Studies on Adsorption and Wetting Phenomena Associated with Solid Surfaces in Aqueous Synthetic and Natural Surfactant Solutions

By: Biswal, Nihar RanjanContributor(s): Paria, Santanu [Supervisor] | Department of Chemical EngineeringMaterial type: TextTextLanguage: English Publisher: 2012Description: 222 pSubject(s): Engineering and Technology | Chemical Engineering | Chemical Process ModelingOnline resources: Click here to access online Dissertation note: Thesis (Ph.D)- National Institute of Technology, Rourkela Summary: Adsorption of surfactants at air-liquid and solid-liquid interface and wetting of solid surfaces are closely interdependent. The performance of many physicochemical process and fundamental understanding depends on these two important phenomena. Because of the importance of these phenomena this study mainly focuses on adsorption of different surfactants at air-water and solid-water interfaces and wetting of those surfactant solutions at flat solid surfaces. The main emphasis of this study is plant-based natural surfactants; however some synthetic surfactants are also studied as a reference for comparison. The effects of electrolytes, alcohols, and naturalsynthetic surfactants mixtures are also studied. Electrolytes are most powerful inexpensive additive enhances the adsorption capacity of ionic surfactants at interfaces which in turn also enhances the interfacial behaviour. Adsorption kinetics and isotherm of anionic (dodecylbenzene sulfonate, SDBS), cationic (cetylpyridinium bromide, CPB), and non-ionic (TX-100) surfactants in the presence and absence of electrolytes on PTFE-water interface are studied. The kinetics of adsorption fits well pseudo-second-order kinetic model for the three surfactants studied here. Adsorption isotherms of TX-100 follow Langmuir type, whereas SDBS and CPB follow Freundlich type. However, in the presence of electrolytes both the ionic surfactants show better fitting with Langmuir type isotherm. The effect of electrolytes on the surfactant concentration far below the CMC shows there is a linear increase in amount adsorbed with the increase in ionic strength of the electrolyte mainly due to reduction in headgroup repulsion and finally reaches a plateau level when the equilibrium concentration reaches CMC at that electrolyte concentration. The structure of tailgroup of non-ionic surfactants also plays an important role in both adsorption and wetting behaviour. To get some insight about the fact, the adsorption and wetting behavior of two nonionic surfactants (TX-100 and Igepal CO-630) having the same headgroup but structurally different tailgroups has been compared. The change in contact angle with the concentration of surfactant follows a trend similar to that for adsorption onto a PTFE surface. At low surfactant concentration, Igepal CO-630 shows a slightly higher adsorption density and better wetting properties than TX-100. Both surfactants show lower adsorption densities at the PTFE–water interface than at the air–water interface. vi The wetting of hydrophobic and hydrophilic solid surfaces by surfactant solutions of better efficiency is an important research topic recently because of its profound practical applications. The wettability of two double-chain surfactants (cationic, didodecyldimethylammonium bromide or DDAB, and anionic, aerosol OT or AOT) solutions on PTFE and glass surfaces has been investigated here. Different physicochemical parameters such as critical micelle concentration (CMC) and surface tension, contact angle, surface excess at air−water and solid-water interfaces, work of adhesion, and free energy of wetting have been estimated for two double-chain surfactants solutions and compared with the reported results of single-chain surfactants. The double-chain surfactant solutions showed maximum lowering of surface tension values (24.36 and 26.35 mN/m for DDAB and AOT, respectively) and a change in contact angle values from pure water on PTFE (∼38° for DDAB and AOT) and glass (∼26.5 and 24° for DDAB and AOT, respectively) surfaces compared to the conventionally studied single-chain surfactants. The surfactant molecules mostly formed a monolayer adsorption on both surfaces during the wetting process. The reduction in synthetic surfactant consumption in any process may lead to a significant reduction in environmental pollution. As a result, in many applications substitution of synthetic surfactants by biodegradable environmentally friendly surfactants is a latest trend. The adsorption and wetting behaviour of biodegradable, most easily and abundantly available three plant-based surfactants, Reetha, Shikakai, and Acacia on PTFE and glass surfaces have been studied here to get some idea about their efficiency compared to commonly used synthetic surfactants. The adsorption kinetics shows all the surfactants are adsorbed within 20 minutes on PTFE surface and the amount adsorbed at equilibrium of Shikakai is more in compare to Reetha and Acacia. A Langmuir-type isotherm best fits for all the surfactants. The change in contact angle on PTFE surface by the surfactant solutions also follow similar trend to that of adsorption density; the final contact values for Reetha, Acacia, and Shikakai are 109.88°, 109.02° and 98.13° respectively. The wetting studies indicate plant surfactants are inferior to the conventionally used synthetic surfactants. The adsorption studies show the density of adsorption at the PTFE-water interface is lower than the air-water interface for all three surfactants, which is also independently supported by the contact studies. The contact angle on glass surface shows that there is an increase in contact angle from 47° (pure water) to 67.72, 65.57, 68.84, and 68.79° vii for Reetha Acacia, Shikakai, and Triton X-100 respectively at the saturation level with the increase in surfactant concentration. Shikakai has shown to be better surface-active agent compared to Ritha and Acacia. To further enhance the efficiency of Shikaki effect of two different alcohols (C1: methanol and C5: amyl alcohol) was also studied. The addition of methanol and amyl alcohol to the Shikakai solution show there is synergistic interaction between the alcohol and Shikakai molecules and that is more for amyl alcohol. Since the interaction is more for amyl alcohol consumption of alcohol is also 1000 times lower than methanol to get similar surface tension reduction. When the concentration of Shikakai is constant with the increasing concentration of alcohols up to a certain concentration of alcohol reductions in surface tension and contact angle are more than that of pure solutions of similar concentrations because of synergistic interaction. Further, to see efficiency of plant-synthetic mixed surfactant system, a double-chain surfactant DDAB was mixed with Shikakai. Pure Shikakai is having higher surface tension and contact angle values at CMC than that of DDAB, indicates inferior than the commonly used synthetic surfactant. Addition of DDAB on Shikakai shows there are gradual lowering of CMCs, surface tension and contact angle values at CMC. When the concentration of synthetic surfactant is ~ 50 mole % in the mixture, the final surface tension and contact angle values are close to that of pure DDAB. The mixed surfactant solutions show highly non-ideal behaviour because of interaction between two molecules which surely has some practical importance. The wetting property of Shikakai on PTFE surface increases significantly in the presence of DDAB. As the wetting property of a plant surfactant enhances in the presence of synthetic surfactant, the use of plant-synthetic mixed surfactant system may be useful in several wetting applications to reduce the surfactant based environmental pollution.
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Thesis (Ph.D)- National Institute of Technology, Rourkela

Adsorption of surfactants at air-liquid and solid-liquid interface and wetting of solid surfaces are
closely interdependent. The performance of many physicochemical process and fundamental
understanding depends on these two important phenomena. Because of the importance of these
phenomena this study mainly focuses on adsorption of different surfactants at air-water and
solid-water interfaces and wetting of those surfactant solutions at flat solid surfaces. The main
emphasis of this study is plant-based natural surfactants; however some synthetic surfactants are
also studied as a reference for comparison. The effects of electrolytes, alcohols, and naturalsynthetic
surfactants mixtures are also studied.
Electrolytes are most powerful inexpensive additive enhances the adsorption capacity of
ionic surfactants at interfaces which in turn also enhances the interfacial behaviour. Adsorption
kinetics and isotherm of anionic (dodecylbenzene sulfonate, SDBS), cationic (cetylpyridinium
bromide, CPB), and non-ionic (TX-100) surfactants in the presence and absence of electrolytes
on PTFE-water interface are studied. The kinetics of adsorption fits well pseudo-second-order
kinetic model for the three surfactants studied here. Adsorption isotherms of TX-100 follow
Langmuir type, whereas SDBS and CPB follow Freundlich type. However, in the presence of
electrolytes both the ionic surfactants show better fitting with Langmuir type isotherm. The
effect of electrolytes on the surfactant concentration far below the CMC shows there is a linear
increase in amount adsorbed with the increase in ionic strength of the electrolyte mainly due to
reduction in headgroup repulsion and finally reaches a plateau level when the equilibrium
concentration reaches CMC at that electrolyte concentration.
The structure of tailgroup of non-ionic surfactants also plays an important role in both
adsorption and wetting behaviour. To get some insight about the fact, the adsorption and wetting
behavior of two nonionic surfactants (TX-100 and Igepal CO-630) having the same headgroup
but structurally different tailgroups has been compared. The change in contact angle with the
concentration of surfactant follows a trend similar to that for adsorption onto a PTFE surface. At
low surfactant concentration, Igepal CO-630 shows a slightly higher adsorption density and
better wetting properties than TX-100. Both surfactants show lower adsorption densities at the
PTFE–water interface than at the air–water interface.
vi
The wetting of hydrophobic and hydrophilic solid surfaces by surfactant solutions of
better efficiency is an important research topic recently because of its profound practical
applications. The wettability of two double-chain surfactants (cationic,
didodecyldimethylammonium bromide or DDAB, and anionic, aerosol OT or AOT) solutions on
PTFE and glass surfaces has been investigated here. Different physicochemical parameters such
as critical micelle concentration (CMC) and surface tension, contact angle, surface excess at
air−water and solid-water interfaces, work of adhesion, and free energy of wetting have been
estimated for two double-chain surfactants solutions and compared with the reported results of
single-chain surfactants. The double-chain surfactant solutions showed maximum lowering of
surface tension values (24.36 and 26.35 mN/m for DDAB and AOT, respectively) and a change
in contact angle values from pure water on PTFE (∼38° for DDAB and AOT) and glass (∼26.5
and 24° for DDAB and AOT, respectively) surfaces compared to the conventionally studied
single-chain surfactants. The surfactant molecules mostly formed a monolayer adsorption on
both surfaces during the wetting process.
The reduction in synthetic surfactant consumption in any process may lead to a significant
reduction in environmental pollution. As a result, in many applications substitution of synthetic
surfactants by biodegradable environmentally friendly surfactants is a latest trend. The
adsorption and wetting behaviour of biodegradable, most easily and abundantly available three
plant-based surfactants, Reetha, Shikakai, and Acacia on PTFE and glass surfaces have been
studied here to get some idea about their efficiency compared to commonly used synthetic
surfactants. The adsorption kinetics shows all the surfactants are adsorbed within 20 minutes on
PTFE surface and the amount adsorbed at equilibrium of Shikakai is more in compare to Reetha
and Acacia. A Langmuir-type isotherm best fits for all the surfactants. The change in contact
angle on PTFE surface by the surfactant solutions also follow similar trend to that of adsorption
density; the final contact values for Reetha, Acacia, and Shikakai are 109.88°, 109.02° and
98.13° respectively. The wetting studies indicate plant surfactants are inferior to the
conventionally used synthetic surfactants. The adsorption studies show the density of adsorption
at the PTFE-water interface is lower than the air-water interface for all three surfactants, which is
also independently supported by the contact studies. The contact angle on glass surface shows
that there is an increase in contact angle from 47° (pure water) to 67.72, 65.57, 68.84, and 68.79°
vii
for Reetha Acacia, Shikakai, and Triton X-100 respectively at the saturation level with the
increase in surfactant concentration.
Shikakai has shown to be better surface-active agent compared to Ritha and Acacia. To
further enhance the efficiency of Shikaki effect of two different alcohols (C1: methanol and C5:
amyl alcohol) was also studied. The addition of methanol and amyl alcohol to the Shikakai
solution show there is synergistic interaction between the alcohol and Shikakai molecules and
that is more for amyl alcohol. Since the interaction is more for amyl alcohol consumption of
alcohol is also 1000 times lower than methanol to get similar surface tension reduction. When
the concentration of Shikakai is constant with the increasing concentration of alcohols up to a
certain concentration of alcohol reductions in surface tension and contact angle are more than
that of pure solutions of similar concentrations because of synergistic interaction.
Further, to see efficiency of plant-synthetic mixed surfactant system, a double-chain
surfactant DDAB was mixed with Shikakai. Pure Shikakai is having higher surface tension and
contact angle values at CMC than that of DDAB, indicates inferior than the commonly used
synthetic surfactant. Addition of DDAB on Shikakai shows there are gradual lowering of CMCs,
surface tension and contact angle values at CMC. When the concentration of synthetic surfactant
is ~ 50 mole % in the mixture, the final surface tension and contact angle values are close to that
of pure DDAB. The mixed surfactant solutions show highly non-ideal behaviour because of
interaction between two molecules which surely has some practical importance. The wetting
property of Shikakai on PTFE surface increases significantly in the presence of DDAB. As the
wetting property of a plant surfactant enhances in the presence of synthetic surfactant, the use of
plant-synthetic mixed surfactant system may be useful in several wetting applications to reduce
the surfactant based environmental pollution.

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