Stability Properties of Surfactant-Free Thin Films at Different Ionic

Feb 17, 2015 - films of pure water with a radius of 100 μm also show remarkable stability when created in closed Sheludko cells. To understand thin f...
0 downloads 0 Views 1MB Size
Subscriber access provided by UNIVERSITY OF MICHIGAN LIBRARY

Article

Stability properties of surfactant free thin films at different ionic strengths: Measurements and modelling Frederik Jürgen Lech, Peter A. Wierenga, Harry Gruppen, and Marcel B.J. Meinders Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504933e • Publication Date (Web): 17 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

1

Stability properties of surfactant free thin films at different ionic strengths:

2

Measurements and modelling

3 4

Frederik J. Lecha,b, Peter A. Wierengaa,b, Harry Gruppenb, Marcel B.J. Meindersa,c*

5

a

Top Institute Food and Nutrition, Wageningen, the Netherlands

6

b

Wageningen University, Laboratory of Food Chemistry, Wageningen, the Netherlands

7

c

Wageningen UR, Food and Biobased Research, Wageningen, the Netherlands

8 9

*Corresponding author

10

[email protected]

11

P.O. Box 17, 6700 AA, Wageningen, The Netherlands

12

Telephone: +31 317 480165

13

14

Abstract

15

Foam lamellae are the smallest structural elements in foam. Such lamellae can experimentally be

16

studied by analysis of thin liquid films in glass cells. These thin liquid films usually have to be

17

stabilized against rupture by surface active substances, like proteins or low molecular weight

18

surfactants. However, horizontal thin liquid films of pure water with a radius of 100 µm also show

19

remarkable stability when created in closed Sheludko cells. To understand thin film stability of

20

surfactant free films, the drainage behaviour and rupture times of films of water and NaCl solutions

21

were determined. The drainage was modelled with an extended DLVO model, which combines DLVO

22

and hydrophobic contributions. Good correspondence between experiment and theory is observed,

23

when hydrophobic interactions are included, with fitted values for surface potential (ψ0, water) of −60 ±5

24

mV, hydrophobic strength (Bhb,water) of 0.22 ±0.02 mJ/m2 and a range of the hydrophobic interaction

25

(λhb, water) of 15 ±1 nm in thin liquid films.

1 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 16

26

In addition, Vrij’s rupture criterion was successfully applied to model the stability regions and rupture

27

times of the films. The films of pure water are stable over long time scales (hours) and drain to final

28

thickness > 40 nm if the concentration of electrolytes is low (resistivity 18.2 MQ). With increasing

29

amounts of ions (NaCl) the thin films drain to < 40nm thickness and the rupture stability of the films is

30

reduced from hours to seconds.

31

Keywords

32

Surfactant free air water interface, surface potential, extended DLVO theory, foam stability

33

Introduction

34

An important aspect in the stability of foams is the life time of the thin liquid films that separate the

35

gas bubbles in the foam.1 Usually, surface active materials, like proteins and low-molecular weight

36

surfactants (LMW), are used to stabilize the films. The stability of LMW surfactant stabilized films

37

has been studied extensively and was recently reviewed.2 Thin films of LMW can quantitatively be

38

described by the DLVO.3 In addition, Vrij developed a criterion to describe how the growth of small

39

surface perturbations leads to spontaneous rupture of the film.4 This criterion was used by others to

40

determine the critical thickness in foam films.5

41

Because it is impossible to make foam of pure water, it is expected that thin liquid water films are

42

unstable. Still, horizontal thin liquid films of pure water and dilute NaCl solutions in closed Sheludko

43

cells have been described by several authors.6-8 The films described are usually very unstable since

44

90% of the films ruptured immediately after creation.6 However, stable, free standing (vertical) thin

45

water films in small glass capillaries are also observed. The stability of the films is ascribed to ionizing

46

x-ray photons, thereby inducing some charge between the two interfaces.8 Wang and Yoon showed the

47

possibility to create horizontal thin films of dilute NaCl solutions that were stable for minutes.7 They

48

argue that the stability of the films originates from equilibrium between hydrophobic and electrostatic

49

forces that is mediated by the electrolytes in solution.

2 ACS Paragon Plus Environment

Page 3 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

50

The electrostatic forces in the thin films are considered the result of a surface wall potential of the air-

51

water interface. This surface wall potential was for instance shown using experiments where the

52

electrophoretic mobility of air bubbles in an electric field was measured.9 The origin, polarity and

53

magnitude of the charge, however, is still under heavy debate. Several studies proposed the structuring

54

of water layers close the hydrophobic interface as a source of the charge.10,

55

however, would only be possible on short length scales up to 20 nm.12 The ordering of water over 40

56

nm is far shorter than the equilibrium thickness (120 nm) of the films reported.

57

explain the stability of the films by itself. For the stabilization of the films by structured water, the

58

ordered layers should overlap to create repulsion. Other studies suggest that the surface charge of

59

water could be due to the preferred adsorption of hydroxide ions to the interface.13-15 Typical values

60

for this surface potential, obtained by different experimental techniques (i.e. Volta potential difference

61

measurements or calomel electrodes) and simulations, were recently summarized, and showed values

62

ranging from -1.1 V to +0.5 V.16

63

Next to the surface charge of the water interfaces, the repulsive hydration interaction has been

64

suggested to attribute to the stabilization of a thin film of clean water.17, 18 Whether or not hydrophobic

65

forces play a role in the stability of thin liquid films is still under debate. 19, 20

66

The aim of this study is to gain a generic understanding of the drainage and stability of surfactant free

67

thin liquid films. To achieve this, drainage behaviour and rupture times of surfactant free thin films,

68

created in Sheludko cells, were described with a model using extended DLVO (including hydrophobic

69

interactions) and the stability criterion of thin films. 4

70

Experimental Section

71

Materials

72

Ultra-pure water (Millipore, EMD Millipore Corp., Billerica, MA, USA) with a surface tension of 72.6

73

± 0.3 mN/m at 20 °C stable over 1 h, a resistivity of 18.6 MΩ*cm and residual total organic carbon of

74

3 ppb was used as is to make the films and the NaCl solutions. NaCl was purchased from Sigma

75

Aldrich (Zwijndrecht, The Netherlands) and roasted at 700 °C for three days to eliminate possible

11

7

This structuring,

Hence, it cannot

3 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 16

76

organic contaminants. A stock solution of 0.1 M NaCl was prepared and diluted to make solutions for

77

the thin film experiments. The solutions were prepared on the day of the experiment and kept at the

78

same temperature (20°C) as the experimental set up before making the thin film. All glassware was

79

soaked in dilute (0.1 M) NaOH solution, thoroughly rinsed with ultra-pure water and dried in an oven

80

at 60°C before use.

81

The total ionic strength of water and NaCl solutions is given by (equation 1)

82

 =  ∑  

83

with the sum of all ions i in the solution, ρi∞ is the molar concentration of the ion in bulk at infinite

84

distance from the interface and z is the valence of the ions. Note that this includes the OH- and H+ ions

85

determining the pH of the water films, which was measured to be pH 5.6, for all experiments. The

86

total ionic strength (INaCl + IpH 5.6) of a 0.700 mM NaCl solution is 0.703 mM.

87

Methods

88

Thin liquid films (radius = 100 µm) were made in a modified Scheludko cell as described elsewhere.1

89

A schematic drawing, including relevant dimensions of the setup is depicted in figure 1. The Sheludko

90

cell is encased in glass, closed with a glass cover. The casing has a reservoir of water at the bottom to

91

ensure a relative humidity of 100 % in the cell during the experiments.

92

The thickness of the thin films was determined by image analysis obtained by micro-interferometry

93

using a microscope (Axio plan 01, Zeiss, Jena, Germany), in reflected light mode, equipped with a

94

five mega pixel CCD camera (Mightex Systems, Pleasanton, CA, USA). Images of the thin films,

95

recorded with the CCD camera, were processed by a software script (developed at the Laboratory of

96

Food Chemistry) for Matlab (Software version 2013b, MathWorks. Natick, MA, USA). It calculated

97

the average intensity of light in a defined area (at the centre) of the thin film. The area is typically

98

1963 µm², which is ~6 % of total area of the thin film. The thickness was calculated according to

99

equation (2):

100



(1)



ℎ =   ± √Δ

(2)

4 ACS Paragon Plus Environment

Page 5 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

101

with λ as the wavelength of the light (546 nm), n the refractive index of water, l the order of

102

interference and ∆ as (I-Imin)/(Imax-Imin) where is I is the average of the light intensity at each time

103

point, Imin, the minimum intensity (which is measured in the Sheludko cell without a water film

104

present) and Imax as the maximum intensity.

105

The film thickness was measured from the moment it reaches a radius of 100 µm up to 1 hour of

106

lifetime. If a film did not break within one hour, it was considered stable. In this case, the life time will

107

be given as one hour.

108

If the thin films ruptured during this time, at least 20 repetitions were used to calculate the average and

109

standard deviation of the rupture time and thickness at which rupture occurred. The rupture time is the

110

time from reaching a 200 µm wide film until the film breaks. In case of stable films, solutions were

111

measured 4 times, in case of rupturing films; the experiment was repeated at least 20 times.

112

The whole setup, thin film cell, pure water and salt solution were equilibrated to room temperature (20

113

°C) on the microscope table for at least 1 hour prior to the measurement. Liquid was drawn into the

114

capillary by a syringe (500 µL, Hamilton, Reno, NV, USA) and left to equilibrate for 10 minutes

115

before a thin film was made by drawing more solution into the capillary.

116

Modelling and theoretical background

117

Surface forces

118

When two bubbles come into contact, the thin liquid film between the bubbles will drain due to the

119

capillary pressure, caused by the difference in curvature of the thin film and the adjacent meniscus (or

120

Plateau border in foam). When the film thickness becomes smaller than about 100 nm, attractive and

121

repulsive interactions between the interfaces of the film become important and determine the stability

122

of the thin liquid film. Attractive interactions, enhancing film thinning and destabilizing the film,

123

include long range dispersion forces summarized as van der Waals (vdW) forces. Repulsive

124

interactions, acting against film thinning and thereby stabilizing the film, include electrostatic

125

interactions (ES) between two similarly charged surfaces and steric interactions. The vdW and ES

126

interactions are described quantitatively by the DLVO theory. Besides these DLVO forces, other

5 ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 16

127

interactions are recognized that might play a role in film stability. The interactions include steric

128

repulsion (excluded volume), hydration, repulsive and attractive hydrophobic forces as well as

129

oscillatory solvation forces.2, 11, 18 The sum of these surface forces determine the disjoining pressure

130

Π(h), which depends on the film thickness h.

131

The van der Waals contribution is calculated from (equation 3)

132

Πvw = − " #! $

133

with AH as the compound Hamaker constant of a water film in air AH = 4*10-20 J. 18, 21

134

The contribution of the electrostatic interaction to the disjoining pressure is approximated by equation

135

(4)18

136

Πel = 64)* + ,  e-.#

137

with

138

, = tanh 378 6:;

139

and

140

? @AB 4

(6)

9

C6 CD 89:

*

hb

#

(7)

hb

6 ACS Paragon Plus Environment

Page 7 of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

152

The theoretical individual attractive and repulsive contributions for an air water interface as well as the

153

resulting disjoining pressure as function of film thickness are calculated (figure 2).

154

Film drainage

155

The drainage of the film is modelled by considering the flow between two circular plane-parallel

156

plates. 23, 24 The change of the film thickness h with time t was described by (equation 8)

157

d# dL

158

where cf is a factor that accounts for the interaction of the fluid with the interface. For an immobile

159

interface, as we will consider here, cf = 1. Furthermore, µ = 10-3 kg/m/s is the viscosity of the water

160

phase, Rf is the radius of the film (in our experimental set-up Rf = 100 µm), and

161

ΔS = Q − Π

162

is the driving capillary pressure, with γ is the surface tension of the water-air interface (in the

163

calculation we used γ= 72 mN/m), 1/Rm is the (mean) curvature of the meniscus and Π is the total

164

disjoining pressure (Π= ΠVW+Πel+Πhb). Rm~1.25 mm, assuming complete wetting of the inner wall of

165

the Sheludko cell. (see figure 1).

166

Starting from an initial thick film with thickness h0, the film will drain. Depending on the precise

167

shape of Π(h), the film can drain to a stable equilibrium thickness where the disjoining pressure equals

168

the capillary pressure (figure 2 B). If the disjoining pressure is smaller than the capillary pressure the

169

thin film can rupture spontaneously.

170

Film rupture

171

Film rupture of surfactant stabilized films has successfully been described by the theory developed by

172

Sheludko and Vrij.4,

173

curve. In these regions of instability, small spontaneous surface perturbations grow in time until both

174

interfaces touch each other and the film ruptures. The unstable regions are given by the Vrij criterion

175

(10)

176

VW V#

M # $

= − OPN QR ΔS

(8)

N

T

(9)

U