用户:Schenad/朗缪尔-布洛杰特薄膜

布儒斯特角显微镜下浮于水面上的，由磷脂构成的朗缪尔薄膜。

朗缪尔-布洛杰特薄膜（Langmuir–Blodgett film），或LB薄膜、LB膜，得名于欧文·朗缪尔和凯瑟琳·B·布洛杰特，是一类由人工制备的单分子层。朗缪尔-布洛杰特薄膜的制备需先在某液体表面形成单分子层；将一固体从单分子层区域缓慢浸入液体（或从液体中抽出）就可以让单分子层转移到固体表面。通过这一方式附着在固体表面的这一单分子层即为朗缪尔-布洛杰特薄膜。需要注意的是，朗缪尔-布洛杰特薄膜特指已被转移到固体表面的单分子层；而朗缪尔薄膜（Langmuir film）指的是漂浮于液体表面的单分子层。通过反复浸入与抽出的操作，液体表面的朗缪尔薄膜可以被一层层地转移到固体表面，而不同的操作次数将制备出不同厚度的单分子层。单分子层既可以是两亲分子（包含亲水的头部和疏水的尾部，例如脂肪酸），也可以由纳米颗粒构成。

历史背景

早在朗缪尔和布洛杰特的发现之前，本杰明·富兰克林就已于1773年完成了相关的实验。当时，富兰克林将大约一汤匙的油倒入一个小池塘之后，约有半英亩水面的涟漪在他倒入油的一瞬间消失了。然而，富兰克林并没有意识到水面上的油膜是单分子层。一个多世纪之后，约翰·斯特拉特，第三代瑞利男爵根据富兰克林的实验计算出当时形成的油膜的厚度约为1.6纳米。

阿尼亚斯·泡克耳斯通过在自家厨房水槽中做的一系列实验展现了如何用屏障控制薄膜的面积。She added that surface tension varies with contamination of water. She used different oils to deduce that surface pressure would not change until area was confined to about 0.2 nm². This work was originally written as a letter to Lord Rayleigh who then helped Agnes Pockels become published in the journal, Nature, in 1891.

Sarfus image of one Langmuir–Blodgett monolayer of stearic acid (thickness=2.4nm).

Agnes Pockels’ work set the stage for Irving Langmuir who continued to work and confirmed Pockels’ results. Using Pockels’ idea, he developed the Langmuir (or Langmuir–Blodgett) trough. His observations indicated that chain length did not impact the affected area since the organic molecules were arranged vertically.

Langmuir’s breakthrough did not occur until he hired Katherine Blodgett as his assistant. Blodgett initially went to seek for a job at General Electric (GE) with Langmuir during her Christmas break of her senior year at Bryn Mawr College, where she received a BA in Physics. Langmuir advised to Blodgett that she should continue her education before working for him. She thereafter attended University of Chicago for her MA in Chemistry. Upon her completion of her Master's, Langmuir hired her as his assistant. However, breakthroughs in surface chemistry happened after she received her PhD degree in 1926 from Cambridge University.

While working for GE, Langmuir and Blodgett discovered that when a solid surface is inserted into an aqueous solution containing organic moieties, the organic molecules will deposit a monolayer homogeneously over the surface. This is the Langmuir–Blodgett film deposition process. Through this work in surface chemistry and with the help of Blodgett, Langmuir was awarded the Nobel Prize in 1932. In addition, Blodgett used Langmuir–Blodgett film to create 99% transparent anti-reflective glass by coating glass with fluorinated organic compounds, forming a simple anti-reflective coating.

物理原理

LB films are formed when amphiphilic molecules like surfactants or nanoparticles interact with air at an air–water interface. Surfactants (or surface-acting agents) are molecules with hydrophobic 'tails' and hydrophilic 'heads'. When surfactant concentration is less than critical micellar concentration (CMC), the surfactant molecules arrange themselves as shown in Figure 1 below. This tendency can be explained by surface-energy considerations. Since the tails are hydrophobic, their exposure to air is favoured over that to water. Similarly, since the heads are hydrophilic, the head–water interaction is more favourable than air–water interaction. The overall effect is reduction in the surface energy (or equivalently, surface tension of water).

Figure 1: Surfactant molecules arranged on an air–water interface

For very small concentrations, far less than critical micellar concentration (CMC), the surfactant molecules execute a random motion on the water–air interface. This motion can be thought to be similar to the motion of ideal-gas molecules enclosed in a container. The corresponding thermodynamic variables for the surfactant system are, surface pressure ($\Pi$), surface area (A) and number of surfactant molecules (N). This system behaves similar to a gas in a container. The density of surfactant molecules as well as the surface pressure increases upon reducing the surface area A ('compression' of the 'gas'). Further compression of the surfactant molecules on the surface shows behavior similar to phase transitions. The ‘gas’ gets compressed into ‘liquid’ and ultimately into a perfectly closed packed array of the surfactant molecules on the surface corresponding to a ‘solid’ state.

The condensed films is subsequently deposited on a solid substrate to create highly organized thin film coatings. Langmuir–Blodgett troughs are used for this purpose.

Besides LB film from surfactants depicted in Figure 1, similar monolayers can also be made from inorganic nanoparticles.

Pressure–area characteristics

Adding a monolayer to the surface reduces the surface tension, and the surface pressure, $\Pi$ is given by the following equation:

$\Pi =\gamma _{0}-\gamma$

where, $\gamma _{0}$ is equal to the surface tension of the water and $\gamma$ is the surface tension due to the monolayer. But the concentration-dependence of surface tension (similar to Langmuir isotherm) is as follows:

$\gamma _{0}-\gamma$ = RTK_HC = – RT$\Gamma$

Thus,

$\Pi =RT\Gamma$ or,
$\Pi A=RT.$

The last equation indicates a relationship similar to ideal gas law. However, it should be noted that the concentration-dependence of surface tension is valid only when the solutions are dilute and concentrations are low. Hence, at very low concentrations of the surfactant, the molecules behave like ideal gas molecules.

Experimentally, the surface pressure is usually measured using the Wilhelmy plate. A pressure sensor/electrobalance arrangement detects the pressure exerted by the monolayer. Also monitored is the area to the side of the barrier which the monolayer resides.

Figure 2. A Wilhelmy plate

A simple force balance on the plate leads to the following equation for the surface pressure:

$\Pi =-\Delta \gamma =-\left[{\frac {\Delta F}{2(t_{p}+w_{p})}}\right]\approx -{\frac {\Delta F}{2w_{p}}}$,

only when $w_{p}\gg t_{p}$.

Here, $l_{p},w_{p}$ and $t_{p}$ are the dimensions of the plate, and $\Delta F$ is the difference in forces. The Wilhelmy plate measurements give pressure – area isotherms that show phase transition-like behaviour of the LB films, as mentioned before (see figure below). In the gaseous phase, there is minimal pressure increase for a decrease in area. This continues until the first transition occurs and there is a proportional increase in pressure with decreasing area. Moving into the solid region is accompanied by another sharp transition to a more severe area dependent pressure. This trend continues up to a point where the molecules are relatively close packed and have very little room to move. Applying an increasing pressure at this point causes the monolayer to become unstable and destroy the monolayer.

Figure 3. (i) Surface pressure – Area isotherms. (ii) Molecular configuration in the three regions marked in the $\Pi$-A curve; (a) gaseous phase, (b) liquid-expanded phase, and (c) condensed phase. (Adapted from Osvaldo N. Oliveira Jr., Brazilian Journal of Physics, vol. 22, no. 2, June 1992)

研究应用

• 朗缪尔-布洛杰特图案生成（Langmuir–Blodgett patterning）是一种用于大面积的、介孔结构图案生成（large-area patterning）的新范式。
• 最近，在制备大面积的二维层状材料的超薄膜时，人们发现朗缪尔-布洛杰特方法不失为一种有效的方式。

另见

• 布儒斯特角显微镜
• 朗缪尔-布洛杰特槽
• 纳米粒子沉积
• 自组装单分子层
• 威氏平板

参考书目

• R. W. Corkery, Langmuir, 1997, 13 (14), 3591–3594
• Osvaldo N. Oliveira Jr., Brazilian Journal of Physics, vol. 22, no. 2, June 1992
• Roberts G G, Pande K P and Barlow, Phys. Technol., Vol. 12, 1981
• Singhal, Rahul. Poly-3-Hexyl Thiopene Langmuir-Blodgett Films for Application to Glucose Biosensor. National Physics Laboratory: Biotechnology and Bioengineering, p 277-282, February 5, 2004. John and Wiley Sons Inc.
• Guo, Yinzhong. Preparation of poly(N-alkylmethacrylamide) Langmuir–Blodgett films for the application to a novel dry-developed positive deep UV resist. Macromolecules, p1115-1118, February 23, 1999. ACS
• Franklin, Benjamin, Of the stilling of Waves by means of Oil. Letter to William Brownrigg and the Reverend Mr. Farish. London, November 7, 1773.
• Pockels, A., Surface Tension, Nature, 1891, 43, 437.
• Blodgett, Katherine B., Use of Interface to Extinguish Reflection of Light from Glass. Physical Review, 1939, 55,
• A. Ulman, An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly, Academic Press, Inc.: San Diego (1991).
• I.R. Peterson, "Langmuir Blodgett Films ", J. Phys. D 23, 4, (1990) 379–95.
• I.R. Peterson, "Langmuir Monolayers", in T.H. Richardson, Ed., Functional Organic and Polymeric Materials Wiley: NY (2000).
• L.S. Miller, D.E. Hookes, P.J. Travers and A.P. Murphy, "A New Type of Langmuir-Blodgett Trough", J. Phys. E 21 (1988) 163–167.
• I.R.Peterson, J.D.Earls. I.R.Girling and G.J.Russell, "Disclinations and Annealing in Fatty-Acid Monolayers", Mol. Cryst. Liq. Cryst. 147 (1987) 141–147.
• A.M.Bibo, C.M.Knobler and I.R.Peterson, "A Monolayer Phase Miscibility Comparison of the Long Chain Fatty Acids and Their Ethyl Esters", J. Phys. Chem. 95 (1991) 5591–5599.

[[Category:纳米技术]] [[Category:物质状态]] [[Category:薄膜]]