Langmuir 1996,11, 3054-3060
3054
Reorientation of Polyethylene Glycol Octylphenyl Ether in Nonequilibrium Adsorption Layers at the WaterlAir Interface. Role of Molecular Weight and Temperature V. B. Fainerman,? R. Miller,*?$and A. V. Makievskit Institute of Technical Ecology, 25 blv. Shevchenko, Donetsk, 34001 7,Ukraine, and Max-Planck-Institut f i r Kolloid- und Grenzflachenforschung,Rudower Chaussee 5, 0-12489 Berlin-Adlershofi Germany Received November 8, 1994. In Final Form: March 20, 1995@
The dynamic surface tensions of aqueous Triton X solutions (X-45, X-100,X-114,X-165,X-305,and X-405)were measured by using a maximum bubble pressure technique in a broad concentration and temperature interval. Using a special measuring cell for the MPTVLAUDA, studies in the time interval from 100ps up to 50 s were achieved. The results at very short adsorptiontimes simulate a faster surface tension decrease than expected from a common diffusion-controlled mechanism. The deviation from that theory increases with temperature and the length of the ethylene oxide chains. The experimental results are in agreement with a theoretical model taking into consideration different orientations of the EO chain in diluted and compressed adsorptionlayers and the temperature effect on the interfacial water structure.
Introduction In a recent paper1 the kinetics of adsorption of Triton X-100 and Triton X-405 studied by dynamic surface tensions was found to be faster than expected from a common diffusional transport model. The interpretation of the observed effects was made on the basis of a model similar to the idea of a Langmuir mechanism. This model assumes, in line with molecular models of the surfactant at a surface, two different molecular states at the interface: a horizontal and a normal orientation of the hydrocarbon and ethylene oxide (EO) chains. Assuming a diffusion-controlledadsorption after the surface has been covered by flat-oriented molecules, a coexistence of both orientations sets in. From then on, the coverage of the interface is continuously close to saturation and only the composition of the adsorption layer changes from a completely flat to a completely normal orientation of the molecules. The phenomenon of “super diffusional” adsorption kinetics is of both theoretical and practical interest. This phenomenon and the so-called barrier-controlled adsorption processes are connected with rather complex dynamic behavior ofthe surfactant adsorption layers.2 The growing interest in such models is a strong stimulus for further development. Its importance for practical purposes is difficult to evaluate, as it is a tool to control the kinetics of surface tension decrease in many processes where surfactants are i n ~ o l v e d . ~ This paper focuses on a detailed analysis of the surface tension decrease of surfactant solutions with special emphasis on concentration, hydrophobidhydrophilic balance, and temperature. Most of all, the studies will analyze the reliability of experimental results at very short times. It is kn0wn~9~ that the rate of an adsorption process
* To whom all correspondence should be addressed. Institute of Technical Ecology. Max-Planck-Institut fur Kolloid- und Grenzflachenforschung. Abstract Dublished in Advance ACS Abstracts. June 1. 1995. (1)Fainermin, V. B.; Makiewski, A. V.; Joos, P. Colloids Surc A 1994,90,213. (2)Baret, J.F. J.Phys. Chem. 1968,72,2755; J.Chem. Phys. 1968, 65. .. 895. (3) Miller, R.; Fainerman, V. B.; Joos, P. Adv. Colloid Interface Sci. 1994,49, 249. (4)Mysels, K. J. Langmuir 1986,2,423. @
can be faster than diffusion only under the following conditions: in the presence of convection or under the condition that at zero time a significant amount of surfactant is already adsorbed. The convective diffusion is negligible at short adsorption times in both maximum bubble pressure and drop volume studies. The surfactant’s initial adsorption at either the bubble or drop interface plays a major role in these studies and needs special analysis. Dynamic surface tensions of aqueous solutions of Tritons with different ethylene oxide chain lengths are studied here in order to determine the effect of the molecular geometry. Measurements in a broad temperature interval are suitable to learn about orientation processes of such nonionics at a liquid interface.
Materials and Methods Octylphenyl polyethylene glycol ethers C14H21O(C2H40),H (purchased from Serva and Aldrich) with different EO units (Triton X-45(n = 4.51,Triton X-100(n = lo),Triton X-114(n = 11.4),Triton X-165(n = 16.51, Triton X-305(n = 30.5)and Triton X-405(n = 40.5))were used without further purification. The n-decanol was purified by vacuum distillation. Aqueous surfactant solutions were prepared from a stock solution using doubly distilled water. Surface active impurity traces were removed by distillation over alkaline KMn04 in a quartz still. Chromic acid was used to wash the measuring cell, the capillary, and glassware, followed by a distilled water rinse. The dynamic surface tension measurements were performed with an automatic maximum bubble pressure method, MPT1, from Lauda, Germany. In this method the initial bubble surface is always a part of the preceding detached b ~ b b l e . ~ The , ~ time of fast bubble growth is called dead time, t d . In the critical point of the pressure-gas flow rate dependence t d is exactly the time interval between two bubbles.6 The relative expansion 8
I
S.J. Colloid Sci. 1961,16,549. (6)Fainerman, V. B.; Miller, R.; Joos, P. Colloid Polym. Sci. 1994, 272,731. ( 5 ) Hansen, R.
0743-746319512411-3054$09.0OlO 0 1995 American Chemical Society
Langmuir, Vol. 11, No. 8, 1995 3055
Reorientation of Polyethylene Glycol Octylphenyl Ether
1
0
1
3
2 Sqr(tl
4
5
[SI
Figure 1. Dynamic surface tension as a function of tm of Triton X-45 solutions at 20 "C(CO = 0.071 (m), 0.118 (El+), 0.19 (0) x mol/cm3n-decanol (A). 10-6 mol/cm3) and of 0.2 x 75 T
t 6
65
E
50
5
561
45
I
0
1
2
3
SqrM Is1
Figure 2. Dynamic surface tension as a function of tu2 of Triton X-100 solutions at 20 "C(CO = 0.077 (mu);0.124 (+), 0.155 (0) mol/cm3n-decanol (A). x 10-6 moVcm3) and of 0.2 x
of the bubble surface areaA in the moment of detachment is given by 697
The values of the coefficient 6 range between 213 (ideal sphere) and 1(conditions in the MF'T19, andit couldexceed 1 in the moment of bubble detachment. Hence, the effective time of fast bubble growth (effective dead time) results as6''
In the standard design of the MF'T1, t d FZ 80 ms. In the present experiments a special measuring cell was designed using capillaries of different lengths (7mm instead of 15 or 10 mm as commonly used with the MF'T1) and avariable distance between the capillary tip and the electrode. This construction,described in more detail elsewhere: provides dead times of about 10 ms. To calculate the surface pressure n(t) the short time approximation of the diffusion-controlled adsorption model can be used:lOJ1
(3) where D is the diffusion coefficient, co is the surfactant bulk concentration, R is the gas law constant, and Tis the absolute temperature. The shorter the dead time, the smaller is the initial surface pressure caused by a
remaining adsorption layer on the surface of the residual bubble. Thus, extremely short adsorption times can be reached only withvery short dead times and at high bubble formation frequencies. The special measuring cell discussed aboveg works with the common program of the MPTl without any changes and yields adsorption times from 100 ps up to 50 s, obtained when the stopped gas flow regime is used.12 It is worth noting that, in the drop pressure and drop volume methods under constant liquid flow, the initial adsorption at the surface of a fresh drop is significantly higher than in the bubble pressure method.13 The reason is the compressibility and low viscosity of gas. For other methods used at short adsorption times a completely bare surface is postulated at time t = 0 (oscillating jet5 or inclined plane14). This of course is only an approximation. A more accurate model should have to take into consideration a Marangoni flow caused by the surface tension gradient along the oscillating jet or flowing liquid film.
Results and Discussion The experimental results are shown in Figures 1-6. The dynamic surface tensions y are plotted as a function (7)Makiewski,A.V.;Faineman, V. B.; Joos, P. J.Colloid ZnteTface Sci. 1994,166, 6. (8)Joos, P.; Rillaerts, E. J . Colloid Znterfuce Sci. 1981,79,96. (9) Faineman, V. B.; Miller, R. J. Colloid Interface Sci., in press. (10) Hansen, R. S.J.Phys. Chem. 1960,64, 637. (11)Faineman. V.B.: Makievski, A. V.: Miller, R. Colloids Surf., . .A 1994,.87, 61. (12) Faineman, V. B.;Miller, R. Colloids Surf.,A, 1995,97,65. (13)MaeLeod, C.A.; Radke, C. J. J.Colloid Interface Sci. 1993,160, 435. (14) Van den Bogaert, P.; Joos, P.J. Phys. Chem. 1979,83, 2244.
Fainerman et al.
3056 Langmuir, Vol. 11, No. 8, 1995 75 T
45
1 0
I
0,2
0,4
0,6
SqrM
03
1
[SI
Figure 3. Dynamic surface tension as a function of tm of Triton X-305 solutions at 20 "C (CO = 0.09 (W,0.226 (0),0.452(e),0.678 (0)x mol/cm3)and of 0.2 x mol/cm3n-decanol (A).
Figure 4. Dynamic surface tension as a function of tuz of Triton X-114 solution, co = 0.423 x (01, 50 "c (e),60 "c (O), and 70 "c(A).
low6mol/cm3,at 30 "C (m), 40 "C
!
cn 4 0 1 32
4 0
4
0,1
02
0,3 Sqr(t)
OA
0,5
[SI
Figure 6. Dynamic surface tension as a function of tvz of Triton X-165 solution, co = 0.536 x low6mol/cm3,at 40 "C (m), 50 "C (O), 60 "C (e),70 "C (0).
of tu2,the coordinates of the diffusion-controlledadsorption at short adsorption times. The slope of this plot derived from eq 3 has the form (4)
From the given results we can see that in all cases of low surface pressure (ll 3 mN/m) the initial part of the y(tm) dependencies are linear. Consequently at allstudied temperatures at short times and low surface pressures the Tritons adsorb in a diffusion-controlled manner as expected (cf. Figures 1 and 2). The values (dyldtllzh, obtained from experiments and calculated from eq 4 using
the parameters of Table 1agree fairly well. In all other cases (II=- 3 mN/m) the experimental values are higher than the calculated ones, and the differences increase with an increasing number of EO groups and temperature. For example, for Triton X-305 and X-405 the ratio of experimental and theoretical values of (dy/dtv2)t+,is 10 to 15; for Triton X-100 and X-114, it is 3 to 5. Except in the range ll < 3 mN/m, the Tritons show an adsorption rate significantly higher than expected from eq 4, at each concentration and temperature. This can be clearly seen when compared with the y(t1I2)dependencies of decanol given in Figures 1-3. At comparable concentrations the decanol adsorbs significantly more slowly than the Tritons. Decanol adsorption kinetics is described completely by
Langmuir, Vol. 11, No. 8, 1995 3057
Reorientation of Polyethylene Glycol Octylphenyl Ether
32
I
0
0,1
03
02 SqrM
0,4
Figure 6. Dynamic surface tension as a function of tu2 of Triton X-305 solution, co = 0.452 x (01, 50 "C (e),60 "C (O), and 70 "C (A). 65
35
03
(81
mol/cm3,at 30 "C (M), 40 "C
T
:
0
6
4
2
8
10
12
l/Sqrft) [ l b l
Figure 7. Dynamic surface tension as y(l/tllz)plots of Triton X-405 solution, co = 0.508 x 50 "C (e),60 "C (01,and 70 "C (A). Table 1. Characteristic Adsorption Parameters of Tritons at Different Temperatures Triton adsorptnparams 2008b X-45 X-100 X-114 X-165 X-305 X-405
D (10-6cm2/s) 0.95 w2(109cm2/mol) 2.1 w1(109cm2/mol) 8.1 D cm2/s) 0.88 w2 (10gcm2/mol) 3.0 w1(109cm2/mol) 13 D cm2/s) 0.86 02 (109cm2/mol) 3.2 wl(10gcm2/mol) 14 D(10-6cm2/s) 0.73 02 (10gcm2/mol) 3.6 w1 (10gcm2/mol) 18.4 D (10-6cm2/s) 0.73 w2 (10gcm2/mol) 5.6 w1 (logcm2/mol) 30.6 D(10-6cm2/s) 0.7 w ~ ( l O ~ c m ~ / m o l 7.1 ) w1(109cm2/mol) 40
30b
40b
0.95 1.14 2.0 2.2 8.1 8.1 0.88 1.1 3.0 3.15 13 13 0.86 1.1 3.2 3.4 14 14 0.83 1.0 3.6 3.8 18.4 18.4 0.73 0.9 5.5 5.5 30.6 30.6 0.7 0.84 7.1 7.1 40 40
50b
1.4 2.3 8.1 1.3 3.3 13 1.3 3.5 14 1.2 4.0 18.4 1.1 5.5 30.6 1.0 7.0 40
60*
70b
1.6 1.85 2.4 2.5 8.1 8.1 1.5 1.7 3.45 3.6 13 13 1.5 1.7 3.7 3.9 14 14 1.35 1.6 4.2 4.4 18.4 18.4 1.25 1.4 5.5
5.5
30.6 1.2 7.0 40
30.6 1.36 6.8 40
Data from Fainerman et al.ll Temperature in "C. the common diffusion-controlled adsorption mode1.15-19A perfect agreement (corrected for the slight concentration difference) was reached between our data at co = 2 x 1O-I mol/cm3 and M c L e o d ' ~ obtained ,~~ with a growing drop technique a t co = 1.7 x 1O-I mol/cm3. Extrapolation of the linear parts of the y ( P ) dependencies to t = 0 in Figures 1-6 leads in all cases to the a
(15) Fainerman, V. B.; Lylyk, S. V. Koll. Zh. 1982,44,598. (16) Kufher, R. J. J. Colloid Sci. 1961,16, 497. (17)Defay, R.; Hommelen, J. R. J. Colloid Sci. 1959,14,401. (18) Lin, S.-I.;McKeigue, K.; Maldarelli, C. Langmuir 1991,7,1055. (19) MacLeod, C.A.; Radke, C. J. J . Colloid Interface Sci. 1994,166, 73.
moVcm3,at 30 "C(M), 40 "C (O),
surface tension value of pure water. This does not hold true for measurements with a capillary of 15 mm length. For capillaries of this length, the dead time is of the order of 80 ms and therefore (IT(t"?)m t 0, as discussed in detail in ref 9. For concentrated Triton solutions (Figures 3,5, and 6) part of the data are in the time interval from 100 ps and 1 ms. We are the pioneer of presenting such data.g The temperature has shown a strong effect on the dynamic surface tension behavior of Triton solutions. In the studied time interval, the decrease of surface tension with temperature far exceeds changes known for pure water (Figures 4-6). For example, the surface tension of water changes by 6.75 mN/m when the temperature is increased from 30 to 70 "C. In the same temperature interval the surface tensions of the Triton solutions (Figures 4-6) decrease by 10-20 mN/m (at a timet % 0.1 8).
Results on Triton solutions at comparatively long adsorption times already have been discussed elsewhere.12 In that time interval the approximation of Hansen and Joosll is very suitable for our data interpretation,
(&))-=%&I
From this extrapolation, the equilibrium surface tension values y- can be obtained. For the two Tritons with higher molecular weights (X-305 and X-405)the y- values decrease with increasing temperature (Figure 7). For Triton X-165,X-114,and X-100(Figure 8) the curves for different temperatures meet almost a t one point. The y-
Fainerman et al.
3058 Langmuir, Vol. 11, No. 8, 1995
8
m
en
0
*
A 00 A
30 0
1
3
2
5
4
l/Sqr(t) 11/81
Figure 8. Dynamic surface tension as y ( l / t u z ) plots of Triton X-100 solution, co = 0.155 x 50 "C (+), 60 "C (01,and 70 "C (A).
64
32
mol/cm3,at 30 "c(B), 40 "c(01,
I
4 0
I
0,s
1
2
1,5
1/Sqr(t) 11Is]
Figure 9. Dynamic surface tension as y(lltu2) plots of Triton X-45 solution, co = 0.118 x and 70 "C (e).
mol/cm3,at 30 "C
(W), 50 "C (O),
Table 2. Equilibrium Surface Tensions y- of Triton Solutions at Different Temperatures (Obtained from y ( l / P ) Extrapolations)
surfactant Triton x-45 x-100 X-114 X-165 X-305 X-405 a
concn (10-6 mol/cm3) 0.118 0.155 0.423 0.536 0.452 0.508
204
30b
26.0 33.5
25.0 34.0 29.4
44 50 53
48.6 52.9
40b
50b
33.1 29.1 40.6 46.3 49.1
26.0 30.1 27.6 36.7 43.3 46.4
60b
70b
29.4 26.7 35.9 40.8 43.4
27.0 29.4 26.6 34.1 38.2 41.3
Data from Fainerman et al.ll Temperature in "C.
values obviously depend only slightly on temperature. For Triton X-45 (Figure 9) an opposite trend is observed. The curves cross each other at about t = 100 s. The results are summarized in Table 2 (extrapolation accuracy is not better than 1mN/m). It is worth mentioning that analogous effects of the oxyethylene chain length on the temperature dependence of y- have also been observed by AbramzonZ0for other nonionic surfactants with EO groups. This peculiar behavior is not observed for nonionics without oxyethylene chains, for example, fatty alcohols. The temperature dependence of y- at a larger surface pressure of short chain alcohols is the same as observed for Triton X-45.21 The increasing surface activity of the higher molecular weight Tritons and the faster surface tension decrease with increasing temperature can be explained by a change in the water structure. At higher temperatures, hydrogen bonds between water molecules are weaker and the soluble nonpolar group of the surfactant molecule behaves in a
less hydrophobic manner. Thus the surface activity would decrease with temperature.22 At the same time, the hydrogen bonds between water and the hydrophilic oxyethylene groups become weaker, which increases the surface activity of the surfactant molecule. The final temperature effect depends on the ratio of the hydrophilic and hydrophobic parts in the surfactant molecule, so that for Triton X-405 the equilibrium surface tension ydecreases with increasing T while for Triton X-45 it increases. On the basis of this explanation the effect of added structure former or structure breaker on y- and y(t) in aqueous Triton solutions should be predictable. For example, the addition ofthe structure breaker urea should increase dyldt and decrease y- of Triton X-305 and X-405 solutions. The effect of addition of urea to aqueous solutions of Triton X-45 or nonionics without EO groups should be the opposite. As shown by Joos and Ser15en,~~
(20)Abramzon, A. A. Surfactants; Khimija: Leningrad, 1979. (21)Posner, A. M.;Anderson, J. R.; Alexander, A. E. J.Colloid Sci. 1962,7,623.
(22)Erday-Gruz, T. Transport Phenomena in Aqueous Solutions; Academiani Kiado: Budapest, 1974. (23)Joos, P.;Serrien, G. J. Colloid Interface Sci. 1989,127, 97.
Langmuir, Vol. 11, No. 8, 1995 3059
Reorientation of Polyethylene Glycol Octylphenyl Ether
I
03
0
1
116
2
SqrW
23
33
3
[SI
Figure 10. Dynamic surface tension as a function oft" of Triton X-405solution,co = 0.508 x (W) and absence (0)of 0.4 M urea. small amounts of urea in short chain alcohol solutions lead to higher y- values and slightly slower adsorption rates. Dynamic surface tensions of a Triton X-405solution with and without urea are shown in Figure 10. As expected and in contrast to the results obtained in ref 23, the opposite effect can be observed for the higher molecular weight Triton X-405. For a further interpretation of the measured dynamic surface tensions we recall a model for a two-state adsorption of a surfactant at an interface.l The following relationships and approximations form the basis of this model: 1. The surfactant adsorption r for short adsorption times teffis described by the asymptotic equation
2. The adsorption is given by the sum of adsorbed molecules in the two orientations (indices 1, 2):
r = rl + r2
(7)
3. When the adsorption layer is close to saturation, the adsorption of component 2 is given by
r2= ( r o o ,
- i ) / ( o o l - oo2)
(8)
where wol = lK1- and wop = WZ..are the partial molar areas of the two adsorption states (w"1 > mop)and rl.. and r2..are the maximum adsorptions of the two adsorption states. The relationship between rl and T2 has the form
d ln(T /r ) 1 2 = dlT
wol - wo2 -
RT
(9)
which has been obtained by Fainerman et a1.l on the basis of the Butler equation and the principle of Braun-Le Chatelies4at an interface. This equation serves for fitting the experimental data to verify the chosen values of 0'1 and w"2. For the fitting eq 9 is used in the form
-x
RT
dl- d 2=
n
x n
(n - l ) ! i = l j = Z j > i
ln(I'liK2i) - ln(TlJT2j)
llj- ni
(10)
where w'1 and w'p are the mean experimental values of (24) Joos, P.;Semien, G. J. Colloid Interface Sci. 1991,145, 291.
4
mol/cm3,at 20 "C in the presence
mol and wop and we assume wol - 0'2 x dl - w'2 to be a criterion for the correct choice of the parameters. Equation 8 for the determination of Tp is valid under the given experimental conditions at II > 5 mN/m. A summary of the data ofdiffusion coefficientsand molar areas of the Tritons used for further calculations is given in Table 1. The values at 20 and 30 "C are more or less the same as experimentsin this temperature interval were performed at intermediate values and no accurate data at these specific temperatures exist (cf. experiments and parameter values given by Fainerman et al.l). The values ofcoopat 20 "C for the different Tritons are also taken from Fainerman et al.,I while those at higher temperatures are calculated from the present experimental data. The values of wol are calculated from the geometric size of the Triton molecules. For all Tritons the experimental results are in very good agreement with the value calculated from eqs 6-10. In Figure 11 a comparison is given of the geometrically calculated values of 0'1 and those obtained by fitting the experimental results. The lower experimental values of w o l for Triton X-305and X-405are probably caused by the larger number of possible conformations of the molecule at the interface. The pressure intervals ofhorizontal (rx rl)and normal (r5 T2) orientations of Tritons and the coexisting region (r= rl T z )are displayed in Figure 12 as a function of the EO chain length n. With increasing EO groups the interval of coexistence becomes more narrow. Note that at very low pressures ll (up to 1 mN/m for n = 40.5 and 3 mN/m for n = 4.5) a diluted adsorption layer exists and the state of adsorbed molecules does not influence the surface pressure, which is given by the Henry isotherm J3 = RZT. Only in this pressure interval, as it was shown above, do the experimental data follow the dependence given by eq 4.
+
Conclusions Studies of different Tritons in a broad temperature interval allow the following conclusions on both the measuring technique and the structure of the adsorption layer: 1. The maximum bubble pressure technique allows reliable dynamic surface tension measurements at adsorption times even below 1 ms. 2. At short adsorption times and low surface pressures ll .C 3 mN/m all studied Tritons show pure diffisioncontrolled adsorption kinetics. At longer times and higher surface pressures (ll > 3 mN/m) deviations of the experiments from a diffision mechanism become signifi-
Fainerman et al.
3060 Langmuir, Vol. 11, No. 8, 1995 40
oJ
I
10
0
30
20
40
50
n
Figure 11. Partial molar area of Tritons as a function of the number n of EO groups: theoretical values o0i(molecular geometry) (m);experimental values at 30 "C (01, 50 "C (01, and 70 "C (+).
z
60 46 -. 40 ..
E 35 -.
L
-.
g
30
t
20 -.
Q)
$ 15
5
v)
I
25 --
t
m
--
lo-. 5 -.
04 0
B
4
a
8
a
30
40
I
10
20
50
Number of EO Groups n
Figure 12. Surface pressure range n for different molecular orientationsas a function of the number n of EO groups: upper curve (Be&,normal orientation (TzL 0.95r);lower curve (U+A),horizontal orientation (rlL 0.95r), maximum surface pressure at the CMC (0)from ref 11. cant. The deviations increase with temperature and number of EO groups. 3. Rates of adsorption apparently higher than expected from a conventional diffusion model can be well explained by considering two adsorption states of the Triton molecules in the diffusion kinetics model. 4. The different character of interaction of water molecules with the hydrophobic and hydrophilic parts of the Tritons explains the effects of temperature on the adsorption properties.
Acknowledgment. We thank S. V. Lylyk for the excellent performance of a part of the experiments. The work was financially supported by a project of the European Community (INTAS 93-2463)and the Fonds der Chemischen Industrie (R.M., 400429). A research fellowship (V.B.F.) from the Max-Planck-Institut fur Kolloid- und Grenzflachenforschung Berlin-Adlershof is also gratefully acknowledged. LA9408883
