Modelling of Formulated Products Used in Industrial Chemistry and Chemical Engineering Applications to Understand the Interactions between the Components and Their Non-equilibrium Properties

Abstract

Formulated products in the form of liquid solutions have many applications in our daily life. They are available as pharmaceuticals, detergent liquids, cosmetics, paints, food and lubricants. These formulations are not simple chemicals. They contain different components which come in different phases as a structure. One of the main components for formulated products is surfactants. It is important to understand how the dissolution of surfactants is governed by their structure and components. The main goal of present thesis is to provide further insight into surfactant dissolution process. To achieve this goal dissipative particle dynamics simulations (DPD) technique was used. Dissipative particle dynamics simulation is a mesoscale technique based on simulation of soft beads in which their motion is governed by Newton’s laws of motion. These beads interact with each other through sum of three effective pairwise forces which are conservative, dissipative and random forces. Within this thesis the key aspects examined were the effects of the degree of surfactant hydrophobicity, changing length of the surfactant tail and ratio of head to tail lengths, and adding additives on the dissolution process of surfactants. To explore the effect of changing hydrophobicity the coefficient of the interaction potential in DPD was changed from 126 to 161.To examine the effect of changing the surfactant molecular structure, the surfactant length ratio was changed by varying the tail beads from 3 to 7 beads and the head beads from 2 to 4 beads. Mixtures of 4A2BC and 3A2BC systems were examined and additives of oil beads varied from 1-4 beads for the 3A2BC system were examined. By changing the degree of surfactant hydrophobicity or hydrophilicity, it was found that the dissolution of surfactant system with higher hydrophobicity produces micelles which are more elongated and worm-like in nature. It was also found that it takes longer after the initial lamellar break-up for it to dissolve across the box to an even concentration. The dissolution process is defined as a two-stage process where the first part is transformation from lamellar phase to concentrated micelles and the second part is diffusion of concentrated micelles. It was found that surfactants with longer hydrophobic tails take longer for the lamellar phase to break down and to dissolve fully into the box. The effect of changing the molecular structure of the surfactant was examined. It was found that as the proportion of the hydrophobic part increased, the dissolution process gets slower but increasing the hydrophilic part makes the dissolution process faster. The effect of adding more on the hydrophobic part was found to be larger than effect of adding more of the hydrophilic part. It was also found that the diffusion parameters could be obtained by knowing micelle size and when the micelle size gets larger the dissolution process become slower. By having mixtures of surfactants or adding additives to the surfactants the link between the micelle size and diffusion parameters could be broken. For example, adding additives like oil makes the surfactant system act like longer surfactant chain systems. The kinetics of the early stages of the dissolution process of surfactant lamellar phases were studied in this thesis. It was found that the dissolution process starts after a critical time when some molecules from the first lamellar layer start to move out to the water by forming a micellar shape. This movement repeats until the whole lamellar layer dissolves into micelles and micelles diffuse into the box. The shorter the tail of the surfactant system, the faster the dissolution process happened to be and the longer the tail of the surfactant the more delay occurred until the dissolution process started.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Thomas Rodgers (Supervisor) & Andrew Masters (Supervisor)