New insights into the surface functionalities and adsorption evolution of water molecules on silica gel surface: A study by second derivative near infrared spectroscopy

2014

AMR

Self Cite

Abstract. Silica gel, a material that is produced from the condensation polymerisation of silicic acid, contains surface silanol groups formed during the condensation. The silanol groups on the surface are mostly of free and vicinal silanol groups. These silanol groups can be modified in several different ways. Thermal treatment and hydrothermal treatment can be carried out to alter the concentration proportions between free and hydrogen bonded silanol groups on the surface. They can also be chemically treated with suitable chlorosilanes to modify the silanol groups into polar or non polar materials that can be used in separation science.This article explores the chemical nature of silanol groups on the surfaces of different materials. Near infrared reflectance spectroscopy was used as the instrumental technique in this study. The silanol groups classifications were made by analyzing the near infrared spectra obtained during the adsorption of water molecules. Absorption of the combination frequencies of water molecules in the region 5500-5000 cm -1 were used in characterizing the silanol groups on the surfaces. Second derivative technique was employed in the resolution and detailed analysis of these absorptions.The study reveals that the materials contain free, vicinal and gem silanol groups. Silica gel contains free and vicinal silanol groups, thermally treated silica gel contains fewer vicinal silanol groups compared to the base silica gel, and hydrothermally treated silica gel contains higher concentrations of vicinal silanol groups compared to the base silica gel. Furthermore, the chemically modified silica gel contains vicinal or geminal silanol groups depending on the type of functionality introduced. IntroductionSilica gel, a material used in several different fields including catalysis, separation science and polymers, is prepared by the condensation polymerisation of silicic acid. The material so produced contains free and hydrogen bonded silanol groups on the surface. The surface silanol groups are polarised due to hydrogen bondings and the exhibit three different degrees of acidity. The acidic nature of the silanol groups is responsible for the adsorption properties of silica gel. The polarised silanol groups on the surface can adsorb water molecules and other polarised molecules by hydrogen bonding.

Section: Introductionmentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

The Nature of Silanol Groups on the Surfaces of Silica, Modified Silica and Some Silica Based Materials

2014

AMR

Self Cite

“…Near infrared spectroscopy was chosen as the preferred analytical technique. Christy [5][6][7] has used near infrared spectroscopy in combination with second and fourth-derivative techniques and gravimetry to show the water adsorption properties of silicagel and silicagel related materials [5][6][7]. A similar approach was used in following the adsorption behavior of water on cellulose and CNCs surface.…”

Section: Introductionmentioning

confidence: 99%

Comparison of Water Adsorption Properties of Cellulose and Cellulose Nanocrystals Studied by Near-Infrared Spectroscopy and Gravimetry

Angkuratipakorn

Singkhonrat

2017

KEM

Self Cite

Abstract. The adsorption properties of water molecules on cellulose and cellulose nanocrystals (CNCs), isolated from defatted rice bran (DRB) by 55% sulfuric acid hydrolysis under sonication were investigated. The powdered samples of cellulose and CNCs were analysed by using near infrared spectroscopy (NIR) and gravimetry at 38% and 55% humidities. Small amounts of samples were dried under vacuum at 120 o C and the NIR spectra of the dry samples and their spectra during the adsorption water molecules were measured by using an NIR spectrometer equipped with a transflectance accessory and a DTGS detector. The quantitative adsorption of water molecules by the samples was determined by gravimetry. Second and fourth derivative profiles of the NIR spectra were used in understanding the chemistry of adsorption of water molecules and the adsorption processes by the samples.The results show that the adsorption of water molecules by the cellulose samples gives rise to three prominent peaks that can be related to the water molecules engaged in hydrogen bonding with C2, C3 and C6-OH groups on the glucose units of the cellulose polymers. Furthermore, the cellulose nanocrystals adsorb twice as much of water as the cellulose polymer. It is also clear from the results that C2 and C3-OH groups in the glucose units adsorb water molecules at a faster rate than the C6-OH group and responsible for nearly 50% of the water adsorption.

“…Since this absorption is purely from water molecules adsorbed on to the OH groups on the surface of the adsorbent, the band structure of the above peak revealed by the second and fourth derivative techniques can be directly connected to the different types of surface OH groups. With the help of near infrared reflection spectroscopy combined with second and derivative techniques, Christy has published a series of articles [7][8][9][10] regarding the nature of OH groups and their affinity to water molecules in different substances. Christy was able to distinguish the near infrared absorption bands arising from the water molecules hydrogen bonded silanol groups and water molecules hydrogen bonded to vicinal silanol groups [7].…”

Section: Introductionmentioning

confidence: 99%

“…With the help of near infrared reflection spectroscopy combined with second and derivative techniques, Christy has published a series of articles [7][8][9][10] regarding the nature of OH groups and their affinity to water molecules in different substances. Christy was able to distinguish the near infrared absorption bands arising from the water molecules hydrogen bonded silanol groups and water molecules hydrogen bonded to vicinal silanol groups [7]. Furthermore, the near infrared absorption bands arising from the water molecules hydrogen bonded to different OH groups in the carbohydrate polymers were identified [10].…”

Section: Introductionmentioning

confidence: 99%

Comparison of Water Adsorption Characteristics of Oligo and Polysaccharides of α-Glucose Studied by Near Infrared Spectroscopy

2014

AMR

Self Cite

Abstract. The adsorption properties of water molecules on oligo and polysaccharides are attributed to the OH groups of the glucose rings in the molecules. The water molecules are adsorbed onto OH groups by hydrogen bond formation. Near infrared spectroscopic and gravimetric techniques were used in analysing the adsorption characteristics of malto-oligosaccharides and some polysaccharides. Near infrared spectra of the dry oligo and polysaccharides were acquired during the adsorption of water molecules at a relative humidity of 50-55%. The amounts of water adsorbed by the samples were also recorded by an analytical balance. Second derivative techniques were used in decomposing the OH combination frequencies of the adsorbed water molecules in the region 5300-5000 cm -1 .The results indicate that the water molecules are adsorbed on to C2 and C3-OH groups at a higher rate compared to the adsorption onto C1-OH groups in the molecules. Adsorption also takes place onto the ethereal oxygen atoms in the glucose rings in malto-oligosaccarides. The gravimetric results show that the adsorption of water molecules increases with the number of glucose units in the malto-oligosaccharides except maltotriose which has the highest adsorption over a period of 75 minutes. Furthermore, the adsorption characteristics of amylose and amylopectin are similar.