Langmuir 1995,11,2090-2097
2090
Destabilization of Monoglyceride Monolayers at the Air-Aqueous Subphase Interface. 2. The Role of Film Elasticity Julia de la Fuente Feria and Juan M. Rodriguez Patino" Departamento de Ingenieria Quimica, Facultad de Quimica, Universidad de Sevilla, c / o Professor Garcia Gonzalez, s l n , 41012 Seville, Spain Received June 8, 1994. I n Final Form: December 29, 1994@
The aim of this work is to establish a quantitative relationship between the destabilization of a monoglyceride (monomyristin)monolayer at the air-water interface as a function of subphase composition (ethanol and sugar solutions),temperature, and surface pressure. The destabilization of monomyristin films has been followed kinetically by observing changes in the film area with time at constant surface pressure. It has been observed that the film stability is a phenomenon related to the cohesive forces in the film and to the interactions in the subsurface region. Film elasticity-expressed by the modulus (-dn/dA)-is proposed as the parameter which describes this relationship. Its value has been related to the mechanisms that control film destabilization. The most common relaxation mechanisms found are desorption caused by dissolution and diffusion in the subphase and collapse caused by nuclei formation and further growth. The diffusion-controlled mechanism only appears when the film has a condensed structure and with low values of the elasticitymodulus. For monolayers with a liquid-condensed or liquidexpanded structure-with high values of the elasticity modulus-the relaxation mechanism of collapse with nuclei formation appears. The values ofthe kinetic parameters for the destabilizationcan be discussed in terms of interactions between subphase and film molecules.
Introduction Dynamic surface properties come into play in all cases in which for whatever reason a quiescent dispersion is locally not in complete thermodynamic equilibrium or in which, as a result of external disturbance, the whole system is not in complete equi1ibrium.l This applies to all kinds of dispersions, such as fluid-fluid or solidfluid, which are of interest in industrial processes or products concerned with suspensions, emulsions, foams, or aerosols. An understanding of the structure and stability of the monolayers is essential for the prediction of the properties of food colloids stabilized by surfactants2 and macromole c u l e ~ . This ~ is because the stability of foams and emulsions is governed to a large extent by the stability of the film around the bubbles or the droplet^.^ In the past, it has been shown that monolayers exhibit some kinetic effects such as hysteresis and time dependence on the shape of isotherms, as well as relaxation of the surface p r e s ~ u r e . ~ -Several ~ possible mechanisms-which have been proposed to explain kinetic processes occurring in monolayers-were analyzed in a previous work.1°
* To whom correspondence should be addressed. @Abstract published in Advance ACS Abstracts, May 15, 1995. (l)Prins, A.; Bergink-Martens, D. J. M. In Food Colloids and Polymers: Stability and Mechanical Properties; Dickinson, E., Walstra, P., Eds.; The Royal SocietyofChemistry; Cambridge, U.K., 1993;p 291. (2)Krog, N. J.;Riison, T. H.; Larsson, K. InEncyclopediaofEmulsion Technology; Becher, P., Ed.; Dekker; New York, 1985;Vol. 2, p 321. (3)Dickinson, E.; Murray, B. S.; Stainsby, G. In Advances in Food Emulsions and Foams; Dickinson, E., Stainsby, G., Eds.; Elsevier: London, 1988;p 123. (4)Halling, P.J. CRC Crit. Rev. Food Sci. Nutr. 1961,15,155. ( 5 ) Tomoaia-Cotisel, M.; Zsako, J.; Chifu, E.; Cadenhead, D. A. Lanpmuir 1990.6. 191. (6)Binks, B. P. Adv. Colloids Interface Sci. 1991,34,343. (7)Mesini, Ph.; Lebeau, L.; Ondet, P.; Mioskowski, Ch. Chem. Phys. Lioids 1992.63.27. -(8) McFate, C.; Ward, D.; Olmsted, J. Langmuir 1993,9,1036. (9)Lo Nostro, P.; Gabrielli, G.Langmuir 1993,9,3132. 0743-746319512411-2090$09.0010
In part 1of this series we concluded that the monolayer stability of emulsifiers depends on different variables such as surface pressure, temperature, and subphase composition.1° These factors also affect the film ~ t r u c t u r e . l l - ~ ~ For monoglyceride monolayers, it was observed that the main causes of instability are the desorption in subphase competing with collapse followed by nuclei formation.lo The aim of this work is to establish a quantitative relationship between structure and monolayer stability. The relationship between the kinetics of adsorption of protein molecules at the interface and the structure of the film (governed by the protein concentration) has been studied previ0us1y.l~ In this work it was observed that those proteins which give highly cohesive interfacial films adsorb slowly, and they are more resistant to mechanical deformation. It has also been found that the collapse kinetics of stearic acid monolayers depends on the monolayer structure.16 Film elasticity-expressed by the modulus (-h/dA)-is a parameter which represents this relationship. Film elasticity could be a parameter which quantifies the intermolecular and film-subphase interactions. Elasticity is a measurement of the resistance to a change in the film area.l' From another point of view, dynamic surface pressure and elasticity play an important (10)de la Fuente Feria, J.;Rodriguez Patino, J. M. Langmuir 1994, 10,2317. (11)Rodriguez Patino, J.M.; de la Fuente Feria, J.;G6mez Herrera, C. J . Colloid Interface Sci. 1992,148,223. (12)Rodriguez Patino, J.M.; Ruiz Dominguez, M.; dela Fuente Feria, J. J . Colloid Interface Sci. 1992,154,146. (13)Rodriguez Patino, J. M.; Ruiz Dominguez, M. Colloid Surf A: Physicochem. Eng. Aspects 1993,75,217. (14)Rodriguez Patino, J.M.; Ruiz Dominguez, M.; de la Fuente Feria, J. J. Colloid Interface Sci. 1993,157,343. (15)Graham, D.E.; Phillips, M. C. J . Colloid Interface Sci. 1979,70, 403. Phillips, M. C.; Evans, M. T.; Graham, D. E.; Oldani, D. Colloid Polym. Sci- 1975,253,424. (16)Rabinovitch, W.; Robertson, R. F.; Mason, R. F. Can. J . Chem. 1960,38,1881. (17)Kim, S. H.; Kinsella, J.E. J . Food Sci. 1985,50,1526.German, J.0.; O'Neill, T. E.; Kinsella, J. E. J . A m . Oil Chem. SOC.1985,62,1358. Djabbarah, N.F.; Wasan, D. T. AIChe J . 1985,31,1041.
0 1995 American Chemical Society
Destabilization of Monoglyceride Monolayers role in many processes, such as emulsification, foaming, extraction, distillation, or chemical and electrochemical surface reactions.18-20
Materials and Method Monomyristin (1-ruc-monotetradecanoylglycerol),more than 99%pure, was acquired from Sigma. This monoglyceride is used as an emulsifier in food formulations.2 Ethanol, sugars (glucose and sucrose), and hexane of analytical grade were obtained from Merck and used without further purification. The water used was purified by means of a Millipore filtration device (Milli-Q). The absence of active surface contaminants in both the water and the spreading solvent (a mixture of hexane and ethanol of 9:l (v/v))was verified. A compression of the subphase without surfactant was carried out before the monolayer was applied. No sign of impurities was observed. Both the structure and stability of monomyristin monolayers spread on water or aqueous solutions of ethanol (0.5 mom), glucose (1 moVL), and sucrose (0.5 moW) were studied in a commercial fully-automated balance (Lauda) described e1~ewhere.lO-I~The solutes chosen as subphase componentsethanol, glucose, and sucrose-are typical ingredients in food formulations.21 The temperature of the system formed by the spread film and its subphase was maintained constant within k0.2 "C at 15 "C < T < 30 "C and to within &0.4 "C at T < 15 "C and T > 30 "C by a Lauda k2R electronic thermostat. The film structure was derived from the surface pressure-area plots (n-A isotherms). The film elasticity was generated from the slope of the curves obtained with a constant compression rate of nm2 molecule-I min-l. The value of the compression 6.2 x rate chosen ensures reproducibility in the n-A isotherms. The monomyristin solutions were spread on the subphase by means of a micrometric syringe at the working temperature. Aliquots of 250pL (7.16 x 10I6molecules)were spread in each experiment. Between 7 and 15 min elapsed before measurements were taken to allow for evaporation of the spreading solvent, as a function of the temperature. All isotherms were recorded and then analyzed off-line. Each isotherm consisted of data records of surface pressure (mN/m) and barrier position, automatically corrected for the number of monomyristin molecules spread using a built-in analog correcting device of the film balance (nm2/ molecule). Monolayer stability was derived from the relaxation experiments in which the surface pressure was constant, as this is the preferred method and capable of being interpreted in a kinetic sense.6Jo,22For constant surface pressure stability data, the change in the area of the monolayer is monitored with time. The results of area relaxation versus time were transformed in N/No, where N and NOare the number of molecules in the monolayer that remain on the surface at time 6 and those at the initial moment, respectively.lO
Results Monomyristin Monolayer Structure. The effect of temperature-for a temperature range between 20 and 30 OC-on the monomyristin monolayer spread on water is shown in Figure 1. This monoglyceride on water has both liquid-condensed (LC) and liquid-expanded (LE) structures. The latter was observed at the higher temperatures or at the lower pressures. A displacement of the isotherm toward the pressure axis and decreases in the collapse pressure can be observed when the temperature increases. This behavior was found with fatty acidsll and their monoglycerides12 previously-when they are (18)Davies, J. T. 2'urbulencePhenomena;AcademicPress: NewYork, 1972. (19) Hennenberg, M.; Sanfeld, M.; Bisch, P. M. MChE J . 1981,27, 1002.
(20)Prevost, M.; Bisch, P. M.; Sanfeld, M. J . Colloid Interface Sci. 1982,88, 353. (21) Leadbetter, S. L. Food Focus, No. 9, Leatherhead Food R.A., 1990. Banks, W.;Nur, D. D. InAdvances in Food Emulsions and Foams; Dickinson, E., Stainsby, G., Eds.; Elsevier: London, 1988; p 257. (22) Smith, R. D.; Berg, J. C. J . Colloid Interface Sci. 1980,74,273.
Langmuir, Vol. 11, No. 6, 1995 2091 ww
50 40 -
30 -
20 10 -
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
A (nn?/molec)
Figure 1. n-A isotherms of monomyristin monolayers spread on water.
spread on water or aqueous ethanol solutions-and it was attributed to the molecular loss from the monolayer or to a decrease in the interactions between hydrocarbon chains. Figure 2 shows the effect of the subphase composition on the monolayer structure at 20 "C. These monolayers have liquid-condensed and liquid-expanded structures as a function of the surface pressure. The plots in Figure 2 fit the experimental data to eqs 1and 2,where liquid-expanded structure:
A =mA + n
liquid-condensed structure:
In n = m'A
+ n'
(1) (2)
m, n, m', and n' are the regression coefficients. The linear regression coefficient was better than 0.990 in all cases. From eqs 1and 2, the elasticity moduli can be calculated directly, using the same values of surface pressure and temperature as in the kinetics experiments (Table 1).It follows that both the monolayer structure (MS) and elasticity modulus depend on the surface pressure, the temperature, and the subphase composition. It can be seen that the elasticity modulus increases with pressure-for both liquid-condensed and liquid-expanded structures-and its value is higher with a liquid-condensed structure. This means that the film has a strongly packed structure on the surface a t the higher values of surface pressure. Different values for the elasticity modulus can be observed with the same film structure. So, the mechanical properties of the film give a complementary description of the monolayer characteristics. In Table I, it can be seen that, at 35 mN/m and 20 "C, the values of the elasticity modulus follows this order: ethanol, 0.5 m o m > water > glucose, 1m o m > sucrose, 0.5 m o m That means that the presence of ethanolin the subphase makes the monolayer less compressible, whereas the sugars have the opposite effect. A similar behavior has been found with other monoglyceride~~~J~ and fatty acids.ll The ethanol molecules in the subphase can adsorb at the air-water interface due to its surfactant character.23These molecules can interact with the monomyristin at the interface-by hydrophobic interactions between hydro(23) Rodriguez Patino, J.M.; Martin Martinez, R. J . Colloid Interface Sci. 1994,267, 150.
de la Fuente Feria and Rodriguez Patino
2092 Langmuir, Vol. 11, No. 6, 1995
1
WATER
A
3c I
ETHANOL 0.5 M
1
b
b
h
\ 0A
I b
h
A
\A \
0
0 0
I
I
I
I
I
0.1
0.2
0.3
0.4
0.5
A
0.6
0
0.1
0.5
0.6
A (nm 2/molec)
A (nm */molec)
4
GLUCOSE1 M
A
0.4
0.3
0.2
SUCROSE 0.5 M
AA
4
3
4 4
Q
?A
4 %A
b
*e QA
b
0
0.1
0.2
0.3
0.4
0.5
0.6
0
0.1
0.2
0.3
0.4
0.5
4At
0.6
A (nm 2/molec) A (nm 2/molec) Figure 2. n-A isotherms for monomykin monolayers on aqueous solutions at 20 "C.The broken and soiid lines represent the data calculated according to eqs 1 and 2, respectively. carbon chains or by electrostatic interactions between can increase, as is observed in Table 1. The hydrophobic interactions are not possible between monomyristin and polar groups. As a consequence, the rigidity of the film
Destabilization of Monoglyceride Monolayers
Langmuir, Vol. 11, No. 6, 1995 2093
Table 1. Structural Characteritics of Monomyristin Monolayers
N/No
-dn/dA x
T subphase
water water
water
water ethanol, 0.5 M glucose, 1 M sucrose, 0.5 M
("C)
10 20
30 40 20 20 20
JL
10-2 ((mN
A (nm2/
(mN/m) molecule) 35 0.200 15 0.423 35 0.203 45 0.177 55 0.175 15 0.409 35 0.253 45 0.185 35 0.258 35 0.160 35 0.234 35 0.230
MS
molecules)/ (nm2/m))
LC LE LC LC LC LE LE LE LE LC LC LC
3.5 1.1 2.9 3.7 4.5 0.8 2.1 2.9 2.6 3.7 1.3 0.9
sugar m o l e ~ u l e s , ~and ~ J ~as, ~a ~consequence the monolayer is more expanded (Figure 2) and the film is more compressible. Monomyristin Monolayer Stability. The temperature, surface pressure, and subphase composition exert a significant influence on the film stability. Two relaxation mechanisms fit the experimental data (eqs 3 and 4)from the experiments at constant surface pressure,where a and
0.8
TEMPERATURE
0.6
10%
* 20 o c 0.4
0 30 O C 40 O C
'
,
0.2 0
25
50
J
too
75
125
150
8 (min)
Figure 3. Destabilization of monomyristin monolayers spread on water at 35 mN/m. UINo
J 0.8 l
.
35 mN/m
45 mN/m
b are coefficients that depend on the studied variables. Equation 3 fits the rate of film molecular loss during an initial, non-steady state of desorption (dissolution m e ~ h a n i s m )and , ~ ~eq~4~represents ~ the rate of molecular loss in the monolayer reached in a steady state (diffusion m e c h a n i ~ m )or~ the ~ , ~transformation ~ of a homogeneous monolayer phase into a heterogeneous monolayer-collapse phase due to the formation of nuclei.22This equation also represents the destabilization due to evaporation.26Similar reasoning used in a previous work with monostearin, monopalmitin, and monoolein monolayers spread on the same aqueous subphaseslo applies here. So, in this work the monolayer instability due to evaporation can be ignored. In Figures 3-5 the effects of variables upon the monolayer stability are represented by means of a continuous line for the regressions obtained from eq 4 and with a dashed line for the regressions from eq 3. Effect of Temperature. The effect of temperature on the instability of a monomyristin monolayer spread on water-measured in experiments in which the surface pressure is kept constant at 35 mN/m-is shown in Figure 3. The plots of experimental points in Figure 3 verify eqs 3 and 4. The values of the kinetic coefficients ( a and b), the linear regression coefficient (LR), and the characteristic times of change between mechanism (@*)are included in Tables 2 and 3, where the experiments have been separated as a function of the monolayer structure and shown in order of increasing value of the elasticity modulus ( -dddA). It can be seen that monolayer instability does not depend on the temperature between either 10 and 20 "C or 30 and 40 "C. However, the rate of molecular loss is greater at (24) McArthur, B. W.; Berg, J. C. J . Colloid Interface Sci. 1979,68,
0 55 mN/m
I
0.2 0
20
40
60
80
100
120
140
180
160
8 (mln) "0
I
1
0.21 0
20
60
40
8
80
100
(mini
Figure 4. Destabilization of monomyristin monolayers spread on water at (A) 20 and (B) 30 "C.
the higher temperatures. Between 20 and 30 "C, there is a transition in the monolayer structure (Figure 1).That is, a change between liquid-condensed and liquidexpanded structure takes place at this temperature. The change in the rate of molecular loss with temperature may be associated with the structural state of the m0nolayer.~,~~J8 Effectofpressure. The effect of surface pressure on the instability monomyristin spread on water at 20 and 30 "C is shown in Figure 4. At 20 "C it was possible to carry out kinetic experiments at up to 55 mN/m, whereas at 30 "C
201.
(25)Ter Minassian-Saraga, L. J . Colloid Sci. 1966, 11, 398. (26) Brooks, J. H.; Alexander, A. C. In Retardation of Evaporation by Monolayers; La Mer, V. K., Ed.; Academic Press: New York, 1962.
(27) Ivanova, T. 2.; Panaiotov, I.; Georgiev, G.; Launois-Surpas,M. A.; Proust, J. E.; Puisew, F. Colloids Surf. 1991, 60, 263. (28) Joos, P.;Van Uffelen, M. J . Colloid Interface Sci. 1998,155,271.
2094 Langmuir, Vol. 11, No. 6,1995
de la Fuente Feria and Rodrguez Patino nuclei occurring concurrently. In these cases, the effect of pressure on kinetic parameters (Tables 2 and 3) is not simple.1° Effect of Subphase Composition. For short times ( 8 < 10 min), the film molecular loss is similar no matter what the subphase composition is (Figure 5). However, after this period the number of molecules that remain on the surface for a value of 8 follows the order
N/No
0.8
0.6 Sucrose 0.5 M
1
\
0.4
water > glucose, 1m o m > sucrose, 0 , 5 m o m > ethanol, 0 , 5 m o m
Ethanol 0.5 M
T = 20 OC
I
T=35 mN/m
~
0' 0
25
50
75
I00
125
150
,
175
I
200
225
0 (min) Figure 5. Destabilizationof monomyristin monolayers spread on different aqueous solutions at 20 "C and 35 mN/m. Table 2. Destabilization of Liquid-Condensed Monomyristin Monolayers elasticity modulus increasing -dnldA
sucrose, glucose, 0.5M 1M 20 T("C) 20 n(mN/m) 35 35 O(min) 112.6 203.5 NINQ 0.51 0.54 ~ o ~ ~ ( L20R ) 16 0.990 0.995 O* (min) 18.5 39.0 105bl (LR) 210 97 0.995 0.998 0, (min) 10%~ (LR)
water
subphase
20 35 145 0.80 17 0.990 6.4 34.2 0.999
ethanol 0.5 M 20 35 48.4 0.56
10 35 95 0.80 7.9 0.995 14.2 500 79 0.997 0.994
. water
20 45 36.5 0.28
20 55 109 0.61
610 0.994 21.0 2.47 0.980
580 0.994 23.3 0.09 0.950
Table 3. Destabilization of Liquid-Expanded Monomyristin Monolayers
That is, monolayer stability is highest on water. The monolayer instability is higher on ethanol aqueous solutions. In this regard the behavior of monomyristin is similar to that observed with other monoglycerides.1° The values of the kinetic coefficients (aand b ) , and the characteristic times of the mechanism change are shown in Tables 2 and 3. It can be seen that the behavior of monomyristin monolayers on sugar aqueous solutions is different from that observed with the same lipid spread on ethanol aqueous solutions. In the first case the monolayer molecular loss could be quantified by a desorption mechanism. This loss takes place in two steps corresponding to dissolution and diffusion into the subphase. With ethanol in the subphase, one step appears following eq 4. If monomyristin spread on aqueous solutions behaves in a fashion similar to that previously observed with different mono- and diglyceride~,~~ the equilibrium surface pressure of monomyristin on ethanol aqueous solutions must be lower than that on water. So, the mechanism for the monolayer molecule loss on ethanol aqueous solutions could be due to either diffusion or collapse. Afull explanation of this behavior is not possible with the data available. Rate of Monolayer Molecule Loss. The rate of monolayer molecule loss can be quantified by means of eqs 3 and 4. When the controlling mechanism is governed by eq 3-dissolution mechanism-the value of the rate by the unit surface is given by eq 5.29whereAois the molecular
elasticity modulus increasing -dnldA
water
subphase
T ("0 n(mN/m) 0 (min) N/NQ lo5& (LR) Be (min) 10'bz (LR)
t
30 15 85.8 0.598 264 0.992
20 15 177 0.795 62 0.985
30 35 12 0.392 3230 0.997
40 35 18.45 0.274 2500 0.999 13.95 4.4 0.999
30 45 36.4 0.302 2420 0.990 5.16 1.29 0.997
the highest value of ~twas 45 mN/m, due to collapse of the monolayer. The effect of surface pressure on the monolayer molecular loss depends on the value of the surface pressure. In a first step, ifthe pressure increases between 15 and 35 mN/m, the rate of molecular loss increases too. The effect at higher pressures is different-as is observed between 45 and 55 mN/m at 20 "C (Figure 4A)and between 35 and 45 mN/m at 30 "C (Figure 4B). The molecular loss can be quantified by a mechanism with two steps where log(N/NO) vs e are two linear relationships. In these experimental conditions the surface pressures are greater than the equilibrium surface pressure.2 As a consequence, the two steps in the mechanism of molecular loss could correspond to diffusion and collapse with formation of
area a t the initial time (e = 0), e is the numerical constant, N i s the number of molecules a t time 8,S is the total area occupied by the monolayer, and a and b are the regression coefficients from eqs 3 and 4. If the mechanism is dissolution controlled-determined by eq 5-the rate falls with time. The lowest value, &is, is reached when 8 = 8*. If the mechanism is diffusion or collapse controlled-eq 6-the rate of molecular loss is constant. In this case the rate of molecular loss is diffusion (&f) and collapse (Reo) controlled if in eq 6 i = 1and i = 2, respectively. The values of the characteristic rates of molecular loss are shown in Table 4. In Figure 6, the rate of molecular loss is represented for monomyristin monolayers spread on water a t 20 "C and at 15 mN/m (Figure 6A) and at 35 mNlm (Figure 6B), as an example. It can be observed that when the mechanism controlling the kinetic process follows eq 3, there is a decrease in the rate a t the beginning of the relaxation process. However, the (29) Chaiko, D. J.; Osseo-Asare, K. J. Colloid Interface Sei. 1988, 121, 13.
Destabilization of Monoglyceride Monolayers Table 4. Dissolution, Diffusion, and Collapse Rate by Unit Area &is X lo-'' R&if x lo-'' Real X lo-''
2' iz (molecules/ (molecules/ (molecules/ subphase ("C)(mN/m) (cm2s)) (cm2s)) (cm28)) water 10 35 1.92 1.44 water 20 15 0.585 35 6.73 0.648 45 13.4 5.37 55 12.7 2.03 water 30 15 2.48 35 49.2 45 50.3 26.6 water 40 35 37.5 92.0 ethanol, 20 35 12.1 0.5 M elucose. 20 35 2.09 1.6 1M sucrose, 20 35 4.03 3.63 0.5 M
Langmuir, Vol. 11, No. 6, 1995 2095 . dN/dB
lO"(molec/mln)
2501
..! .. ....
-""I
200
0
A
.
-
3500
B
3000
rate of monolayer molecular loss is practically constant if the kinetics follows the mechanism represented by eq 4. In Figures 7 and 8 the effect of temperature and subphase composition on the rate of molecular loss is represented. The presence of solutes in the subphase (at 20 "C and 35 mN/m) that can interact with the monolayer molecules-at the interface or in the subsurface zoneaffects the monolayer stability (Figure 8). So, the presence of ethanol and sugars in the subphase increases the rate of molecular loss from the monolayer. However, a monomyristin monolayer spread on ethanol aqueous solutions is more unstable that one on sugar. The effect of temperature-with water as the subphase and at 35 mN/m-is more complex (Figure 7). The rate ofmonolayer molecular loss increases with temperature up to 30 "C, whereas at 40 "C the observed effect is the opposite. This behavior can be due to the fact that diffusion and collapse by nuclei formation occur concurrent1y.l'
Discussion It can be observed from the data (Figures 3 and 4) that monolayer instability depends on the monolayer structure. Data in Tables 2 and 3 show that the mechanism that controls the film molecular loss and the kinetic coefficients also depends on the elasticity values. Data in Tables 2 and 3 show that, as general behavior, the monolayers with liquid-expanded structure do not exhibit the mechanism of desorption by dissolution in the subphase-the mechanism represented by eq 3. On the other hand, the competition between desorption by diffusion and collapse by nuclei formation only appears at the higher values of the elasticity modulus. The relationship between the structure and stability of the monolayers is discussed as a function of the monolayer state. Destabilizationof Liquid-CondensedMonolayers. Acondensed structure in monolayers is produced at lower values of temperature and higher values of surface pressure. The presence of solutes in the subphase does not affect the structure of the monolayer whatever the values of pressure and temperature are. However, interactions between monolayer molecules are fewer with sugar in the subphase than when on water or aqueous ethanol solutions. In the latter subphases the n-A isotherms appear at lower surface areas. For low values of the elasticity modulus, monolayer destabilization is controlled by a dissolution-diffusion mechanism. Two steps appear in the global kinetic process, accounted for by eqs 3 and 4. This occurs when monomyristin is spread on sugar aqueous solutions-at
i
2500
2000 5 1500;
1000 500
0
-i
-'. ~ " i " " ' ' , . . ~ . r
. , , . ~ . . . . . l . . . . r ~ . . . l .
20 "C and 35 mN/m-or on water-at 35 mN/m and at temperatures of 10 and 20 "C. In these cases the value of the a coefficient in eq 3 is practically independent of the subphase composition. However, the coefficient b decreases in the order sucrose, 0.5 moVL > glucose, 1moVL > water (this is the same order in which the elasticity modulus increases). That is, coefficient b is higher for destabilization of a monomyristin monolayer spread on sucrose aqueous solutions. In these systems diffusion could be the mechanism that controls the monolayer molecular loss. This behavior could be associated with the molecular interactions both at the interface and in the subsurface zone.10p30 The elasticity modulus is greater with ethanol in the subphase for the same conditions of surface pressure and temperature. The role of the ethanol molecules in the monomyristin monolayer destabilization could be explained by the interactions between monolayer and ethanol molecules at the interface. So, this way diffusion of the monolayer molecules and monomyristin-ethanol complexes is facilated by the presence of ethanol, a phenomenon similar to that observed previously with different monoglycerides.lo In these systems the desorption mechanism does not include a step with an equilibrium between the subsurface zone and ,the interface. With water in the subphase the higher values of the elasticity modulus appear at the higher values of surface pressure. In this case, the monolayer molecules are more (30) Demel, R. A,; Yim, C. C.; Lin, B. Z., Hanser, H. Chem. Phys. Lipids 1992,60,209.
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2096 Langmuir, Vol. 11, No. 6, 1995
Figure 8. Rate of molecular loss of monomyristin monolayers spread on different aqueous subphases at 20 "C and at 35 mN1
of the b coefficient. In Tables 2 and 3 the different values of the coefficient are indicated by the subindex 1,for the first step (from 0 = 0 to e = 0,) and 2 for the second step (for 0 > 0J. Because the formation of critical nuclei needs an activation en erg^,^^,^^ the second step of the relaxation process must be due to collapse and the first one to the diffusion toward the subphase. That is, under our experimental conditions, collapse and diffusion compete. It must be remembered that in these conditions surface pressures are higher than the equilibrium surface pressures. When diffusion and collapse occur concurrently-as can be seen with monomyristin monolayers on water at 20 "C, and at surface pressures of 45 and 55 mN/m (Table 2)-the rate of collapse-determined by the value of the b2 coefficient-is lower at the highest value of surface pressure. The explanation of this fact is that at higher pressures, when the elasticity modulus values are higher, the monolayer is near fracturing. If this level of pressure is reached, the molecules in the monolayer could be organized in a two-dimensional phase or clusters.32 If these clusters at the interface exist, the diffusion of monolayer molecules to the subphase bulk will decrease as the surface pressure increases (Table 2 and Figure 4). That is, the two-dimensional clusters at the interface must be destroyed before the three-dimensional clusters can be formed by collapse. The effect of temperature is more complex, as can be seen with monomyristin monolayers spread on water at 35 mN/m, at 10 and 20 "C (Figure 3 and Table 2). In these experimental conditions coefficient a increases while coefficient b decreases as the temperature increases. The dependence of coefficient a on temperature could be explained by either reduction of the van der Waals attraction between molecules with increasing temperature or by the temperature effect33on the physical properties of the system (density, viscosity, and diffusivity) which are of interest in the process of monolayer destabilization. According to either of the two previous reasons, the rate of molecular loss by the dissolution mechanism increases with temperature. The temperature dependence on the b coefficientis not so easy to explain. The decreasing rate of molecular loss with increasing temperature could be due to film-subphase interactions-such as hydrationdehydration of the polar head group as a function of t e m p e r a t ~ r e . ~ From , ~ ~ , ~another ~ point of view, it is possible that the second step of the monolayer molecular loss process is not a pure diffusion mechanism. The relaxation experiment was made a t 35 mN/m which is a pressure higher than the equilibrium surface pressure. So, it is possible that the b coefficient accounts for both the dissolution and collapse mechanisms. In this case the anomalous thermal behavior has already been reported. Destabilizationof Liquid-ExpandedMonolayers. The attraction between molecules decreases with a liquidexpanded structure. Destabilization of monolayers with liquid-expanded structures does not occur with an equilibrium between the surface and the subsurface zone, according to a dissolution-diffusion mechanism. In these experimental conditions, the molecules leave the surface a t a constant rate from the initial time. Depending on
packed, and the Interactions between hydrocarbon chains are higher. The monolayer becomes more inflexible, and the emulsifier molecules could diffise toward the subphase and-at the same time-could associate forming nuclei. So, two steps in the global process appear, both of them characterized by eq 4, but with a variation in the value
(31)Pezron, E.;Claesson, P. M.; Berg, J. M.; Vollhardt, D. J.Colloid Interface Sci. 1990,138, 245. (32)Tomoaia-Cotisel, M.; Zsako, J.;Mocaw, A.; Lupea, M.; Chifu, E. J . Colloid Interface Sci. 1987,117, 464. (33)Reid, R. C.; Rausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids,5th ed.; McGraw-Hill: New York, 1987. (34)Caminati, G.; Senatra, D.; Gabrielli, G. Langmuir 1991,7,604. (35)Iwanishi, M.; Toyoki, K.; Watanabe, T.;Muramatsu, M.J. Colloid Interface Sci. 1981,79, 21.
-dN/d 8 (1 E15)
10
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I
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OC
6
4
2
0 0
5
I
I
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''
,
,
,
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Figure 7. Rate of molecular loss (molecules/min) of monomyristin monolayers on water at 35 mN/m as a function of
temperature.
2000 -
*WATER
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-"SUCROSE 0.5 M +GLUCOSE
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Langmuir, Vol. 11, No. 6, 1995 2097
Destabilization of Monoglyceride Monolayers the elasticity modulus value, this can occur (Table 3) in one step-for low values of the elasticity modulus-or in two steps-for high values of the elasticity modulus-both of them according to eq 4. All the experiments with liquid-expanded structure refer to monomyristin monolayers on water. In the absence of collapse-T < 35 "C and ~t < 35 mN/m-the kinetic diffusion coefficient ( b l ) increases both with pressure, at T = 30 "C, and with temperature, at n = 15 mN/m (Table 3). These results are similar to those previously discussed with liquid-condensed structure. Collapse appears at the higher values of the elasticity modulus. The rate of collapse falls when the value of the elasticity modulus increases. Similar explanations cited previously with liquid-condensed structure are applied in this case: for temperature and pressure values near fracture, the monolayer loses its homogeneity and it is more difficult to break up the two-dimensional clusters to form three-dimensional nuclei.
Conclusions We have studied the destabilization of monomyristin monolayers spread on aqueous solutions of ethanol and sugars. On the basis of the experimental results, it is
seen that the influence of temperature, surface pressure, and subphase composition depend on the film structure. Different values for the elasticity modulus can be observed with the same film structure. So, the mechanical properties of the film give a complementary description of the monolayer characteristics. The mechanism that controls the monolayer molecular loss is due mainly to the monolayer structure and the value of the elasticity modulus. It is possible to establish the following: (1) For low values of the elasticity modulus, both mechanisms of desorption (dissolution and further diffusion) with diffusion control only appears when the film has a liquid-condensed structure, whereas with a liquidexpanded structure only the diffusion mechanism appears. (2)For high values ofthe elasticity modulus, with liquidcondensed and liquid-expanded structures, two steps appear in the global process, both of them following eq 4 with a change in the value of the kinetic coefficient. In these cases diffusion and collapse occur concurrently because of the strong packing of the molecules at the interface for each structure. LA9404494
