given area change, i.e., leading to more significant inhomogeneities in the film. However, with the current apparatus, frequencies much higher than 0.1 Hz are not accessible, due to problems of wave reflection inside the barrier.

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Conclusions A Langmuir trough technique was used to investigate the surface pressure isotherms and the dilatational properties of Suc18, Lac18, Glu18, and Gal18. Suc18 and Lac18 exhibit similarities in their properties at both the A-W and O-W interfaces. Brewster angle microscopy indicated that, at increasing surface pressures, Suc18 and Lac18 undergo a LE/LC transition which is not first order. In many instances domains far larger than typical colloidal dimensions (i.e., greater than several micrometers) were formed within the interface. Also, the interfacial texture was found to be strongly dependent on the rate of deformation of the interface. These observations are significant with regard to colloidal stability controlled by such low molecular weight surfactants, where the interface-interface interactions act at the microscopic scale and thus the dynamic interfacial structure at the microscopic scale is important. Glu18 and Gal18 form more aggregated films at the higher surface pressures. Gal18 presents solid-film-like behavior at the A-W interface, which is characterized by


a high collapse pressure, a steep surface pressure isotherm, and very high values of the dilatational modulus. At the O-W interface, however, the behavior of the two monosaccharide esters is different. No solidlike behavior is exhibited and a surface pressure plateau appears on their π-A isotherms, which is taken as an indication of a first order phase transition, possibly LE/LC. Inter-headgroup interactions are probably mostly responsible for the aggregation of Glu18 and Gal18 at the O-W interface, contrary to the case of the A-W interface, where the major attractive force is the hydrophobic interactions between the fatty acid chains of the surfactant molecules. Acknowledgment. The authors gratefully acknowledge the financial support of the Commission of the European Communities (FAIR-CT-985015) and the BBSRC (Grant 24/BI11184). Many thanks also to Dr. Doug Sarney (IFR, Reading, UK) for the gift of the samples of enzymatically synthesized sugar esters. LA011784C

Surface Pressure Isotherms, Dilatational Rheology, and Brewster

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