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Chinese Science Bulletin © 2007
SCIENCE IN CHINA PRESS
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Investigation of adsorption of surfactant at the air-water interface with quantum chemistry method CHEN MeiLing1, WANG ZhengWu2†, WANG HaiJun1, ZHANG GeXin1 & TAO FuMing3 1
School of Chemistry and Materials Engineering, Southern Yangtze University, Wuxi 214122, China; Department of Food Science & Technology, School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai 201101, China; 3 Department of Chemistry and Biochemistry, California State University, Fullerton, California 92834, USA 2
Density functional theory (DFT) of quantum chemistry was used to optimize the configuration of the −
anionic surfactant complexes CH3(CH2)7OSO 3 (H2O)n (n=0―6) and calculate their molecular frequencies −
at the B3LYP/6-311+G* level. The interaction of CH3(CH2)7OSO 3 with 1 to 6 water molecules was investigated at the air-water interface with DFT. The results revealed that the hydration shell was formed in −
the form of H-bond between the hydrophilic group of CH3(CH2)7OSO 3 and 6 waters. The strength of H-bonds belongs to medium. Binding free energy revealed that the hydration shell was stable. The increase of the number of water molecules will cause increases of the total charge of hydrophilic group and S10-O9-C8 bond angle, but decreases of the alkyl chain length and the bond lengths of S10-O11, S10-O12 as well as S10-O13, respectively.
A surfactant molecule has an amphiphilic structure and a tendency to escape from solution. Therefore, surfactants can concentrate from the solution and adsorb each other together to form a parallel single molecular layer easily at the interface when dissolved in water. This is a speculation from molecule structure of surfactant[1]. By measuring the interfacial tension and using the Gibbs adsorption equation, we can conduct experiment analysis of the status of adsorption of surfactants at the air-water interface[1,2]. And by using molecular dynamics simulation, the adsorption of surfactants at the interface also can be ― predicted[3 5]. But neither of the above described the changes of molecular structure of surfactants adsorbed at the interface and the essential of interaction between surfactant and solvent at the molecular level. Because of the complicated molecular structure of the surfactant, which generally consists of from dozens of atoms to hundreds of atoms and possesses the amphiphilic structure, and very little work has been reported on the adsorption of the surfactant at the interface with quanwww.scichina.com
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tum chemistry method so far. Ryszard and his co-workers[6] investigated the interaction of an alkyl ammonium surfactant with only three water molecules. Yan et al.[7] investigated the characteristics of electronic structure for surfactants in solution by using Onsager model. But neither of them described the mechanism of interaction between surfactant and solvent at the molecular level. Hence, if the interaction of surfactant with solvent at the air/water interface can be investigated with quantum chemistry method, it will not only provide theoretical reference for explaining the adsorption of surfactant at the interface, but also enlarge the application of the quantum chemistry method in surfactant area. In the present work, with the help of quantum chemistry, the interactions of a widely used anionic surfactant, Received May 19, 2006; accepted January 28, 2007 doi: 10.1007/s11434-007-0201-5 † Corresponding author (email:
[email protected]) Supported by the National Natural Science Foundations of China (Grant Nos. 20676051 and 20573048) and the Important Construction Project (Category A) of Shanghai Jiao Tong University (Grant No. AE150085)
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PHYSICAL CHEMISTRY
anionic surfactant, quantum chemistry method, density functional theory
−
CH3(CH2)mOSO 3 (m=8), with 6 water molecules were discussed. The changes of the charge of hydrophilic group, the alkyl chain length and the bond distance were also investigated, respectively. For the complexes of −
CH3(CH2)7OSO 3 (H2O)n (n=1―6), the binding energy, the dipole moments and the effect of each water molecule on the complexes were studied，respectively. And these calculated results show the change of the surfactant structure and the adsorption mechanism at the interface phase.
1 Method Density functional theory (DFT) has been proved to be an effective method for H-bonded complexes in recent ― years[7 13]. Therefore, B3LYP/6-311+G(d) was used to −
optimize cluster geometries of CH3(CH2)7OSO 3 (H2O)n (n=1―6) in this work. Harmonic frequencies were calculated to confirm the optimized complexes geometry corresponding to the minimum energy. The binding energy D0 (ZPE-corrected binding energy) was given as a difference between the total energy of the complexes −
and the sum of energy of isolated CH3(CH2)7OSO 3 and individual H2O molecules. Incremental binding energy ΔD0 was calculated to investigate the effect of additional water molecule on the hydration shell. The binding energy and the incremental binding energy were obtained according to the following expressions: D0 = −ΔE = ECH ΔD0 = D0CH
The
− 3 (CH 2 )7 OSO3
+ nEH 2O − ECH
− 2 (CH 3 )7 OSO3 (H 2 O) n
isolated
molecule
− D0CH
− 3 (CH 2 ) 7 OSO3 (H 2 O) n
− 2 (CH 3 ) 7 OSO3 (H 2 O) n-1
energy
for
surfactant
− CH3(CH2)7OSO 3
and H2O was respectively calculated with the basis set 6-311+G(d). Atomic charges were calculated using the Milliken population analyses. All calculations were performed with the Gaussian03 program package[14].
2 Results and discussion The structure of the hydration shell of surfactant −
CH3(CH2)7OSO 3 with 6 labeled water molecules is presented in Figure 1 and the corresponding data of H-bond structure of the shell are listed in Table 1. 1452
Figure 1
The structure of the hydration shell of surfactant −
CH3(CH2)7OSO3 with 6 water molecules. Table 1
The H-bonds data of hydration shell
Y―H…X Y―X Hydrogen bonds Dist. (Å) Hydrophilic-water hydrogen bonds 2.71 O(W6)－H…O11 2.96 O(W5)－H…O12 2.99 O(W1)－H…O13 3.01 O(W3)－H…O13 Water-water hydrogen bonds 2.92 O(W4)－H…O(W1) 2.93 O(W1)－H…O(W6) 2.89 O(W2)－H…O(W6) 2.86 O(W5)－H…O(W2) 2.83 O(W3)－H…O(W5) 2.89 O(W4)－H…O(W3)
H…X Dist. (Å)
Y―H…X Angle (°)
1.74 2.10 2.05 2.07
166.7 146.3 162.6 163.7
1.97 2.01 1.97 1.98 1.89 1.94
163.9 157.6 158.1 148.9 159.9 164.3 −
2.1 H-bonds between CH3(CH2)7OSO 3 and water molecules As shown in Table 1, all H-bonds lengths (Y―X distance) are in the range of 2.71―3.01 Å, the H…X bond length is 1.74―2.01 Å, and the O―H…O bond angle, 146°―167°. The strength of all H-bonds belongs to medium[15]. As shown in Figure 1 and Table 1, the six waters forming H-bonds may be divided into two groups: one is the waters forming H-bonds directly with −
CH3(CH2)7OSO 3, which include water molecules W1, W3, W5 and W6; the other is the waters forming H-bonds just with the first type of water molecules, such as water molecules W2 and W4, which act as bridges in the shell. Accordingly, all H-bonds also can be divided −
into two groups: (1) H-bonds between CH3(CH2)7OSO 3
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H…O12) broken, then forming three new H-bonds: O(W3)―H…O(W5), O(W5)―H…O12 and O(W5)― H…O(W2). We can conclude from Table 2 that configurations of water molecules and hydrophilic group would be affected by the increase of water molecule number. Consequently, the orientation of water molecules around surfactants, the binding energy, the charge on S atom and the total charge of hydrophilic group also have been much affected.
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and water molecules. The H-bond distances are 2.71, 2.96, 2.99 and 3.01 Å, respectively, and their average distance of H-bonds is 2.92 Å. A phenomenon should be emphasized, that is a much stronger H-bond, O(W6)― H…O11, was formed between W6 and O11 in this group. The reason is that the addition of the sixth water molecule W6 makes the H-bonds (O(W2)―H…O11 and O(W1)―H…O11) broken, and consequently forms a much stronger H-bond (O(W6)―H…O11). (2) H-bonds between water molecules, which gives the corresponding average H-bond distance 2.89 Å. The average dis−
tance of H-bonds between CH3(CH2)7OSO 3 and water molecules is longer (0.03 Å) than that between water molecules, and we can conclude from the difference that −
−
CH3(CH2)7OSO 3 (H2O)n (n=4) was different from that when n = 1, 2 and 3. Such special change also appeared on the total negative charge of the hydrophilic group when n = 5. A comparison between Figures 2 and 3 indicates that the addition of the fifth water molecule W5 makes H-bonds (O(W3)―H…O12 and O(W2)― Table 2
−
Figure 2
The structure of CH3(CH2)7OSO3 (H2O)4.
Figure 3
The structure of CH3(CH2)7OSO3 (H2O)5.
−
It was worth noting that the O9 atom did not form H-bond with any water molecule. The reason is that this atom is different from the atoms O11, O12 and O13. As shown in Table 2, the net charge on the O9 atom is less than that of any other O atom in the hydrophilic group, which makes the interaction between the O9 atom and water molecules weaker, and therefore water molecules firstly form H-bonds with the atoms O11, O12 and O13.
The total charge of the hydrophilic group and charges of each atom in this group with different water molecules n
q(SO
0
1
2
3
4
5
6
)
−0.722
−0.718
−0.813
−0.947
−1.004
−0.811
−0.912
q(O9) q (S10) q(O11) q(O12) q(O13)
−0.158 0.649 −0.428 −0.357 −0.428
−0.111 0.630 −0.475 −0.286 −0.475
−0.040 0.563 −0.552 −0.344 −0.440
0.048 0.445 −0.525 −0.402 −0.513
−0.007 0.477 −0.493 −0.375 −0.606
−0.004 0.609 −0.504 −0.410 −0.502
−0.022 0.482 −0.464 −0.399 −0.509
− 4
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H-bonds between CH3(CH2)7OSO 3 and water molecules are weaker than those between water molecules. Table 2 lists the total charge of the hydrophilic group and charge of each atom in this group with different water molecules. It is seen that though the charge on S atom decreases from 0.649 to 0.482, the charge is always positive. Thus S atom is considered as an electron donor, which transfers electrons to the adjacent O atoms. The total charge of hydrophilic group increases from −0.722 to −0.912 and is negative indicating that the hydrophilic group is an electron donor (or proton acceptor) for H-bonds and water is an electron acceptor (or proton donor) for H-bonds. This result proves that the function of the hydrophilic group is to provide interaction force between the surfactant molecules and the solvent molecules when surfactants were adsorbed at the interface[15]. It also can be seen from Table 2 that the change tendency of charge on S atom in the hydrated complex
2.2 The structure of hydrophilic group and the changes of hydrophobic group length −
The optimized structure of CH3(CH2)7OSO 3 is shown in Figure 4, along with the labeling of the atoms. The changes of bond angle of S10-O9-C (in degree) and some chain lengths (Å) with different water molecules are presented in Table 3. As shown Table 3, the chain length of R (C1―C8) decreases by 0.015 Å from 8.997 to 8.982 Å. But the total chain length of R (C1―S10) decreases by 0.051 Å from 11.630 to 11.579 Å. It is thus clear that the main change of chain length is in the distance between C8 and S10. Table 3 also shows that the bond distance of R (C8―O9), on the contrary, increases by 0.021 Å, but the bond distance of R (O9― S10) is shortened by 0.066 Å. It fully proves that the C8 －O9 bond is weakened and the O9―S10 bond is strengthened because of the interaction between the hydrophilic group and H2 O molecules. Table 3 also shows that the bond distances of R(S10―O11), R(S10―O12) and R(S10―O13) are shortened by 0.025, 0.026 and 0.028 Å, respectively, which indicates that these bonds are strengthened in the presence of water molecules.
actions of interaction of hydrophilic group with water molecules and the van der Waals interaction of the hydrophobic groups, surfactants are titled easily and closely packed at the adsorption layer. This result is in accord with the results of molecule dynamic simulation of ref. [3] and in good agreement with the results of ref. [16]. 2.3 The changes of binding energy Table 4 lists the total energy, binding energy, binding free energy and dipole moments of the hydrated com−
plex CH3(CH2)7OSO 3 (H2O)n. The stability of the hydration shell can be described with ZPE-corrected binding energy D0. Total energy E represents the minimum energy of optimized structure. It can be seen from the analysis of 2.1 that there is a steady net of H-bonded in the hydration shell. If we remove any one of water molecules, the H-bonded stable net will be destroyed. It can be seen from Table 4 that the binding energy D0 increases from 54.27 to 279.41 kJ/mol, which indicates that the stabilization of studied six hydrated complexes −
CH3(CH2)7OSO 3 (H2O)n (n=1―6) and then the stabilization of the hydration shell increase as the H2O molecule number increases. The incremental binding energy ΔD0 can be used to describe the effect of the additional one water molecule on the binding energy. The larger the value of ΔD0 is, the stronger effect on binding energy the water molecule has. The value of ΔD0 is related to the hydrated sites of water molecules. As shown in −
Figure 4
−
Structure formula and the partial number of CH3(CH2)7OSO3.
The obvious change was also found at the bond angle of ∠S10-O9-C8 that increases from 114.99° to 116.10° (Table 3). It reveals that because of the interaction between surfactant and solvent, the hydrophobic chain has the tendency to be titled at the interface. Therefore, it can be predicted that under the combined Table 3
The changes of bond angle of ∠S10-O9-C (in degree) and chain lengths (Å) with different numbers of H2O molecules (n)
n R(C1―C8) R(C8―O9) R(O9―S10) R(C1―S10) R(S10―O11) R(S10―O12) R(S10―O13) ∠S10-O9-C8
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Table 4，for the hydrated complexes CH3(CH2)7OSO 3 (H2O)n with the number n = 1, 2, 5 of H2O, the increase of ΔD0 is relatively large, respectively. Accordingly, the stabilization of hydrated complexes with the number n=1, 2, 5 of H2O is relatively poor, respectively; similarly, for the hydrated complexes with the number n=3, 4, 6 of H2O, ΔD0 is relatively large and the stabilization is relatively poor, respectively; binding free energy
0 8.997 1.419 1.717 11.630 1.479 1.469 1.479 114.99
1 8.994 1.426 1.695 11.614 1.486 1.463 1.486 115.46
2 8.989 1.431 1.679 11.602 1.492 1.468 1.479 115.76
3 8.987 1.436 1.666 11.593 1.485 1.472 1.485 116.12
4 8.984 1.437 1.659 11.587 1.483 1.471 1.491 116.18
5 8.983 1.439 1.657 11.583 1.455 1.441 1.452 117.98
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6 8.982 1.440 1.651 11.579 1.454 1.443 1.451 117.99
−
E −2.86×106 −3.06×106 −3.27×106 −3.47×106 −3.67×106 −3.87×106
De 64.81 121.59 173.30 229.03 294.89 350.87
ΔZPE −10.50 −19.75 −28.58 −42.43 −58.58 −71.46
−ΔG increases progressively as the number of H2O molecules increases. This further improves that the hydration shell is stable. Table 4 also shows that binding energy has significant change from n=4 to 5 and from 5 to 6. This phenomenon can be explained with the changes of configurations of hydrated complexes. The reason of the changes of binding energy (for n=4 to 5 ) are already discussed in the earlier section (2.1); when the sixth water molecule W6 is added, the configuration of hydrated complexes is also changed (Figure 3 and Figure 1), the (O(W2)―H…O11 and O(W1)―H…O11) H-bonds are broken and then three new H-bonds (O(W2)―H…O(W6), O(W6)―H…O11 and O(W1)―H…O(W6)) are formed. So the binding energy has obvious change between n=5 and 6. 2.4 Relationship between dipole moment and binding energy It can be found from Table 2 that there is similar change tendency for the dipole moment μ and the incremental binding energy ΔD0. There is a minimum for both dipole moment and incremental binding energy at n = 4. The larger the dipole moment is, the stronger the polarity of hydrate, the greater the interaction of surfactant with 1 2 3 4 5 6
7
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Zhao G X. Surfactant Physical Chemistry (revised) (in Chinese). Beijing: Peking University Press, 1991. 75 Gang C. Interaction decay of nonionic surfactants at water surfaces. Chem Phys Lett, 2003, 376(5): 758―760 Dominguez H, Berkowitz M L. Computer simulations of sodium Dodecyl sulfate at liquid/liquid and liquid/vapor interfaces. J Phys Chem B, 2000, 104(22): 5302―5308 Wei Y L, Rong Z M, Liu H L. Dynamic lattice Monte Carlo simulation of adsorption of Nonionic-surfactant s on oil-water interface. J Chem Ind Eng (in Chinese), 2005, 56(5): 894―899 Dong F L, Li Y, Zhang P. Mesoscopic simulation study on the orientation of surfactants adsorbed at the liquid/liquid interface. Chem Phys Lett, 2004, 399(1): 215―219 Ryszard Z, Henryk S. Structure of stable double-ionic model water clusters of quaternary alkyl ammonium surfactants with some monovalent counterions as derived by the DFT method. Int J Quant Chem, 2004, 99(5): 724―734 Yan X C, Luo D M, Zeng H, et al. Study on characters of electronic structures for anionic surfactants with different hydrophobic bases in gas and solvent using onsager model and ab initio method. Acta Chim Sin (in Chinese), 2004, 62(19): 1948―1950 Christian T, Daniel B A, Handy N C. Predicting the binding energies of H-bonded complexes: A comparative DFT study. Phys Chem
ΔD0 54.27 47.53 42.89 41.88 49.74 43.10
D0 54.27 101.84 144.72 186.61 236.31 279.41
−ΔG 75.81 146.48 212.55 273.47 334.59 394.05
μ
7.41×10−29 6.40×10−29 6.29×10−29 5.86×10−29 7.72×10−29 7.54×10−29
water molecules and the larger the increment of binding energy will be.
3 Conclusions By using the density functional theory of quantum −
chemistry, the interaction of CH3(CH2)7OSO 3 with 6 water molecules has been investigated at a molecular level. The results revealed that a hydration shell was formed between hydrophilic group and water molecules in the form of H-bonds. The H-bonds between CH3−
(CH2)7OSO 3 and water molecules are weaker than those formed just by water molecules only. The interaction of surfactants with water molecules makes the chain length of hydrophobic group shortened, the total charges of hydrophilic group and the bond angle of S10-O9-C8 increased and the bond distances of S10―O11, S10―O12 as well as S10―O13 are shortened. Such information can hardly be obtained by molecule simulation. The approach of investigating the interaction between surfactant and water molecules by using quantum mechanical method in this work may provide a new idea for deeply researching interaction of surfactants with different solvents. 9 10 11 12
13 14 15 16
Chem Phys, 1999, 1(17): 3939―3947 Alavi S, Thompson D L. H-bonding and proton transfer in small hydroxylammonium nitrate clusters: A theoretical study. J Chem Phys, 2003, 119(8): 4274 Cai Z L, Jeffrey R R. The first singlet (n,π*) and (π,π*) excited states of the hydrogen-bonded complex between water and pyridine. J Phys Chem A, 2002, 106(37): 8769―8778 Zhang B, Cai Y, Mu X L, et al. Multiphoton ionization and density functional studies of pyrimidine-(water)n clusters. J Chem Phys, 2002, 117(8): 3701―3710 Ahmed D, Ludwik A, Guido M. Density functional theory study of the hydrogen-bonded pyridine-H2O complex: A comparison with RHF and MP2 methods and with experimental data. J Phys Chem A, 2000, 104(10): 2112―2119 Oleg V S, Leonid G, Jerzy L. Does the hydrated cytosine molecule retain the canonical structure? A DFT Study. J Phys Chem B, 2000, 104(22): 5357―5361 Frisch M J, et al. GAUSSIAN 03, Revision C. 01, Gaussian, Inc., Wallingford, CT, 2004 Zhou G D, Duan L Y. The Base of Structural Chemistry (in Chinese). 3rd ed. Beijing: Peking University Press, 2003. 225 Rosen M J. Surfactants and Interfacial Phenomena. New York: Wiley, 1987. 210
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n 1 2 3 4 5 6
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Total energy E (kJ/mol), binding energy De (kJ/mol), zero-point energy correction ΔZPE (kJ/mol), ZPE-corrected binding energy D0 (kJ/mol),
Table 4
incremental binding energy ΔD0, binding free energy −ΔG (kJ/mol) and dipole moments μ (C·m) of hydrated complexes CH3(CH2)7OSO3 (H2O)n(n=0―6)
