Langmuir 1994,10, 2317-2324
2317
Destabilization of Monoglyceride Monolayers at the Air-Aqueous Subphase Interface. 1. Kinetics Julia de la Fuente Feria and Juan M. Rodriguez Patino* Departamento de Ingenieria Quimica, Facultad de Quimica, Universidad de Sevilla, c l Prof. Garcia Gonzcilez, s l n . 41012 Sevilla, Spain Received November 1, 1993. In Final Form: March 9, 1994@ In this work the effects of the interactions between monoglycerides (monostearin,monopalmitin, and monoolein) and solutes in subphase (ethanol, glucose, and sucrose)on the monolayer stability have been studied. The destabilization of monoglyceride monolayers has been followed kinetically by observing changes in the film area with time at constant surface pressure, or by measuring the surface pressure decrease as a function of time when the area is kept constant, using a Langmuir-type film balance. Sugars or ethanol in subphase increase the instability of monolayers. Temperature and surface pressure exert a significant influence on the loss of film molecules. Existing relaxation mechanisms have been tested by comparing the experimentaldata with the predicted relationshipsbetween the loss of monolayer molecules and time. There is competition between the collapse and desorption mechanisms for monolayer destabilization as a function ofthe monoglyceride, subphase composition,temperature, and surface pressure.
Introduction Nonequilibrium processes occur in many systems containing surfactants a t the liquidgas or liquid/liquid interfaces. These processes are important to the understanding of technological procedures, such as emulsification, foaming, detergency,flotation, extraction, etc. Spread monolayers at the air-water interface can show relaxation phenomena mainly because of instability due to desorption or collapse. The loss of molecules by desorption or collapse has great importance from a practical point of view in emulsifier films which stabilize food emulsions and foams. Monoglycerides are a kind of polar lipid of biological relevance and they also rank among the most commonly used emulsifiers in the food industry.l Previous works using the film balance from this laboratory have shown that when fatty acids2or monoglycerides are spread on ethanol3and glycerol4an anomalous behavior of isotherms dependent on temperature appears. That behavior, characterized by contraction of the areas with an increase in temperature, could give rise to an irreversible displacement of the isotherms by continuous compression-expansion cycles at a constant temperature. These phenomena were attributed to a loss of film molecule^.^^^ However, from the experiments under these conditions carried out with a film balance, it is impossible to ascertain the true degree of molecular loss reached. Quantification of the displacement toward the x-axiswhen the temperature increases cannot be immediate, since this takes place with film expansion. The purpose of this paper is to analyze the degree and the mechanism of monoglyceride molecular loss from monolayers spread on aqueous solutions containing ethanol and sugars. By including these solutes in the subphase, we aim to obtain information about the influence of film-subphase interactions on the relaxation phenomena. On the other hand, with these solutes in subphase *To whom correspondence concerning this work should be addressed. Abstract published in Advance ACS Abstracts, J u n e 1,1994. (1)Charalambous, G.; Doxatakis, G. Food Emulsifiers: Chemistry, Technology, Functional Properties and Applications; Elsevier: Amsterdam, 1989. (2)Rodriguez Patino, J. M.; de la Fuente Feria, J.;G6mez Herrera, C. J . Colloid Interface Sci. 1992,148,223. (3)Rodriguez Patino, J. M.; Ruiz Dominguez,M.; dela Fuente Feria, J. J. Colloid Interface Sci. 1992.154.146. (4)Rodriguez Patino, J. M.; Ruiz Dominguez, M. Colloids Surf. A @
1993,75,217.
the behavior of model and real food systems could be approached.
Relaxation Mechanisms Two kinds of experiments have been done traditionally for the analysis of relaxation mechanism^.^ First, the area (A) is kept constant and the surface pressure (x) decreases. This decrease is measured as a function of time (e). In the second type of experiment, as utilized in the great majority of these works, the surface pressure is kept constant by further compression of the monolayer, measuring the area A as a function of time. Obviously, other experiments are p ~ s s i b l e . ~Several -~ relaxation mechanisms can be fitted to the results derived from these experiments, as is shown in Table 1. The first desorption experiments at constant surface pressure of slightly soluble monolayers were performed by Ter Minassian-Saraga.15 The exchange of matter for diffusion-controlled adsorption or desorption can be (5)Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interface; Wiley: New York, 1966. (6)de Keyser, P.; Joos, P. J. Colloid Interface Sci. 1983,91,131. (7)Motomura, R.;Shibata,A.; Nakamura, M.; Matuura, R. J . Colloid Interfie Sci. 1969,29,623. (8)Miller, R.;Loglio, G.; Tesei, U.; Schano,IC-H.Adu.Colloid Interface Sci. 1991,37,73. (9)Miller, R.;Kretzschmar, G. Adv. Colloid Interface Sci. 1991,37, 97. (lo)Smith, R.D.; Berg, J. C. J. Colloid Interface Sci. 1980,74,273. (11)Tomoaia-Cotisel, M.;Zsako, J.; Chifu, E.; Cadenhead, D. A. Langmuir 1990, 6, 191. (12)Pezron, E.; Claesson, P. M.; Berg, J. M.; Vollhardt, D. J. Colloid Interface Sci. 1990,138,245. (13)Retter, U.;Vollhardt, D. Langmuir 1992,8, 1963. (14)Vollhardt, D.; Retter, U. Langmuir 1992,8, 309. (15)Ter Minassian-Saraga, L. J. Colloid Sci. 1966,11, 398. (16)Patlack, C. S.;Gershfeld, N. L. J.Colloid Interface Sci. 1967, 25,503. (17)Baret, J. F.; Bois, A. G.; Casalta, L.; Dupin, J. J.; Firpo, J. L.; Gonella, J.; Melion, J. P. J. Colloid Interface Sci. 1976,53,50. (18)Gershfeld, N. L. Annu. Rev. Phys. Chem. 1976,27,349. (19)Gershfeld, N. L. J. Colloid Interface Sci. 1982,85,28. (20)Chaiko, D. J.; Osseo-hare, K. J. Colloid Interface Sci. 1988, 121,13. (21)Brooks, J. H.;Alexander, A. E. In Retardation ofEuaporation by Monolayers; La Mer, V. IC,Ed.; Academic Press: New York, 1962. (22)Gardner, J. W.; Addison, J. V.; Schechter, R. S.MChE J . 1978, 24,400. (23)Smith, T.J. Colloid Interface Sci. 1967,25,443. (24)Gabrielli, G.; Puggelli, M.; Ferroni, E.; Carubia, G.; Pedochi, L. Colloid Surf. 1989,41,1. (25)Ivanova, T.Z.;Panaiotov, 1.; Georgiev, G.; Launois-Supras, M. A.; Proust, J. E.; Puisieux, F. Colloids Surf. 1991,60, 263. (26)Bois, A. G.; Ivanova, M. G.; Panaiotov, I. I. Langmuir 1987,3, 215.
0 1994 American Chemical Society
de la Fuente Feria and Rodriguez Patino
2318 Langmuir, Vol. 10, No. 7, 1994
Table 1. Relaxation Mechanisms in Spread Monolayersa mechanisms desorption dissolution diffusion collapse formation of critical nuclei nuclei growth evaporation surface rheology surface chemical reaction polar group hydration Marangoni effect change in conformatiodorganization a
equations -log -log -log -log -log -log -log
pressure
(NI”)= aem + ble (1) (NIN~) = aem (2) (NIN~) = ble (3)
Z ne
+
(NINo)= bz0 cBz (4) (NIN~) = bze (5) (NINo) = c02 ( 6 ) (NI”)= bse (7)
II
’ne
refs 6,10 12,15-20
10-14 10,12,21 18;22 16,23 24 25,26 25
See text for discussion and explanation of the symbols.
described on the basis of Fick’s diffusion laws. She proposed that desorption of spread monolayer at any constant surface pressure involves two processes: (A) One process is dissolution into the bulk aqueous phase to a saturated aqueous layer of molecular dimensions located immediately beneath the spread monolayer (the subsurface). During the initial non-steady-state period of desorption, the rate of film molecular loss can be expressed by eq 2 in Table 1 (penetration theory). (B)Another process is molecular diffusion. M e r a time the concentration gradient within the diffusion layer becomes constant and desorption reaches a steady state. The rate of film molecular loss is then given by eq 3 in Table 1 (film theory). In eqs 2 and 3, a and bl are coefficients depending on the surface pressure and on the temperature. Diffusion was found to be the rate-limiting step for film desorption in the studies of Ter Minas~ian-Saraga.’~ Patlak and Gershfeld16 utilized Ter Minassian-Saraga’s model but imposed the condition that the dissolution process was also rate-limiting and developed analytical methods for determining the activation energy of dissol~tion.’~ Baret et al.17subsequently demonstrated that the Patlak-Gershfeld model for film desorption was better able to describe the time course of desorption than any of the other proposed models, which relied solely on diffusion as the rate-limiting step for film desorption. However, controversy still exists in the literature. In the opinion of de Keyser and Joos6 the treatment of Baret et al. is erroneousand leads to the false conclusion that desorption is not controlled by diffusion. Recently, a desorptiondiffusion-controlled model was adopted by Chaiko and Osseo-Asare.20This mechanism has been used to quantify the protein desorption from the film.27Data from previous papers have been compared with the theory presented by Joos and Van Uffelen.28 At a surface pressure greater than the equilibrium spreading pressure (ne),with insoluble surfactants, the relaxation phenomena are due to the transformation of a homogeneous monolayer phase into a heterogeneous monolayer-collapsephase system. It has been shown that monolayer collapse may occur in two different ways: 10~12,29 either by macroscopic film “fracture”characteristic of films compressed to a sufficient extent at a sufficient rate or by a process of nucleation and growth of bulk surfactant fragments, occurring whenever a characteristic critical surface pressure, ni,is exceeded. The fracture is the most obvious film instability. It occurs at a high surface pressure, the fracture pressure (nf),and leads to folding and bending.30 Unfortunately, these fracture pressures are highly dependent on the rate of compressiong1and ~
~~
(27)MacRitchie, F.Adv. Colloid Interface Sci. 1986,25,341. (28)Joos,P.;Van Uffelen, M. J . ColloidZnterface Sci. 1993,155,271. (29)Ternes, R.L.;Berg, J. C. J . Colloid Interface Sci. 1984,98,471. (30)fies, H.E.;Swift, H.Langmuir 1987,3,353.
often not reproducible at a given rate of compression. For films of solid surfactants, nf> q > n,,whereas for those of liquid surfactants Jtf x ni x ncx ne,where x, is the collapse p r e s ~ u r e . ’However, ~ ~ ~ ~ in many cases monolayers are unstable even at pressures well below nf.12The collapse rate should follow the relationships (4) to (6)in Table 1, where bz and c are coefficients depending on the surface pressure and on the number of surfactant molecules in the nuclei. The coefficient bz accounts for the formation of nuclei and c for the growth rate of nuclei.1° Other approaches consider the nature ofthe collapse phase and whether or not it involves the buildup of multilayer^^^ or the formation ofliquid lenses. They attempt to produce a correlation between their geometry and ne.33Recently, the idea of modeling monolayer collapse by homogeneous nucleation and growth of bulk surfactant nuclei has also been analyzed using the Prout and Tompkins’ law34 including the effects ofbranchingchainsll and overlapping of the chains.13J4 Evaporationis another important cause of film molecule loss for saturated fatty alcohols,l0Pz1but opinions concern~~”~ ing the magnitude of this effect with fatty a ~ i d s and its estersg7differ from one work to another. Bikaldi and N e ~ m a argued n ~ ~ that at atmospheric humidities below the saturation point an additional desorption mechanism involving complex hydrodynamic convection in the surface region (Marangonieffect) due to evaporation could be also possible. However, no strong evidence has yet been available for quantitative estimation of the evaporated amount. As the same relationship fits the rate of film moelcular loss by evaporation and diffusion in the treatment hereafter, it is convenient to consider evaporation to be included in diffusion. Other relaxation mechanisms,such as surface rheology, surface chemical reaction, polar group hydration, the simultaneous motion of the monolayer and the liquid substrate as a result of the surface pressure gradient (Marangoni effect), or the relaxation process in the monolayer itself-such as change in the conformation of the molecules, or an organization of two-dimensional clusters, etc.-are difficult t o quantify. Some of these relaxation mechanisms could be caused by continuous compression or expansion experiments, such as the methods used for measurement of the n-A isotherms.38 (31)Rabinovitch, W.; Robertson, F. R.; Mason, S. G. Can. J . Chem. 1960,38,1881. (32)Shuquian, X.; Miyano, K.; Abraham, B. M. J . Colloid Interface Sci. 1982,89,581. (33)De Keyser, P.;Joos, P. J . Phys. Chem. 1984,88, 274. (34)Prout, E.G.; Tompkins,F.C . Trans. Faraday Soc. 1944,40,488. (35)Good, P.A.; Schechter, R. S. J.Colloid Interface Sei. 1972,40, 99. (36)Bikaldi, 2.;Newman, R. D. J . Colloid Interfuce Sci. 1981,82, 480. (37)Muramatsu,M.; Ohno, T. J . ColZoidInterfaceSci. 1971,35,469. (38)Rolan, C.M.;Zuckermann, M. J.;Georgallas,A. J.Chem. Phys. 1987,86,5852.
Destabilization of Monoglyceride Monolayers Unfortunately, despite the amount of theoretical and experimental work performed to date, the question as to whether the different mechanisms associated with molecular loss control monolayer destabilization cannot be answered nor can we quantify the molecular loss. So the answers to these questions must be based to a great extent on experimental work. This is especially true in the cases when there are solutes in subphase which could potentially interact with the monolayer molecules. Existing relaxation mechanisms (Table 1) are tested in this work by comparing the experimental data with the predicted relationships between the loss of monolayer molecules and time.
Experimental Section
Langmuir, Vol. 10, No. 7, 1994 2319 NINo
........ *. ...............
.. . . .........* 'p.%.. m..
A I
. # . .#
+ .*- Monostearln -I Monopalmltln
.*Monoolein o'OO
20
40
80
80
100
120
140
Chemicals.
Synthetic ruc-1-monooctadecanoylglycerol (monostearin),ruc-1-monohexadecanoylglycerol(monopalmitin), and ruc-1-mono(cis-9-octadecanoy1)glycerol (monoolein)of purity greater than 99%were acquired from Sigma. The sugars (glucose and sucrose) and ethanol of analytical grade used as subphase components were obtained from Merck and used without further purification. The water used was purified by means of a Millipore filtration device. The absence of surface-active contaminants in aqueous subphase components and hexane-ethanol (mixture 9:l (v/v)) used as spreading solvent was checked by measuring the surface pressure in the entire area interval in the absence of amphiphilic substances. Surface Film Balance. The kinetics of film destabilization was studied in a commercial, fully automated Langmuir-type film balance (Lauda Filmwaage) elsewhere d e s ~ r i b e d . A ~~~ Plexiglas cover over the trough provided a seal against dust and foreign matter. The film balance was placed in an isolated roomair conditioned-free from impurities in the air and free from temperature changes and mechanical vibrations. The most satisfactory vibration-free conditionswere obtained using a table built with three-layers of sand between layers of expandedpolystyrene. The sensitivity of the film pressure is 0.1 mN/m, and of 0.005 nmVmolecule for the molecular area. The experiments were carried out a t constant temperature maintained within &O.l "C by water circulating through a electronic thermostat (Lauda k2R) on the bottom of the trough. Procedure. Plots showing the time dependence ofthe number of molecules in the monolayer at a constant surface pressure were obtained for several pure surfactant films-each over arange of surface pressures and temperatures. The ethanol concentrations in the subphase were 0.1, 0.5, 1.0, and 5.0 mol&, and the sugar (glucose or sucrose) concentration was 0.5 mol&. In the constant surface pressure runs, the surfactant was deposited with a hexane-ethanol mixture as spreading solvent following a 15-minwait to allow for solvent evaporation. During this time the movable barrier was placed a t the maximum area, and the surface pressure was practically zero. The film was then compressed t o the desired surface pressure level where it was maintained automatically. This precompression usually required from 30 s to 5 min, depending on the surfactant and the surface pressure level sought. However, long time periods are required in some of the measurements. From the plots the change of surface area with time for each surface pressure was obtained. The values of the apparent area were transformed to a molecule film number. It is assumed that (i)when B = 0, the surfactant molecules on the film are the ones previously spread and (ii)the molecular area is constant. So the decrease in area with time will be due either t o the loss of molecules from the film or to the formation of three-dimensional structures by monolayer collapse. With these considerations NAo = N/NQ,where N and the subindex 0 are the number of molecules on the monolayer and the initial moment, respectively. Experiments a t constant area were also carried out following a compression of the monolayer as used to measure the n-A isotherm. The spreading of the monolayer and the waiting time were as in the constant surface runs. The film was compressed at a rate of 3.3 cm-min-1 up t o the collapse pressure. In these experiments the collapse pressure is the highest surface pressure that corresponds to the slope change ofthe compressionisotherm a t the lowest molecular area. At this point the movable barrier was stopped and the change of surface pressure with time was obtained.
N/No
I 0,85
x 35
, A 0
20
O C
35mN/m
30,OC 35p1N/m
40
80
0
25 OC 35mNIm
v 35 "C 12mS(lm 80
100
120
, 140
8 (" Figure 1. Destabilization of monoglyceride monolayers on water: (A) destabilization at 20 "C and 35 mN*m-l; (B) destabilization of monopalmitin monolayers. The broken and solid lines represent the data calculated according to a desorption mechanism by dissolution (eq 2) and diffusion (eq 31, respectively. The equilibrium spreading pressure (ne)is a key parameter for the description of the mechanisms that give use to the destabilization process in surfactants that are spread on the gasliquid interface, such as desorption or collapse.5 The procedure for determining neand results of monoglyceridesin function of temperature and subphase composition have been described before.39
Results and Discussion Destabilizationof Monoglyceride Monolayers on Water. The results obtained with monostearin, monopalmitin, and monoolein monolayers spread on water are shown on Figure 1. These results can be used as a reference for the other systems with ethanol and sugars in subphase. At 20 "C the monostearin and monoolein monolayers on water are stable. The initial molecular loss of about 1-2% that is noticeable in some cases, such as monostearin monolayers on water (Table 2), is not significant and could be attributed to structural rearrangements of molecules in the film.1° For the same experimentalconditionsthe monopalmitin monolayers are unstable. The loss of molecules from the monolayer could be quantified by a desorption mechanism. This loss takes place in two steps corresponding to dissolution and diffusion into the subphase (eqs 1 to 3). The plots of experimental points in Figure 1verify the kinetic model. In Table 2 the kinetic parameters, the characteristic time (e*) corresponding to mechanism change, and the value (39) Rodriguez Patino,J. M.; Martin Martinez,R. J.Colloidhterface Sci., in press.
de la Fuente Feria and Rodriguez Patino
2320 Langmuir, Vol. 10, No. 7, 1994
Table 2. Characteristic Parameters for Destabilization of Monoglyceride Monolayers on Water T ("C) x(dim) ne( d / m ) * MS' 1 0 3 ~(LR) 105b1(LR) NINo emin
monoglyceride
monostearin4
B*(min)
20 20 20 20 20 25 30 35 35 20
35 27.1 Sd 0.996 66.1 Sd 0.986 69.5 47 27.1 53 27.1 Sd 1.000 60.0 SICd 1.000 30.0 59 27.1 monopalmitin 35 31.0 S' 2.97 (0.991) 11.6(0.997) 0.961 200.0 12.0 35 31.3 S' 2.95 (0.996) 6.40(0.995) 0.952 150.7 14.0 35 31.8 S' 2.75 (0.993) 31.7 (0.995) 0.896 155.0 56.8 35 33.7 LCe 6.97(0.982) 3.02(0.980) 0.907 200.5 28.4 12 33.7 LE' 1.83(0.982) 0.953 145.6 monooleha 35 45.4 LE' 0.989 120.6 a Without dissolution. Data from ref 39.c Monolayer structure (MS): collapsed (C), solid (S), liquid-condensed(LC), liquid-expanded (LE). Data from ref 3. e Data from ref 4.
ofN/No at the maximum experimental time ( 0 )are shown. The characteristic time was obtained analytically as the cut point of the linear fit corresponding to each step of the desorption mechanism. The fit of the experimental data to the mechanism was made at a time interval based on the best regression linear coefficient that is included in Table 2. Although in the monopalmitin experiments the surface pressure is close to ne,the monolayer instability caused by a collapse mechanism can be rejected at these temperatures. The reason that a monolayer can be compressed to surface pressures above the ne is that an activation energy is required in order to transform the monolayer into bulk c r y ~ t a l s . ~ J ~ J ~ Moreover, the molecule loss from the monolayer is an irreversible process. When the experiment a t 35 mN/m was repeated (data not shown), after a wait time of 24 h at the maximum area, the molecules did not return to the interface. After the rest period the monolayer disclosed an unusual initial pressure ( x = 11.2 mN/m) at large molecular area before the recompression. This effect could be related to the hydration ofthe monopalmitin head group a t the interface. The initial pressure was observed recently0 with fatty acids spread on subphase containing alkaline hydroxides when the acids were fully ionized. But this pressure dissappeared with time or with continuous compression. However, the most important aspect of this work is that the kinetic model that quantified the molecule loss with time is similar in both cases-before and after the rest period. Nevertheless the loss of molecules increases after the rest period and the recompression. At 35 "C and 12 mN/m (n< ne)the molecule loss in monopalmitin monolayer can be quantified using eq 2. This behavior has not been observed in other experiments. The rate of molecular loss could be due to a desorption mechanism with dissolution into the subphase as the step that controls the process, at least during the experimental time. The influence of temperature and pressure on the rate of molecule loss can be studied from the experiments with monopalmitin monolayers. The monolayer instability increases when temperature and/or pressure increases (Table 2). These results are in agreement with those of other a u t h o r ~when ~ ~ ~ the~ ~ monolayer instability is described by a diffusion-controlled mechanism. Monostearin monolayer stability at 20 "C was observed at surface pressure higher than ne,such as 35,47,53, and 59 mN/m (Table 2). The pressure of 59 mN/m is close to the collapse pressure observed from compression experim e n t ~ .The ~ monolayer can remain in a metastable condition for considerable periods of time before collapsing. This is a typical behavior of surfactant monolayers that are solids.1° (40)Hasmonay, H.; Hochapfel, A.; Peretti, P. J. Colloid Interface Sci. 1992, 149, 247.
I 5 10 15 20 25 30 35
Or80
1
40
45
50
55
8 (min) Figure 2. Destabilization of monostearin and monopalmitin monolayers at the collapse point. Lines drawn fit the models to the data (for details see text).
A measurement has been also obtained with monostearin monolayer spread on water at the collapse area, A, = 0.23 nm2*molecule-l. Relaxation phenomena were followed a t 20 "C by recording the pressure at the collapse area as a function of time. The measurement was performed after the experiment a t constant pressure of 35 mN/m, without loss of molecules (Table 2). Apparently the results (Figure 2) contradict those obtained in earlier experiments a t constant pressure. This phenomenon has been studied with slightly soluble monolayer^.^^^^ Constant area relaxation studies are difficult to interpret due to the interference of other relaxation processes and the fact that the monolayer is continuously passing through the various monolayer states during the course of the e x ~ e r i m e n t . If ~ ~in experiments at constant surface pressure there is an equilibrium between the monolayer and the subphase, desorption is not possible. Figure 2 shows that monolayers-that appear stable in experiments at constant surface pressure-can be removed from the interface by a diffusion-controlled mechanism or by the appearance of a new collapse phase in an experiment at constant area. Processing of experimental data according to the Prout-Tompkins equation34showed that it was possible to obtain a linear fit (Figure 2) l0g[(nt.,- ~t.)/n] = 0.256 log 8 - 1.207
(LR = 0.997)
where n and no are the pressures at time 0 and at initial moment, respectively. The results indicate that the relaxation phenomena in these conditions could be due to nucleation and growth of critical nuclei. That is, the formation of the new bulk (41)Ter Minassian-Saraga, L. J. Chem. Phys. 1966, 52, 181. (42)Binks, B. P.Adu. Colloid Interface Sci. 1991, 34, 343.
Langmuir, Vol. 10, No. 7, 1994 2321
Destabilization of Monoglyceride Monolayers
at 5 mol& (Figure 3B and Table 3). In these experiments
NlNo
0,99
OIQ7
n > ne.The instability is shown by the monolayer spilling
1%. '
t '
0,96 0
20
40
60
80
I
100 120 140 160 180 200
e win) N/No
0,s;
'
20
40
80
60
I
100 120 140 160 180 200
8 (min) Figure 3. Destabilization of monostearin monolayers spread on ethanol aqueous solutions: (A) concentration 0.5 mom, (B) concentration 5 m o m JC = 10 mNm-l. Lines drawn fit the models to the data (for details see text).
collapse phase is predominantly responsible for removing monostearin from the monolayer. Destabilization of Monoglyceride Monolayers on Ethanol Aqueous Solutions. Overall, the monoglyceride monolayers spread on aqueous solutions containing ethanol have a greater molecular loss than when the same monoglycerides are spread on water at similar temperature and pressure. The loss of molecules and the kinetic mechanism responsible for it depend on the monoglyceride. Monostearin Monolayer. The stability of monostearin monolayers on aqueous ethanol solutions at 0.5 and 5 m o m is shown in Figure 3. The characteristic parameters for destabilization ofthe monolayers are included in Table 3. At 25 "C and at pressures of 10 and 25 mN/m, the monostearin monolayers on 0.5 mol& ofethanol in aqueous solution are stable. When the pressure is 44 mN/m (n > ne),at the same temperature, the monolayer is unstable and spilled over the edges. In this case it is not possible to investigate the kinetics of molecular loss. As the monolayer has a solid structure at these experimental conditions (25 "C and 44 mN/m) and n > ne,the instability may be due to collapse by film fracture. At 30 "C and the same concentration of ethanol in the subphase, the rate of molecular loss from the monolayer can be fit using a desorption-diffusion-controlled model, although at this temperature n > ne. The kinetic parameters for the film destabilization are included in Table 3. From Figure 3A and Table 3 it can be seen that when the pressure increases the characteristic time (e*) decreases, and the values of the kinetic parameters increase, especially the bl parameter. The monolayer instability increased significantly when the monostearin was spread on aqueous ethanol solutions
over the edges at the higher pressure (25 mN/m) or by a molecular loss. In the latter case the loss rate can be quantified by a mechanism with two steps where log (Nl No)vs 8 are two linear relationships (Figure 3B). The first step would correspond to a molecular loss by diffusion (eq 3). The desorption rate increases with temperature. The second step (after 8*) would fit a mechanism of monolayer collapse with formation of critical nuclei (eq 5). The growth of nuclei would not be a cause for monolayer destabilization, at least during the time of the experiment. From Figure 3B it can be seen that when 8 > 8*, the slope of the log(N/No)vs e relationship depends on the temperature: At 10 and 15 "C the slope increases with temperature. While desorption is a rate process which decreases with time until a steady state is reached, the rate of condensation accelerates with time due to the ever-increasing number of critical nuclei.10*20 So monolayer collapses could be the mechanism for the monolayer instability. At 20 and 25 "C the slope decreases when the temperature increases. From n-A isotherms, a reversal of the temperature effect is commonly ascribed43 to the different effects that temperature exerts on the hydrophobic chains (increasing their mobility) and on polar headgroups (hydration-dehydration processes). A full explanation of this behavior is not possible with the data available. The dependence of the rate constants of the first and second order for the collapse process is not simple. Collapse due to nucleation can have a significant effect on the n-A isotherms, as we have observed in previous workn3Monolayer expansions following compressions to surface pressures near fracture reveal noticeable hysteresis, which is greater if compressions are carried out at this higher surface pressure for a longer period of time before reexpansion is commenced. In agreement with Ternes and Berg,29a monolayer which has experienced some degree of collapse a t n > nemay thus be populated with small nuclei which continue to grow at the expense of the monolayer. In the experiments with monostearin spread on aqueous ethanol solutions at 5 mol& (Table 31, the loss of molecules could be an irreversible p r o ~ e s s . ~ Monopalmitin Monolayers. The stability of monopalmitin monolayers spread on ethanol aqueouos solutions at 0.5and 1 m o m is shown in Figure 4. The characteristic parameters for destabilization of the monolayer are included in Table 3. At 20 "C and 35 mN/m the monopalmitin monolayer on 0.5 m o m of ethanol in aqueous solution is stable. When the temperature increases to 30 "C, at the same pressure, the monolayer is unstable. The molecular loss can be quantified by a mechanism with two steps. The first step would correspond to a loss of molecules by dissolution (eq 2). In the second step the rapid increase in molecular loss could be due to the fact that diffusion and collapse occurred concurrently because n > ne (Table 3). This reasoning was found to agree with the results obtained at 25 mN/m at 30 "C. It can be seen that the molecular loss increases with pressure, and it is the second step of the mechanism that causes that behavior. The pressure of 25 mN/m is close to neand the two steps in molecular loss could fit a diffusion-controlled mechanism at this experimental condition. These results are similar to those of monostearin monolayer at the same experimental conditions (Table 3). The value of the bl parameter increases with the pressure, whereas the value of the a parameter is (43)Iwahashi, M.; Toyoki, K ; Watanabe, T.;Muramatsu, M. J. Colloid Interface Sci. 1981,79,21.
de la Fuente Feria and Rodriguez Patino
2322 Langmuir, Vol. 10, No. 7, 1994
Table 3. Characteristic Parameters for Destabilization of Monoglyceride Monolayers on Ethanol Aqueous Solutions monostearin
monopalmitin
monoolein
e
LCS
0.5" 0.5b 0.5 0.5 5.0
25 25 30 30 10
25 44 25 40 10
22.6 22.6 23.6 23.6 2.4
5.0
15
10
2.3
SB
5.0
20
10
2.3
SB
5.0
25
10
2.0
SB
5.0b 0.5O! 0.5 0.5 1.0 1.06 5.0b 0. l e 0.5c 0.5
25 20 30 30 20 30 20 20 20 20
25 35 25 35 35 35 35 35 25 35
2.0 23.7 25.0 25.0
SB
41.4 41.4
LEh LEh
0.5
30
25
40.0
LEh
5.w
20
35
1.0
SB SB SB
1.46 (0.997) 1.76 (0.980)
1.56 (0.990) 2.46 (0.999) 11.5 (0.973) 72.8 (0.984) 64.3 (0.980) 248 (0.998) 124.6 (0.997) 88.5 (0.999) 110 (0.993) 34.7 (0.970)
54
Sh
LCh LCh
27.9 (0.991) 67.6 (0.998) 3.37 (0.991)
6.77 (0.995) 4.33 (0.992) 2.74 (0.996)
60
0.966 0.977 0.838
312.4 101.9 185.2
54.9 19.4 95.1
0.548
181.7
27.6
0.688
169.0
28.8
0.470
358.4
0.987 0.813 0.783 0.968
184.2 245.8 149 103.3
17.2 14.0 18.1
0.997 0.203
120 30 52.5
35.0
0.702
61.7
25.0
110
LEh
1042 (0.994) 2439 (0.930) 505 (0.993) 64.7 (0.983)
a Without dissolution. b Spilled over the edges: unstable. The barrier moved toward greater area. Instantaneous monolayer loss. Data from ref 39. f Monolayer structure (MS): collapsed (C), solid (S),liquid-condensed (LC),liquid-expanded (LE). Data from ref 3. Unpublished results.
N/No
N/No 1
0 20 O C :35 mN/m x ) OC:
25 mN/m
a 30 *C; 25 mN/m
20
40
60
80
100 120 140 160 180 200
0' 0
"
5
10
"
15
20
"
25
30
35
'
40
"
45
50
' ' 1
55
60
8 (min) Figure 4. Destabilization of monopalmitin monolayers spread on ethanol aqueous solutions. The broken and solid lines represent the data calculated according to a desorption mechanism by dissolution (eq 2) and diffusion (eq 3), respectively.
practically independent of the presure, and the value of e* decreases. The stability of the monostearin is greater than the monopalmitin monolayer, due to the hydrocarbon chain length. Monolayer instability increases when the ethanol concentration in subphase is 1 mom. At 20 "C and 35 mN/m there is a molecular loss that can be quantified by a mechanism with two steps that fits a desorption process. When the temperature increases to 30 "C, the film instability is indicated by spilling over the edges. A similar behavior was observed with monopalmitin monolayers spread on ethanol aqueous solution a t 5 mom. So it was not possible to assign a mechanism for monolayer loss at these experimental conditions. Monopalmitin monolayers on ethanol aqueous solution a t 1 m o m and 20 "C have also been studied at the collapse area (A, = 0.21nmLmolecule-l). The plot of pressure vs time is included in Figure 2. As with monostearin on
Figure 6. Destabilization of monoolein monolayers spread on ethanol aqueous solutions at 0.5 m o a . Lines drawn fit the models to the data (for details see text).
water, the experimental data can be fitted to the ProutTompkins equation I O ~ U Z-~ Xc)/ni = 0.103log
e - 1.044
(LR = 0.984)
So the formation of the new bulk collapse phaseby a mechanism of nucleation and growth of critical nuclei-could be the cause for loss of molecules at the higher pressure close to the collapse pressure. Monoolein Monolayers. The stability of monoolein monolayers spread on aqueous ethanol solutions was studied a t 20 and 30 "C and a t pressures lower than ne at these temperatures. The loss of molecules from the monolayer and the characteristic parameters for destabilization of monoolein monolayers on ethanol aqueous solutions are included in Figure 5 and Table 3, respectively. The monolayer behavior depends on the experimental conditions. At 20 "C and ethanol concentrations of 0.1 m o m (at 35 mN/m), and 0.5 m o m (at 25 mN/m), the
Langmuir, Vol. 10, No. 7, 1994 2323
Destabilization of Monoglyceride Monolayers barrier moves toward greater area with time. That is, the area apparently increases with time. The phenomenon is the opposite to that corresponding to a loss of molecules by dissolution or collapse. This phenomenon is shown a t the beginning of the compression with 0.1 m o m of ethanol in subphase and after 30 min with 0.5 mol& of ethanol in subphase. A surface chemical reaction could be the cause of this phenomenon. Oleic acid or its esters can be autooxidized by exposure to atmospheric air yielding various oxyacids. Such autoxidation can be triggered or even be enhanced by spreading oleic acid on the airwater i n t e r f a ~ e .These ~ products can have an area greater than that of the oleic acid or its esters. It is even possible that the bonds break and oxidize to fragmental CSacids, followed by desorption. These phenomena can hardly be distinguished from other relaxation mechanisms in Table 1, based only on the log(NIN0) vs 8 relationship. All of these processes were analyzed kinetically by Iwahashi et al.43with a radiotracer technique. Only with oleic acid spread on dilute acidic permanganate solutions (at low pressure values) was it observed that the area apparently increases with time, reaches a maximum, and then decreases. The behavior observed could also be attributed to the film-subphase interactions. These interactions may take place between polar groups and hydrocarbons chains and produce an expansion of the monolayer, as has been discussed in prevous study of n-A isotherms with the monostearin-ethanol ~ y s t e m .The ~ location of ethanol at the aqueous interface between the monolayer head groups can be facilitated by intermolecular hydrogenbonded complexes between the monoglyceride head group and ethanol. The ethanol penetration at the interface is easier for monoolein than for monostearin and monopalmitin because the monoolein monolayer structure is more expanded due to its double bond. These results agree with previous works.4 That is, when the film structure is expanded or when there is the possibility of formation of a network a t the interface, the film stability increases. Monolayer instability increases when monoolein is spread on ethanol aqueous solutions a t 0.5 m o m and increases the pressure or the temperature. In both cases, a mechanism with two steps where logW/N~)vs 8 are linear relationships appears. The effect of the pressure is more important than that of temperature (Table 3). In these cases, the monolayer instability may be due to a diffusion mechanism. If the formation of a network in the subphase between ethanol and monoolein exists, the two steps may be in agreement with a diffusion of monoolein and monoolein-ethanol clusters, because of n < Xe.
When the ethanol concentration in subphase is 5 mol/ L, the monolayer instability is greatest. In this case the monolayer disappears and the surface pressure is practically zero during the film compression process. The monoolein monolayer instability in these conditions may be due to its instantaneous desorption in subphase. This behavior is different from that observed with monostearin and monopalmitin monolayers. Destabilization of Monoglyceride Monolayers on Sugar Aqueous Solutions. The loss of molecules from the monolayer €or monostearin, monopalmitin, and monoolein spread on glucose and sucrose aqueous solutions at 0.5 m o m is shown in Figure 6. The characteristic parameters for destabilization of monoglyceride monolayers on sugar aqueous solutions are included in Table 4. General observations are as follows: The monoglyceride monolayers are more unstable on sucrose than on glucose aqueous solutions. There is an increase in monolayer instability when the number of -CHZ- groups in the hydrocarbon chain
N/No
0,75
1 0
10
20
30
40
50
80
70
80
e "n) NlNo
Monoobarln A MonopalmlUn
0,7s
I
0
, 10
20
30
40
SO
80
70
80
e "n) Figure 6. Destabilization ofmonoglyceridemonolayers spread on aqueous solutions of (A) sucrose 0.5 m o m and (B) glucose 0.5mol&. T = 20 "C and II = 35 "am-'. The broken and solid lines represent the data calculated according to a desorption mechanism by dissolution (eq 2) and diffusion (eq 3), respectively.
decreases. It can be seen in Figure 6 that the molecular loss from the monolayer is greater for monopalmitin than for monostearin. With one unsaturation in the hydrocarbon chain, as in the case of the monoolein molecule, the stability of the monolayer spread on aqueous sugars solutions increases. This behavior is similar to that observed with the same lipid spread on diluted aqueous ethanol solutions, such as 0.1 m o m (Table 3). The barrier moves toward greater area with time at the beginning of the compression with glucose in the subphase and after 22 min with sucrose in subphase. Similar reasoning to that used with ethanol in subphase could be utilized to explain the monolayer stability opposite to desorption. In both cases n < ne.Of the two explanations given, autoxidation and the penetration of solute molecules at the interface, the second one applies here. In fact, the diffusion of molecules from the subphase and the formation of clusters at the interface between sugar, water, and monoolein could be more likely. From experiments performed at 20 "C and 35 mN/m on water, apparent monolayer expansion was not observed (Table 2). With ethanol and sugars in the subphase the barrier moves toward greater area with time (Tables 3 and 4). In every experiment the monolayer structure is liquid-expanded. So the solute in subphase has no influence on the oxidation of the double bond in the monoolein hydrocarbon chain. When the molecular loss from the monolayer (as with monostearin-sucrose, monopalmitin-glucose, and monopalmitin-sucrose aqueous solutions) is significant, the desorption could be the mechanism that controls the
de la Fuente Feria and Rodriguez Patino
2324 Langmuir, Vol. 10,No. 7, 1994
Table 4. Characteristic Parameters for Destabilizationof Monoglyceride Monolayers on Sugar Aqueous Solutions monoglyceride subphase T ("C) n (mN/m) ne(mN/mY MSc 103a(LR) 105bl(LR) N/No e(-) O*(,,,in) monostearha monostearin monopalmitin monopalmitin monooleinb monoolein6
glucose, 0.5 M sucrose, 0.5 M glucose, 0.5M sucrose, 0.5 M glucose, 0.5M sucrose, 0.5 M
20 20 20
20 20 20
35 35 35 35 35 35
29.9 32.9 31.2 24.6 40.5 46.1
LCd LCd LCe
0.977 14.5(0.999) 40.7(0.986) 0.876 8.92 (0.97) 6.97(0.99) 0.921 18.5 (0.993) 36.4 (0.994) 0.790
51.7 53.7 60.0 72.3
LEe LEe
0.971
21.5
LD
7.3 13.3 20.4
solid (SI,liquid-condensed a Without dissolution. The barmier moved toward greater area. Monolayer structure (MS): collapsed (C), (LC),liquid-expanded (LE). Data from ref 44. e Unpublished results. !Data from ref 39.
process, although in the three cases n > ne. The loss rate can be fit using a mechanism with two steps corresponding to dissolution and diffusion in subphase. The kinetic parameters for the film destabilization are included in Table 4.
Summary The experimental results showed that the stability of monoglycerides monolayers spread on aqueous solutions depends on the film and subphase composition, the temperature, and the pressure. When a loss of molecules from the monolayer exists, relaxation mechanisms have been tested by comparing the experimental data with the predicted relationships between the loss of monolayer molecules and time. It is unlikely that evaporation contributed significantly to the loss of monoglyceride molecules from the monolayers. In the present study the rate of film loss was nearly zero for monostearin or monoolein on water (Table 2) and with certain solutes in the subphase (Tables 3 and 4) at the same temperature. The feasible formation of complexes between monoglyceride and solute a t the interface-due to film-subphase interactions-should retard evaporation, or, having no effect on evaporation, actually enhance film loss. Moreover, the apparent increase in area with time-as is shown with monoolein monolayers on ethanol (Table 3) and sugars (Table 4) in subphase-disagrees with the evaporation effect. Finally, the monolayer molecular loss at high temperatures is lower than that at low temperatures-as is shown either in Figure 1B with monopalmitin monolayers on water at 35 mN/m, at temperatures of 30 and 35 "C,or in Figure 3B with monostearin monolayers on 5 M aqueous ethanol solutions at temperatures of 1 5 2 0 , and 25 "C-especially at the longer times. These data also disagree with the evaporation effect. At n < nethe monoglyceride monolayers are stable in most cases. An exception in that behavior can be observed with monoolein monolayers spread on aqueous ethanol solutions. In this case the instability has been attributed to the film-ethanol interactions, with diffusion or by the barrier moved toward greater area. At n > nethere is competition between the desorption and collapse mechanisms. When the loss of molecules from the monolayer can be fit by a mechanism with two steps corresponding to dissolution and diffusion in subphase, the rate of molecular loss increases with temperature or pressure. The molecular loss can be also quantified vs 8 are by a mechanism with two steps where log(",) two linear relationships, which could correspond to diffusion and collapse with formation of critical nuclei occurring concurrently. In these cases, the effects of temperature and pressure on kinetic parameters are not simple. There are exceptions in such behaviors as with monostearin monolayers on water. When there are high interactions between hydrocarbon chains in monolayer molecules, as with monostearin on water a t n > n e , the monolayer structure is solid, and the activation energy for monolayer collapse increases. A different behavior can be observed when the monolayer structure is expanded
as in the case of monoolein on ethanol aqueous solutions. In this case the monolayer collapses at n x ne. At higher pressures (n >> ne),the instability is characterized by the monolayer spilling over the edges. In the discussion section that behavior has been assigned to monolayer collapse. In fact, this mechanism was observed from measurements at the collapse area. From the results, we conclude that the monolayer stability depends on the film-subphase interactions. These interactions can operate concurrently with the desorption and collapse mechanisms. The concurrency of these effectscould exist, in which case the interpretation of the overall kinetics is more complex. So the anomalous behavior with temperature-as with monostearin on ethanol aqueous solutions at 5 m o m (Figure 3B) or monopalmitin on water (Figure 1) or ethanol aqueous solutions at 0.5 m o m (Figure 4)-could be related to this. The film-subphase interactions depend on the subphase composition: With sugars in the subphase there is the possibility of formation of a network at the interface, by intermolecular hydrogen-bonding between monoglyceride and sugar.44 The decrease in area occupied by the monolayer with time may be due to the fact that interactions between hydrocarbon vanish. This mechanism of molecular loss is different to that observed with the same lipids on ethanol aqueous solutions. With ethanol in the subphase the effect is more complex. Ethanol acts as a surfactant and it is able to adsorb at the interface. So hydrophobic interactions between ethanol and monoglyceride are possible. When the ethanol concentration is low (0.1 mol&) there is an expansion in the monolayer structure. However, at the highest concentrations there is a significant loss of molecules by desorption that can be instantaneous, as is observed with monoolein monolayers spread on aqueous ethanol solutions at 5 mol&. It is possible that ethanol and lipid make up clusters at the highest ethanol concentrations in subphase. This would be equivalent to a reduction of the lipid hydrophobic characteristics which will facilitate lipid dissolution. When the monoglyceride-ethanol cluster takes part in the relaxation process, the mechanism is more complex, and the influences of temperature and pressure on monolayer instability are difficult to quantify. The fdm-subphase interactions, together with the effect of the film-film interactions, can have effects on the monolayer structure. So the influence of pressure and temperature on monolayer stability depends on the monolayer structure. The am-subphase interactions can be quantitatively important with a liquid-expanded structure, as with monoolein monolayers on ethanol or sugars aqueous solutions. In these cases, if ~t < xe the film-subphase interactions can stabilize the monolayer and can also produce an increase in the apparent area. The effects of these interactions on the relaxation mechanisms in spread monolayers on aqueous solutions of ethanol and sugars are oftheoretical and practical interest and will be studied in a forthcoming paper. (44)Rodriguez Patino,J. M.; Ruiz Dominguez,M.; de la Fuente Feria, J. J . Colloid Interface Sci. 1993,157, 343.
