Langmuir 1986,2, 538-540
538
Letters Porous Glass Plate Immobilized with the Adsorbed Monolayer of Dialkylsilane Amphiphiles. Permeation Control by a Phase Transition of the Adsorbed Monolayer' Yoshio Okahata,* Katsuhiko Ariga, and Osamu Shimizu Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro- ku, Tokyo 152, Japan Received February 6,1986. In Final Form: April 7,1986 Adsorbed monolayers of dialkylsilane amphiphiles were immobilized in a porous glass plate (average pore diameter, 10 nm). The permeation rate of NaCl across the porous glass plate occluded with amphiphiles was less than that acro88 the original porous glass plate. The rate could aLs0 be regulated by phase transitions from solid states to fluid liquid crystalline states of the immobilized monolayers in the pores, depending on the hydrophobic nature of the monolayer surface. The monoalkylsilane amphiphiles reduced the permeability only slightly and did not show the permeation change by the phase transition. Recently, various types of permeability-controllable membranes, in which the immobilized, well-oriented molecular aggregates such as synthetic lipid multibilayers2s3 and nematic liquid crystals4 act as permeation valves, have been developed in connection with a study about the transport mechanism in biological membranes. We have reported that permeability of the bilayer-immobilized film is reversibly controlled by an electric field across the membrane5 or an electrochemical redox reaction in the membrane.6 Sagiv and co-workers reported that the well-organizedmonolayer could be prepared on a flat glass plate by a spontaneous adsorption of monoalkylsilane compounds, as well as by the Langmuir-Blodgett techniq~e.~ In this paper, we prepared the covalently bonded, adsorbed monolayer of dialkylsilane amphiphiles in the porous glass plate and controlled the permeability of NaCl by the phase transition of the adsorbed monolayer, depending on the hydrophilic or hydrophobic nature of the monolayer surface. A schematic illustration of the adsorbed monolayer-immobilized glass plate is shown in Figure 1. Two different types of mono- or dialkylsilane ammonium amphiphiles were newly prepared? one bears the (1)Permeability-Controllable Membranes. 4. For part 3, see: Okahata, Y.; Takenouchi, K. J. Chem. SOC.,Chem. Commun. 1986, 558. (2)Kajiyama, T.; Kumano, A.; Takayanagi, M.;Okahata, Y.;Kunitake, T. Chem. Lett. 1979,645. Kajiyama, T.; Kumano, A.; Takayanagi, M.; Kunitake, T. Chem. Lett. 1984, 915. (3)Okahata, Y.Acc. Chem. Res. 1986,19,57. Okahata, Y.; Iiiuka, N.; Nakamura, G.; Seki, T. J. Chem. SOC.,Perkin Trans. 2 1985, 1591 and references therein. (4) Kajiyama, T.; Nagata, Y.; Washizu, S.; Takayanagi, M. J. Membr. Sci. 1982, 11, 39. Shinkai, S.; Nakamura, S.; Tachiki, S.; Manabe, 0.; Kajiyama, T. J.Am. Chem. SOC.1985,107, 3363. ( 5 ) Okahata, Y.; Taguchi, K.; Seki, T. J.Chem. Soc., Chem. Commun. 1985, 1122. (6) Okahata, Y.; En-na, G.; Taguchi, K.; Seki, T. J. Am. Chem. SOC. 1985.107. 5300. (7) Netzer, L.; Sagiv, J. J.Am. Chem. SOC.1983,105,674. Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 99, 235. Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 101, 201.
Si group a t the end of the hydrophobic alkyl chain (compounds 1 and 2) and the other bears it near the hydrophilic ammonium head group (compounds 3 and 4). The immobilization of adsorbed amphiphiles into the porous glass plate (0.9 X 10 X 10 mm; average pore size, 10 nm; average pore volume, 0.44mL g-l) was carried out as follows. The glass plate was immersed into a toluene solution (0.1 g/ mL) of mono- or dialkylsilane compounds a t room temperature for 4 h under an intermittent ultrasonic irradiation. The glass plate was picked up, gently dried at room temperature, and heated a t 100 OC for 20 min. The plate was washed several times with hot chloroform and then hot water in order to remove the unimmobilized amphiphiles. The surface area of the porous glass plate (0.9 X 10 X 10 mm, 97.8 mg) obtained by N2 adsorption method was slightly decreased from 6.72 to 5.47 m2 by the immobilization of the dialkyl amphiphile 1. The adsorbed amount of 1 was calculated to be 2.65 Fmol from the elemental analysis of C and N of the glass. Thus, the amphiphile 1 was estimated to be able to cover 0.90 m2as a monolayer form? which is ca. 14% of the surface area of pores obtained by N2adsorption method (6.72 m2) and ca. 5 X lo3 times the area of the surface of the glass plate (2 X m2). The X-ray photoelectron spectroscopy (XPS) of both the surface and the cross section of the amphiphile-plugged glass plate showed that amphiphiles were mainly immobilized near the external surface pores but not near the internal pores. These results indicate that amphiphiles can not penetrate into the center through small pores (10-nm diameter) and can form adsorhed monolayers on ~
~
~~~
~
(8)The amphiphile 1 or 2 was prepared from [3-[(w-bromoundecanoyl)amino] propyl]triethoxysilane and Nfl-dimethyloctadecylamine or trimethyamine in ethanol, respectively. Amphiphile 3 was prepared from (3-bromopropyl)triethoxysilaneand a-(dimethylamino)Nfl-didodecylacetoamide in ethanol. Compound 4 was commercially available (Petrarch Systems Inc.). The structures were confirmed by NMR spectra, thin-layer chromatography, and elemental analysis (C, H, N). (9)The area per molecule of 1 was obtained to be 6.0 nm2 from pressure-area isotherms of the monolayer spread on a water surface.
0743-7463/86/2402-0538$01.50/0 0 1986 American Chemical Society
Langmuir, Vol. 2, No. 4, 1986 539
Letters
t/ "C
Porous glass plate ( (average pore diameter :lOnm )
/I
70 60 50 40 30 20 10
NaCl
U A d s d r r b e d immobilized amphiphile monolayers
t
I
-8
3.0
3.2
5
3.4
T - ~ ~ O -I- ~ K
L
Arrphiphiles with hydrophobic surface Et0 CH3 E t O - & & * c O w Ed
Figure 2. Arrhenius plots of NaCl permeation through the porous glass plate occluded by amphiphiles with hydrophilic surfaces. Apparent permeation rates, P/cma s-l, were obtained from the initial slopes of the NaCl permeation as described in previous papers (see ref 1,5, and 6). (a) The original porous glass plate; (b) the glass occluded by monoalkyl amphiphiles 2; (c) the glass occluded by dialkyl amphiphiles 1.
CH3
t/ "C
3 Et? , y 3 E t O - : i W N " W Et0
tH3
70 60 50 40 30 20 IO
4
Figure 1. Schematic illustration of the cross section of the porous glass plate in which mone or dialkylsilane ammonium amphiphiles are immobilized as monolayers in the pores by an adsorption method. the pores near the surface (ca. 14% of all pores) of the porous glass plate, as illustrated in Figure 1. The dialkyl amphiphiles 1 or 3 immobilized in the porous glass plate showed a sharp endothermic peak a t 42 or 37 OC, respectively, from measurements of differential scanning calorimetry (DSC) in aqueous media. The results indicate phase transitions from solid to fluid liquid crystalline states of the adsorbed monolayer. The glass plates covered with monoalkyl amphiphiles 2 and 4 showed no DSC peaks. These results indicate that monolayers of monoalkylsilane amphiphiles do not have well-oriented structures. Permeation of NaCl across the monolayer-immobilized glass plate was measured as follows. The glass plate was positioned between an aqueous solution of NaCl (0.2 mol dm-3) and distilled water. The permeation rate (P,cm2 s-l) of NaCl was obtained by the increase of electric conductance on the distilled water side a t various temperatures. Arrhenius plots of NaCl permeation are shown in Figures 2 and 3. The original, porous glass plate (average pore diameter, 10 nm) was highly permeable to NaCl and gave the usual straight Arrhenius plot. In contrast, when the porous glass plates covered with the dialkylsilane amphiphile 1 or 3 were employed, NaCl permeation was decreased 10-100 times relative t o that of the original glass. Also Arrhenius plots gave inflections, with or without a jump, near the phase transition temperature (T,) of monolayers obtained from DSC measurements. Thus, the adsorbed monolayer in the pores can regulate the permeability of small substances such as NaCl by the phase transition from solid to fluid liquid crystalline state. The glass plates plugged with monolayers of monoalkyl am-
3.0
32
3.4
3.t
T - ~ ~ O-I- ~ K
Figure 3. Arrhenius plots of NaCl permeation through the porous glass plate occluded by amphiphiles with hydrophobic surfaces. (a) The original porous glass plate; (b) the glass occluded by monoalkyl amphiphiles 4; (c) the glass occluded by dialkyl amphiphiles 3. phiphiles 2 and 4 did not reduce the permeability very much. They probably gave the simple straight Arrhenius plots because of the disordered structures of monolayers. Permeability was reduced more when the hydrophobic surface of the occluded amphiphiles was in contact with an aqueous phase (compounds 3 and 4, Figure 3) rather than the hydrophilic surface (compounds 1 and 2, Figure 2). Arrhenius plots of the occluded dialkyl amphiphiles with the hydrophobic surface (compound 3) showed a discontinuous disruption a t T,in contrast with the continuous inflection a t T, with the hydrophilic surface (compound 1). Thus, hydrophilic, charged ions such as Na+ and C1- seem to penetrate hydrophilic surfaces more easily than do hydrophobic ones. Their permeability can be regulated by temperature control near the T,of the dialkyl amphiphile monolayer immobilized in the pores with the hydrophobic surface.
540 Langmuir, Vol. 2, No. 4, 1986 The contact angles against water of the original glass plate and the plates plugged with 1 (hydrophilic surface) and 3 (hydrophobic surface) were 1l0, 61°,and B O O , respectively. This is consistent with both the above permeation results and the XPS analyses, which show that amphiphiles to be mainly immobilized near the surface of the porous glass. In conclusion, the dialkylsilane amphiphiles, which were covalently bonded as a monolayer form in the pores of the glass plate, could regulate its permeability by the phase transition of the amphiphiles. The treated glass plate can
Letters
be used repeatedly without damaging the adsorbed lipid surface because it is covalently bonded to the glass.
Acknowledgment. We thank Central Glass Co., Tokyo, and Asahi Glass Co., Tokyo, for their presents of porous glass plates. We are grateful to profs. T. Kajiyama and N. Higashi (Kyushu University) for the XPS analysis (instruments, ESCA 750, Shimazu Co., Ltd., Kyoto). Registry No. 1, 102630-43-3;2, 102630-44-4;3, 102630-45-5; 4, 102630-46-6; NaCl, 7647-14-5.
