Langmuir Monolayers of Diacyl Glycerol Amino Acid-Based

10878

Langmuir 2003, 19, 10878-10884

Langmuir Monolayers of Diacyl Glycerol Amino Acid-Based Surfactants. Effect of the Substitution Pattern of the Glycerol Backbone Rosa Albalat,† Josep Claret,† Jordi Igne´s-Mullol,*,† Francesc Sague´s,† Carmen Mora´n,‡ Lourdes Pe´rez,‡ Pere Clape´s,‡ and Aurora Pinazo‡ Departament de Quı´mica Fı´sica, Universitat de Barcelona, Martı´ i Franque´ s 1, E-08028 Barcelona, Spain, and Institut d’Investigacions Quı´miques i Ambientals de Barcelona, Jordi Girona, 18-26, E-08034 Barcelona, Spain Received July 15, 2003. In Final Form: October 3, 2003 The Langmuir monolayers of diacyl glycerol amino acid-based surfactants with different substitution patterns of the glycerol backbone have been studied by means of surface pressure versus molecular area isotherms and Brewster angle microscopy (BAM). The isotherms show that all the studied compounds form stable monolayers on a water subphase. Concerning the compounds with alkyl chains in adjacent positions, the effect of the nature of the headgroup and the length of alkyl chains on their monolayer behavior is similar to what is observed in naturally occurring phospholipids. BAM images of the condensed phase show initially lobulated domains that relax to rounded-shaped structures with a singularity at the center (star defect) or at the boundary of the condensed droplets. The compound with alkyl chains bound to the ends of the glycerol backbone shows a markedly different behavior. Although condensed domains are favored in this case, some hindrance in the interaction between alkyl chains of different molecules leads to the formation of compact and rough growing droplets, reminiscent of those predicted by the Eden model of cluster growth. This hindrance is attributed to the lower hydrophobic interaction between intra-alkyl chains due to the carbon atom separating them.

1. Introduction Naturally occurring amino acids are of particular interest in the field of biocompatible surfactants. Owing to their antimicrobial activity, surfactant molecules from renewable raw materials that mimic natural lipoamino acids are the preservatives of choice for food, cosmetic, and pharmaceutical applications.1,2 Given their natural and simple structure, they show low toxicity and quick biodegradation.3,4 In this context, we have recently synthesized diacyl glycerol amino acid-based surfactants.5,6,9 Preliminary results on physicochemical and biological properties show that these novel compounds combine the advantages of both partial glycerides and lipoamino acids.7 We have observed that they exhibit an antimicrobial activity similar to that of long-chain NRacyl amino acid derivatives5,8 and form lamellar phases * Author to whom correspondence should be addressed. † Universitat de Barcelona. ‡ Institut d’Investigacions Quı´miques i Ambientals de Barcelona. (1) Krog, N. J. In Food Emulsions, 3rd ed.; Friberg, S. E., Larsson, K., Eds.; Marcel Dekker: New York, 1997; Vol. 81, pp 141-188. (2) Xia, J.; Nnanna, I. A.; Sakamoto, K. Amino acid surfactants: Chemistry, synthesis and properties; Marcel Dekker: New York, 2001; Vol. 101. (3) Infante, M. R.; Pe´rez, L.; Pinazo, A.; Clape´s, P.; Mora´n, C. In Novel Surfactants. Preparation, Application and Biodegradability; Holmberg, K., Ed.; Marcel Dekker: New York, 2003; in press. (4) Pe´rez, L.; Garcı´a, M. T.; Ribosa, I.; Vinardell, M. P.; Manresa, A.; Infante, M. R. Environ. Toxicol. Chem. 2002, 21, 1279-1285. (5) Pe´rez, L.; Pinazo, A.; Vinardell, M. P.; Clape´s, P.; Angelet, M.; Infante, M. R. New J. Chem. 2002, 26, 1221-1227. (6) Mora´n, C.; Infante, M. R.; Clape´s, P. J. Chem. Soc., Perkin Trans. 1 2002, 1124-1134. (7) Infante, M. R.; Pinazo, A.; Seguer, J. Colloids Surf., A 1997, 123124, 49-70. (8) Mora´n, C.; Clape´s, P.; Comelles, F.; Garcı´a, T.; Pe´rez, L.; Vinardell, P.; Mitjans, M.; Infante, M. R. Langmuir 2001, 17, 5071-5075. (9) Pe´rez, L.; Infante, M. R.; Angelet, M.; Clape´s, P.; Pinazo, A. Prog. Colloid Surf. 2003, in press.

and vesicles,9 which are characteristic of partial glycerides and phospholipids. It is known that the particular polymorphic phase formation of phospholipids depends on their tripartite structure (Chart 1), namely, the hydrophobic aliphatic double chain, the hydrophilic headgroup, and the hydrogen belt of the glyceryl ester groups. Diacyl glycerol amino acid surfactants have also this tripartite structure in which the polar group has been substituted for an amino acid (Chart 2). In this work, surface pressure (π)-molecular area (A) isotherms and Brewster angle microscopy (BAM) were used to analyze the influence on the monolayer behavior of the structure and position of the polar group in diacyl amino acid-based surfactants. We have studied seven different compounds. Two were monocationic derivatives: 1,2-diacyl-rac-glycero-3-O-(NR-acetyl-L-arginine) (1212RAc, 1414RAc). Two were dicationic derivatives: 1,2diacyl-rac-glycero-3-O-(L-arginine) (1212R, 1414R). Two were nonionic derivatives: 1,2-dilauroyl-rac-glycero-3-O(NR-acetyl-L-glutamine) (1212QAc). The last one was the 1,3-dilauroyl-glycero-2-O-(NR-acetyl-L-glutamine) (12QAc12; Chart 2). The results are compared with dimyristoyl phosphatidylethanolamine (L-DMPE), dilauroyl phosphatidylcholine (rac-DLPC), and dimyristoyl phosphatidylcholine (rac-DMPC; Chart 1). The paper is organized as follows. First, we analyze the monolayer behavior of the compound 1414RAc, which we will use as a reference. We report π-A isotherms and BAM images of condensed droplets, which reveal inner textures compatible with the existence of hexatic order. Next, the effect of the headgroup is analyzed by comparing the reference compound with 1414R and with analogous natural phospholipids. Then, we assess the effect of the chain length by studying the monolayer behavior of compounds with lipid chains shorter than that of 1414R.

10.1021/la035281d CCC: $25.00 © 2003 American Chemical Society Published on Web 11/19/2003

Diacyl Glycerol Amino Acid Surfactant Monolayers Chart 1

Chart 2

Langmuir, Vol. 19, No. 26, 2003 10879

2. Materials and Methods The glycero amino acid-based surfactants shown in Chart 2 were synthesized following the methods described elsewhere.5,9 The purity of the products was assessed with high-performance liquid chromatography (HPLC) and elemental analysis. The results indicate that the purity is higher than 99%. Standard phospholipids (DMPE, DLPC, and DMPC) were obtained from Sigma with a purity of 99% and were used as received. Unless otherwise stated, the subphase used was ultrapure water (resistivity ≈ 18.2 MΩ cm) supplied by a Purelab USF system. Spreading solutions (≈1 mM) were prepared using a 9:1 chloroform-methanol mixture as the solvent. Both solvents were HPLC grade, were obtained from Baker (chloroform) and Merck (methanol), and were used as received. Monolayers were prepared by depositing drops of the appropriate solution on the water subphase contained in a homemade Teflon Langmuir trough. The control of the monolayer area is achieved by means of two spring-loaded motor-driven Teflon barriers. The trough area was 8 × 24 cm with a 4-mm depth. The temperature in the subphase was controlled to within (0.5 K using a combination of a recirculating water bath (Julabo) and thermoelectric Peltier elements. The temperature of the water subphase was monitored with a Teflon-encapsulated thermistor, and the surface pressure was monitored using a filterpaper Wilhelmy plate and an R&K electrobalance. Imaging of the monolayer was performed by means of a custombuilt Brewster angle microscope.10,11 This device is based on a reflection polarizing microscope with incidence at the Brewster angle of the air/water interface and is sensitive to changes in the local index of refraction at the interface. Moreover, the presence of anisotropic media, such as a condensed lipid monolayer, leads to a reflectivity that depends on molecular ordering at mesoscopic length scales. This results in BAM images with textures that may be analyzed to extract information about the molecular configuration and relevant material parameters of the system. In what follows, light travels north to south in the images and the geometrical BAM distortion and illumination inhomogeneities have been corrected. The optical configuration is such that the brightest (respectively darkest) areas correspond to aliphatic chains tilted toward the right (respectively left) of the reader. To minimize air convection and the effect of dust particles, the whole experimental setup was enclosed in a sealed Plexiglas box. To limit the drift of domains in the monolayer, the environmental temperature was kept approximately 5° higher than that of the monolayer. After spreading, the monolayer was allowed to equilibrate and the solvent was allowed to evaporate for 10 min. The π-A isotherms were subsequently recorded under compression rates on the order of 10-2 nm2 molecule-1 min-1.

3. Results and Discussion

Finally, we study a compound with a 1,3-substitution pattern (12QAc12). Both the isotherms and the BAM images reveal remarkable differences with respect to the compound with the standard substitution pattern (1212QAc). On the basis of our experimental results and on already known thermodynamic properties, we suggest a rationale for the behavior observed.

3.1. Glycero Arginine-Based Surfactants. 3.1.1. Reference Compound: 1,2-Di-O-myristoyl-rac-glicero-3O-(NR-acetyl-L-arginine) (1414RAc). 1414RAc forms stable monolayers at the air-water interface, as evidenced in Figure 1 with a family of π-A isotherms. They display the features commonly found in phospholipids of intermediate chain lengths (18 > n > 12),12,13 including a plateau of coexistence between an isotropic expanded (LE) and a tilted condensed (LC) phase after a first-order phase transition. The nature of the phases and the transition are confirmed by BAM images of nucleating LC droplets. At our usual compression rates, LC nuclei are lobulated, with a maximum diameter of about 50 µm (Figure 2). Larger droplets, needed to properly resolve the inner texture, may be obtained by slow compression (rates of 0.01 nm2 molecule-1 min-1 or less) followed by constant area relaxation so that the initially lobulated boundary (10) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936-939. (11) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590-4592. (12) Weidemann, G.; Vollhardt, D. Thin Solid Films 1995, 264, 94103. (13) Weidemann, G.; Vollhardt, D. Biophys. J. 1996, 70, 2758-2766.

10880

Langmuir, Vol. 19, No. 26, 2003

Albalat et al.

Figure 1. Surface pressure versus molecular area isotherms of 1414RAc at T ) 7, 10, 15, 25, 30, and 35 °C. Compression rate: 0.02 nm2 molecule-1 min-1. Figure 3. Monolayer of 1414RAc in the LC-LE coexistence region at T ) 18 °C and π ) 8mN/m after 3 h of isothermal relaxation at a constant area. BAM analyzer is turned 60° counterclockwise to maximize the contrast in the inner texture.

Figure 2. Monolayer of 1414RAc in the LC-LE coexistence region at T ) 18 °C and π ) 8mN/m after compression at 0.02 nm2 molecule-1 min-1. BAM observation with the analyzer at 0°.

smoothens and round droplets are obtained. Up to 5 h are allowed for this relaxation, more than in natural phospholipids such as DMPE (where relaxation is completed typically in 1 h),12 suggesting a much lower LE-LC line tension in 1414RAc monolayers. After relaxation, BAM images reveal that LC domains are subdivided in circular sectors differing in their reflectivity, which is uniform inside each sector (Figure 3). This is commonly attributed to a different azimuthal orientation of the aliphatic chains inside each sector (provided the molecular tilt is uniform inside each droplet, as it is usually proven to be the case).14 The darkest sector (lowest reflectivity) is always found on the left half of the droplet. Considering the configuration of our BAM experimental setup,15 we deduce that the hydrophobic chains are roughly oriented along the bisector and directed toward the expanded LE phase (splay configuration).17 Finally, it is worth noting that the existence of a star defect structure is a clear signature of hexatic order in the 1414RAc condensed droplets, commonly found in lipidic Langmuir mononolayers.17 This is a clear example of how BAM image analysis can be used to qualitatively assess the molecular ordering. The contact point between sectors of different reflectivities may be either inside the droplet (in this case six sectors are (14) Mo¨hwald, H.; Kenn, R. M.; Degengardt, D.; Kjaer, K.; Als-Nielsen, J. Physica A 1990, 168, 127-139. (15) Igne´s-Mullol, J.; Claret, J.; Sague´s, F. J. Phys. Chem. B 2003, in press. (16) Fischer, T. M.; Bruinsma, R. F.; Knobler, C. M. Phys. Rev. E 1994, 50, 413-428. (17) Kaganer, V. M.; Mohwald, H.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779-819.

Figure 4. Blow-up of the 10 °C isotherm of 1414RAc (solid line) showing the compressibility change at about 43.0 Å2 molecule-1 (+). Compression rate: 0.02 nm2 molecule-1 min-1.

observed) or at its boundary (in this case four or five sectors exist). Moreover, droplets with the defect on the boundary adopt a cardioid-like shape rather than circular. This result is relevant in the study of condensed droplet shapes in Langmuir monolayers in terms of the interplay between the boundary conditions and the configuration of the azimuth field inside the droplet.18 Compression past the LC-LE coexistence region leads to the coalescence of the LC droplets in the surface pressure range 30-35 mN m-1 and to loss of contrast in the BAM images. This pressure range coincides with a decrease in the compressibility of the monolayer, κ (defined in the usual manner as κ ) (-1/A)[∂A/∂π)T] evidenced in the π-A isotherm by a sharp change in the slope (see Figure 4). These observations suggest a second-order phase transition into an untilted LC phase, similar to what is observed in natural phospholipids such as DMPE.12 At higher pressures (π ≈ 50 mN m-1), the collapse of the monolayer leading to three-dimensional structures on the water surface is indicated by a new plateau in the π-A isotherm, only partially shown in Figure 1. The temperature dependence of the surface pressure corresponding to the onset of the tilted LC phase allows us to calculate the temperature below which the LE phase does not exist, that is, the temperature of the LC-LE-G triple point of the monolayer, Tp. The linear extrapolation of these π values down to π ) 0, depicted in Figure 5, yields a value for Tp close to 0 °C. 3.1.2. Effect of the Headgroup: 1,2-Di-O-myristoyl-racglicero-3-O-(L-arginine) (1414R). The general monolayer (18) Loh, K. K.; Rudnick, J. Phys. Rev. E 2000, 62, 2416-2427.

Diacyl Glycerol Amino Acid Surfactant Monolayers

Figure 5. Temperature dependence of the surface pressure corresponding to the onset of the tilted LC phase for 1414RAc and 1414R. Compression rate: 0.02 nm2 molecule-1 min-1.

Figure 6. Effect of the headgroup on surface pressure versus molecular area isotherms. Compression rate: 0.02 nm2 molecule-1 min-1. T ) 15 °C.

behavior of 1414R is similar to that just reported for 1414RAc, featuring the two phase transitions mentioned in the previous section, although with a shift of the onset surface pressure toward lower values for both processes (Figure 6). The temperature of the triple point of the monolayer, Tp, is close to 7 °C, which is significantly higher than the one obtained for 1414RAc (see Figure 5). The effect of the charge and size of the headgroup in lipid monolayers has been addressed in the literature. Generally, larger headgroups lower the interaction between hydrophobic chains, resulting in a shift to higher surface pressures of the different phase transitions. Examples of such studies include the methylation of the amino group of diacylphosphoethanolamines13,19 and the addition of ethyleneoxide groups of different lengths between the phosphocholine head and the glycerol backbone in diacylphosphatidylcholines.20 Additionally, Brezesinski et al.21 reported that the methylation of triplechain phosphoethanolamines decreases their critical temperature and the temperature below which the LE phase is not found (triple point). These authors report as well an increase of the tilt angle in the zero surface pressure limit, and as a consequence, larger surface pressures are needed to obtain the untilted LC phase, in agreement with the general behavior just reported. (19) Sandez, M. I.; Suarez, A.; Gil, A. J. Colloid Interface Sci. 2002,250, 128-133. (20) Baltes, H.; Schwendler, M.; Helm, C. A.; Mohwald, H. J. Colloid Interface Sci. 1996, 178, 135-143. (21) Brezesinski, G.; Bringezu, F.; Weidemann, G.; Howes, P.; Kjaer, K.; Mohwald, H. Thin Solid Films 1998, 327-329, 256-261.

Langmuir, Vol. 19, No. 26, 2003 10881

On the other hand, the effect of the headgroup charge has been addressed in the literature to explain the differences between π-A isotherms of DMPE and DMPC using a theoretical model based on the different P--N+ dipole interactions for different headgroup sizes.22 In other experiments, the effect of the electric charge of the headgroup has been studied for fatty acid Langmuir monolayers. In this case, the ionization of the amphiphilic molecule at high pH values leads to more expanded phases and to a shift of the isotherm to higher surface pressures.23 It is clearly observed in Figure 6 that the acetylation of the NR group of the arginine in 1414R (presumably implying a larger headgroup size but at the same time the loss of one electric charge in 1414RAc) shifts the isotherm to higher surface pressure values. Both this behavior and the decrease of the Tp value (Figure 5) are in good agreement with the aforementioned effect of the headgroup size. However, these results do not agree with the reported effect concerning the role of the charge of the headgroup in fatty acid monolayers.23 It is clear that, for the family of compounds investigated in this work, differences in the size of the headgroup are dominant over charge differences. Fatty acids have a much smaller headgroup size and, consequently, a much larger charge density as compared with that of our compounds, and this may be the source of the discrepancy. It is noteworthy that our findings agree with models in which the size of the headgroup of the amphiphile is considered the main parameter affecting the properties of the Langmuir monolayer.24 The morphology, size, and texture of the LC domains in the 1414R monolayer, as rendered by BAM imaging, are very similar to those of 1414RAc reported previously. We have also compared the monolayer behavior of 1414R and 1414RAc with the natural phospholipids DMPE and DMPC, whose π-A isotherms are also included in Figure 6. For DMPC and DMPE, there is no effect of the charge of the headgroup because at the pH of the water subphase (≈5.5) they are zwitterionic. The isotherms of 1414R and 1414RAc are found between those of the natural phospholipids, suggesting that the acetylation of the amino group (1414R f 1414RAc) modifies the thermodynamic properties of the monolayer in a lesser extent than the full methylation does (DMPE f DMPC).13,19 The limiting area (defined as the area where the transition to an untilted LC occurs) is 41-42 Å2 molecule-1 for DMPE and 42-44 Å2 molecule-1 for 1414R and 1414RAc. This gives an area per chain of 21-22 Å2 molecule-1 for our compounds, slightly larger than what one would expect for aliphatic tails without rotational symmetry, as should correspond for phospholipids and phospholipid analogues. Although an accuracy of better than 95% in the determination of area values is hard to achieve using π-A isotherms, X-ray studies performed in DMPE monolayers confirm higher values than expected,25 which is attributed to disorder frozen in the monolayer due to a mismatch between the headgroup ordering and tail ordering. 3.1.3. Effect of the Chain Length: 1,2-Di-O-lauroyl-racglicero-3-O-(NR-acetyl-L-arginine) (1212RAc) and 1,2-DiO-lauroyl-rac-glicero-3-O-(L-arginine) (1212R). Monolayers of 1212R and 1212RAc, the analogues of the two compounds reported previously with two methylene groups less per chain, have been studied to address the effect of the chain length in this family of compounds. It (22) Bowen, P. J.; Lewis, T. J. Thin Solid Films 1983, 99, 157-163. (23) Johann, R.; Vollhardt, D. Mater. Sci. Eng., C 1999, 8-9, 35-42. (24) Schmid, F.; Lange, H. J. Chem. Phys. 1997, 106, 3757-3760. (25) Kenn, R. M.; Kjaer, K.; Mohwald, H. Colloids Surf., A 1996, 117, 171-181.

10882

Langmuir, Vol. 19, No. 26, 2003

Figure 7. Effect of the length of the alkyl chain on the surface pressure versus molecular area isotherms. Compression rate: 0.02 nm2 molecule-1 min-1. T ) 15 °C.

Figure 8. Effect of the headgroup on the surface pressure versus molecular area isotherms. T ) 10 °C. Compression rate: 0.02 nm2 molecule-1 min-1.

is important to point out that the higher water solubility of the short chain length analogues makes it difficult to study the monolayer at temperatures above 20 °C and introduces a higher experimental uncertainty in their π-A isotherms than those obtained at lower temperatures. As expected, the shorter the alkyl chain the larger the tendency to form expanded phases and the higher the π values required for the LE-LC phase transition (Figure 7). These results are attributed to the smaller hydrophobic interaction between aliphatic chains as their length decreases, following the usual behavior of lipid Langmuir monolayers.17 Figure 8 depicts the π-A isotherms at 10 °C of 1212R and 1212RAc. Formation of condensed phases only occurs at a high compression, although they are obtained at lower pressures for the nonacetylated arginine derivative (1212R) than for the acetylated one (1212RAc). This behavior is consistent with the effect of the headgroup size reported previously. In Figure 8, the π-A isotherm of the phospholipid DLPC is included for comparison with those of 1212R and 1212RAc, showing that these surfactants are able to lower the interfacial tension more efficiently than DLPC in the low areal density region of the π-A isotherm (A g 70 Å2 molecule-1). In all cases, the headgroups have a lower relative impact on the shape of the isotherms than for the dimirystoyl derivatives (compare with Figure 6). 3.2. Effect of the Position of the Polar Group of the Glycero Amino Acid-Based Surfactants: 1,2-DiO-lauroyl-rac-glicero-3-O-(NR-acetyl-L-glutamine) (1212QAc) and 1,3-Di-O-lauroyl-glicero-2-O-(NRacetyl-L-glutamine) (12QAc12). To elucidate the effect of the position of the polar group in the glycerol backbone,

Albalat et al.

Figure 9. Surface pressure versus molecular area isotherms of 12QAc12 at T ) 5, 10, 15, and 25 °C. Compression rate: 0.02 nm2 molecule-1 min-1.

Figure 10. Effect of the position of the headgroup on the surface pressure versus molecular area isotherms. T ) 5 °C. Compression rate: 0.03 nm2 molecule-1 min-1.

two compounds were synthesized, differing in their backbone substitution pattern, 1212QAc and 12QAc12 (see Chart 2). To eliminate the effect of the charge in the polar group, nonionic derivatives were obtained using the amino acid glutamine instead of arginine. Figure 9 shows a set of π-A isotherms of 12QAc12 at different temperatures. We can observe remarkable differences when compared with the typical behavior of 1,2 compounds. At our usual compression rates, isotherms exhibit a maximum, indicating that the monolayer does not attain its equilibrium state during compression. BAM observation confirms that this maximum occurs at areas per molecule where a condensed phase nucleates (see the following). A second compression of the monolayer following an expansion to molecular areas in excess of 80 Å2 molecule-1 results in the suppression of the maximum (Figure 10). BAM images show that more droplets are present in the second compression step than in the first one. Nevertheless, if the expanded monolayer is allowed to relax long enough before the second compression sweep, the maximum appears again in the π-A isotherm. In all cases, lowering the compression rate results in a decrease of the height of this maximum. All these observations suggest the existence of a kinetically limiting step in the nucleation and growth of the condensed phase. Isotherms of 1212QAc and 12QAc12 are compared in Figure 10 (1212QAc is also compared with the other 12,12 analogues studied here in Figure 8). The surface pressure of the 12QAc12 monolayers is sensibly higher in the high surface area limit. Even more remarkable is the fact that the transition into a condensed phase occurs in 12QAc12

Diacyl Glycerol Amino Acid Surfactant Monolayers

Figure 11. Condensed nucleus of 12QAc12 grown at a compression rate of 0.03 mm2 molecule-1 min-1, at T ) 15 °C and π ) 22 mN/m. BAM observation with the analyzer at 0°.

at areas per molecule much higher than for the 1,2 analogue and at much lower surface pressures. Although some kinetic hindrance in the condensation of 12QAc12 monolayers exists, this process becomes eventually more favorable than that for 1212QAc. The difference between the monolayer behaviors of the two analogues is more dramatic when analyzed in terms of the BAM images. Unlike 12QAc12, we have not observed condensed domains in 1212QAc monolayers (actually, this is the expected result for 12,12 analogues, because condensation takes place at high surface pressure values resulting in a low reflectivity contrast).26 It is worth highlighting the following features of 12QAc12 LC droplets (Figure 11): (i) the absence of an inner texture suggesting the lack of a long-range orientational order of the alkyl chains; (ii) a compact but very rough LE-LC boundary; (iii) large domains with surface areas that may exceed 1 mm2; and (iv) a growing rate larger than that found for glycerolipids with the 1,2-substitution pattern. The absence of an inner texture indicates that neither the hexatic structure nor the long-range orientational order commonly found in condensed lipid monolayers17 are present in Langmuir monolayers of 12QAc12. This increased disorder is a consequence of the different conformation or orientation of the two alkyl chains in a single molecule and can be understood as a propagation of the disorder from the molecular level to the mesoscopic one (see the following). The LC domains of 12QAc12 can be viewed as a high-density pseudo-frozen expanded phase. The small number of nuclei is clear evidence of the kinetic hindrance, probably of a steric nature, due to the interaction between 12QAc12 molecules. Moreover, the high growing rate of these domains is also consistent with the existence of a kinetic limiting step at the early stages of the nucleation process. When the steric hindrance to form the initial nuclei is overcome, the additional pressure is invested in a faster growth of the LC domains. Finally, it is worth noticing that the morphology of these droplets, and in particular the roughness of their boundaries, is reminiscent of that obtained by computer simulations according to the Eden model27 for cluster growth. This is a classical model in the context of interfacial pattern formation phenomena, and it is reported to be most (26) Tamada, K.; Kim, S.; Yu, H. Langmuir 1993, 9, 1545-1550. (27) Vicsek, T. Fractal Growth Phenomena; World Scientific: Singapore, 1992.

Langmuir, Vol. 19, No. 26, 2003 10883

suitable to reproduce the morphology of domains whose growth is controlled by the kinetics of a reaction taking place at the domain boundary.28 Although further analysis is required to quantitatively test this analogy, the morphology of the LC droplets supports our arguments for a kinetic-limited condensation. The different monolayer behaviors of 1212QAc and 12QAc12 described previously should be attributed to the different substitution patterns of the glycerol backbone of the amphiphile, which will lead to a different hydrophobic interaction between the alkyl chains inside each molecule. For the 1212QAc substitution pattern, we can consider that the interaction and coupling between chains is strong enough to force both alkyl chains to have the same orientation. Conversely, the one carbon atom separation between the alkyl chains in the 12QAc12 molecule may hinder their coupling, and both the orientation and the conformation of each alkyl chain in a single molecule can be different, thus rendering the packing of alkyl chains more difficult than in the case of 1212QAc, where both chains have the same orientation.29 In other words, the nucleation leading to the LC phase in the 12QAc12 monolayers requires overcoming additional steric effects and possibly unfavorable orientations or conformations of the alkyl chains. This conclusion is supported by the absence (or decrease) of the maximum (Figure 10) and the increase in the number of nuclei in a second compression of the monolayer. It is likely that some nuclei still remain in the monolayer after full expansion, and the extra surface pressure to create them is no longer needed. Long enough relaxation in the expanded phase, however, effectively eliminates all nuclei, and the initial situation is recovered. The higher tendency of 12QAc12 to form LC domains can be qualitatively understood using some thermodynamic considerations. We have to analyze first the initial and final states of the LE-LC phase transition. We assume that, given the large intermolecular distance, LE phases of 1212QAc and 12QAc12 are not significantly different. For the LC phases, a more disordered bidimensional system is expected for 12QAc12 than for 1212QAc, and this implies that the entropy decrease in the phase transition of 12QAc12 should be smaller than for 1212QAc. Considering that the energetic contributions to form the condensed phase (chain-chain interactions) are roughly similar in both cases, the different entropic contributions make the transition into a condensed phase more favorable for 12QAc12 than for 1212QAc at a given temperature and surface pressure. 4. Conclusion We have shown that a novel family of 1,2-diacyl glycerol amino acid-based surfactants exhibits a monolayer behavior similar to that found in natural phospholipids, suggesting that they may be viable substitutes for these compounds in industrial applications, which need multifunctional compounds. The use of BAM image analysis of the inner textures has revealed that condensed phases of the dimyristoyl glycerol compounds exhibit hexatic order. Variations in the chain length introduce similar changes as those commonly found in lipid monolayers. The effect of a different polar head is dominated by their different size, rather than by their different charge. Generally, compounds with a 1,2-substitution pattern in the glycerol backbone exhibit a tendency to form continu(28) Smith, R. L.; Collins, S. D. Phys. Rev. A 1989, 39, 5409-5413. (29) Small, D. M. Handbook of Lipid Research; Plenum Press: New York, 1986; Vol. 4.

10884

Langmuir, Vol. 19, No. 26, 2003

ous monolayers (expanded phases) at significantly lower molecular densities than their natural phospholipid counterparts. Our investigations with the compound bearing a 1,3substitution pattern (12QAc12) shows that this feature plays a crucial role in the characteristics of Langmuir monolayers of this family of surfactants. They display the formation of rough and compact LC domains, very different from those observed in phospholipid monolayers (longrange orientational order is absent) but reminiscent of those obtained by computer simulations according to a kinetically limited model of cluster growth. We attribute this abnormal growth to a kinetic barrier originated in the steric interaction between alkyl chains of different

Albalat et al.

molecules, which are likely to be misaligned as a result of a lower hydrophobic intra-alkyl interaction when compared to phospholipid analogues. The lack of longrange orientational order in these LC phases favors their existence at low molecular densities. Acknowledgment. This work was supported in part by SEID, project BXX2000-0638-C02-01, DURSI, project 2001SGR00045, and by C.I.C.Y.T., project PPQ2000-1687C02-01. J. I-M. acknowledges financial support from Generalitat de Catalunya, RED-2000 program. LA035281D