Surface Tension and Adsorption Kinetics of Volatile Organic Amphiphiles in Aqueous Solution

Amphiphiles that possess a dual character, hydrophobic and hydrophilic, are employed in many chemical, pharmaceutical and biological applications. Amphiphile molecules that include a hydrophilic head and a hydrophobic tail can easily adsorb at a liquid/vapour interface, to reach to a minimum free energy and hence a most thermodynamically stable state. Surface tension is a key parameter for understanding such behavior of an amphiphile, or a surfactant. This thesis represents a comprehensive study on adsorption and surface tension of slightly volatile, organic amphiphiles in aqueous solution. Although for a vapor-liquid interface, adsorption from both liquid and vapor phases should be considered, they have been almost always considered exclusive of one another. When a volatile surfactant is dissolved in the liquid phase, it also applies a finite partial pressure in the vapor phase. Recently, dynamic surface tension experiments showed that adsorption from both sides of a vapor/liquid interface must be studied simultaneously. It is noted that surface tension phenomena are often dynamic, in particular when the surface under consideration is perturbed. With the newly discovered importance of adsorption from both sides of a vapor/liquid interface, one may have to ask the question: how dynamic surface tension is influenced and responding to the surface perturbation and environment changes, and whether both sides of the interface play a role in surface tension responses. In this research, axisymmetric drop shape analysis-profile (ADSA-P) is used for surface tension measurement. The experiments are performed in a closed chamber where the effects of surfactant concentrations of both liquid and vapor phases on the surface tension can be studied. The partial vapor pressure of surfactant is controlled with an environment solution containing the same surfactant as the sample solution. The environment solution is to facilitate adsorption from the vapor side of the interface by creating a surfactant vapor phase. The effects of surface perturbation, environment condition (i.e., temperature and pressure) and carbon chain length on the surface tension and adsorption kinetics are studied in detail. The surface tension response of 1-octanol aqueous solution to surface area perturbation is investigated. Upon surface compression, the surface tension decreases followed by a gradual increase back to the value prior to compression. On surface expansion, two categories of surface tension response are observed: First, when the change in surface area is smaller than 5%, the behavior similar to that of conventional surfactants is observed. The surface tension increases followed by a gradual decrease back to the value prior to expansion. Second, when the change in surface area is greater than 5%, and the drop concentration is sufficiently larger than the environment concentration, the surface tension initially slightly increases, but after a time delay, it sharply decreases, followed by a gradual increase back to the value prior to expansion. Previous studies showed that at steady-state condition a network of hydrogen bonding between surfactant and water molecules near the surface is created. The unique surface tension response after large expansion might be related to the momentarily destruction of this hydrogen bonding network and gradually making a new one. The effect of temperature on the surface tension and adsorption kinetics of 1-octanol, 1-hexanol and 1-butanol aqueous solutions is studied. The steady-state surface tension is found to decrease upon an increase in temperature, and a linear relationship is observed between them. The modified Langmuir equation of state and the modified kinetic transfer equation are used to model the experimental data of the steady-state and dynamic (time-dependent) surface tension, respectively. The equilibrium constants and adsorption rate constants are evaluated through a minimization procedure for temperatures ranging from 10°C to 35°C. From the steady-state modelling, the equilibrium constants for adsorption from vapor phase and liquid phase are found to increase with temperature. From the dynamic modelling, the adsorption rate constants for adsorption from vapor phase and liquid phase are found to increase with temperature too. The influence of carbon dioxide pressure on the surface tension and adsorption kinetics of the aforementioned surfactant aqueous solutions is investigated. To consider the effect of adsorption/desorption of the two species (surfactant and carbon dioxide) from both sides of a vapor/liquid interface on the surface tension, the modified Langmuir equation of state and the modified kinetic transfer equation are derived. The steady-state and dynamic surface tension data are modelled using the modified Langmuir equation of state and the modified kinetic transfer equation, respectively. The equilibrium constants and adsorption rate constants of surfactant and carbon dioxide are evaluated through a minimization procedure for CO2 pressures ranging from 0 to 690 KPa. From the steady-state modelling, the equilibrium parameters for surfactant and carbon dioxide adsorption from vapor phase and liquid phase are found unchanged for different pressures of carbon dioxide. From the dynamic modelling, the adsorption rate constants for surfactant and carbon dioxide are found to decrease with carbon dioxide pressure. The role of carbon chain length of amphiphiles in aqueous solution is also studied. It is illustrated that the equilibrium constants for adsorption from both sides of a vapor/liquid interface increase from 1-butanol to 1-octanol. The modelling results show that the ratio of the equilibrium constant for adsorption from vapor phase to the equilibrium constant for adsorption from liquid phase declines from 260 to 26 as the chain length is increased from 1-butanol to 1-octanol. Therefore, the contribution to adsorption from liquid phase augments as the chain length is increased. The adsorption kinetics for this group of short carbon chain surfactants is modelled using a kinetic transfer equation. The modelling results show that the adsorption rate constants from vapor phase and liquid phase (kag and kal) increase from 1-butanol to 1-octanol. Steady-state and dynamic modelling also reveals that the maximum surface concentration increases with carbon chain length. These results may be due to the higher hydrophobicity character of a surfactant molecule at longer carbon chain length

Controlling out-of-equilibrium phase transitions in complex surfactant systems

Surfactants in solutions self-assemble in a range of structures that are academically and industrially important. In particular, liquid crystalline phases have energies near kBT and are generally responsive to external fields, including shear flows and small temperature changes. Lyotropic lamellar Lα phases can undergo a transformation into multilamellar vesicles (MLVs), which have wide ranging application in protocell models and particle encapsulants. The Lα-to-MLV transformation exhibits a rich behaviour when co-surfactant and salt is introduced to the system, and thermal energy provided, ranging from vesicle formation to shear-controlled hexagonal vesicle packing. In this thesis, I select sodium dodecyl sulfate (SDS)/octanol/brine (20 g/L NaCl in H2O) as a model system to investigate the equilibrium and non-equilibrium structures attainable by these state and process variables, employing continuous and oscillatory microfluidic flows. Chapter 1 provides an overview of the Lα-to-MLV transformation in non-ionic and charged surfactant systems and reviews the microfluidic approaches employed in flow-induced transformations. Chapter 2 introduces the experimental techniques used in this work, namely polarised optical microscopy (POM), small angle neutron scattering (SANS), nuclear magnetic resonance (NMR) alongside microdevice design and integration with SANS. Our research sought to answer the following three questions: 1) what are the accessible SDS/octanol/brine solution microstructures? 2) what effect the membrane elastic properties have on the flow induced textures in linear flows? 3) what effect the microfludic flow field has on SDS/octanol/brine flow induced textures. In Chapter 3, I investigate the solution structure of SDS/octanol/brine system across the lamellar (Lα), vesicle (L4) and micellar (L1) phases employing small angle neutron scattering (SANS), optical microscopy and nuclear magnetic resonance (NMR). Chapter 4 quantifies the effect of Lα elastic properties on the shear induced Lα-to-MLV transformation by utilising continuous flow microfluidics on a long serpentine chip. Chapter 5 focuses on the effect of continuous and oscillatory microfluidic contraction-expansion flows on the formation of MLVs. Chapter 6 describes an outreach activity at the intersection of surfactant science and art, first developed for the Great Exhibition Road Festival 2022 and refined since. The final chapter reflects on the key findings of this PhD project and provides an outlook for future research.Open Acces

Constrained Drop Surfactometer for Studying Interfacial Structure and Rheology.

Ph.D. Thesis. University of Hawaiʻi at Mānoa 2017

Surface Tension and Adsorption of Volatile Organic Amphiphiles in Aqueous Solution

The surface tension of an interface separating two bulk phases is one of the most widely studied properties in surface science research. The importance of surface or interfacial tension is reflected in the diverse number of applications which are influenced by surface tension related effects. This thesis represents a comprehensive experimental and theoretical investigation on molecular adsorption and surface tension from a class of organic compounds in aqueous solutions. The research illustrates the effect of both liquid and vapor phase adsorption on the interfacial properties. Adsorption from both sides of the vapor/liquid interface is considered simultaneously rather than exclusive of one another, which has been the conventional practice. In the experimental study, the surface tension of a number of different volatile organic compounds is measured using the Axisymmetric Drop Shape Analysis-Profile (ADSA-P) method. The experiments were performed in a controlled environment under conditions where the surface tension can be affected by both vapor and liquid phase adsorption. The vapor phase was exerted by the presence of an environment solution containing the same organic component as in the drop solution. The results show that initially the surface tension is influenced by the organic concentration in both the liquid and the vapor phase. At the final steady-state the liquid phase becomes less important and the primary factor influencing the surface tension is the vapor phase concentration. The ADSA-P technique is verified by reproducing a select number of cases using the Wilhelmy plate method. A possible consequence of the surface tension phenomenon is illustrated through time-dependent contact angle experiments. The behavior of the interface at steady-state conditions is investigated by measuring the surface tension response to a change in drop volume. It is concluded that the organic compounds considered in the current study may represent a rather general group of molecules whose surface behavior is unique to that of many conventional surfactants. In the theoretical study an empirical model is proposed to describe the relation between the steady-state surface tension and the concentration of the environment and drop solutions. The results confirm the experimental observation that the final steady-state surface tension is determined primarily by the organic concentration in the vapor phase. In addition, a modified adsorption isotherm is developed to account for simultaneous adsorption from both sides of the vapor/liquid interface at steady-state conditions. The derivation is based upon the classic Langmuir analysis, and the new equation is consistent with the Langmuir isotherm under traditional conditions where adsorption occurs from one side of the interface. The modified isotherm is shown to be consistent with the experimental data and is used to generate the equilibrium parameters for three of the systems studied in this research. The adsorption isotherm is then extended to model the dynamic adsorption process through the creation of a new kinetic transfer equation. As with the adsorption isotherm, the transfer equation is based on Langmuir kinetics and is capable of simulating adsorption from both sides of the interface during surface equilibration. The kinetic transfer equation is validated against experimental data from two systems which exhibit a transfer-controlled adsorption mechanism. The theoretical predictions from the transfer equation fit well with the experimental data for both systems. However, significant variability is observed in the least squares estimates of the kinetic rate constants. The variability is attributed to the limitations of empirical models that utilize adjustable fitting parameters to optimize the model predictions, and the wide range of surfactant concentrations studied. Specific concentration regions are identified where the variability in the rate constants is minimal and thus, where the model is most appropriate

Simulation of heat and mass transfer phenomena in the critical elements of H2O-LiBr absorption cooling machines. Experimental validation and application to design

Degut a la tendència a l'increment del preu de la energia, i el seu ús cada cop més estès per aire condicionat en els paisos desenvolupats, els sistemes de refrigeració basats en energia solar tenen cada cop més atractiu. El objectiu final d'aquesta tesi és el desenvolupament d'eines de simulació numèrica pel disseny de màquines de refrigeració per absorció que tinguin la possibilitat de funcionar amb energia solar. Malgrat existeixen en el mercat màquines d'absorció d'aquestes característiques des de fa anys, hi ha una deficiència en el desenvolupament de sistemes de petita capacitat. Els sistemes de petita capacitat impliquen problemes addicionals en el seu disseny (sistemes refrigerats per aire, compacitat ...) que només es poden abordar fent ús d'eines de disseny adequades, tant pel sistema com pels seus components. Tanmateix, hi ha també certa deficiència en la literatura especialitzada en el desenvolupament de models matemàtics adequats per la descripció dels processos de transferència de calor i de massa en les màquines de refrigeració per absorció: àrea mullada en les superfícies d'intercanvi de calor i de massa, paper dels additius, etc. Per aquestes raons aquest treball ha estat enfocat en aquests objectius:- Estudi de processos bàsics de transferència de calor i de massa juntament amb els fenomens fluid-dinàmics implicats en absorbidors de màquines d'absorció. Aquest estudi ha estat fet mitjançant simulacions detallades resolent les equacions de Navier-Stokes sota ertes hipòtesis.- Desenvolupament d'eines de simulació numèrica pel disseny i predicció de sistemes de refrigeració per absorció, aprofitant la informació donada per models més detallats.- Desenvolupament d'eines de simulació numèrica pel disseny dels elements crítics d'intercanvi de calor i de massa de sistemes de refrigeració per absorció (absorbidor, generador, evaporador, condensador) mantenint el càlcul en un raonable temps de CPU. Aquest model recolza el mencionat en el punt anterior.- Desenvolupament de un prototipus de màquina d'absorció, refrigerada per aire, fent servir H2O-LiBr com a fluid de treball, amb les eines numèriques desenvolupades. - Contrastació experimental dels models desenvolupats.- Estudi del funcionament de la màquina d'absorció anteriorment mencionada. - Avaluació dels resultats per millorar els criteris de disseny i optimització del mateix de cara a prototipus de segona generació.Després del desenvolupament d'aquestes eines de simulació numèrica que s'han fet servir per problemes específics sortits en el procés d'estudi d'una màquina en concret, un marc de treball ha estat creat per l'estudi d'altres sistemes de refrigeració per absorció.Due to the increasing trend of the price of the energy, mainly obtained from fossil combustibles, and its also increasing use for air-conditioning in developed countries, solar cooling has been becoming more attractive from the point of view of economics and environment conservation. The final aim of this thesis is the development of numerical simulation tools for the design of absorption machines with the possibility of being driven by solar energy. Although there are available in the market absorption chillers of such characteristics for years, there is a lack in development of small capacity systems. Small capacity systems imply additional problems of design (air-cooled systems, compactness ...) that only can be afford with adequate design tools for system and components. Moreover, there is also a lack in the specialised literature in the development of adequate mathematical models for the description of the heat and mass transfer processes in absorption machines: wetted area of the heat and mass transfer surfaces, role of additives, complex geometries etc.For these reasons this work has been focused on the following detailed objectives: - Study of basic heat and mass transfer processes together with the fluid-dynamic phenomena implied in absorbers of absorption chillers. This study has beencarried out by means of detailed simulations solving the Navier-Stokes equations under certain hypotheses. - Development of numerical simulation tools for design and prediction of absorption systems, taking advantage of information given by more detailed models. - Development of numerical simulation tools for design of the heat and mass exchange components of absorption systems keeping the calculation in a reasonable CPU time. This model provides of the necessary information for the model mentioned in the previous point.- Development of a prototype of an air cooled absorption machine based on the numerical results obtained from the models.- Validation of the models developed by means of comparison of numerical results and experimental data obtained from the prototypes developed.- Study of the performance of the above mentioned absorption system. - Evaluation of the results in order to improve the design criteria for a second generation of prototypes.After the development of these numerical simulation tools and their applicationin specific problems, a framework has been created for the study of other type of absorption systems.Postprint (published version

Surface Activity of Poly(ethylene glycol)-Coated Silver Nanoparticles in the Presence of a Lipid Monolayer

We have investigated the surface activity of poly(ethylene glycol) (PEG)-coated silver nanoparticles (Ag-PEG) in the presence or absence of lipid monolayers comprised of monounsaturated dioleoylphosphocholine and dioleoylphosphoglycerol (DOPC/DOPG; 1:1 mol ratio). Dynamic measurements of surface pressure demonstrated that Ag-PEG were surface-active at the air/water interface. Surface excess concentrations suggested that at high Ag-PEG subphase concentrations, Ag-PEG assembled as densely packed monolayers in the presence and absence of a lipid monolayer. The presence of a lipid monolayer led to only a slight decrease in the excess surface concentration of Ag-PEG. Surface pressure–area isotherms showed that in the absence of lipids Ag-PEG increased the surface pressure up to 45 mN m–1 upon compression before the Ag-PEG surface layer collapsed. Our results suggest that surface activity of Ag-PEG was due to hydrophobic interactions imparted by a combination of the amphiphilic polymer coating and the hydrophobic dodecanethiol ligands bound to the Ag-PEG surface. With lipid present, Ag-PEG + lipid surface pressure–area (π–A) isotherms reflected Ag-PEG incorporation within the lipid monolayers. At high Ag-PEG concentrations, the π–A isotherms of the Ag-PEG + lipid films closely resembled that of Ag-PEG alone with a minimal contribution from the lipids present. Analysis of the subphase silver (Ag) and phosphorus (P) concentrations revealed that most of the adsorbed material remained at the air/lipid/water interface and was not forced into the aqueous subphase upon compression, confirming the presence of a composite Ag-PEG + lipid film. While interactions between “water-soluble” nanoparticles and lipids are often considered to be dominated by electrostatic interactions, these results provide further evidence that the amphiphilic character of a nanoparticle coating can also play a significant role

Internal Combustion Engines and Powertrain Systems for future Transport 2019

Internal Combustion Engines and Powertrain Systems for Future Transport 2019 provides a forum for IC engine, fuels and powertrain experts, and looks closely at developments in powertrain technology required to meet the demands of the low carbon economy and global competition in all sectors of the transportation, off-highway and stationary power industries