Langmuir 1996,11,2889-2892
2889
Hydration and Structural Properties of a Homologous Series of Nonionic Alkyl Oligo(ethy1ene oxide) Surfactants G. Klose,* St. Eisenblatter, J. Galle, A. Islamov, and U. Dietrich Department of Physics, University of Leipzig, Linnistrasse 5, 04103 Leipzig, Germany Received December 19, 1994. I n Final Form: April 14, 1995@ The water sorbed by oligo(ethy1eneoxide) monododecyl ether of the homologous series from one up to eight oxyethylene groups and the repeat distance in the case of the lamellar phase at constant relative humidities (RH = 84.2 and 97.0%)were determined by the isopiestic method and X-ray diffraction, respectively, at 25 "C. Further, deuterium NMR relaxation of deuterium oxide in large concentration range of surfactant dispersions in heavy water was measured at the same temperature. In the case of the lamellar phase the surface area of surfactant and the thickness ofthe hydrophobic core were estimated from the repeat distanceusing the compositionofthe samples and the molecularvolumes. For all surfactant dispersions the effective water binding energy was deduced from the deuterium relaxation times. It was found that the dependence of the water sorbed and of the surface area at constant relative humidity as well as of the effective water binding energy on the number of the oxyethylene groups can be described by linear functions. These findings are discussed with respect to the parameters which exhibit gradual changes along the ethylene oxide chain and in the light of a dynamic picture. The water sorbed increases by 0.7 and 2.1 molecules of water at the relative humidities of 84.2 and 97.0%(25 "C), respectively, the surface area of the surfactant by 3.7 A2 at the relative humidity of 97.0%(25 "C), and the effective water binding energy by 7 kJlmol by adding one oxyethylene group.
1. Introduction The amount of hydrate water and the surface requirement per surfactant are fundamental characteristics of lyotropic aggregates. In the case of nonionic surfactants of the type ClzH2dOCHzCHz),OH (CIZE,)they depend on the length of the oxyethylene chain and on the external conditions. Various results are available from the literature for both quantities for surfactants with two to eight oxyethylene units. They were determined under different external conditions by different techniques or theoretical estimated under different assumptions (see, e.g, refs 1- 13). Not surprising, the values ofsome ofthem differ considerably. The main aim of this paper is to report on the hydration and some structural properties of surfactants of the homologeous series C12E, with n = 1-8 at well-defined conditions. So, the surfactants were always hydrated at constant relative humidity (RH = 84.2%or 97.0%)at 25 "Cand the properties were determined in dependence on the ethylene oxide chain length. The amount of water sorbed was gravimetrically measured by the isopiestic method and the repeat distance Abstract published in Advance ACS Abstracts, June 1, 1995. (1)Volkov, V. A.Kolloidn. Zh. 1974,36,941. (2)Zulauf, M.; Weckstrom,K.; Hayter, J. B.; Degiorgio, V.; Cortb, M. J . Phys. Chem. 1986,89,3411. (3)Carvell, M.; Hall, D. C.; Lyle, I. G.; Tiddy, G. J. T. Faraday Discuss. Chem. SOC.1986,81, 223. (4)Lee, E.M.; Thomas, R. K.; Cummins, P. G.; Staples, E. J.;Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989,196. ( 5 ) Jonstromer, M.; Jonsson, B.; Lindman, B. J. Phys. Chem. 1991, 95,3293. (6)Rosen, M. J.;Cohen, A. W.; Dahanayake, M.; Hua, X. J. Phys. Chem. 1982,86,541. (7)Nisson, P. G.; Wennerstrom, H.; Lindman, B. J. Phys. Chem. 1983,87,1377. (8) Paz, L.; Di Meglio, J. M.; Dvolaitzky, M.; Ober, R.; Taupin, C. J. Phys. Chem. 1984,88,3415. (9)Rosen, M. J.; Murphy, D. S. Langmuir 1991, 7,2630. (10)Almgren, M.; Alsins, J. Isr. J. Chem. 1991,31, 159. (11)Lu, J. R.; Li, 2. X.; Thomas, R. K.; Staples, E. J.; Tucker,I.; Penfold, J . J. Phys. Chem. 1993,97,8012. (12)Clunie, J. S.;Goodman, J. F.; Symons, P. C. J. Chem. SOC., Faraday Trans. 1967,65,287. (13)Claesson, P.M.; Eriksson, J. Ch.;Herder, Ch.; Bergenstahl, B. A,; Pezron, E.; Pezron, I.; Stenius, P. Faraday Discuss. Chem. Soc. 1990, 90,129. @
0743-746319512411-2889$09.00/0
was determined by X-ray diffraction in the case of the lamellar phase. Further, the deuterium NMR relaxation times TI of samples prepared with heavy water were measured. For additional information the effectivewater binding energy of the surfactants was determined by a method applied already to C12E4.l~ Note, that there is a serious interest in the data presented here also in order to be able to understand properties of surfactant-modified lipid membranes. Such systems are fhding growing interest as appropriate model systems for investigating the interaction between water and polar surfaces (see refs 15-19 and references cited there).
2. Materials and Methods The ethyleneoxide monododecyl ethers C~ZHZ~(OCH~CH~),OH (C1zEn)with n = 1-8 were purchased from Nikko Chemicals (Tokyo,Japan)and used without furtherpurification. They were dried by fresh Pz05. The amount of water sorbed by the surfactants at RH = 84.2% or 97.0% was gravimetrically determined. The relative humidities were controlled by saturated salt solutions ofKCl and KzS04,20respectively. Specialattention was paid to high-temperature stability (0.1 K) and very small temperature gradients across the hydration vessel. The small gradients were realized by inserting the whole vessel, which containedthe surfactant (about20 mg) as well as the salt solution, in a basin surrounded by two others. The outer basin was temperaturecontrolled with a stability of 0.1K. The temperature stability in the sorption vessel was 0.02 K. Note, several days were necessary to reach thermodynamic equilibrium (seeFigure 1). The hydrated surfactantswere filled in special thin-walled markcapillaries for X-raydiffraction(1mm outer diameter)which were sealed after filling. X-ray diffraction was applied for determining the repeat distance d from the first-order peak of Bragg reflection in the (14)Eisenbliitter, S.;Galle, J.; Volke, F. Chem. Phys. Lett. 1994,228, 89. (15)Wnig, B. PhD Thesis, Leipzig, 1992. (16)Kijnig, B.; Hose, G. Prog. Colloid Polym. Sci. 1993,93,279. (17)Miidler, B.; Hose, G.; Mops, A,;Richter, W.; Tschierske, C. Chem. Phys. Lipids 1994,71, 1. (18)Naumann, C.; Dietrich, C.; Lu, J. R.; Thomas, R. K.; Rennie, A. R.; Penfold, J.; Bayerl, T. M. Langmuir 1994,10, 1919. (19)Hose, G.; Eisenblltter, S.;Konig, - B. J . Colloid Interface Sci., in press. (20)Young, J . F.J. Appl. Chem. 1967,17, 241.
0 1995 American Chemical Society
Klose et al.
2890 Langmuir, Vol. 11, No. 8, 1995
Table 1. Amount of Water (Water/Surfactant Molar Ratio RW/Aand Weight % of Surfactant) Sorbed by ClZE, at the Relative Humidities RH = 84 and 97% at 25 "C RH = 84% RH = 97% n RwiA wt % RWIA wt %
9-
s
1 2 3 4 5 6 7 8
8-
p!
72
4
6
8
1
0.7 1.1 1.5 2.1 2.9 3.6 4.7 5.5
0
1.2 2.1 5.2 6.9 8.5 9.9 13.6 16.0
91.4 84.8 77.1 74.5 72.6 71.6 66.9 65.1
Table 2. Properties of the Lamellar Phasea RH = 97% RWIA moymol d/A AA/& dhdA
tld Figure 1. Hydration kinetic of (20 mg) at RH = 97.0% (T = 25 "C). The solid line is the result of a fit of the data yielding: RWIA= 9.39 - 5.23 exp (-t/1.53) where RWIAis the waterlsurfactant molar ratio.
94.6 93.3 92.2 90.6 88.6 87.5 85.4 84.5
n 4 5 6 7
39.2 42.0 43.6 44.8
6.9 8.5 9.9 13.6
42.6 45.4 48.0 54.2
15.4 14.5 13.7 12.1
a Water sorbed per C12E, surfactant and repeat distanced both measured at 25 "C as well as calculated surface area and the thickness of the hydrophobic core.
that probably only aggregates without mesophases exist (cf. the remark in ref 22). The notation of the phases according to ref 24 was used. The dependences of the waterlsurfactant molar ratio RWIA on the number of the oxyethylene groups n can be described by a linear function
+
Rw,A= awn b,
(1)
where 2
4
8
6
n Figure 2. Amount of water per surfactant RWIA (moumol) sorbed by the nonionic surfactants &E, at RH = 84.2% (A) and 97.0% (v),and at excess water (0)las a function of the number of the oxyethyleneunits n at T = 25 "C. The solid lines are the result of a linear fit of the data. case of the lamellar phase (nonoriented stacks of layers) using a self-mademeasuring chamber for temperature and humidity controls and a position sensitive detector (Brown,Germany)for registration (for details see ref 21). The error of the repeat distance measured is not larger than h0.05 nm. For deuterium NMR the samples were prepared by direct addition ofheavy water to the water-freesurfactantin the molar ratio wanted and subsequent vortexing. After the NMR sample tubes were sealed (7.5 mm outer diameter), the samples were stored at 25 "C at least 2 days before measuring. The deuterium relaxation time TIof heavy water was determined by the inversion recovery pulse sequence x-r-nI2 using a Bruker MSL 300 spectrometer operating at the deuterium NMR frequency of 46.073 MHz. 3. Results Figure 2 shows the amount ofwaterRwlA (molesofwaterl moles of surfactant) sorbed at RH = 84.2% and 97.0% as function of the number of oxyethylene groups n. The data obtained by Volkov at excess water (RH 100%)' are included. The phase states are taken from literature ( n = 3, 4, 5, and 6 from ref 22, n = 2 and 8 from ref 23) or were determined by us (n = 7) using deuterium NMR of heavy water and X-ray diffraction. For n = 1we assume ~
~~~~
(21) Hartenstein, K. Diploma work, Leipzig, 1992. (22) Mitchell,D. J.;Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983,19, 975. (23) Conroy, J. P.; Hall, C.; Leng, C. A.; Rendall, K.; Tiddy, G. J. T.; Walsh, J.; Lindblom, G. Prog. Colloid Polym. Sci. 1990,82, 253.
b, = -0.4
(RH = 84.2%)
a, = 2.1, b, = -1.3
(RH = 97.0%)
a, = 4.0, b, = -6.6
(RH * 100.0%)
U, = 0.7,
and
(according to ref 1). The experimental data obtained with RH = 84.2% and 97.0% are summarized in Table 1 (cf. also Figure 2). In the case of the lamellar phase the surface area of surfactant and the thickness of the hydrophobic core were estimated from the composition of the sample RWIAand the repeat distance d by a well-developed formalism,25 which is explicitly described in the Appendix. The results are collected in Table 2. The surface areas obtained are in good agreement with those published, e.g., in refs 3,12, and 13. Obviously, the dependence of the surface area of the surfactants AA in the bilayers on the number of the oxyethylene groups n can be approximated by a linear function too
A,
(2ATl-k
bA
(2)
where aA = 3.7 Az and b A = 27.0 k (RH = 97.0%) (correlation coefficient 0.975). Note, Rosen et aL6determined the minimum area per surfactant molecule at excess water from surface tension measurements and described its dependence on the number ofoxyethylene units n in contrast to us byA,hn-m (24) Tiddy, G. J. T. Phys. Rep. 1960, 57 (l), 1. (25) Nagle, J. F.; Wiener, M. C. Biochim. Biophys. Acta 1988, 942, 1.
Hydration and Structure of Surfactants I
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t t
.
I
'
.
I
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hangmuir, Vol. 11, No. 8, 1995 2891
1
0 I
I
2
4
I
6
.
1
1
0
n
10
0
Figure 3. Surface area of surfactant AA in aqueous C12En dispersions as function of the number of oxyethylene units n estimated from repeat distance d (La-phase)and composition R W I A (RH = 97%) (cf. Table 1)(0)and determined at excess water by Rosen et a1.6(0)using surface tension measurements (25 "C). Solid lines obtained by fitting procedures of the experimental A*. = constant as proposed by Hsiao et aLZ6and by L a ~ ~ g e . ~ ' For comparison we have fitted the data of Rosen obtained at 25 "C assuming a function of the form AA = aAnC bA. A nonlinear fitting procedure yields c = 1.0 f 0.1. That supports a linear fit as done by us. (Cf. also Figure 3.) The 2H NMR relaxation times T1 of heavy water were measured in large water concentration ranges (usually betweenRwlA = 4 and 200). Figure 4a shows one example. The effective water binding energy of the surfactant E , (free energy of hydration) deduced from the dependence of the relaxation time on water concentration by the method of ref 14 are presented in Figure 4b as a function of the number of oxyethylene units n. A strong linear dependence is found (correlation coefficient 0.998). The slope amounts to 7 k J per mole of oxyethylene group. Note, that Olofsson2sfound the same value for the enthalpy of hydration per mole of oxyethylene group using microtitration calorimetry. Considering that the entropic terms of the free energy of hydration are small compared with the enthalpic ones, what holds for short ethylene oxide chains,29this is not surprising and supports our findings.
+
4. Discussion We studied the amount ofwater sorbed by the surfactant RWIA and the surface area& at constant relative humidity RH. Both RwIA and AA as well as the effective water binding energy E , of the surfactant were investigated as a function on the number of oxyethylene groups n. increases by 0.7 and 2.1 (mollmol) at RH = 84.2% and 97%, respectively, adding one oxyethylene group to the ethylene oxide moiety regardless the length of the headgroup at least for n = 1-8 and the phase state. AA determined in the case of lamellar phase (RH = 97%, n = 4-7) grows by 3.7 k and E, by 7 kJ/mol ( n = 2-8) per additional oxyethylene group. These findings are surprising considering gradual changes observed for some quantities along the oxyethylene chain. The order of the oxyethylene groups of &E, (for n = 4,30for n = 2 and 6, unpublished results) and of Triton TX-10031determined by deuterium NMR of deuterated oxyethylene chains decreases rapidly toward the hydroxyl group. There is (26)Hsiao, L.;Dunning, H. N.; Lorenz, P. B. J.Phys. Chem. 1966, 60,657. (27)Lange, H.Kolloid 2. 1986,201,131. (28)Olofsson, G.J. Phys. Chem. 1985,89 (81, 1473. (29)Englander, T.Personal communication. (30)Ward, A. J. I.; Hao Ku; Phillippi, M. A,; Mane, C. Mol. Cryst. Liq. Cryst. 1988,154, 55.
20
40
30
50
RWIA
60
-
40
-
20
-
. 3 , 5 E
u"
0
I
I
I
#
2
4
6
8
n
Figure 4. (a) Plot of the deuterium NMR relaxation time TI of heavy water (25 "c)across the water concentration RWIA in aqueous dispersions of C12E3. Solid line was obtained by fitting the data on the basis of the model of ref 14. (b) Effective water binding energy per mole of surfactant C12Enaccording to the model of ref 14 (25 "C). The solid line is the result of a linear fit of the data yielding 7 kJ mol-' per oxyethylene group.
also a dynamic gradient along the (deuterated) oxyethylene chain according to deuterium TI NMR measurements (ref 31 and unpublished results). Further, several chemically shifted proton NMR peaks for oxyethylene groups have been d e t e ~ t e d . ~ ~ , ~ ~ There is a simple explanation for these apparent contradictions. The order parameter, the NMR TI relaxation time and the NMR chemical shift ofthe individual methylene groups of the oxyethylene chain describe local properties. In contrast the water sorbed by the surfactant, the surface area and the deuterium NMR TI relaxation time of heavy water characterize averaged properties. This can be understood by analyzing NMR results in more detail. In all systems studied by deuterium NMR (La-phase)only one pake d ~ b l e t ~ with J ~ one - ~ relaxation ~,~~ time T114of heavy water was observed. Hence, the water is in fast exchange between sites of different motional characteristics in the NMR time scale (tc
