University of Amsterdam
Faculty of Science
Van der Waals-Zeeman Institute
Report Bachelor Project Physics and Astronomy,
size 15 EC, conducted between 02-03-2016 and
11-07-2016
Surface tension analysis of
oppositely charged
polyelectrolytes and surfactants
Author:
Bas Berend Kluft
10369058
Daily Supervisor:
dr. Bijoy Bera
Supervisor:
prof. dr. D. Bonn
Second assessor:
dr. N.F. Shahidzadeh
July 11, 2016
Abstract:
Surface tension measurements can be used to study surface properties of SDS and PDADMAC solutions. However, (Hu, D., & Chou, K. C. (2014))6 claimed that surface tension measurements are
inaccurate indicators for studying surface tension properties of SDS-PDADMAC mixtures. This study showed by doing surface tension analysis that surface tension measurements remain as reliable as ever for investigating surface properties. Furthermore, this study found that electrostatic interactions and subsequent aggregates between SDS and PDADMAC influences the surface tension. The aqueous solution of SDS and PDADMAC have been studied for various temperatures and no significant influences of temperature on the surface tension has been found.
1
Table of Contents
ABSTRACT: ... 0
INTRODUCTION: ... 2
THEORETICAL FRAMEWORK: ... 4
CHARACTERISTICS OF SURFACTANTS AND POLYELECTROLYTES ... 4
BASIC THERMODYNAMICS ... 7
METHODS ... 9
MATERIALS ... 9
SURFACE TENSION MEASUREMENTS ... 9
TEMPERATURE CHANGE ...11
MICROSCOPIC PICTURES...11
RESULTS ... 12
SURFACE TENSION MEASUREMENTS AT ROOM TEMPERATURE (T=293K) ...12
SURFACE TENSION MEASUREMENTS AT VARIOUS TEMPERATURES ...16
DISCUSSION ... 16
CONCLUSION ... 17
ACKNOWLEDGMENT ... 17
BIBLIOGRAPHIC REFERENCES ... 18
2
Introduction:
Surfactants are amphiphilic compounds, meaning that they contain a non-polar tail or hydrophilic group and a polar head or hydrophobic group. Therefore, the tail is water-insoluble and the head water-soluble.1 These coupled hydrophilic group and hydrophobic group make surfactants unique.
Surfactants have many interesting properties, such as the wetting and penetration effect, emulsification, dispersion, foaming and detergency. Due to these vast range of properties, surfactants have sparked interest from both scientists and industry in investigating the mechanisms behind these phenomena and their implications.
Polyelectrolytes are charged polymers and have the same properties as both polymers and electrolytes.2 When polyelectrolytes and surfactants of opposite charge are added to the same
solution, polyelectrolytes can interact with the surfactants, which leads to changes in both surface and bulk properties of the solution. In particular, polyelectrolytes and oppositely charged surfactants can form hydrophobic complexes at the critical aggregation concentration (cac), which is a lower surfactant concentration than the critical micelle concentration (cmc),3 due to the strong
electrostatic interactions between the hydrophobic group of the surfactant and the main chain of the polymer. Mixtures of surfactants and polyelectrolytes can also create surface gels at the air-water interface at low or at high surfactant concentrations 4 and are used as thickeners in water
based formulations such as paints, drilling muds etc.5 These properties of polyelectrolytes and
surfactants complexes and their industrial applications has made electrostatic interactions between surfactants and polyelectrolytes a rapidly growing field of interest in colloid science. In this study we focus on the aqueous solution of sodium dodecyl sulfate (SDS) and polydiallyldimethylammonium chloride (PDADMAC). The molecular structures of SDS and PDADMAC are shown in figure 1 (inset)6.
SDS is an anionic surfactant carrying negatively charged ions in their polar head and PDADMAC is a cationic polyelectrolyte, meaning SDS and PDADMAC are oppositely charged molecules, which can
1
Falbe, J. (Ed.). (2012). Surfactants in consumer products: Theory, Technology and Application. Springer Science & Business Media.
2
Hara, M. (Ed.). (1992). Polyelectrolytes: science and technology. CRC Press. 3
Chu, D. Y., & Thomas, J. K. (1986). Effect of cationic surfactants on the conformation transition of poly (methacrylic acid). Journal of the American Chemical Society, 108(20), 6270-6276.
4
Monteux, C., Williams, C. E., Meunier, J., Anthony, O., & Bergeron, V. (2004). Adsorption of oppositely charged polyelectrolyte/surfactant complexes at the air/water interface: formation of interfacial gels. Langmuir,20(1), 57-63.
5
Goddard, E. D., & Ananthapadmanabhan, K. P. (1993). Interactions of surfactants with polymers and proteins. CRC press.
6
Hu, D., & Chou, K. C. (2014). Re-Evaluating the Surface Tension Analysis of Polyelectrolyte-Surfactant Mixtures Using Phase-Sensitive Sum Frequency Generation Spectroscopy. Journal of the American Chemical Society,136(43), 15114-15117.
3 lead to electrostatic interactions between SDS and PDADMAC as described above. These electrostatic interactions have been studied with several techniques, such as surface tension measurements, ellipsometry, and electrode measurements for various mixtures of opposite charged surfactants and polyelectrolytes.4 However an underlying meaning or understanding of the
interactions between the oppositely charged ions in SDS and PDADMAC is still not clear, because “there has been no good analytical tool to obtain molecular-level information at the liquid surface”.6
(Hu, D., & Chou, K. C. (2014))6 have studied the surface tension behavior of aqueous solutions of SDS
and PDADMAC. They found a sharp decrease in surface tension when reaching the 0,1 mM shortly followed by a sharp increasing in surface tension and above 1mM a subsequent decrease in surface tension as shown in figure 1.6 This drop in surface tension between 0,1 mM and 1 mM is not in
agreement with previous surface tension analysis of opposite charged polyelectrolyte-surfactant mixtures.4,7 From this drop in surface tension Dan Hu and Keng Chou claimed that surface tension
measurement are “inaccurate indicators” for studying surface activities of aqueous solutions of SDS and PDADMAC.6
Figure 1: The surface tension of aqueous PDADMAC solution (50 ppm) for various SDS concentrations and in the insets molecular structures of SDS and PDADMAC at room temperature (T=293 K) are shown.6
7
Monteux, C., Llauro, M. F., Baigl, D., Williams, C. E., Anthony, O., & Bergeron, V. (2004). Interfacial microgels formed by oppositely charged polyelectrolytes and surfactants. 1. Influence of
4 This study aims to further investigate the real nature of surface tension for SDS-PDADMAC mixtures, by doing surface tension measurements and investigate the influences of SDS concentration on polymer-surfactant bulk aggregation. Furthermore, this study investigates the influences of temperature on the aqueous solution of SDS and PDADMAC, by surface tension measurements for various SDS concentrations at different temperatures with surface tension analysis.
Theoretical Framework:
Characteristics of surfactants and polyelectrolytes
Surfactants are amphiphilic mixtures and adsorb at air/water or oil/water interfaces, due to their surface properties as mentioned in the introduction. The hydrophobic tail will be extended out of the bulk into the air as shown in figure 2 and the hydrophilic head will remain on the water surface, since the air molecules are non-polar as the hydrophobic compounds. The surfactant molecules located at the interface have a lower free energy than those molecules located in the bulk phase. The minimum amount of work required to create an interface is called the “interfacial free energy”. This interfacial free energy (W) is related to the surface tension (γ) as follows𝑊𝑊 = 𝛾𝛾 ∗ 𝛥𝛥𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 , where the change in area of the interface equals 𝛥𝛥𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖.
Surfactants reduce the interfacial free energy and with that the surface tension.8
Figure 2: A sketch of a cationic surfactant arrangement on an air-water interface. The nonpolar tails are orientated to minimize the contact with the water molecules and pointing towards the air, because of the non-polar nature of air molecules. The non-polar head remains on the water surface.
8
5 Surfactants form a self-assembled aggregate above a certain concentration, due to interactions between them known as micelles. These aggregates may be spherical, globular, or rod like or have the structure of spherical bilayers as shown in figure 3, depending upon the type of surfactant and the solution conditions such as temperature, pH and ionic strength.9 Surfactants form micelles at a
certain concentration called the “critical micelle concentration” (CMC). Above this concentration there is no new surfactant present at the surface, because the space of the interface is completely occupied. The surface tension remains relatively constant as there is no more surfactant presented at the surface reducing the surface tension. The CMC varies from medium to medium and depends on the pressure, temperature and presence of other surface active substances such as polyelectrolytes.8
Figure 3: A sketch of the structures of spherical, globular, rod like and spherical bilayers micelles in dilute aqueous solutions. The surfactant molecules cluster shielding their hydrophobic tails from the water molecules by using their hydrophilic heads as protection.9
9
Nagarajan, R., & Ruckenstein, E. (1991). Theory of surfactant self-assembly: a predictive molecular thermodynamic approach. Langmuir, 7(12), 2934-2969.
6 Surfactants can be classified in four groups; anionic, cationic, non-ionic and zwitterionic. They carry, respectively, a net negative charge, a net positive charge, no charge and both negative and positive charge in their polar compound.10 Surfactants can have strong electrostatic interactions with
oppositely charged molecules, such as polyelectrolytes. As mentioned already in the introduction, strong electrostatic interactions occur above CMC and CAC as shown in figure 4. The electrostatic interactions are influenced by the concentration of both the surfactant and the polyelectrolyte. The higher the concentrations of both surfactants and polyelectrolytes, the more likely electrostatics interactions between opposite charged polyelectrolyte and surfactants occur.11
Figure 4: A sketch of polyelectrolyte-surfactant mixture of opposite charge before reaching the cac and after the cac. After reaching the cac bulk aggregation occurs and polymer-surfactant aggregates are formed.
10
Asnacios, A., Langevin, D., & Argillier, J. F. (1996). Complexation of cationic surfactant and anionic polymer at the air-water interface. Macromolecules,29(23), 7412-7417.
11
Bain, C. D., Claesson, P. M., Langevin, D., Meszaros, R., Nylander, T., Stubenrauch, C., ... & Von Klitzing, R. (2010). Complexes of surfactants with oppositely charged polymers at surfaces and in bulk. Advances in colloid and interface science, 155(1), 32-49.
7
Basic Thermodynamics
Surface tension (γ) is the force that causes surface molecules of a liquid to be pushed together and form a boundary. Surface tension is measured in force per unit length and is related to the Gibbs free energy (G) as shown in equation 1. The Gibbs free energy is the energy needed to create a system out of nothing with constant pressure P minus the heat present in the environment at a constant temperature T and is an extensive quantity. Extensive quantities doubles, when the amount of matter doubles. So, if the number of molecules doubles, the Gibbs free energy also doubles. The Gibbs free energy depends on the extensive quantities internal energy (U), volume (V) and entropy (S) and the intensive quantities pressure (P) and temperature (T), see equation 2. Intensive do not double, when the amount of molecules doubles. It is not possible to add an extensive quantity with an intensive quantity, but when two intensive quantities are multiplied one gets an extensive quantity. The P times V can be interpreted as the work needed to make room for the system if the pressure of the environment remains constant. The heat present in the environment at temperature T equals 𝑇𝑇∆𝑆𝑆.
𝛾𝛾 = �𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑�
𝑃𝑃,𝑇𝑇 (1)
𝑑𝑑 = 𝑈𝑈 + 𝑃𝑃𝑃𝑃 − 𝑇𝑇𝑆𝑆 (𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑃𝑃, 𝑇𝑇)
(2)
The first law of thermodynamics states that the change in internal energy is related to the heat added to the system a constant temperature T plus the work done on the system with a constant pressure P and is an application of the conservation of energy principle, see equation 3.
𝑑𝑑𝑈𝑈 =𝑇𝑇𝑑𝑑𝑆𝑆 − 𝑃𝑃𝑑𝑑𝑃𝑃 (3)
A change of Gibbs free energy is given by equation 4 and can be rewritten by plugging the first law of thermodynamics into equation 4.
𝑑𝑑(𝑑𝑑) = 𝑑𝑑(𝑈𝑈) + 𝑑𝑑(𝑃𝑃𝑃𝑃) − 𝑑𝑑(𝑇𝑇𝑆𝑆) (4)
𝑑𝑑(𝑑𝑑) = 𝑑𝑑(𝑈𝑈) + 𝑃𝑃𝑑𝑑(𝑃𝑃) + 𝑃𝑃𝑑𝑑(𝑃𝑃) − 𝑆𝑆𝑑𝑑(𝑇𝑇) − 𝑇𝑇𝑑𝑑(𝑆𝑆) (5)
8
The surface tension can also be expressed in terms of enthalpy (H) or Helmholtz free energy (F) as shown in equation 9 and 11. Enthalpy is the energy needed to create a system out of nothing, when the pressure is held constant and Helmholtz free energy is the energy needed create a system minus the heat present in the environment at temperature T. The Helmholtz free energy becomes in practice when the systems temperature is kept constant. Enthalpy (H) and Helmholtz free energy are related to the Gibbs free energy as shown in equation 7 and 8. Equation 9 and 11 are obtained by plugging in respectively equation 7 and 8 into equation 1.
𝑑𝑑 = 𝐻𝐻 − 𝑇𝑇𝑆𝑆 (𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑃𝑃) (7) 𝑑𝑑 = 𝐹𝐹 + 𝑃𝑃𝑃𝑃 (𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑇𝑇) (8)
𝛾𝛾
=�
𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑�
𝑃𝑃,𝑇𝑇 =�
𝑑𝑑𝐻𝐻 𝑑𝑑𝑑𝑑�
𝑃𝑃,𝑇𝑇− 𝑇𝑇�
𝑑𝑑𝑆𝑆 𝑑𝑑𝑑𝑑�
𝑃𝑃,𝑇𝑇 (9) �𝑑𝑑𝑇𝑇�𝑑𝑑𝛾𝛾 𝐴𝐴,𝑃𝑃 = − � 𝑑𝑑𝑆𝑆 𝑑𝑑𝑑𝑑�𝑃𝑃,𝑇𝑇 (10) 𝛾𝛾 = �𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑� 𝑃𝑃,𝑇𝑇 = � 𝑑𝑑𝐹𝐹 𝑑𝑑𝑑𝑑�𝑃𝑃,𝑇𝑇− 𝑃𝑃 � 𝑑𝑑𝑃𝑃 𝑑𝑑𝑑𝑑�𝑃𝑃,𝑇𝑇 (11) �𝑑𝑑𝑃𝑃�𝑑𝑑𝛾𝛾 𝐴𝐴,𝑇𝑇= − � 𝑑𝑑𝑃𝑃 𝑑𝑑𝑑𝑑�𝑃𝑃,𝑇𝑇 (12)9
Methods
Materials
The anionic surfactant, sodium dodecyl sulfate (C12H25NaO4S) used in this study was purchased from
Sigma-Aldrich (99%) and has a molecular weight of Mw=288.38 g/mol. Sodium dodecyl sulfate was
dissolved in water before use. The cationic polyelectrolyte, polydiallyldimethylammonium chloride ((C8H18CIN)n) was purchased from Aldrich and has an average molecular weight of Mw=
200,000-350,000, 20 wt. % in H2O. The solutions of 50 ppm PDADMAC and SDS were prepared by adding the
polyelectrolyte PDADMAC to the surfactant solution SDS made prior.
Surface tension measurements
Kruss Force Tensionmeter K100 is used to measure the surface tension of an aqueous solution of SDS and PDADMAC, for various SDS concentrations and 50 part per million (ppm) PDADMAC. Surface tension was first measured at room temperature and later at different temperatures to investigate the influence of temperature on the surface tension of the SDS-PDADMAC mixtures.
The surface tension is measured with Du-Nuoy ring method. When the platinum ring touches the surface, a force acts on the force sensor. Platinum has a very high surface free energy and therefore forms a contact angle (𝜃𝜃) of 0°, with liquids. The force sensor registered the force and if the wetted length of the platinum ring is known this force can be used to calculate the surface tension.
𝛾𝛾 = 𝐹𝐹𝑚𝑚𝑖𝑖𝑚𝑚 𝐿𝐿 ∗ cos (𝜃𝜃)
𝛾𝛾 = 𝑐𝑐𝑠𝑠𝑠𝑠𝑠𝑠𝑐𝑐𝑐𝑐𝑠𝑠 𝑐𝑐𝑠𝑠𝑐𝑐𝑐𝑐𝑡𝑡𝑐𝑐𝑐𝑐; 𝐹𝐹𝑚𝑚𝑖𝑖𝑚𝑚= 𝑚𝑚𝑐𝑐𝑚𝑚𝑡𝑡𝑠𝑠𝑚𝑚𝑠𝑠𝑚𝑚 𝑠𝑠𝑐𝑐𝑠𝑠𝑐𝑐𝑠𝑠; 𝐿𝐿 = 𝑤𝑤𝑠𝑠𝑐𝑐𝑐𝑐𝑠𝑠𝑑𝑑 𝑙𝑙𝑠𝑠𝑙𝑙𝑐𝑐ℎ; 𝜃𝜃 = 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑐𝑐𝑐𝑐𝑙𝑙𝑙𝑙𝑠𝑠
The maximum force is experienced when the liquid film produced beneath the ring is stretched. The following illustration shows the experimental setting of the ring method.
10 Figure 5: A sketch of the ring method, where the liquid film is stretched producing a maximum force on the platinum ring. 12
The measured values are corrected since the weight of the liquid increases the amount of force detected by the force sensor and maximum force acting on the ring does not occur at the same time on the inner and the outer side of the ring.
11
Temperature change
The temperature was regulated with the Thermo scientific Phoenix II, which is a water bath. The water bath was connected to the Kruss Force Tensionmeter K100 (see figure 6) and changed the temperature of the metallic holder of the Kruss Force Tensionmeter K100. After the temperature of the metallic holder of the Kruss Force Tensionmeter K100 was stable the liquid equilibrated for 30 minutes before preforming the measurement. The temperature was ramped in steps of 10, started at 283 Kelvin and stopped at 323 Kelvin.
Figure 6: A sketch of the experimental settings, where the Thermo scientific Phoenix II is connected to the Kruss Force Tension meter K100.
Microscopic pictures
Bulk aggregation images were taken by the Leica DM IRB, which is a microscope. The microscope was connected to a computer, which took the images.
12
Results
Surface tension measurements at room temperature (T=293 K)
Before looking into the aqueous solution of SDS and PDADMAC a dilution series with the aqueous solution of SDS is performed, which is used to compare the results of the SDS-PDADMAC mixture with. In figure 7, the measured surface tension values of various SDS concentrations are plotted. The surface tension for low SDS concentrations, such as 0,05 mM are around 71 mM, which is the surface tension of water. The CMC is measured to be 8.5 mM (see figure 1), which is comparable to the CMC value measured in previous studies.13,14
Figure 7: Graphs shows the surface tension of water for various SDS concentrations at room temperature (T=293 K).
Polymer-Surfactant aggregates were formed when adding more than 50 ppm PDADMAC to the SDS solution or when mixing the aqueous solution too aggressive (rpm 350) on the 2mag magnetic motion. The polymer-surfactant bulk aggregation is more for higher concentrations of SDS and was observed for SDS concentration within the region of 1.0-10,0 mM. In figure 8, microscopic pictures of SDS-PDADMAC mixtures are shown for SDS-PDADMAC mixtures.
13
Mukerjee, P., & Mysels, K. J. (1971). Critical micelle concentrations of aqueous surfactant systems (No. NSRDS-NBS-36). National Standard reference data system.
14
Cifuentes, A., Bernal, J. L., & Diez-Masa, J. C. (1997). Determination of critical micelle concentration values using capillary electrophoresis instrumentation. Analytical Chemistry, 69(20), 4271-4274.
13
(a) (b)
(c) (d)
(e)
(e) (f)
Figure 8: Microscopic photographic images of surfactant-polymer bulk aggregates (figure 7a-7e) from aqueous solutions of 50 ppm PDADMAC and various concentrations of SDS. Each picture corresponds to a different SDS concentration dissolve in the solution: (a) 10 mM; (b) 7,5 mM; (c) 5,0 mM; (d) 2.5 mM; (e) 1.0 mM. (f) Microscopic photographic images of an aqueous solution of 50 ppm PDADMAC and 0,05 mM SDS, where no bulk aggregation occurred.
14 In figure 9, the measured surface tension values of PDADMAC mixtures are plotted. The SDS-PDADMAC mixtures above the SDS concentration of 1.0 mM have polymer-surfactant bulk aggregation and the SDS-PDADMAC mixtures below the SDS concentration of 1.0 mM do not. The surface tension values in region of 0,1-10,0 are not in agreement with previous studies and are the form of the surface tension curve that is similar to the surface tension measurements done by (Hu, D., & Chou, K. C. (2014))6. 0.01 0.1 1 10 30 35 40 45 50 55 60 65 70 75 80 SDS Concentration (mM) Sur fa ce T ens ion (mN /m )
Figure 9: Graphs shows the surface tension of aqueous PDADMAC solution (50 ppm) for various SDS concentrations at room temperature (T=293 K). The PDADMAC-SDS mixtures were exposed to polymer-surfactant bulk aggregation for various concentration of SDS. Polymer-polymer-surfactant bulk aggregation occurred for all SDS concentration expect for 10, 0,05 and 0,01 mM.
15 In order to get rid of the polymer-surfactant bulk aggregates we made sure that no more than 50 ppm PDADMAC is add to the aqueous solution of SDS and by stirring the aqueous solution of 50 ppm PDADMAC and SDS on the 2mag magnetic motion at 100 rpm for more than 24. In figure 10, the measured surface tension values of 50 ppm PDADMAC with various SDS concentrations are plotted, when there was no bulk aggregation for all SDS concentrations.
0.01 0.1 1 10 30 35 40 45 50 55 60 65 70 75 80
S
ur
fac
e
T
ens
ion
(m
N
/m
)
SDS Concentration (mM)
Figure 10: Graphs shows the surface tension of aqueous PDADMAC solution (50 ppm) for various SDS concentrations at room temperature (T=293 K), with no polymer-surfactant bulk aggregation.
16
Surface tension measurements at various temperatures
In figure 11, the measured surface tension values of 50 ppm PDADMAC with various SDS concentrations are plotted, when there was no polymer-surfactant bulk aggregation for all SDS concentrations and for various temperatures. The surface tension values at 293 K are in agreement with the surface tension done previously and showed in figure 8. Furthermore, surface tension values are lower for higher temperatures at low concentrations (0,01-0,25 mM). The CMC is measured to be in the region of 7,5 mM-10mM (see figure 9) and is the same for the measured temperatures. 0.01 0.1 1 10 30 40 50 60 70 80
S
ur
fac
e
T
ens
ion
(m
N
/m
)
SDS Concentration (mM)
T= 283 K T= 293 K T= 303 K T= 313 K T= 323 KFigure 11: Graphs shows the surface tension of aqueous PDADMAC solution (50 ppm) for various SDS concentrations and various temperatures.
Discussion
Surface tension measurements seems to represent the aqueous solution of PDADMAC for various concentrations of SDS quite accurately, as opposed to what (Hu, D., & Chou, K. C. (2014))6 claimed.
However, a similar form of surface tension curve is found with polymer-surfactant bulk aggregates in the aqueous solution of 50 ppm and various concentrations of SDS. The polymer-surfactant bulk aggregates occurred for SDS concentration higher than 1.0 mM and before the critical micelle concentration, which is in agreement with the literature.1 This still not explains the drop in surface
tension measured by (Hu, D., & Chou, K. C. (2014))6 in the region of 0,1-1,0 mM. The
polymer-surfactant bulk aggregation is hard to control and occurred randomly, so further research is needed to better control and therefore understand the polymer-surfactant bulk aggregation from
SDS-17 PDADMAC mixtures. However, when stirring at a low speed (100 rpm on 2mag magnetic motion) polymer-surfactant bulk aggregation was rare and if it occurred it was for concentration lower than the critical micelle concentration. The surface tension from the aqueous solution of PDADMAC for various concentrations of SDS is in agreement with the reference articlesof opposite charged polyelectrolytes and surfactants. 4,5 It is clearly visible that measured surface tension values satisfy
thermodynamic principles.
The temperature dependent analysis for surface tension of SDS-PDADMAC mixtures was not in agreement with what was expected from the thermodynamic literature.10 When temperature
dependent measurements at high SDS concentration are done a pattern in surface tension curve cannot be found. However, it is clear that temperature and entropy are influences by one and other (see equation 11), but should still be implemented for our system of SDS and PDADMAC.
Conclusion
Surface tension measurements of the aqueous solution of PDADMAC and SDS showed that surface tension measurements remain as reliable as ever for investigating thermodynamic processes and surface tension properties, as opposed to what (Hu, D., & Chou, K. C. (2014))6 claimed. The
influences of temperature on SDS-PDADMAC mixtures is insignificant and a thermodynamic model on how entropy and temperature influences each other still needs to be implemented for our system. Furthermore this study found that electrostatic interactions and subsequent aggregates between SDS and PDADMAC influence the surface tension.
Acknowledgment
I would like to thank Daniel Bonn and his group for introducing me in the world of soft matter and especially for introducing me to surfactants. Furthermore, I would like to thank Mohsin Jahan Qazi for helping me understand the Kruss Force Tension meter K100. Finally, would I like to thank Bijoy Bera for his help, inspiration and motivation during the whole research.
18
Bibliographic references
Falbe, J. (Ed.). (2012).Surfactants in consumer products: Theory, Technology and Application. Springer Science & Business Media.
Hara, M. (Ed.). (1992).Polyelectrolytes: science and technology. CRC Press.
Chu, D. Y., & Thomas, J. K. (1986). Effect of cationic surfactants on the conformation transition of poly (methacrylic acid).Journal of the American Chemical Society,108(20), 6270-6276.
Monteux, C., Williams, C. E., Meunier, J., Anthony, O., & Bergeron, V. (2004). Adsorption of oppositely charged polyelectrolyte/surfactant complexes at the air/water interface: formation of interfacial gels.Langmuir,20(1), 57-63.
Goddard, E. D., & Ananthapadmanabhan, K. P. (1993).Interactions of surfactants with polymers and proteins. CRC press.
Hu, D., & Chou, K. C. (2014). Re-Evaluating the Surface Tension Analysis of Polyelectrolyte-Surfactant Mixtures Using Phase-Sensitive Sum Frequency Generation Spectroscopy.Journal of the American Chemical Society,136(43), 15114-15117.
Monteux, C., Llauro, M. F., Baigl, D., Williams, C. E., Anthony, O., & Bergeron, V. (2004). Interfacial microgels formed by oppositely charged polyelectrolytes and surfactants. 1. Influence of
polyelectrolyte molecular weight.Langmuir,20(13), 5358-5366.
Rosen, M. J., & Kunjappu, J. T. (2012).Surfactants and interfacial phenomena. John Wiley & Sons.
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19 Bain, C. D., Claesson, P. M., Langevin, D., Meszaros, R., Nylander, T., Stubenrauch, C., ... & Von Klitzing, R. (2010). Complexes of surfactants with oppositely charged polymers at surfaces and in bulk.Advances in colloid and interface science,155(1), 32-49.
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20
Popular scientific summary
Surfactants gebruiken we elke dag bijvoorbeeld als we onze handen wassen met zeep of als we de afwas doen met afwasmiddel. Surfactants verlagen de oppervlakte spanning van water door zich te positioneren op het wateroppervlak, waardoor er minder kracht nodig is om het vuil van je handen of bord af te wassen. Surfactants positioneren zich daar, omdat ze een hydrofiele kop hebben en een hydrofobe staart, als weergeven in figuur 1. De hydrofobe staart wil natuurlijk niet omgeven worden door polaire watermoleculen, maar door non-polaire zuurstof moleculen. Voor de hydrofiele kop geld precies het tegenovergestelde die wil juist graag omgeven worden door polaire watermoleculen.
Figuur 1: Een schets van hoe surfactants zich ordenen op een wateroppervlak.
Als het wateroppervlak volledig bezet is door surfactants worden er micellen gevormd. Waarbij de hydrofobe staart van de surfactant wordt beschermd door de hydrofiele kop. De oppervlakte spanning van de wateroplossing blijft dan constant, omdat er geen nieuwe surfactant zich op het wateroppervlak positioneert. Deze micellen kunnen verschillende vormen aannemen, zie figuur 2. De vorm die ze aannemen hangt af van de temperatuur, druk en zuurgraad van de oplossing en of er een andere oppervlakte actieve stof aanwezig is, zoals een polyelectrolyte.
21 Figuur 2:Een schets van de verschillende vormen die micellen kunnen aannemen.
Polyelectrolytes zijn geladen polymeren en kunnen wanneer ze een tegenovergestelde lading hebben ten opzichte van de surfactant een sterke elektrostatische interactie hebben met deze surfactant wat kan leiden tot een opeenhoping van de surfactant en de polyelectrolyte. De surfactant concentratie waarbij dit gebeurt wordt aan geduid als de “critical aggregation concentration”. Dit mengsel van surfactants en polyelectrolyte wordt gebruikt om op water gebaseerde substanties te verdikken, denk hier bij aan verf of boorvloeistoffen. Door deze toepassingen en de interessante oppervlakte eigenschappen van surfactant en polyelectrolyte opeenhopingen wordt er zowel door wetenschappers als door het industrie veel onderzoek naar deze mengsels gedaan.
Een van deze onderzoeksmethodes is oppervlakte spanning metingen. De oppervlakte spanning van een surfactant polyelectrolyte mengsel neemt af naar maten de surfactant concentratie toe neemt, zoals beschreven hierboven. Echter werd er in 2014 een onderzoek gepubliceerd door Hu en Chou6,
waaruit volgde dat oppervlakte spanning metingen geen goede methode zou zijn om oppervlakte eigenschappen van surfactant en polyelectrolyte mengsels te beschrijven. Zij gebruikten voor hun onderzoek de negatief geladen surfactant SDS en positief geladen polyelectrolyte PDADMAC en vonden met behulp van oppervlakte spanning metingen een daling in oppervlakte spanning tussen 0,1-1,0 mM SDS, zie figuur 3. Deze daling in oppervlakte spanning is tegenstrijdig met de thermodynamica en eerder gepubliceerde onderzoeken.
22 Figuur 3: Oppervlakte spanning metingen van Hu en Chou voor 50 ppm PDADMAC en verschillende SDS
concentraties.
Wij hebben het onderzoek van Hu en Chou gerevalueerd en hebben oppervlakte spanning metingen gedaan voor een mengsel van 50 part per million PDADMAC en verschillende concentraties SDS. Verder hebben wij gekeken naar de invloed van temperatuur op de oppervlakte spanning van het SDS-PDADMAC mengsel. Bij onze oppervlakte spanning metingen deed zich geen daling in oppervlakte spanning tussen 0,1-1,0 mM SDS voor, zie figuur 4. Hieruit concludeerde wij dat oppervlakte spanning metingen nog steeds een goede methode is om de eigenschappen van SDS en PDADMAC mengsels te onderzoeken.
0.01 0.1 1 10 30 35 40 45 50 55 60 65 70 75 80
S
ur
fac
e
T
ens
ion
(m
N
/m
)
SDS Concentration (mM)
23 Tijdens het onderzoek namen wij ook waar dat opeenhopingen van SDS en PDADMAC de oppervlakte spanning beïnvloeden. De SDS en PDADMAC opeenhopingen verlaagde oppervlakte spanning, zie figuur 5. De SDS en PDADMAC opeenhopingen konden worden voorkomen door het SDS en PDADMAC mengsel voor vierentwintig uur lang rustig te mengen op een magnetische plaat.
0.01 0.1 1 10 30 35 40 45 50 55 60 65 70 75 80 SDS Concentration (mM) Sur fa ce T ens ion (mN /m )
Figuur 5: Onze oppervlakte spanning metingen voor 50 ppm PDADMAC en verschillende SDS concentraties, waarbij opeenhopingen van SDS en PDADMAC plaatsvonden.
Verder volgende uit dit onderzoek dat temperatuur een insignificant effect heeft op de oppervlakte spanning van een SDS-PDADMAC mengsel. In figuur 6 zijn onze resultaten geplot, waaruit geen patroon kan worden afgeleid tussen temperatuur en oppervlakte spanning
0.01 0.1 1 10 30 40 50 60 70 80
S
ur
fac
e
T
ens
ion
(m
N
/m
)
SDS Concentration (mM)
T= 283 K T= 293 K T= 303 K T= 313 K T= 323 KFiguur 6:Onze oppervlakte spanning metingen voor 50 ppm PDADMAC en verschillende SDS concentraties bij verschillende temperaturen.
