24529044 langmuir blodgett films

8/13/2019 24529044 Langmuir Blodgett Films

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LANGMUIR-BLODGETT FILMS

NAUMAN MITHANI Student ID: 301016320

Performed: 6thOctober 2009 Submitted: 20thOctober 2009

ABSTRACT

Molecular areas (zero-pressure molecular area (Ao)) of four fatty acids were determined by the aid

of Langmuir-Blodgett films using a Langmuir-Blodgett Film apparatus. The analytes and the respective

determined values are 19.9 4.3 (21.5%) 2 for arachidic acid, 27.5 0.2 (0.6%) 2 for docosanoic acid,

56.2 0.9 (1.6%) 2 for oleic acid, and 68.2 0.5 (0.7%) 2 for cholesterol. The obtained values may be

considered experimentally valid since the uncertainty margins are low for all but one analyte. It is also

reasoned that the orientations of the molecules of the analyte fatty acid films were not absolutely vertical

but rather orientated in a slanted position at the interface.


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PURPOSE

Some of the properties and behaviour of Langmuir films of selected compounds will be studied

using a Langmuir-Blodgett Trough apparatus. Quantitative information can be obtained as to the effect

of chain length on the packing size of the molecules studied as well as the effect of the subphase (bulksolvent) composition on the film properties.

1. INTRODUCTION THEORY

A surfactant is a substance that resides at the interface between two dissimilar phases e.g.water & air. It does so because one section of the molecule is hydrophilic, residing in the (polar) aqueous

phase, whilst the other section is hydrophobic and avoids water by positioning itself in the air {consult

Figure 1 for illustration}. Hence the molecule resides at the boundary. (Such molecules that possess

both, hydrophilic and hydrophobic groups are called amphiphilic molecules.)

If a suitable balance, termed the amphiphatic balance, is obtained between the hydrophilic

and -phobic interactions, the result is a stable surface monolayer. If the hydrophobic section of the

molecule is too short then the substance dissolves; if no hydrophilic end is present then the result is

disorganised multi-layer films.

Suitable compounds for Langmuir films:

Generally, Langmuir films may be obtained from long-chain fatty acids and alcohols. The polar

carboxyl !COOH( ) and hydroxyl !OH( ) terminal groups have an affinity for water; thus, this end of

the molecule anchors itself in the (polar) aqueous sub-phase [ref. 2], whilst (the rest of the molecule)

the alkyl chain CH3! CH

2( )

n!( ) , which is hydrophobic orientates itself upwards, (into the air), away

from the aqueous sub-phase. A summary of other functional groups which may yield similar films is

provided below [ref. 2].

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Table 1: Effectiveness of various functional groups to form Langmuir films

Experimental application:

If a surface area is reduced, by sweeping a barrier over the surface, the surfactant molecules are

forced together and eventually form an ordered, compressed monolayer. Thus, a Langmuir film is

formed, as schematically illustrated below:

A monolayer may be formed by slowly depositing the surfactant, dissolved in an appropriate

volatile, water-insoluble solvent, onto the surface of the water (sub-phase). The solvent evaporates and

thus the surfactant spreads as a monolayer on the surface of the water until the equilibrium spreading

pressure is reached; the surfactant cannot spread beyond this point, and as a result, f loating lenses of

the surfactant form.

Quantification and measurements:

Molecules of a dissolved substance i.e. molecules existing in the bulk of a solution experience

forces from surrounding molecules that are equal in/from all directions. In the case of a surfactants

molecules, which are exist at the interface, this is not true. The forces arising from either side of the air-

water interface are unequal, and the molecule is drawn towards the bulk of the aqueous phase. This

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phenomena gives rise to the surface tension (which is also defined as the work required to expand the

surface isothermally by one unit of area [ref. 1, 2] ).

Presence and accumulation of such (surface influencing) molecules tend to reduce the surface

tension. Therefore, the surface tension (surface pressure) is dependant on the surface density of the

molecules (no. of molecules per unit area), which is altered when the monolayer is compressed. The

surface tension is measured by sweeping closed a barrier in order to reduce the area occupied by the f ilm

under observation.

The plot of surface pressure versus area occupied (at a certain temperature) is called the pressure-

area isotherm; any substance that behaves as a surfactant has its own characteristic isotherm. A typical

pressure-area isotherm for amphiphilic surfactants is provided below:

If the monolayer is not subjected to any external pressure, the molecules behave as a two-

dimensional gas and obey the state equation:

!A = KT {eq. 1}

where:

! is thesurface pressure

A is the molecular area

K is the Boltzmann constant

T is the thermodynamic temperature

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Thesurface pressure rises as the barrier closes and causes a partial ordering of the film thus

producing a two-dimensional liquid; as the barrier closes further, and the monolayer is further

compressed orderly, the film behaves as a quasi-solid. Each state of the film has a characteristic trace on

the pressure-area isotherm, marked by a sharp transition at the point of the change of state (requirement:

absence of contaminants that may influence the surface tension).

Eventually (as previously mentioned), the collapse pressure, !c , is reached; the collapse pressure

may be defined as the pressure that a monolayer can tolerate before expulsing molecules from the Langmuir

film [ref. 2]. If this threshold is exceeded, formation of multi-layers occurs. The actual magnitude of !c is

dependant upon the following factors: the substance that comprises the film and experimental conditions such

as temperature, rate of compression, annealing of film, etc. Typically, the magnitude of !cis 50 to 100 mN/m.

Quantitative information about the dimension and shape of the molecule (of the substance) in

question may be obtained from the isotherm. Specifically, the zero-pressure molecular area, Ao. In this

context,Aois defined as the hypothetical area occupied by one molecule in the condensed phase at zero

pressure [ref. 2].

Effect of sub-phase composition:

The characteristics of a monolayer may be affected by presence of dissolved species in the sub-

phase. In this scenario, where the sub-phase is water, pH, dissolved ions and temperature shall have an

effect.

In the case of long-chain alkaloid acids, the pH affects the degree of ionisation, and

consequently, the (net) repulsion experienced amongst the adjacent acid groups. In the case of alkanoic

acids, the presence of divalent ions e.g. Zn2+lead to such interactions:

R

O O-

R

O O-

+ Zn2+

R

O O

Zn

O

R

O

The pH controls the equilibrium:

Zn2++ 2OH-Zn(OH)2

R-COOHR-COO-+ H+

Experimental apparatus:

Thesurface pressure is obtained from the force acting upon a Wilhelmyplatein a NIMATMtrough

{consult [ref. 2] for further details}.

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2. EXPERIMENTAL PROCEDURE

The experiment was performed as per the prescribed procedure of [ref. 2]. Exceptions and

customisations are however, described here.

The acids chosen for the experiment were arachidic, docosanoic and oleic acidsof concentrations of

1.0 0.1 (!10%) mg/mL for arachidic and docosanoic acids and 1.0 0.2 (!20%) mg/mL for oleic

acid. The toluene based solution of cholesterol used was of a concentration of 0.20 0.02 (!10%) mg/

mL.

The experiment was carried out at 24 C (297 K).

3. RESULTS &CALCULATIONS

"4.1: Equation used for calculation of uncertainties

if y = f(x1,x2 , ...,xn)

then

!y = y !x

1

x1

"#$

%&'

2

+!x

2

x2

"#$

%&'

2

+... +!x

n

xn

"#$

%&'

2

{eq. 2}

! if y = f x( )

i.e. when the output is dependant on a single variable

then

"y

y=

"x

x {eq. 3}

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"4.2: Pressure-area isotherms of analytes and respectivezero-pressure area, Ao

Figure 3:surface pressure (mN/m) vs. molecular area (2) pressure-area isotherm of arachidic acid

Figure 4:surface pressure (mN/m) vs. molecular area (2) pressure-area isotherm of docosanoic acid

0

10

20

30

40

50

60

0 10 20 30 40 50

y = -17.541x + 349.39

R!= 0.8894

pressure-area isotherm of arachidic acid

surfacepressure(mN/m)

molecular area (^2)

0

10

20

30

40

50

0 10 20 30 40 50

y = -5.3609x + 147.42

R!= 0.9996

pressure-area isotherm of docosanoic acid

su

rfacepressure(mN/m)

molecular area (^2)

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Figure 5:surface pressure (mN/m) vs. molecular area (2) pressure-area isotherm of oleic acid

Figure 6:surface pressure (mN/m) vs. molecular area (2) pressure-area isotherm of cholesterol

0

10

20

30

40

0 20 40 60 80

y = -1.1592x + 65.172

R!= 0.9948

pressure-area isotherm of oleic acid


molecular area (^2)

0

10

20

30

40

0 50 100 150 200 250 300

y = -0.9726x + 66.402

R!= 0.9994

pressure-area isotherm of cholesterol


molecular area (^2)

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Summary of the least-squares best-fit lines of the solid phase of the analytes:

y = (m)x + c

where:

y is the surface pressure

x is the molecular area (2) m is the gradient of the best-fit line

c is the y-intercept

arachidic acid: y = (-17.541)x + 349.3 ; !m = 2.815, !c = 49.91

docosanoic acid: y = (-5.360)x + 147.2 ; !m = 0.027, !c = 0.5799

oleic acid: y = (-1.159)x + 65.17 ; !m = 0.015, !c = 0.5690

cholesterol y = (-0.9726)x + 66.40 ; !m = 0.006, !c = 0.2107

{uncertainty in the calculations of gradients and intercepts is not explicitly shown}

Sample calculation of zero-pressure molecular area, Ao:

Arachidic acid is used as the example

y = mx +c

y = !17.541x + 349.3

for zero !pressure molecular area,Ao

0 =!17.541x + 349.3

x =!349.3

!17.541

x = 19.913

Ao = 19.9132

calculation of respective uncertainty

if y = f(x1,x2 , ...,xn )

then

!y = y !x1x1

"#$

%&'

2

+ !x2x2

"#$

%&'

2

+... + !xnxn

"#$

%&'

2

!Ao =Ao!mm

"#$

%&'

2

+ !c

c"#$

%&'

2

where :(m =(17.5412.815c = 349.349.91

Ao = 19.913

!Ao = 19.913 2.815

(17.541"#$

%&'

2

+ 49.91

349.3

"#$

%&'

2

!Ao =4.2782

#Ao= 19.913 4.278 (!21.48%) 2

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Summary of zero-pressure molecular area, Ao, of the analytes:

analyte formulae Ao (2)

arachidic acid C19H39COOH 19.913 4.278 (21.48%)

docosanoic acid C21H43COOH 27.4626 0.1756 (0.6284%)

oleic acid C17H33COOH 56.2295 0.8959 (1.557%)

cholesterol C27H45OH 68.2706 0.4632 (0.6784%)

Table 2:summary of analytes and respective experimentally ascertained Aovalues

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4. DISCUSSION

Notes on experimental procedure

Only 15"L of oleic acid was injected onto the apparatus vs. the prescribed 30 "L for the other two

acids. Reason: the isotherm with 30 "L was not appropriate and it was reasoned that the concentration

of oleic acidwas too high.

Formulae and structures of the analytes

arachidic acid

formulae: C20H40O2

2-D structure:

ball-and-stick structure:

docosanoic acid

formulae: C22H44O2

2-D structure:


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oleic acid

formulae: C18H34O2

2-D structure:


cholesterol

formulae: C27H46O

2-D structure:


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Comparison of experimentally obtained molecular areas and average orientation

Arachidicand docosanoicacids are quite similar, with docosanoic acid possessing two more -CH2- groups

in its main carbon chain. At zero-pressure, when the molecule of the analyte is not under any external force, it

may be assumed to be, on average (the word average is used since the molecule is under some degree ofmotion), in a slightly slanted position and not absolutely vertical. Since the molecule of docosanoic acidis larger

and heavier than that of arachidic acid, it may be assumed to be more slanted than the latter due to gravity; it

has, therefore, a greater surface area exposed to the aqueous sub-phase and hence yields a greater molecular

area.

Oleic acidis not a linear molecule, but rather a bent one; it is expected to lie on

the aqueous sub-phase similar to the depiction on the previous page. Given the bent

shape of the molecule, a much greater area shall be exposed to the water sub-phase,

hence a considerably higherAofor oleic acidthan for docosanoic acid$Ao(oleic acid) /Ao

(docosanoic acid) = 2.04 !2;$if the lower arm of oleic acid is assumed to be raised 30

from an imaginary horizontal and docosanoic acidapproximately perpendicular as

believed thensin 90/sin 30 = 2. {These calculations are based on approximations and

should only be considered part of the qualitativeargument.}

Cholesterol has the highestAo. Plausible

reasoning: even in an ordered quasi-solid phase, the

molecule is unlikely to be vertical as shown, there is

only one polar group, the hydroxyl group (2 atoms, 17

Da in mass) in the entire 27-carbon, 387 Da weighing

molecule. The molecule is expected to lean considerably

under gravity; secondly, as shown in the alternate side

view here (figure on right), the molecule has a

measurable thickness; these factors contribute to its

relatively highestAo.

In compressed films, however, the molecules

may be assumed to be more vertical due to the external

forces rendering them physically so.

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Effect of triple-bond in place of double-bond in oleic acid

If the double-bond of oleic acid were to be replaced with a triple-bond, the resulting change in the

geometry of the molecule would render it a linear molecule, it may be likened to arachidicand docosanoicacids.

Since docosanoic acidis C21, arachidic acidC19 and the alkyne-oleic acidC17, (no of carbons in the main chain), the

Ao value of the alkyne-oleic acidis projected to be$Ao(arachidic acid) /Ao(docosanoic acid) !81%

$81% ofAo(arachidic acid) =Ao(alkyne-oleic acid) = 81% "19.91 2

!16.12 2

Thickness of cholesterol film

Application(s) of this technique

Langmuir-Blodgett films are thin films with a highly organised structure and so have many potential

uses e.g. passive layers in metal-insulator semi-conductors and anti-reflective glass that allows through as much

as 99% of visible light.

if ! =m

V a nd V =area " thickness

then

! =m

area " thickness

where

! = 1.052g /cm3

m =amount of cholesterol added = 40L " 0.2g / L = 8g

area # Ao = 68.27062

= 6.827 "10$15 cm2

! =m

area " thickness

thickness =m

area "!=

8 "10$6 g

6.827 "10$15 cm2 "1.052g /cm3

thickness = 1.113 cm # 1.113"108

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5. CONCLUSION

Thezero-pressure molecular area (Ao), as determined experimentally, is 19.9 4.3 (21.5%) 2 for

arachidic acid, 27.5 0.2 (0.6%) 2 for docosanoic acid, 56.2 0.9 (1.6%) 2 for oleic acid, and 68.2 0.5

(0.7%) 2

for cholesterol. The uncertainty margins are low, except for arachidic acidand therefore may beassumed to be valid. Secondly, it was reasoned that the orientation of the analyte molecules was not

absolutely vertical but slanted.

_________________________________________________________________________________________

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6. REFERENCES:

[ref. 1] : Shoemaker, D.P.; Garland, C.W.; Steinfeld, J.I.; Nibler, J.W.; Experiments in physical chemistr y; 4 th

edition; pg 358 369; McGraw-Hill Book Company, Toronto, 1981

[ref. 2]: Johanssion, T., Brodovitch, J.-C.; Chemistry 366W-3 Physical Chemistry Laboratory; August 09

edition; pg X-1 to X-3; SFU Cornerstone, Greater Vancouver, 2009

[ref. 3]: Chemspider.com; arachidic acid; ; 2009-10-19

[ref. 4]: Chemspider.com; docosanoic acid; ; 2009-10-19

[ref. 5]: Chemspider.com; oleic acid; ;

2009-10-19

[ref. 6]: Chemspider.com; cholesterol; ;

2009-10-19

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24529044 langmuir blodgett films

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