Decyl-maltopyranoside Gibbs Monolayers. - American Chemical Society

maltopyranoside (Mal) solutions at temperatures of 8, 22, and 29 °C. Comparison was made ... The adsorption isotherm of Mal and Glu obtained at 22 °...
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n-Decyl-glucopyranoside and n-Decyl-maltopyranoside Gibbs Monolayers. Phase Changes in the Dilute Liquid-Expanded Range Atte J. Kumpulainen,* C. Marcus Persson, and Jan Christer Eriksson Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and The Institute for Surface Chemistry, P.O. Box 5607, SE-100 44 Stockholm, Sweden Received May 10, 2004. In Final Form: September 7, 2004 Surface tension isotherms were recorded for n-decyl-β-D-glucopyranoside (Glu) and n-decyl-β-Dmaltopyranoside (Mal) solutions at temperatures of 8, 22, and 29 °C. Comparison was made with isotherms of n-decyl-β-D-thiomaltopyranoside (S-Mal) at 22 °C. In addition to the transition from the gaseous to the liquid-expanded (LE) state, a second transition was observed in the early stages of the LE regime for Glu, Mal, and S-Mal at room temperature. The adsorption isotherm of Mal and Glu obtained at 22 °C shows the presence of an adsorption step at an average area/molecule of about 79 Å2 between, approximately, 0.02 and 0.1 mM (the critical micelle concentration (cmc) is 2 mM) and 0.015 and 0.03 mM (the cmc is 2 mM), respectively. Similarly, for S-Mal an adsorption plateau is observed at 70 Å2 between 0.01 and 0.03 mM (the cmc is 0.7 mM). From the temperature dependence of the surface tension, we have seen that there are considerable differences in the adsorption of Glu and Mal. For Mal, the adsorption plateau is also observed at 29 °C at around 79 Å2, whereas Glu exhibits no adsorption plateau at this temperature. At 8 °C, both Mal and Glu exhibit saturation behavior in the dilute part of the liquid-expanded range, but at this temperature the average molecular areas are lower than at 22 °C: around 66 Å2 for Glu and 75 Å2 for Mal. Thus, the temperature sensitivity of Glu is considerably greater than for Mal in this range. The saturation regime coincides with a pronounced surface entropy minimum for Mal. The transition in the dilute liquid-expanded range supposedly occurs from a state with deformed surface micelles arranged in a hexagonal pattern, referred to as the granular range, to a true LE monolayer with a fluid hydrocarbon tail layer covering the entire surface.

Introduction Today replacement of the commonly used nonionic surfactants containing the xenobiotic ethylene oxide with degradable carbohydrate headgroups is an important issue. Moreover, sugar headgroups are of vast importance in nature for cell recognition and antibody responses. Hence, our understanding of surfactants with sugar headgroups must be improved not just to meet the demands of a sustainable development. Validated models of the carbohydrate headgroups in water contact, capable of accounting for their physicochemical behavior, are therefore requested. For surfactant solutions, the initial stage of surfactant adsorption at low concentrations generally includes a dilute ideal regime, the Henry range,1,2 where the surfactant molecules do not interact. In the Henry range, the free energy of adsorption can roughly be estimated from the main contribution to the free energy change, namely, the annihilation of the air-water interface at constant total surface area. Moreover, there are sizable contributions from the contact between the hydrocarbon chain and water in the plane of the surface.3 The Henry range is followed by a transition to the liquid-expanded region where the interactions between the surfactant molecules * Corresponding author. Telephone: +46 8 7909922. Fax: +46 8 208998 E-mail: [email protected]; [email protected]. (1) Meguro, K.; Ueno, M.; Esumi, K. In Nonionic Surfactants: Physical Chemistry; Schick, M., Ed.; Surfactant Science Series, Vol. 23; Marcel Dekker: New York, 1987. (2) Eriksson, J. C.; Ljunggren, S. Colloids Surf. 1989, 38, 179. (3) Kumpulainen, A. J.; Persson, C. M.; Eriksson, J. C. Langmuir, submitted.

become important. For sugar-based surfactants, the transition from the Henry region ends at a rather large area/molecule for the studied surfactants: >250 Å2 for S-Mal, >200 Å2 for Mal, and >145 Å2 for Glu at room temperature, indicating that it is excluded that the surface might be fully covered by hydrocarbon chains and rather that it entails a large fraction of air-water interface. To account for these findings, it is natural to invoke the idea of surface micelle formation. Accordingly, small circular quasi-two-dimensional islands of surfactant start to form at a certain concentration which rapidly grow in number and eventually cover the entire surface.3 The formation of a surface micelle effectively lowers the free energy cost of hydrocarbon-surface water contact by bringing the hydrocarbon chains into a dense hydrocarbon patch of liquidlike properties oriented in the surface plane. Since the surface in the dilute micellar phase still contains mainly air-water interface, the orientation of the hydrocarbon chains in the micelles is such as to maximize the annihilation of the air-water interface, that is, chiefly in the plane of the interface. The formation of clusters in the surface was first envisioned by Langmuir.4 In later years, cluster formation at the surface for surfactants has been discussed5,6 and theoretically modeled for Langmuir monolayers for transitions from gaseous to liquid-expanded (LE) state7 and from LE to liquid-condensed8 state. Recently a cluster formation theory with a mixing of orientational states of (4) Langmuir, I. J. Chem. Phys. 1933, 1, 756. (5) Israelachvili, J. Langmuir 1994, 10, 3774. (6) Stoeckly, B. Phys. Rev. A 1977, 15, 2558. (7) Ruckenstein, E.; Li, B. Langmuir 1995, 11, 3510. (8) Ruckenstein, E.; Li, B. J. Phys. Chem. 1996, 100, 3108.

10.1021/la0488497 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/21/2004

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the surfactant molecules, where some molecules will be oriented in the plane of the surface and others will be oriented normal to the surface arranged in dense crystalline clusters, has been used to account for Gibbs monolayers.9,10 This model can predict the transition from the Henry region where the vast majority of molecules will be in the surface plane to the liquid-expanded with a growing population of clusters. However, this model would not predict a transition in the liquid-expanded regime as the growth of the clusters is bound to be unrestricted. Our model on the other hand includes a curvature-dependent line tension acting on the edge of a surface micelle, which contributes to the total micelle free energy in accordance with the Helfrich expression,11 in the end yielding a free energy minimum for a certain micelle size. At a high enough density of surface micelles, the entire monolayer contracts to form the liquid crystalline granular phase. The granular phase is a result of the surface micelle phase at very high packing densities, where the surface micelles are strongly deformed from a disklike shape to fit in a hexagonally packed surface-covering phase. With increasing chemical potential, the density in the surface changes very slightly up until a critical concentration where all lines are annihilated with the following formation of a surface-covering true liquid-expanded phase distinguished by a coherent liquidlike hydrocarbon tail layer. With increasing adsorption, the height of the hydrocarbon tail layer changes, whereas the overall hydrocarbon tail density stays constant.

Figure 1. Surface tension isotherms of n-decyl-β-D-glucopyranoside (Glu) at 8, 22, and 29 °C. Circles represent 8 °C, squares 22 °C, and triangles 29 °C.

Materials and Methods Surface tension was measured with a Kru¨ss K12 tensiometer, employing the Wilhelmy plate method and using a platinum plate, sandblasted to ensure a contact angle of 0° at the threephase contact line. Surface tension values were obtained from

F ) 2(LT + LW)γ cos θ + LTLW∆Fgh

(1)

where F is the force measured, γ is the surface tension of the liquid-vapor interface, θ is the contact angle at the three-phase line, LT and LW are the thickness and width of the plate, respectively, ∆F is the density difference between the liquid and the vapor phase, g is the gravitational constant, and h is the immersion depth of the plate in the liquid. All surface tension isotherms were recorded at least three times. The temperature was controlled to (0.2 °C. The water used in the experiments was obtained from a Millipore RiOs-8 and Milli-Q PLUS 185 purification system and finally filtered through a 0.2 µm Millipak filter. The total organic carbon content of the outgoing water was controlled with a Millipore A-10 unit and did not exceed 6 ppb during any of the measurements. n-Decyl-β-D-glucopyranoside was used as received from Sigma (>98% GC). n-Decyl-β-D-maltopyranoside and n-decyl-β-D-thiomaltopyranoside from Anatrace (Anagrade) were used as received. Comparisons were made with samples purified in the high-performance surfactant purification apparatus,12 and no differences in adsorption could be detected. To obtain the adsorbed amount from the surface tension isotherms, a fitting procedure was implemented. For all three surfactants, the initial stages of the isotherms were fitted with a linear region of the surface tension as a function of bulk concentration, c. After this, polynomials of γ(ln c) were used in the region c ≈ 0.01-0.3 mM and the final course down toward (9) Fainerman, V. B.; Miller, R.; Wu¨stneck, R.; Makievski, A. V. J. Phys. Chem. 1996, 100, 7669. (10) Drach, M.; Rudzinski, W.; Warszynski, P.; Narkiewicz-Michlek, J. Phys. Chem. Chem. Phys. 2001, 3, 5035. (11) Helfrich, W. Z. Naturforsch. 1973, C28, 693. (12) Lunkenheimer, K.; Wantke, K.-D. Rev. Sci. Instrum. 1987, 58, 2313.

Figure 2. Surface tension isotherms of n-decyl-β-D-maltopyranoside (Mal) at 8, 22, and 29 °C. Circles represent 8 °C, squares 22 °C, and triangles 29 °C. the critical micelle concentration (cmc) was fitted with a variant of the Szyszkowski-Langmuir equation,

(

γ ) γ* - kTΓ∞ ln

)

c +1 k1

(2)

where γ* is a constant, usually set to the surface tension for pure water, γ0, k is the Boltzmann constant, T is the absolute temperature, Γ∞ is the surface density at full coverage, and k1 is an adsorption constant. Such a regional fitting procedure was deemed necessary as no single equation of state can yield a satisfactory representation of the data. To accurately obtain the change of the adsorbed amount in a region, a fairly large number of points are necessary. This makes it rather difficult to discuss the detailed course of the adsorption isotherm around transitions, where the adsorption increases rapidly with small increments of the bulk concentration, for a first-order transition stepwise at a certain concentration.

Results and Discussion Surface tension isotherms for Glu and Mal at 8, 22, and 29 °C are displayed in Figures 1 and 2, respectively. The surface tension isotherm for Glu, Mal, and S-Mal at 22 °C is presented in Figure 3. The corresponding adsorbed amounts at 22 °C are presented in Figure 4. The Gibbs surface tension equation at constant temperature for a nonionic surfactant assuming ideality, or at least a constant activity factor, and using the Gibbs diving plane (surface excess of water ) 0) yields

-

1 dγ dγ )) Γ ) Γ1 + 〈n〉Γ〈n〉 dµ kT d ln c

(3)

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Figure 3. Surface tension isotherms of n-decyl-β-D-maltopyranoside (Mal), n-decyl-β-D-glucopyranoside (Glu), and n-decylβ-D-thiomaltopyranoside (S-Mal) at 22 °C. Circles represent Mal, squares Glu, and triangles S-Mal.

Figure 5. Surface tension isotherms of Mal and Glu around the granular regime. Glu is represented by squares, and Mal by triangles. The drawn line corresponds to a constant area/ molecule of 79.5 Å2. The arrows indicate, with increasing concentration, where Glu enters the granular range, where Mal enters the granular range, where Glu enters the true LE range, and finally where Mal enters the true LE range.

surface appears at rather low surface coverages and bulk concentrations and is attributed to surface micelles packed at very high densities; this region is referred to as the granular range. The strong saturation tendency in the surface renders the hard-disk approach unrealistic in the granular range. The Helmholtz free energy per surfactant molecule in the surface can be obtained by

f σ - µ0b ) kT ln(x) + γa Figure 4. Adsorption isotherms for Glu, Mal, and S-Mal at 22 °C. Glu is represented by the dotted line, with the clearly higher adsorption, Mal by the solid line with squares, and S-Mal by the solid line.

where µ denotes the chemical potential of the surfactant, Γ1 the surface density of monomers, and Γ〈n〉 the surface density of clusters of the thermodynamical average size 〈n〉. The material balance in the surface for the formation of regions of locally higher density of size 〈n〉 gives

ΓA ) Γ1(A - AΓ〈n〉 a〈n〉) + ΓDa〈n〉Γ〈n〉 A ) [Γ1(1 - Γ〈n〉 a〈n〉) + 〈n〉Γ〈n〉]A (4) where A is the total surface area, a〈n〉 is the area of the one micelle, and ΓD is the density of molecules in the micelles. There are two separable contributions to the surface tension decrease for the dilute surface states. First we may assume that the increase in surface density of monomers is given by the ideal contributions as calculated by extrapolation from the Henry region. Second, the surface micelles formed will interact by way of excluded area as predicted by the hard-disk equation of state,13,14

1 Πa ) kT φ 2

(5)

w

where Π ≡ γ0 - γ is the surface pressure, a is the area/ molecule, and φw is the surface area fraction not covered by disklike micelles. The surface micelle approach implies that the surface will become densely covered very quickly by raising the surfactant chemical potential just a little. For the surfactants studied here, a strong saturation tendency where the surface tension lowering is well described by a single surface density, γ ∼ ln(c), is observed in a limited concentration interval. The region of dense packing in the (13) Erpenbeck, J. J.; Luban, M. Phys. Rev. A 1985, 32, 2920. (14) Nilsson, U. Thesis, Lund, Sweden, 1992.

(6)

where f σ denotes the molecular Helmholtz free energy in the surface, µ0b is the standard state bulk chemical potential, and x is the bulk fraction of surfactant. The surface tension decrease in the granular range is well accounted for by a constant area per molecule. From eq 6, it becomes evident that the change in Helmholtz free energy throughout the granular range is virtually nil. Thus, we need to have an alternative mechanism to describe the change in the chemical potential in the surface in the granular range. This mechanism primarily stems from compressing the grain boundaries and effectively making them slightly thinner. Surface tension data of Glu and Mal at 22 °C for the granular region are presented in Figure 5. The drawn line corresponds to the surface tension lowering at a constant area per molecule of 79.5 Å2. At 22 °C, Glu will reach the granular region at a slightly lower concentration than Mal, which indicates that the headgroup of Mal in the surface micelles will generate somewhat higher surface pressure than the Glu headgroup. Moreover, the surface tension at which the Glu granular phase disrupts, with the following formation of the true LE phase, is clearly lower than for Mal, 66.5 and 60.5 mN/m, respectively. Thus, the granular phases are likely to be of similar physical nature for the two surfactants, but the formation of the true LE phase occurs at different concentrations. This can be explained by the considerable difference in size of the two headgroups. The formation of the true LE phase, herein defined at a molecular area lower than 65 Å2, yields full coverage of the entire surface with liquidlike hydrocarbon, accompanied by the formation of a phase where headgroups and water mix. A larger headgroup generates considerably more repulsive interactions, whereby the transition should occur at a lower surface tension. Additionally, there could be higher stabilizing contributions from the headgroup in the granular phase from the Mal headgroup than from the Glu headgroup. We picture the granular surface structure to arise by compression and deformation of circular surface micelles.

Monolayer Transitions in the Liquid-Expanded Range

Figure 6. A schematic depiction of the formation and the disruption of the granular phase. Straight lines correspond to ideal surfactant molecules, circles to surface micelles, each hexagon to a grain, and wiggly lines to surfactant molecules in the true LE monolayer.

A schematic depiction of the formation and disruption of the granular phase is presented in Figure 6. Morphologically similar structures have been found for monolayers of partially fluorinated alkanoic acids.15,16 For the grains and surface micelles to be stable against disruption, there will have to be mainly entropic headgroup repulsion between the surface micelle edges, as well as between the grain boundaries. Direct hydrocarbon contacts, on the other hand, would tend to disrupt the grains, resulting in the formation of a true LE monolayer. By considering the surface micelles as small circular islands of fluid hydrocarbon chains with similar properties as in the true LE range, their stability is related to the dilution of the headgroups in a larger accessible volume, simply due to the high curvature. The effect of more efficient hydrocarbon-water shielding of the headgroup in the micelle than in the true LE range might further increase the stability of the micelle. In total, these effects generate micelles that primarily repel each other and at higher bulk surfactant concentrations grains can withstand a considerable chemical potential increase before being annihilated. The formation of surface micelles should, however, not be considered as a phenomenon generally applicable to all nonionic surfactants, as the properties of the headgroup determine the stability against size and shape fluctuations. These fluctuations are determined by the free energy function of the surface micelle formation. For the surface micelles to be thermodynamically stable, a minimum in the free energy function is necessary, but the stability against size and shape fluctuations is set by the increase in free energy upon increase in micelle size. Hence, the free energy maximum following the minimum has to remain high enough when raising the surfactant chemical potential. The surface micelles of sugar-based surfactants are likely to distribute evenly over the surface, being subject to predominantly repulsive forces. However, as the number of micelles increases the crowding in the surface becomes noticeable and the accessible surface area for each micelle decreases considerably which could lead to short-range attractions between the micelles in the phase change from the dilute micellar to the granular monolayer. Short-range attractions are found between hydrophobic surfaces covered with sugar-based surfactants.17,18 Transitions for (15) Kato, T.; Kameyama, M.; Ehara, M.; Iimura, K.-I. Langmuir 1998, 14, 1786. (16) Ren, Y.; Iimura, K.-I.; Kato, T. J. Phys. Chem. B 2002, 106, 1327. (17) Waltermo, Å.; Claesson, P. M.; Johansson, I. J. J. Colloid Interface Sci. 1996, 183, 506. (18) Persson, C. M.; Kumpulainen, A. J. Colloids Surf., A 2004, 233, 43.

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lipids with carbohydrate headgroups have been attributed to short-range attractions between headgroups.19 The range of the attractive force is related to the curvature of the micelle; a high curvature as for the surface micelle will diminish the strength considerably. In the attractive minimum, the surface micelles may have to distort from being disklike into a more hexagonal shape. Hence, the attractive force could compensate for the free energy cost of lengthening of the micelle edge against the line tension acting upon it. Alternatively, we can assume that there are only repulsive forces between the micelles. This will effectively lower the surface tension, and as the density increases there will have to be a distortion of the lines occurring gradually until the surface is completely covered with distorted micelles packed in a hexagonal pattern, as in the granular monolayer. We note that the hard-disk equation will only be valid as long as the micelles do not distort significantly from the disklike shape, which they do approaching the granular range. Regardless of the exact path from the dilute micellar to the granular region, the granular phase will be ordered and have a high elasticity. The elasticity is defined by

E)

dγ (da )a

(7)

T

The molecular area in the monolayer approximately corresponds to the eigen-area of the surfactant molecules in the granular monolayer. The formation of a LE monolayer without grain boundaries at equal molecular areas will be less favorable, probably due to a combination of greater exposure of hydrocarbon to water and headgroup interactions. Hence, the granular film is stable until the chemical potential is increased to a point where the formation of a true LE monolayer is equally energetically favorable. At this point, there is a stepwise increase in the adsorbed amount with the simultaneous annihilation of grain boundaries and the formation of a surface-covering coherent true LE film. The Temperature Dependence of the Surface Tension and the Surface Entropy. From the surface tension change with temperature at constant concentration, one can deduce the excess surface entropy,20 σ

a

∂γ ∂µ (∂T ) ) - Sn - (∂T ) ) -(s σ

c

c

σ

- sb) ) -∆s

(8)

where Sσ is the surface entropy, nσ is the number of surfactant molecules in the surface, and sσ and sb are the partial molecular entropies in surface and bulk, respectively. Equation 8 can be rewritten to a more suitable expression as the molecular bulk entropy increase corresponds to k ln(x). This yields

a

( )

(

)

dµ0b ∂γ + k ln(x) ) - sσ + ∂T c dT

(9)

Thus, we can obtain the change in the surface entropy. Since we have a limited data set with respect to the surface tension change with temperature, we cannot exactly determine the differential ∂γ/∂T. However, assuming that the temperature variation in the true LE regime is gradual and roughly estimated by the linear slope between the measured points, we can make reasonable estimates of the excess surface entropy in this range. In the granular (19) Tamada, K.; Minamikawa, H.; Hato, M.; Miyano, K. Langmuir 1996, 12, 1666. (20) Aratono, M.; Villeneuve, M.; Takiue, T.; Ikeda, N.; Iyota, H. J. Colloid Interface Sci. 1998, 200, 161.

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Figure 7. Surface tension as a function of temperature for a constant bulk concentration of Mal of 0.082 mM. Three distinct regions are observed: between 12 and 20 °C the slope is about 0.1 mN/m‚K, between 20 and 30 °C 0.07 mN/m‚K, and above 30 °C around 0.1 mN/m‚K.

Figure 8. The change in surface entropy for Mal and Glu as a function of molecular area. The absolute scale is arbitrary. Diamonds correspond to Mal, and circles to Glu. The lines serve as a guide for the eye.

range, we have measurements of the surface tension change with temperature at 0.082 mM bulk concentration of Mal; this is presented in Figure 7. γ(T) in this range clearly can be subdivided into three regions. At low temperatures the slope is around 0.1 mN/m‚K, in the range between 20 and 30 °C the slope changes to 0.07 mN/m‚K, and at higher temperatures the slope returns to around 0.1 mN/m‚K. From the surface tension isotherms of Mal in Figure 2, we clearly see that the surface tension change around the granular regime between 22 and 29 °C is very low. The change in surface entropy (the absolute scale is arbitrary) of Mal and Glu as a function of molecular area is presented in Figure 8. Notably, the surface entropy will be at a minimum in the granular range for Mal, but not for Glu. For both Mal and Glu, the formation of the granular phase is accompanied by a decrease in the surface entropy. For Mal, however, the decrease in surface entropy is very much greater, primarily due to the higher bulk concentrations of surfactant required to disrupt the phase, indicative of a considerably more stable granular phase than for Glu. The entropic minimum is related to the strict ordering in the granular regime. The grains cannot grow in size and are hence probably of similar size. Further there is no contribution to the surface entropy from the air-water interface in this range, due to full coverage of the surface. The air-water contact is the dominant contribution by far in the Henry range and the dilute micellar region, which partially explains the strong increase in surface entropy upon increase in the molecular area. As the grain boundaries vanish with the following formation of the true LE phase, a concurrent increase in the thickness of the hydrocarbon layer and probably in the headgroup layer takes place. This yields a great

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Figure 9. Adsorption isotherms for Mal at 8, 22, and 29 °C. The 8 °C adsorption isotherm is represented by the dotted line, 22 °C by the solid line, and 29 °C by the dashed line (with the higher adsorption).

Figure 10. Adsorption isotherms for Glu at 8, 22, and 29 °C. The 8 °C adsorption isotherm is represented by the dashed line, 22 °C by the solid line, and 29 °C by the dotted line.

Figure 11. Surface pressure vs area/molecule isotherms for Mal at 8, 22, and 29 °C. The 8 °C isotherm is represented by the dotted line, 22 °C by the dashed line, and 29 °C by the solid line, with the lowest area/molecule.

increase in the molecular surface entropy for Mal and a weak increase for Glu. The adsorbed amounts of Mal and Glu at 8, 22, and 29 °C are presented in Figures 9 and 10. The corresponding surface pressure versus area/molecule is seen in Figures 11 and 12. The surface pressure behavior for Glu in the granular region is very different from that of Mal. For Glu, the strong surface pressure increase at 8 °C marking the granular region occurs at a low area/molecule of around 65 Å2, which is very close to the molecular area from which we define the true LE regime, which is the molecular area corresponding to the eigen-area of the hydrocarbon chain (roughly 65 Å2 for a decyl chain). At 22 °C, the molecular area for Glu is around 79 Å2 in the granular regime, and at 29 °C a granular region is not detected. For Mal, on the other hand, granular regions are detected at all measured temperatures with an average molecular area of 75 Å2 at 8 °C and 79 Å2 at both 22 and 29 °C. Hence, the granular regime of Glu clearly is very much more temperature

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Figure 12. Surface pressure vs area/molecule isotherms for Glu at 8, 22, and 29 °C. The 8 °C isotherm is represented by the dashed line, 22 °C by the solid line, and 29 °C by the dotted line, with no evident granular range.

Figure 13. Surface pressure vs area/molecule isotherms of Mal, Glu, and S-Mal at 22 °C. The Mal isotherm is represented by the solid line with squares, Glu (with the lowest molecular area) by the dotted line, and S-Mal by the solid line.

sensitive than that of Mal, indicative of greater stabilizing contributions from the Mal headgroup. In the true LE regime, we may note that the surface pressure versus area/molecule is not particularly temperature sensitive for Glu, whereas the behavior of Mal indicates rearrangements with increasing temperature. For Mal, the area/molecule at 29 °C at the cmc is considerably lower than the corresponding molecular area at the two other temperatures. This is indicative of reorientation of the headgroup with increasing temperature. Interestingly, as mentioned above the slope of γ(T) for Mal at 0.082 mM increases at temperatures lower than 20 °C. Further, the area/molecule at 8 °C is somewhat lower in the granular range than at 22 °C. This indicates that the grains may change in structure upon temperature change. Hypothetically we can have adsorption of additional molecules to the grain, making the average surface density of hydrocarbon somewhat higher, or there could be rearrangements on the grain boundaries resulting in closer contact between the liquidlike centers of the grains. Differences in Adsorption between a Maltoside and Thiomaltoside Headgroup. In similarity to Glu and Mal, S-Mal, or n-decyl-β-D-thiomaltopyranoside, also forms a stable granular phase at room temperature. Surface tension isotherms of Mal, Glu, and S-Mal at 22 °C are presented in Figure 3. The corresponding surface pressure versus area/molecule isotherms are presented in Figure 13. The cmc of S-Mal is considerably lower than that of Mal, about 0.7 and 2 mM, respectively. On the basis of the cmc difference, S-Mal is generally considered to be more hydrophobic than Mal, but in the Henry region the slopes of γ(c) are identical. This means that the S-Mal molecule is in fact as hydrophobic as the Mal molecule, as a more hydrophobic molecule would generate a considerably steeper slope. Hence, the difference stems from

Figure 14. (A) Surface tension as a function of concentration (linear scale) of Mal around the granular range at 22 °C. The arrows indicate, in order of increasing concentration, the transition from the Henry to the dilute micellar region and the transition from the dilute micellar to the granular range, with almost constant area/molecule of about 79 Å2. The drawn lines corresponding to the regional optimal fit are extended somewhat for clarity. (B) Surface tension as a function of concentration (logarithmic scale) for Mal around the granular range. The third arrow indicates the transition from the granular to the true LE range.

the more favorable situation of having a sulfur interacting with the hydrocarbon chains instead of an oxygen. Additionally, the sulfur is more polarizable, whereby attractions between the sulfur atoms could lead to a greater adsorption. S-Mal in similarity to Mal and Glu apparently forms stable grains as appears from a linear region of γ ∼ ln c. The average area/molecule for S-Mal in the granular region differs from the corresponding molecular area of Mal. The area/molecule obtained is around 70 Å2. Thus, the substitution from oxygen to sulfur clearly produces a different nature of the grains. As previously mentioned, the difference could stem from a slightly larger grain or from differences in the grain boundaries. One possible mechanism is the donation of headgroups to neighboring grains. This would result in a semicontinuous phase with headgroups participating in the stabilization of other grains and more importantly closer contact between the centers of the grains. In the S-Mal isotherm, a region between 7 and 10 µM corresponds to a constant molecular area of around 80 Å2. This concentration regime, however, is too brief for any extensive conclusions to be drawn. However, it indicates the likeliness of only a few accessible states for the maltoside headgroups in the grains. Assuming that the difference stems from the grain boundaries, the 79 Å2 could represent the dispersed granular state where the headgroups are restricted to one grain. The 70 Å2 could represent the semicontinuous phase where headgroups are donated between the grains. On the other hand, we cannot exclude that the observed change in surface density possibly can stem from the inclusion of more molecules to the center of the grain, without alteration of the grain boundaries. This would lead to a

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Figure 15. (A) Surface tension as a function of concentration (linear scale) of Glu around the granular range at 22 °C. The arrows indicate, in order of increasing concentration, the transition from the Henry to the dilute micellar range, the transition from the dilute micellar to the granular range (with almost constant area/molecule of 79 Å2), and the transition from the granular to the true LE range. The drawn lines corresponding to the regional optimal fit are extended somewhat for clarity. (B) Surface tension as a function of concentration (logarithmic scale) of Glu around the granular range.

higher surface density of hydrocarbon in the grains. At this stage, we have no way of discerning between the two mechanisms. Surface tension data of Mal, Glu, and S-Mal around the granular range are presented on both linear and logarithmic c scales in Figures 14-16, panels A and B. The arrows in the figures represent transition intervals in the surface. The drawn lines correspond to the regional optimal fits of the surface tension data. The transition from the dilute micellar range to 80 Å2, corresponding to the second arrow in Figure 16A,B, for S-Mal clearly indicates the likeliness of attractive forces between the micelles. A great increase in adsorption is observed in a very limited chemical potential interval around 7 µM, where the area/molecule changes very abruptly. This indicates a clear distinction between the dilute surface micellar range and the granular range. The attractive energy could compensate for the free energy cost of lengthening of the micelle lines into the regular hexagonal lines expected in the dense granular surface-covering regime. Conclusions To summarize, the formation of surface micelles will result in an efficient decrease of the unfavorable hydrocarbon-water contact by the removal of such contacts around the “edges” of the hydrocarbon chains oriented in the surface plane. For all of the surfactants investigated here, surface micelles are the probable outcome for the transition from the Henry region, and hence the transition is not first-order. With increasing interactions between the surface micelles, the formation of a granular film takes place. The integrity of the micelles is not lost in this range,

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Figure 16. (A) Surface tension as a function of concentration (linear scale) of S-Mal around the granular range at 22 °C. The arrows indicate, in order of increasing concentration, the transition from the Henry to the dilute surface micellar region, the transition from the dilute surface micellar to the granular range with an average molecular area of 80 Å2, and the transition from the granular region with an average molecular area of around 80 Å2 to the granular region with an average molecular area of around 70 Å2. (B) Surface tension as a function of concentration (logarithmic scale) of S-Mal around the granular range. The fourth and final arrow indicates the transition from the granular to the true LE range. The drawn lines corresponding to the regional optimal fit are extended somewhat for clarity.

but rather the lines keeping the surface micelles intact will distort from circular into a more hexagonal shape. This distortion of the micelle lines comprises a free energy cost; hence the granular film can be reached by two main routes. The first is the gradual deformation of the surface micelles in accordance with mainly repulsive interactions between the micelles. Second, short-range attractions between the micelles may partly compensate for the free energy cost of deforming the line. The first statement seems to agree best for Mal, whereas both Glu and S-Mal at room temperature seemingly exhibit phase change behavior when increasing the surface density from dilute micellar to granular phase. The granular phase is formed by Mal, Glu, and S-Mal. All of these surfactants enter a region of high elasticity with a largely constant area/ molecule of around 79 Å2 at room temperature. In the case of Mal and S-Mal, the granular phase is very stable as seen by the fact that the composition of the surface is almost invariant for a considerable bulk concentration interval for both Mal and S-Mal at room temperature. S-Mal additionally has a region of around 70 Å2 also exhibiting very high elasticity, indicating that there are other states of adsorption for surfactants carrying maltoside headgroups in the granular regime. This effect can be due to the mutual donation of headgroups between the grains, effectively lowering the molecular area by bringing the grains closer to each other, or adsorption of more surfactant molecules to the grains. For Mal and S-Mal, the increase in chemical potential across the granular

Monolayer Transitions in the Liquid-Expanded Range

regime is above 1 kT. For Glu, the chemical potential interval is shorter, around 0.7 kT. Measurements at different temperatures for Mal and Glu show that the granular regime coincides with a minimum in surface excess entropy. This is a strong indication of high order in the monolayer at these surface densities. The granular region of Glu clearly is more temperature sensitive than that of Mal, as a granular regime is not seen at 29 °C for Glu. Further, the surface density shifts considerably more for Glu than for Mal when decreasing the temperature from 22 to 8 °C. The transition from the granular regime to the true LE state could well be first-order,21 since the transition would

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involve all molecules in the surface as the integrity of the grains is disrupted at one critical concentration with the resulting formation of a surface-covering film of liquidlike hydrocarbon. Acknowledgment. A.J.K. acknowledges the financial support from VR (The Swedish Research Council). LA0488497 (21) Rusanov, A. I. Phasengleichgewicthe und Grenzflaechenerscheinungen; Akademie-Verlag: Berlin, 1978; pp 444-449.